Audio Fundamentals Part 1

Transcription

1 Audio Fundamentals Part 1 Objectives Upon completion of Audio Fundamentals Part 1, you will be able to: Explain what audio is, Describe Acoustic Sound Waves as well as Rarefaction and Compression, Define Frequency, Amplitude and db. Compare and Contrast Analog and Digital Audio. What is Audio Simply speaking, audio a very fast moving pressure wave, changing in height (Amplitude or volume) and width (Wavelength or frequency). This wave is generated by the vibration of an object; for example, speakers in a sound system or simply a pencil dropping on the floor. You ear drum "catches" the sound wave and vibrates because of the pressure waves. The height of the sound wave is called Amplitude, represented by the letter A in the image. Amplitude determines the volume. The further away the wave goes from the Zero line, represented by the horizontal line in the image, the louder the sound will be. 1 Audio Fundamentals Part 1 Christie University

2 The width of the sound wave is called wavelength, represented by the letter B in the image. Wavelength determines the frequency. The audible range of human hearing is 20 to 20,000 Hz or 20 khz. This graphic shows you a frequency of 1Hz, or one cycle/ second. The longer the wavelength, the lower the frequency. The shorter the wavelength, the higher the frequency. Audio is two things. 1. A physical phenomenon and 2. A perceptual phenomenon Physical Phenomenon Audio has an acoustic element: which is sound waves moving through physical medium this includes things like a solid, liquid, gas or plasma. Audio also has an electronic or optical component. There are electrons and photons moving through physical medium such as copper, fiber or air. Electronic and optical audio signals can be transmitted in a range of formats such as analog or digital. Perceptual Phenomenon Psycho-acoustic effects of sound waves reaching a specific listener are defined by their human auditory system. This is experiential and highly subjective. The perception of sound waves can vary greatly by listener. For example, put on a piece of music that you like and see how your grandparents or great grandparents feel about it. 2 Audio Fundamentals Part 1 Christie University

3 The ability of listeners to perceive sound waves is both amplitude and bandwidth limited. You have a limited range of frequencies you can hear and a limited level that you can hear. This varies by a lot of factors, such as age. Acoustic Sound Waves Vibration from a physical source causes sound waves to propagate through a medium. For example when you drop a pebble into a pond and watch the ripples in the water. If you were to take a cross section of the ripples in the water - they would look similar to the graph, which is a sine wave decreasing in amplitude. In the first part of the course we mentioned that sound was created by a vibrating object. The vibrations of the object set particles in the medium in vibrational motion. For a sound wave traveling through air, the vibrations of the particles are best described as longitudinal. Longitudinal waves are waves in the displacement of the medium is the same direction as, or opposite direction to, the direction of travel of the wave. A longitudinal wave can be created in a slinky if the slinky is stretched out in a horizontal direction and the first coils of the slinky are vibrated horizontally. In such a case, each individual coil of the medium is set into vibrational motion in directions parallel to the direction that the energy is transported. 3 Audio Fundamentals Part 1 Christie University

4 A vibrating tuning fork is also capable of creating a longitudinal wave. As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally to the right and the backward retraction of the tine creates a lowpressure area allowing the air particles to move back to the left. Because of the longitudinal motion of the air particles, there are regions in the air where the air particles are compressed together and other regions where the air particles are spread apart. These regions are known as compressions and rarefactions respectively. The compressions are regions of high air pressure while the rarefactions are regions of low air pressure. The diagram depicts a sound wave created by a tuning fork and propagated through the air in an open tube. The compressions and rarefactions are labeled. To put this into perspective of a speaker, this image shows the rarefaction and compression of a sound wave from a speaker. 4 Audio Fundamentals Part 1 Christie University

5 In other words, sound waves, propagating through a medium, create alternating bands of high and low pressure, known as rarefaction and compression, at a given frequency and amplitude. Frequency Frequency of waves is measured in Hz (Hertz), which equals Cycles/ Second. Typically accepted bandwidth of human auditory system is about 20Hz - 20kHz. 20Hz is a very low deep sub bass. 20kHz is a very high frequency that a lot of people can t hear and audio systems have a hard time to reproduce. A high quality audio system can produce the harmonics found in the 20kHz range. This can induce an alpha wave state in the brain and a lot of people have an emotional response to that and recognize whether a song is, for example recorded or live. Frequencies below 20Hz are known as infrasonic, above this band known as ultrasonic. Lower frequencies have longer wavelengths, represented by the red wave in this image. Higher frequencies have shorter wavelengths, represented by the other colors. 5 Audio Fundamentals Part 1 Christie University

6 Amplitude of Acoustic Sound Amplitude of acoustic sound waves is measured in db SPL (decibels Sound Pressure Level). One of the most misunderstood concepts in audio is the decibel. A lot people will say something like "that concert was 130 decibels" or "that jackhammer down the street is 120 decibels" or "Calibrate your cinema audio system at 85 decibels". db (decibel) is a logarithmic ratio, between two values. It is not an absolute value, unless tied to a reference level. db SPL is tied to a specific reference level for sound pressure measured in Pa (Pascals) with a typically accepted value for threshold of human hearing is: 0dB 1kHz = 20 µpa. This is the bench mark where we agree there is a standard, just like we agree on what a lumen is. Here s the complex logarithmic formula: Sound Pressure Level (SPL) This graph shows SPL on the side and frequency across the bottom -> 10Hz which is infrasonic up to 100 khz which is ultrasonic. 6 Audio Fundamentals Part 1 Christie University

7 The curve varies a lot. If you have a very low frequency sound it has to be a pretty high level for you to even know that it s there. Our ears are very sensitive to the frequencies of the human voice at or around 1 khz. Through evolution we have evolved to be very sensitive to other peoples voices as a way of communicating. This is why the center channel is the most important channel in cinema. If dialogue doesn t sound correct, people will notice. Threshold of hearing varies by frequency and individual perception. Typically accepted value for threshold of pain is: db SPL. Maximum pressure (undistorted) at 1 Atmosphere = 194 db SPL - you cannot generate anything louder than this. The exception being a nuclear explosion or a sonic boom. Every increase of 3 db SPL, equals twice the acoustic energy - For example - If you turn up the volume by 9dB that s 8x louder. Amplitude Voltage Amplitude of electronic audio signals is measured in db (decibels) tied to a reference voltage dbv (used in a lot of consumer equipment) voltage relative to 1 volt, without consideration of impedance dbu (used in professional applications) u stands for unloaded. AC voltage relative to.775v at any impedance dbm power relative to 1 millwatt, typically at 600 Ohm impedance Standard signal reference level for analog pro audio is +4dBu = 1.23V Every 6 db increase in dbv, dbu or dbm, equals twice the electrical energy Electronic/ Optical Audio Signals Acoustic sound waves can be transformed into electronic or optical audio signals and these signals can be in turn transformed back into acoustic waves, by using a transducer. A Transducer like a microphone or speaker. Microphones convert sound waves into electronic or optical audio signals Speakers convert audio signals from amplifiers into sound waves. 7 Audio Fundamentals Part 1 Christie University

8 Analog Audio Digital Audio Analog Audio Analog audio is an electronic audio signal analogous to patterns of acoustic sound waves There are five primary signal formats in widespread use: Speaker: up to 120V, unbalanced, very low impedance Professional: +4 dbu, balanced, low impedance Consumer: -10dBV unbalanced, high impedance Instrument: ~-20 dbv, typically unbalanced, very high impedance Microphone: ~-40 dbu to -60 dbu can be balanced or unbalanced, low or high impedance Digital Audio Digital audio is an electronic audio signal, which utilizes a series of digital samples, typically with 16 to 24 bit resolution and sample rates between 44.1kHz (same as a CD) and 96kHz, to very closely represent the original waveform of an analog signal. Requires A to D (Analog to Digital) conversion for storage and/ or transmission and D to A conversion for reception and/ or playback. May or may not utilize audio and/ or data compression schemes Some common signal formats currently in widespread use: Atmos Pulse Code Modulation or PCM, up to 128 channels - 64 is the base number of channels, 24 bit audio, 48 or 96 khz DCI-AES PCM, up to 16 channels, 20 or24 bit, 48 or 96 khz AES3 PCM, 2 channels in a pair, bit, khz SPDIF PCM or Dolby Digital or DTS, bandwidth varies HDMI PCM or Dolby Digital or DTS, bandwidth varies Every effort has been made to ensure the information in this document is accurate and reliable, however in some cases changes in the specifications may not be reflected in this document. Christie reserves the right to make changes to specifications at any time without notice. 8 Audio Fundamentals Part 1 Christie University training.christiedigital.com Audio Fundamentals Part1 v1.0 Sept14