86. Appendix 1 Sound System Parameters. Copyright Notice. This work is copyright Record-Producer.com

Transcription

1 Volume 1

2 Contents 3. Microphone Technology 19. The Use of Microphones 35. Loudspeaker Drive Units 42. Loudspeaker Systems 51. Analog Recording 64. Digital Audio 75. Digital Audio Tape Recording 86. Appendix 1 Sound System Parameters Copyright Notice This work is copyright Record-Producer.com You are licensed to make as many copies as you reasonably require for your own personal use.

3 Chapter 1: Microphone Technology The microphone is the front-end of almost all sound engineering activities and, as the interface between real acoustic sound travelling in air and the sound engineering medium of electronics, receives an immense amount of attention. Sometimes one could think that the status of the microphone has been raised to almost mythological proportions. It is useful therefore to put things in their proper perspective: there are a great many microphones available that are of professional quality. Almost any of them can be used in a wide variety of situations to record or broadcast sound to a professional standard. Of course different makes and types of microphones sound different to each other, but the differences don't make or break the end product, at least as far as the listener is concerned. Now, if you want to talk about something that really will make or break the end product, that is how microphones are used. Two sound engineers using the same microphones will instinctively position and direct them differently and there can be a massive difference in sound quality. Give these two engineers other mics, whose characteristics they are familiar with, and the two sounds achieved will be identifiable according to engineer, and not so much to according to microphone type. There are two ways we can consider microphones, by construction and by directional properties. Let's look at the different ways a microphone can be made, to start off with. Microphone Construction There are basically three types of microphone in common use: piezoelectric, dynamic and capacitor. The piezoelectric mic, it has to be said, has evolved into a very specialized animal, but it is still commonly found under the bridge of an electro-acoustic guitar so it is worth knowing about. Piezoelectric The piezoelectric effect is where certain crystalline and ceramic materials have the property of generating an electric current when pressure or a bending force is applied. This makes them sensitive to acoustic vibrations and they can produce a voltage in response to sound. Piezo mics (or

4 transducers as they may be called - a transducer is any device that converts one form of energy to another) are high impedance. This means that they can produce voltage but very little current. To compensate for this, a preamplifier has to be placed very close to the transducer. This will usually be inside the body of the electro-acoustic guitar. The preamp will run for ages on a 9 volt alkaline battery, but it is worth remembering that if an electro-acoustic guitar, or other instrument with a piezo transducer, sounds distorted, it is almost certainly the battery that needs replacing, perhaps after a year or more of service. Dynamic This is dynamic as in dynamo. The dynamo is a device for converting rotational motion into an electric current and consists of a coil of wire that rotates inside the field of a magnet. Re-configure these components and you have a coil of wire attached to a thin, lightweight diaphragm that vibrates in response to sound. The coil in turn vibrates within the field of the magnet and a signal is generated in proportion to the acoustic vibration the mic receives. The dynamic mic is also sometimes known as the moving coil mic, since it is always the coil that moves, not the magnet - even though that would be possible. The dynamic mic produces a signal that is healthy in both voltage and current. Remember that it is possible to exchange voltage for current, and vice versa, using a transformer. All professional dynamic mics incorporate a transformer that gives them an output impedance of somewhere around 200 ohms. This is a fairly low output impedance that can drive a cable of 100 meters or perhaps even more with little loss of high frequency signal (the resistance of a cable attenuates all frequencies equally, the capacitance of a cable provides a path between signal conductor and earth conductor through which high frequencies can leak ). It is not necessary therefore to have a preamplifier close to the microphone, neither does the mic need any power to operate. Examples of dynamic mics are the famous Shure SM58 and the Electrovoice RE20. The characteristics of the dynamic mic are primarily determined by the weight of the coil slowing down the response of the diaphragm. The sound can be good, particularly on drums, but it is not as crisp and clear as it would have to be to capture delicate sounds with complete accuracy. Dynamic microphones have always been noted for providing good value

5 for money, but other types are now starting to challenge them on these grounds. Ribbon Mic There is a variation of the dynamic mic known as the ribbon microphone. In place of the diaphragm and coil there is a thin corrugated metal ribbon. The ribbon is located in the field of a magnet. <img src="/graphics/coles4038.jpeg" border=0 width=69 height=114 align=right hspace=5 vspace=5 alt="">when the ribbon vibrates in response to sound it acts as a coil, albeit a coil with only one turn. Since the ribbon is very light, it has a much clearer sound than the conventional dynamic, and it is reasonable to say that many engineers could identify the sound of a ribbon mic without hesitation. If the ribbon has a problem, it is that the output of the single-turn coil is very low. The ribbon does however also have a low impedance and provides a current which the integral transformer can step up so that the voltage output of a modern ribbon mic can be comparable with a conventional dynamic. Examples of ribbon mics are the Coles 4038 and Beyerdynamic M130. Capacitor The capacitor mic, formerly known as the condenser mic, works in a completely different way to the dynamic. Here, the diaphragm is paralleled by a backplate. Together they form the plates of a capacitor. A capacitor, of any type, works by storing electrical charge. Electrical charge can be thought of as quantity of electrons (or the quantity of electrons that normally would be present, but aren't). The greater the disparity in number of electrons present i.e. the amount of charge the higher will be the voltage across the terminals of the capacitor. There is the equation:

6 Q = C x V or: charge = capacitance x voltage Note that charge is abbreviated as Q, because C is already taken by capacitance. Putting this another way round: V = Q/C or: voltage = charge / capacitance Now the tricky part: capacitance varies according to the distance between the plates of the capacitor. The charge, as long as it is either continuously topped up or not allowed to leak away, stays constant. Therefore as the distance between the plates is changed by the action of acoustic vibration, the capacitance will change and so must the voltage between the plates. Tap off this voltage and you have a signal that represents the sound hitting the diaphragm of the mic. Sennheiser MKH 40 The great advantage of the capacitor mic is that the diaphragm is unburdened by a coil of any sort. It is light and very responsive to the most delicate sound. The capacitor mic is therefore much more accurate and faithful to the original sound than the dynamic. Of course there is a

7 downside too. This is that the impedance of the capsule (the part of any mic that collects the sound) is very high. Not just high - very high. It also requires continually topping up with charge to replace that which naturally leaks away to the atmosphere. A capacitor mic therefore needs power for these two reasons: firstly to power an integral amplifier, and secondly to charge the diaphragm and backplate. Old capacitor mics used to have bulky and inconvenient power supplies. These mics are still in widespread use so you would expect to come across them from time to time. Modern capacitor mics use phantom power. Phantom power places +48 V on both of the signal carrying conductors of the microphone cable actually within the mixing console or remote preamplifier, and 0 V on the earth conductor. So, simply by connecting a normal mic cable, phantom power is connected automatically. That's why it is called phantom because you don't see it! In practice this is no inconvenience at all. You have to remember to switch in on at the mixing console but that's pretty much all there is to it. Dynamic mics of professional quality are not bothered by the presence of phantom power in any way, One operational point that is important however is that the fader must be all the way down when a mic is connected to an input providing phantom power, or when phantom power is switched on. Otherwise a sharp crack of speaker-blowing proportions is produced. A capacitor microphone often incorporates a switched -10 db or -20 db pad, which is an attenuator placed between the capsule and the amplifier to prevent clipping on loud signals. Electret The electret mic is a form of capacitor microphone. However the charge is permanently locked into the diaphragm and backplate, just as magnetic energy is locked into a magnet. Not all materials are suited to forming electrets, so it is usually considered that the compromises involved in manufacture compromise sound quality. However, it has to be said that there are some very good electret mics available, most of which are backelectrets, meaning that only the backplate of the capacitor is an electret therefore the diaphragm can be made of any suitable material. Electret mics do still need power for the internal amplifier. However, this can take the form of a small internal battery, which is sometimes convenient.

8 Electret mics that have the facility for battery power can also usually be phantom powered, in case the battery runs down or isn t fitted.

9 Directional Characteristics The directional characteristics of microphones can be described in terms of a family of polar patterns. The polar pattern is a graph showing the sensitivity in a full 360 degree circle around the mic. I say a family of polar patterns but it really is a spectrum with omnidirectional at one extreme and figure-of-eight at the other. Cardioid and hypercardioid are simply convenient way points. To explain these patterns further, fairly obviously an omnidirectional mic is equally sensitive all round. A cardioid is slightly less obvious. The cardioid is most sensitive at the front, but is only 6 db down in response at an angle of 90 degrees. In fact it is only insensitive right at the back. It is not at all correct, as commonly happens, to call this a unidirectional microphone. The hypercardioid is a more tightly focussed pattern than the cardioid, at the expense of a slight rear sensitivity, known as a lobe in the response. The figure-of-eight is equally sensitive at front and back,

10 the only difference being that the rear produces an inverted signal, 180 degrees out of phase with the signal from the front. All of this is nice in theory, but is almost never borne out in practice. Take a nominally cardioid mic for example. It may be an almost perfect cardioid at mid frequencies, but at low frequencies the pattern will spread out into omni. At high frequencies the pattern will tighten into hypercardioid. The significant knock-on effect of this is that the frequency response off-axis in other words any direction but head on is never flat. In fact the off-axis response of most microphones is nothing short of terrible and the best you can hope for is a smooth roll-off of response from LF to HF. Often though it is very lumpy indeed. We will see how this affects the use of microphones at another time. Omnidirectional Looking at directional characteristics from a more academic standpoint, the omnidirectional microphone is sensitive to the pressure of the sound wave. The diaphragm is completely enclosed, apart from a tiny slowacting air-pressure equalizing vent, and the mic effectively compares the changing pressure of the outside air under the influence of the sound signal with the constant pressure within. Pressure acts equally in all directions, therefore the mic is equally sensitive in all directions, in theory as we said. In practice, at higher frequencies where the size of the mic starts to become significant in comparison with the wavelength, the diaphragm will be shielded from sound approaching from the rear and rearward HF response will drop. Figure-of-Eight At the other end of the spectrum of polar patterns the figure-of-eight microphone is sensitive to the pressure gradient of the sound wave. The diaphragm is completely open to the air at both sides. Even though it is very light and thin, there is a difference in pressure at the front and rear of the diaphragm, and the microphone is sensitive to this difference. The pressure gradient is greatest for sound arriving directly from the front or rear, and lessens as the sound source moves round to the side. When the sound source is exactly at the side of the diaphragm it produces equal pressure at front and back, therefore there is no pressure gradient and the microphone produces no output. Therefore the figure-of-eight microphone is not sensitive at the sides. (You could also imagine that a

11 sound wave would find it hard to push the diaphragm sideways sometimes the intuitive explanation is as meaningful as the scientific one). All directional microphones exhibit a phenomenon known as the proximity effect or bass tip-up. The explanation for this is sufficiently complicated to fall outside of the required knowledge of the working sound engineer. The practical consequences are that close miking results in enhanced low frequency. This produces a signal that is not accurate, but it is often thought of as being warmer than the more objectively accurate sound of an omnidirectional microphone. Cardioid and Hypercardioid To produce the in-between polar patterns one could consider the omnidirectional microphone where the diaphragm is open on one side only, and the figure-of-eight microphone where the diaphragm is completely open on both sides. Allowing partial access only to one side of the diaphragm would therefore seem to be a viable means of producing the in-between patterns, and indeed it is. A cardioid or hypercardioid mic therefore provides access to the rear of the diaphragm through a carefully designed acoustic labyrinth. Unfortunately the effect of the acoustic labyrinth is difficult to equalize for all frequencies, therefore one would expect the polar response of cardioid and hypercardioid microphones to be inferior to that of omnidirectional and figure-of-eight mics.

12 Multipattern Microphones There are many microphones available that can produce a selection of polar patterns. This is achieved by mounting two diaphragms back-toback with a single central backplate. By varying the relative polarization of the diaphragms and backplate, any of the four main polar patterns can be created. It is often thought that the best and most accurate microphones are the true omnidirectional and the true figure-of-eight, and that mimicking these patterns with a multipattern mic is less then optimal. Nevertheless, in practice multipattern mics are so versatile that they are commonly the mic of first choice for many engineers. AKG C414

13 Special Microphone Types Stereo Microphone Two capsules may be combined into a single housing so that one mic can capture both left and right sides of the sound field. This is much more convenient than setting two mics on a stereo bar, but obviously less flexible. Some stereo mics use the MS principle where one cardioid capsule (M) captures the full width of the sound stage while the other figure-of-eight capsule (S) captures the side-to-side differences. The MS output can be processed to give conventional left and right signals. Neumann stereo microphones Interference Tube Microphone This is usually known as a shotgun or rifle mic because of its similarity in appearance to a gun barrel. The slots in the barrel allow off-axis sound to cancel giving a highly directional response. The longer the mic, the more directional it is. The sound quality of these microphones is inferior to normal mics so they are only used out of necessity. Sennheiser interference tube microphone

14 A close relation of the interference tube microphone is the parabolic reflector mic. This looks like a satellite dish antenna and is used for recording wildlife noises, and at sports events to capture comments from the pitch. Boundary Effect Microphone The original boundary effect microphone was the Crown PZM (Pressure Zone Microphone) so the boundary effect microphone is often referred to generically as the PZM. In this mic, the capsule is mounted close to a flat metal plate, or inset into a wooden or metal plate. Instead of mounting it on a stand, it is taped to a flat surface. One of the main problems in the use of microphones is reflections from nearby flat surfaces entering the mic. By mounting the capsule within around 7 mm from the surface, these reflections add to the signal in phase rather than interfering with it. The characteristic sound of the boundary effect microphone is therefore very clear (as long as there are no other nearby reflecting surfaces). It can be used for many types of recording, and can also be seen in police interview rooms where obviously a clear sound has to be captured for the interview recording. The polar response is hemispherical. Crown PZM microphone Miniature Microphone This is sometimes known as a tie-clip mic, although it is rarely ever clipped to the tie these days. This type of mic is usually of the electret design, which lends itself to very compact dimensions, and is almost always omnidirectional. Miniature microphones are used in television and in theater, where there is a requirement for microphones to be unobtrusive. Since the diaphragm is small and not in contact with many air molecules, the random vibration of the molecules does not cancel out

15 as effectively as it does in a microphone with a larger diaphragm. Miniature microphones therefore have to be used close to the sound source; otherwise noise will be evident. Beyerdynamic MCE5 Vocal Microphone For popular music vocals it is common to use a large-diaphragm mic, often an old tube model. A large diaphragm mic generally has a less accurate sound than a mic with a diaphragm mm or so in diameter. The off-axis response will tend to be poor. Despite this, models such as the Neumann U87 are virtually standard in this application due to their enhanced subjective warmth and presence. Microphone Accessories First in the catalogue of microphone accessories is the mic support. These can range from table stands, short floor stands, normal boom stands, tall stands up to 4 meters for orchestral recording, fishpoles as used by video and film sound recordists, and long booms with cable operated mic positioning used in television studios. Attaching the mic to the stand is a mount that can range from a basic plastic clip, to an elastic suspension or cradle that will isolate the microphone from floor noise. The other major accessory is the windshield or pop-shield. A windshield may be made out of foam and slipped over the mic capsule, or it may look like a miniature airship covered with wind-energy dissipating material. For blizzard conditions windshield covers are available that

16 look as though they are made out of yeti fur. The pop-shield, on the other hand, is a fine mesh material stretched over a metal or plastic hoop, used to filter out the blast of air cause by a voice artist's or singer's P and B sounds.

17 Check Questions What is the piezoelectric effect? Where would you find a piezo-electric transducer? What is attached to the diaphragm of a dynamic microphone? What passive circuit component is incorporated in the output stage of all professional microphones? (Note that some microphones use an active circuit to imitate the action of this component). Describe the sound of a dynamic microphone. How does a ribbon microphone differ from an ordinary dynamic microphone? What is the old term for 'capacitor microphone'? Why does the capacitor microphone have a more accurate sound than a dynamic microphone? Why does a capacitor microphone need to be powered (two reasons)? What precaution should you take when switching on phantom power? Can dynamic microphones of professional quality be used with phantom power switched on? What is a pad? Why does an electret microphone need to be powered? Describe the actual polar response of a typical nominally omnidirectional microphone. Describe the proximity effect. What is an 'acoustic labyrinth', as applied to microphones?

18 Why does a boundary effect microphone give a clear sound? Why are large-diaphragm microphones used for popular music vocals? Describe the differences between wind shields and pop shields.

19 Chapter 2: The Use of Microphones Use of Microphones for Speech In sound engineering, as opposed to communications which will not be considered here, there are commonly considered to be three classes of sound: speech or dialogue, music and effects. Each has its own considerations and requirements regarding the use of microphones. There are a number of scenarios where speech may be recorded, broadcast or amplified: Audio book Radio presentation, interview or discussion Television presentation, interview or discussion News reporting Sports commentary Film and television drama Theatre Conference In some of these, the requirement is for speech that is as natural as possible. In an ideal world perhaps it should even sound as though a real person were in the same room. The audio book is in this category, as are many radio programs. There is a qualification however on the term natural. Sometimes what we regard as a natural sound is the sound that we expect to hear via a loudspeaker, not the real acoustic sound of the human voice. We have all been conditioned to expect a certain quality of sound from our stereos, hifis, radio and television receivers, and when we get it, it sounds natural, even if it isn t in objective terms. In the recording and most types of broadcasting of speech there are some definite requirements: No pops on P or B sounds. No breath noise or blasting Little room ambience or reverberation A pleasing tone of voice Popping and blasting can be prevented in two ways. One is to position the microphone so that it points at the mouth, but is out of the direct line of fire of the breath. So often we see microphones used actually in the

20 line of fire of the breath that it seems as though it is simply the correct way to use a microphone. It can be for public address, but it isn t for broadcasting or recording. The other way is to use a pop shield. Ideally this is an open mesh stocking-type material stretched over a metal or plastic hoop. This can be positioned between the mouth and the microphone and is surprisingly effective in absorbing potential pops and blasts. Sometimes a foam windshield of the type that slips over the end of the microphone is used for this purpose. A windshield is really what it says, and is not 100% effective for pops, although its unobtrusiveness visually has value, for example, for a radio discussion where hoop-type pop shields would mar face-to-face visual communication among the participants. The requirement for little room ambience or reverberation is handled by placing the microphone quite close to the mouth around 30 to 40 cm. If the studio is acoustically treated, this will work fine. Special acoustic tables are also available which absorb rather than reflect sound from their surface. A pleasing tone of voice? Well, first choose your voice talent. Second, it is a fact that some microphones flatter the voice. Some work particularly well for speech, and there are some classic models such as the Electrovoice RE20 that are commonly seen in this application. Generally, one would be looking for a large-diaphragm capacitor microphone, or a quality dynamic microphone for natural or pleasing speech for audio books or radio broadcasting. In television broadcasting, one essential requirement is the microphone should be out of shot or unobtrusive. The usual combination for a news anchor, for example, is to have a miniature microphone attached to the clothing in the chest area, backed up by a conventional mic on a desk stand. Often the conventional mic is held on stand-by to be brought on quickly if the miniature mic fails, as they are prone to through constant handling. Oddly enough, the use of microphones on television varies according to geography. In France for example, it is quite common for a television presenter to hand hold a microphone very close to the mouth. Even a discussion can take place with three or four people each holding a microphone. The resultant sound quality is in accordance with French subjective requirements. Radio microphones are commonly used in television to give freedom of movement and also freedom from cables on

21 the floor, leaving plenty of free space for the cameras to roll around smoothly. News Reporting For news reporting, a robust microphone perhaps a short shotgun can be used with a general-purpose foam windshield for both the reporter and interviewee, should there be one. Such a microphone is easily pointable (the reporter isn t a sound engineer) and brings home good results without any trouble. The sound quality of a news report may not be all that could be imagined, but a little bit of harshness or degradation sometimes, oddly, makes the report more authentic. Sports Commentary Sports commentary is a very particular requirement. This often takes place in a noisy environment so the microphone must be adapted to cope with this. The result is a mic that has a heavily compromised sound quality, but this has come to be accepted as the sound of sports commentary so it is now a requirement. The Coles 4104 is an example of a 1950s design that is still widely used. It is a noise-cancelling microphone that almost completely suppresses background noise, and the positioning bar on the top of the mic ensures that the commentator always holds it in the correct position (as, indeed it is always held - sports commentators often like to move around in their commentary box as they work). Film and Television Drama For film and television drama, a fishpole (or boom as it is sometimes known) topped by a shotgun or rifle mic with a cylindrical windshield is the norm. The operator can position and angle the mic to get the best quality dialogue (while monitoring on headphones), while keeping the mic and the shadow of the mic out of shot. Miniature microphones are also used in this context, often with radio transmitters. Obviously they must not be visible at all. However, concealing the mic in the costume can affect sound quality so care must be taken. Sometimes in the studio a microphone might be mounted on a large floor mounted boom that can extend over several meters (we re not in fishing

22 country anymore). In this case the boom operator has winches to point and angle the microphone. Theatre In theatre the choice is between personal miniature microphones with radio transmitters, or area miking from the front and sides of the stage. Personal microphones allow a higher sound level before feedback since they are close to the actor s mouth. For straight drama, it isn t necessary to have a high sound level in the auditorium. In fact in most theatres it is perfectly acceptable for the sound of the actors voices to be completely unamplified. However if amplification, or reinforcement, is to be used then area miking is usually sufficient. Shotgun or rifle mics are positioned at the front of the stage (an area sometimes known for traditional reason as the floats, therefore the mics are sometimes called float mics ) to create sensitive spots on stage from which the actors can easily be heard. The drawback is that there will be positions on the stage from which the actors cannot be heard. The movements of the actors have to be planned to take account of this. Conference I use this term loosely to cover everything from company boardrooms to political party conferences. You will see that there can be a vast difference in scale. In the boardroom it has become common to use gooseneck microphones or boundary effect microphones that are specifically designed for that purpose. This lies beyond what we normally consider to be sound engineering and is categorized in the specialist field of sound installation. The party conference is another matter. To achieve reasonably high sound levels the microphone has to be close to the mouth, yet the candidate for obvious reasons does not want to look like a microphone-swallowing rock star. Therefore the microphone has to be unobtrusive so that it can be placed fairly close to the mouth without drawing undue attention to itself (the cluster of broadcasters microphones in front of the lectern is another matter, but they don t have to be so close). The AKG C747 is very suitable for this application. You will have noticed that in this context microphones are often used in pairs. There are two schools of thought on this issue. One is that the microphones should point inwards from the front corners of the lectern.

23 This allows the speaker to turn his or her head and still receive adequate pickup. Unfortunately, as the head moves, both microphones can pick up the sound while the sound source the mouth is moving towards one mic and away from the other. The Doppler effect comes into play and two slightly pitch shifted signals are momentarily mixed together. It sounds neither pleasant nor natural. The alternative approach is to mount both microphones centrally and use one as a backup. The speaker will learn, through not hearing their voice coming back through the PA system, that they can only turn so far before useful pickup is lost. It is worth saying that in this situation, the person speaking must be able to hear their amplified voice at the right level. If their voice seems too loud, to them, they will instinctively back away from the mic. If they can t hear their amplified voice they will assume the system isn t working. I once saw the chairman of a large and prestigious organisation stand away from his microphone because he thought it wasn t working. It had been, and at the right level for the audience. But unfortunately, apart from the front few rows, they were unable to hear a single unamplified word he said.

24 Use of Microphones for Music The way in which microphones are used for music varies much more according to the instrument than it possibly could for speech where the source of sound is of course always the human mouth. First, some scenarios: Recording Broadcast Public address Recording studio Location recording Concert hall Amplified music venue Theatre The requirements of recording and broadcasting are very similar, except that broadcasting often works to a more stringent timescale, and in television broadcasting microphones must be invisible or at least unobtrusive. There are two golden rules: Point the microphone at the sound source from the direction of the best natural listening position. The microphone will always be closer than a natural comfortable listening distance. So, wherever you would normally choose to listen from is the right position for the microphone, except that the microphone has to be closer because it can t discriminate direct sound from reflected sound in the way the human ear/brain can. It is always a good starting point to follow these two rules, but of course it may not always be possible, practical, or a natural sound may not be wanted for whatever reason. Broadcasters, by the way, tend to place the microphone closer than recording engineers. They need to get a quick, reliable result, and a close mic position is simply safer for this purpose. Ultimate sound quality is not of such importance. The recording studio is a very comfortable environment for microphones. The engineer is able to use any microphone he or she desires and has

25 available. The mic may be old, large and ugly, cumbersome to use perhaps with an external power supply (not phantom) and pattern selector, prone to faults etc., but if it gets the right sound, then it will be used. Location recording is not quite so comfortable and you need to be sure that the microphones are reliable and easy to use, preferably without external power supplies and with a simple stand mount rather than a complicated elastic suspension. As far as comfort goes, the concert hall is a reasonably good place to record in as at least they are used to the requirements of music (the owners of many good recording venues often have higher priorities religious worship being a prominent example). There are however restrictions on the placement of microphones during a concert. Usually it is against fire regulations to have microphones among the audience, unless the mics are positioned in such a way that they don t impede egress and cables are very securely fixed. Generally therefore there will be a stereo pair of mics slung from the ceiling, supplemented by a number of mics on stage, which are closer than the engineer would probably prefer them to be under ideal circumstances. For amplified music, the problem is always in getting sufficient level without feedback. This necessitates that microphones are very much closer than the natural listening position, to the point that natural direction has very little meaning. The ultimate example would be a microphone clipped to the bridge or sound hole of a violin. It wouldn t even be possible to listen from this position. In rock music PA, microphones are used as close to the singer s lips as possible, right against the grille cloth of a guitarist s speaker cabinet and within millimetres of the heads of the drums. Primarily this is to achieve level without risk of feedback. However this has also come to be understood as the rock music sound because it is what the audience expects. In this context, the most distant mics would be the drum overhead mics, which don t need much gain anyway. For string and wind instruments there are a variety of clip-on mics available. There are also contact mics that pick up vibrations directly from the body of the instrument, although even these are not entirely immune to feedback. In theatre musicals, the best option for the lead performers is to use miniature microphones with radio transmitters. The placement of the mic is significant. The original lavalier placement, named for Mme Lavalier

26 who reportedly wore a large ruby from her neck, has long gone. The chest position is great for newsreaders but it suffers from the shadow of the chin and boominess caused by chest resonance. The best place for a miniature microphone is on a short boom extending from behind the ear. Mics and booms are available in a variety of flesh colours so they are not visible to the audience beyond the second or third row. If a boom is not considered acceptable, then the mic may protrude a short distance from above the ear, or descending from the hairline. This actually captures a very good vocal sound. It has to be tried to be believed. One of the biggest problems with miniature microphones in the theatre is that they become sweated out after a number of performances and have to be replaced. Still, no-one said that it was easy going on stage. For the orchestra in a theatre musical, clip on mics are good for string instruments. Wind instruments are generally loud enough for conventional stand mics, closely placed. So-called booth singers can use conventional mics.

27 Stereo Microphone Techniques Firstly, what is stereo? The word stereophonic in its original meaning it suggests a solid sound image and does not specify how many microphones, channels or loudspeakers are to be used. However, it has come to mean two channels and two loudspeakers using as few or as many microphones that are necessary to get a good result. When it works, you should be able to sit in an equilateral triangle with the speakers, listen to a recording of an orchestra and pinpoint where every instrument is in the sound image. (By the way, some people complain that stereophonic, as a word, combines both Greek and Latin roots. Just as well perhaps, because if it had been exclusively Latin it would have been crassophonic!) When recording a group of instruments or singers, it is possible to use just two or three microphones to pick up the entire ensemble in stereo, and the results can be very satisfying. There are a number of techniques: Coincident crossed pair Near-coincident crossed pair ORTF Mercury Living Presence Decca Tree Spaced omni MS Binaural The coincident crossed pair technique traditionally uses two figure-ofeight microphones angled at 90 degrees pointing to the left and right of the sound stage (and, due to the rear pickup of the figure-of-eight mic, to the left and right of the area where the audience would be also). More practically, two cardioid microphones can be used. They would be angled at 120 degrees were it not for the drop off in high frequency response at this angle in most mics. A 110-degree angle of separation is a reasonable compromise. This system was originally proposed in the 1930s and mathematically inclined audio engineers will claim that this gives perfect reproduction of the original sound field from a standard pair of stereo loudspeakers. However perfect the mathematics look on paper, the results do not bear out the theory. The sound can be good, and you can with

28 effort tell where the instruments are supposed to be in the sound image. The problem is that you just don t feel like you are in the concert hall, or wherever the recording was made. The fact that human beings do not have coincident ears might have something to do with it. Coincident crossed pair Separating the mics by around 10 cm tears the theory into shreds, but it sounds a whole lot better. Near-coincident crossed pair The ORTF system, named for the Office de Radiodiffusion Television Francaise, uses two cardioid microphones spaced at 17 cm angled outwards at 110 degrees, and is simply an extended near-coincident crossed pair.

29 The redeeming feature of the coincident crossed pair is that you can mix the left and right signals into mono and it still sounds fine. Mono, but fine. We call this mono compatibility and it is important in many situations the majority of radio and television listeners still only have one speaker. The further apart the microphones are spaced, the worse the mono compatibility, although near-coincident and ORTF systems are still usable. ORTF Mercury Living Presence was one of the early stereo techniques of the 1950s, used for classical music recordings on the Mercury label. If you imagine trying to figure out how to make a stereo recording when there was no-one around to tell you how to do it, you might work out that one microphone pointing left, another pointing center and a third pointing right might be the way to do it. Record each to its own track on 35mm magnetic film, as used in cinema audio, and there you have it! Nominally omnidirectional microphones were used, but of course the early omni mics did become directional at higher frequencies. Later recordings were made to two-track stereo. These recordings stand up remarkable well today. They may have a little noise and distortion, but the sound is wonderfully clear and alive. The same can be said of the Decca tree, used by the Decca record company. This is not dissimilar from the Mercury Living Presence system but baffles were used between the microphones in some instances to create separation, and additional microphones might be used where necessary, positioned towards the sides of the orchestra.

30 Decca tree Another obvious means of deploying microphones in the early days of stereo was to place three microphones spaced apart at the front of the orchestra, much more distant from each other than in the above systems. If only two microphones are used spaced apart by perhaps as much as two meters or more, what happens on playback is that the sound seems to cluster around the loudspeakers and there is a hole in the middle of the sound image. To prevent this, a centre microphone can be mixed in at a lower level so that the hole is filled. There is no theory on earth to explain why this works - being so dissimilar to the human hearing system - but it can work very well. The main drawback is that a recording made in such a way sounds terrible when played in mono. The MS system, as explained previously, uses a cardioid microphone to pick up an all-round mono signal, and a figure-of-eight mic to pick up the difference between left and right in the sound field. The M and S signals can be combined without too much difficulty to provide conventional left and right signals. This is of practical benefit when it is necessary to record a single performer in stereo. With a coincident crossed pair, one microphone would be pointing to the left of the performer, the other would be pointing to the right. It just seems wrong not to point a microphone directly at the performer, and with the MS system you do, getting the best possible sound quality from the mic. It is sometimes proposed as an advantage of MS than it is possible to control the width of the stereo image by adjusting the level of the S signal. This is exactly the same as adjusting the width by turning the mixing console s panpots for

31 the left and right signals closer to the centre. Therefore it is in reality no advantage at all. Binaural stereo attempts to mimic the human hearing system with a dummy head (sometimes face, shoulders and chest too) with two omnidirectional microphones placed in artificial ears just like a real human head. It works well, but only on headphones. A binaural recording played on speakers doesn t work because the two channels mix on their way to the listener, spoiling the effect. There have been a number of systems attempting to make binaural recordings work on loudspeakers but none has become popular. In addition to the stereo miking system, it is common to mic up every section of an orchestra, whether it is a classical orchestra, film music, or the backing for a popular music track. Normally the stereo mic system, crossed pair or whatever, is considered the main source of signal, with the other microphones used to compensate for the distance to the rear of the orchestra, and to add just a little presence to instruments where appropriate. Sectional mics shouldn t be used to compensate for poor balance due to the conductor or arranger. Sometimes however classical composers don t get the balance quite right and it is not acceptable to change the orchestration. A little technical help is therefore called for. Instruments We come back to the two golden rules of microphone placement, as above. It is worth looking at some specific examples: Saxophone There are two fairly obvious ways a saxophone can be close miked. One is close to the mouthpiece, another is close to the bell. The difference in sound quality is tremendous. The same applies to all close miking. Small changes in microphone position can affect the sound quality enormously. There are many books and texts that claim to tell you how and where to position microphones for all manner of instruments, but the key is to experiment and find the best position for the instrument and player you have in front of you. Experience, not book learning, leads to success. Of the two saxophone close miking positions, neither will capture the natural sound of the instrument, if that s what you want. Close mic positions almost never do. If you move the mic further away, up to

32 around a meter, you will be able to capture the sound of the whole of the instrument, mouthpiece, bell, the metal of the instrument, and the holes that are covered and uncovered during the normal course of playing. Also as you move away you will capture more room ambience, and that is a compromise that has to be struck. Natural sound against room ambience. It s subjective. Piano Specifically the grand piano it is common to place the microphone (or microphones) pointing directly at the strings. Oddly enough no-one ever listens from this position and it doesn t really capture a natural sound, but it might be the sound you want. The closer the microphones are to the higher strings, the brighter the sound will be. You can position the microphones all the way at the bass end of the instrument, spaced apart by maybe 30 cm, and a rich full sound will be captured. Move the microphones below the edge of the case and angle them so that they pick up reflected sound from the lid and a more natural sound will be discovered. You can even place a microphone under a grand piano to capture the vibration of the soundboard. It can even sound quite good, but listen out for noise from the foot pedals. Drums The conventional setup is one mic per drum, a mic for the hihat perhaps, and two overhead mics for the cymbals. Recording drums is an art form and experience is by far the best guide. There are some points to bear in mind: You can t get a good recording of a poor kit, particularly cymbals, or a kit that isn t well set up. It is often necessary to damp the drums by taping material to the edge of the drum head to get a shorter, more controlled sound. The mics have to be placed where the drummer won t hit them, or the stands. Dynamic mics generally sound better for drums, capacitor mics for cymbals.

33 The kick drum should have its front head removed, or there should be a large hole cut out so that a damping blanket can be placed inside. Otherwise it will sound more like a military bass drum than the dull thud that we are used to. The choice of beater hard or soft - is important, as is the position of the kick drum mic either just outside, or some distance inside the drum. The snares on the underside of the snare drum may rattle when other drums are being played. Careful adjustment of the tension of the snares is necessary, and perhaps even a little damping. Microphones should be spaced as far apart from each other as possible and directed away from other drums. Every little bit helps as the combination of two mics picking up the same drum from different distances leads to cancellation of groups of frequencies. The brute force technique is to use a noise gate on every microphone channel, and this is commonly done. Noise gates will be covered later. Perhaps this is a brief introduction to the use of microphones, but it s a start. And to round off I ll give away the secret of getting good sound from your microphones: Listen!

34 Check Questions What problem is commonly found in live sports commentary? What does a fishpole operator concentrate on while working? In theater, what is 'area miking'? How is feedback avoided in live sound (the simplest technique)? Why must the speaker at a conference hear his or her own amplified voice at the right level? Write down, copy if you wish, the two golden rules for microphone positioning Why do microphones have to be placed closer than a natural listening position? Where are personal mics worn in the theater? What is stereo? Describe the coincident crossed pair. What is the benefit of separating the microphones (relate this to the human hearing system)? What is the value of mono compatibility? Why is it desirable to mic up every section of an orchestra independently? Pick an instrument other than those mentioned in the text. Describe the effect of two alternative close miking positions. When you look at a grand piano, performed solo, on stage, does the pianist sit on the left or the right? Why? Why do drums often need to be damped?

35 Chapter 3: Loudspeaker Drive Units Loudspeakers are without doubt the most inadequate component of the audio signal chain. Everything else, even the microphone, is as close to the capabilities of human hearing as makes hardly any difference at all. However, amplify the signal and convert it back into sound and you will know without any hesitation whatsoever that you are listening to a loudspeaker, not a natural sound source. Loudspeakers can be categorized by method of operation and by function: Method of operation: Moving coil Electrostatic Direct radiator Horn Function: Domestic Hi-fi Studio PA In this context we will use PA to mean concert public address rather than announcement systems that are beyond the scope of this text. The moving coil loudspeaker, or I should say drive unit as this is only one component of the complete system, is the original and still most widely used method of converting an electric signal to sound. The components consist of a magnet, a coil of wire (sometimes called the voice coil ) positioned within the field of the magnet and a diaphragm that pushes against the air. When a signal is passed through the coil, it creates a magnetic field that interacts with the field of the permanent magnet causing motion in the coil and in turn the diaphragm. It is probably fair to say that % of the loudspeakers you will ever come across use moving coil drive units. The electrostatic loudspeaker (and this time it is a loudspeaker rather than just a drive unit) uses electrostatic attraction rather than magnetism. The electrostatic loudspeaker has the most natural sound quality, but is not

36 capable of high sound levels. Hence it is rarely used in professional audio outside of, occasionally, classical music recording. A moving coil drive unit can be constructed as either a direct radiator or a horn. In a director radiator drive unit, the diaphragm pushes directly against the air. This is not very efficient as the diaphragm and the air have differing acoustic impedance, which creates a barrier for the sound to cross. A horn makes the transition from vibration in the diaphragm to vibration in the open air more gradual, therefore it is more efficient, and for a given input power the horn will be louder. Let's look at these in more detail: Moving Coil Drive Unit Perhaps the best place to start is a 200 mm drive unit intended for low and mid frequency reproduction. This isn't the biggest drive unit available, so why are larger drive units ever necessary? The answer is to achieve a higher sound level. A 200 mm drive unit only pushes against so much air. Increase the diameter to 300 mm or 375 mm and many more air molecules feel the impact. The next question would be, why are 300