SOUND: Nature of sound

Transcription

1 SOUND: Nature of sound: Sound is a mechanical wave. Mechanical waves need a material medium to propagate. The medium can be a gas, liquid or solid. They can not propagate in vacuum because vacuum is not a material medium. That is why sound can not propagate from our Moon to the Earth because there is vacuum in between. On the other hand, we can see our Moon because light ( which is an electromagnetic wave ) is able to travel also in vacuum. Sound waves travelling in gases and liquids can only be longitudinal waves. However, in solids the waves can be both longitudinal and transverse. In a longitudinal sound wave, the particles of the medium vibrate in the same direction as the direction of propagation of the wave. In a transverse sound wave, the particles of the medium vibrate perpendicularly to the direction of propagation of the wave. The waves that reach our ears in our normal air environment travel through air independently of where they were originated. Sound waves in air ( a gas ) are longitudinal waves as mentioned before. They consist of a pattern of compressions and rarefactions of air regions where air molecules vibrate in the same direction as the direction of propagation of the wave. Regions of compression exhibit higher than average air pressure. Regions of rarefaction exhibit lower than average air pressure. Longitudinal sound waves are described as pressure waves for this reason. The top parts of the diagram below schematically represents successive regions of compression and rarefaction in air when a sound wave is present. The bottom parts show a plot of the pressure ( and density ) of air as a function of the position corresponding to the top part of the diagrams.

2 Generation of sound: Sound waves are always generated when an object or medium vibrates. Examples of objects that generate familiar sounds are a drum membrane, a guitar string, and our vocal cords. The vibrating object then transmits the vibrations to the surrounding medium and in this way the wave has been generated and starts to propagate. The diagram below shows a drum with its vibrating membrane generating compressions and expansions ( rarefactions ) in air and therefore, generating sound waves. Speed of sound: The speed of sound waves is strongly dependent on the type of medium and slightly dependent on the temperature of the medium. Sound waves in air at room temperature ( around 70 o F ) travel approximately at 340 m/s. In water, sound waves travel 4 times faster than in air. In steel, they travel about 15 times faster than in air. As the temperature of air increases, so does the speed of sound but the increase is not so dramatic. Sound reflection: Sound waves reflect when they reach a wall in a way similar to how light reflects from a mirror. Clear evidence of this is the echo that is heard when somebody screams loud in front of a mountain or fall wall. Sound reflection effects are taken into consideration when theaters are designed. Sound refraction: Sound refraction also takes place when sound penetrates a second medium where the speed of propagation is different from the speed of propagation in the first medium. This is a phenomenon similar to refraction of light but it is not a phenomenon that concerns us in our daily lives because most sound phenomena involved in our day to day lives happen in the same medium ( air ). Sound diffraction: Diffraction is a wave phenomenon exhibited by all kinds of waves. It consists in the deviation of the direction of propagation of a wave from a straight line direction as it passes through openings or around objects but still propagating through the same medium. In the case of light, the phenomenon is not easily detected in our normal daily lives. In the case of sound, it is at all times being manifested. Diffraction is what allows us to hear a sound that was produced behind a wall even if we were not in the direct line of sight of the sound source. The picture below shows diffraction in action in

3 the case of surface water waves going around an object ( the waves are propagating diagonally from the top-left to the bottom-right corner of the picture. Hearing perception from the human ear: Most of the hearing perception comes from the ears. The ear shape channels the sound waves to the eardrum. The sound waves then force the eardrum into vibrational motion which is transformed at the end into electrical signals that are transmitted and interpreted by the neurological system as shown below.

4 Frequency range for human hearing: Sound waves are observed in the range from 0 to 1 GHz. Normal human hearing detects sound waves approximately in the range from 20 Hz to 20,000 Hz. Sound with frequencies below 20 Hz is called infrasound. Sound with frequencies above 20,000 Hz is called ultrasound. Humans don t hear infrasound or ultrasound but some animals do. For example, dogs hear ultrasound and elephants hear infrasound ( both also hear in the human range ). The frequency range of human hearing is reduced with aging. Ultrasound is used in autofocusing cameras where the travel time of the ultrasonic wave to and from the object is used to estimate the distance from the object as shown below. Ultrasound is also used in medicine for imaging and in oceanography to map the sea floor, to detect schools of fish, in submarines for communication purposes and many other uses. A simple program ( called sweepgen ) is available on the web and allows the user to generate sounds of different frequencies in the range from 20 Hz to 20,000 Hz. It can be used to illustrate the range of human hearing. To use it, two downloads are needed as follows: (SweepGen) ( runtimes-setup-100.zip ) When the program is run, the simplest thing to do is to do an automatic scan from 20 to 20,000 Hz so that the ear sensitivity can be tested experimentally. Sound level and intensity ranges for human hearing: Sound intensity like any other wave is measured in watts per square meter ( W/m 2 ). Due to the huge range of intensities commonly encountered, an alternative quantity called the sound level has been introduced. The sound level compares the intensity of a sound wave with an arbitrary reference intensity value. The units of sound level are decibels ( db ) and some people should be familiar with the unit because it is common in consumer electronics specifications for speakers and other devices. The diagram that follows shows how the values of the thresholds of hearing and pain depend on the frequency of sound. For example, a sound with a frequency of 1000 Hz can not be heard if its sound level is below 0 db ( intensity below W/m 2 ). Also, at this frequency, the sound feels painful for sound levels above 120 db ( intensity above 1W/m 2 ). From the same diagram, it can be seen that a sound with a frequency of 50 Hz, needs at least 50 db ( at least 10-7 W/m 2 ) to be heard. For comparison purposes, this

5 means that a sound with a frequency of 50 Hz, needs an intensity 100,000 times larger than a sound with frequency of 1000 Hz in order to be heard. Speakers and microphones: A speaker is a simple device used to convert an electrical signal into a sound wave. A microphone does the opposite work that a speaker does. The microphone converts a sound wave into an electrical signal. It is interesting to know that a speaker can be used as a microphone and also, a microphone can be used as a speaker. For example, an intercom unit in an apartment works as a speaker when it reproduces the voice of the caller at the entrance of the building. The same intercom unit works as a microphone when it sends the sound signal of the person inside the apartment down to the unit at the entrance of the building. The main components of a simple speaker ( or microphone ) are an elastic membrane ( like a drum membrane ), coils of thin electric wire, and a magnet. The coils can be attached to the elastic membrane and the magnet can be fixed to the structure of the speaker. When an electric signal travels through the coils of a speaker, a magnetic field is generated which interacts with the magnet and the magnetic force puts the coils ( and membrane ) into a vibrational motion that generates sound waves. In a microphone, the process is reversed with respect to the speaker. Sound waves reaching the microphone, put the elastic membrane into vibration and the vibrational motion of the coils attached to the membrane induce an electrical signal due to the presence of the magnetic field of the magnet. The physical phenomena that allows the transformation of the vibrational motion of the membrane into an electrical signal is called electromagnetic induction.