Wave, Motion and Sound - Part II

2. Sound  waves
Sound is a form of energy that travels in the form of a longitudinal wave. Sound is produced by vibration of a physical body. The air around it goes into compression and rarefaction and the sound wave thus travels from its source to farther distances. A plucked guitar string, a vibrating tuning fork, our voices, animal calls are all due to some physical objects set into vibrations. In case of human voice, there is a pair of vocal chords in our throat. When they vibrate, voice is emitted.  Voice, tone and pitch depends on the physical nature of our vocal chords.

Initially the density of air around the tuning fork is undisturbed. As the fork prongs go back and forth or oscillate, the density of the air is pushed and pulled and the air becomes compressed and rarefied alternately.  

Speed of sound depends directly on the elasticity of the medium and inversely on the density of the medium. For a gaseous medium, elasticity can be approximated to the pressure of the gas.  In air, the velocity of sound is about 340 m/s. If we change the air medium to a less denser medium like Helium gas, our voices will sound shrilly like the cartoon character Mickey mouse. On the other hand, if the medium surrounding our vocal chord is dense (say with Argon gas), then our voices will sound hoarse like the cartoon character Goofey.

The following table gives the speed of sound in different mediums at 0°C.  

The table below gives the speed of sound in air at different temperatures.

Since sound waves travel because of local air pressure differences, in addition to temperature, humidity and air pressure also changes the velocity of sound.

Reflection of sound waves
Just as light waves bounce off or reflect from a surface, sound waves too are reflected. Reflection of sound waves can be demonstrated by a simple experiment described below.

Take a bell and a hollow tube.  Let the tube be placed at an angle to a reflecting surface such as a plate. Keep a screen vertical to the plate. Ask a friend to ring the bell. Take another hollow tube on the other side of the screen and keep it near your ear. Change the angle of the second tube with respect to the plate. At a particular angle, you will be able to hear the ringing of the bell clearly. This is the angle of reflection of the sound wave. You will notice that the angle of reflection = angle of incidence. The conclusion of the experiment is that the sound waves are reflected just like the light waves.

Focussing of sound waves occur with concave surfaces just like focussing of light rays with concave mirrors. This is demonstrated by two concave surfaces arranged as shown below.

If a source of sound such as a watch is kept at one focus, the sound of the ticking of the watch is clearly heard at the other focus. Such examples of sound reflections are a source of amazement in science museums where whispering galleries surprise visitors with their own voices reflected back. Also when we are unable to hear, we cup our hands near our ears in a concave geometry, which then gives us a better hearing.

Sound reflection is also seen in echoes.  Since our ears cannot distinguish sounds heard within a gap of 1/10^th of a second. Thus an echo can be heard if the sound reflected back takes more than 0.1 second.  By taking the speed of sound to be about 340m/s at ordinary temperature and pressure, an echo can be heard if the distance of the reflecting object is more than 17 meters.

Measuring velocity of sound
A simple method of determining the velocity of sound is shown below.

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