There are many methods of finding the speed of sound. The one described below can be performed in a school laboratory. For understanding this method considers Fig. 13. 10 where a column of air is shown enclosed in a vertical glass tube. The glass tube is connected to a water reservoir by a rubber tube. By moving the reservoir up and down the length of the air column can be changed.

Hold a vibrating tuning fork horizontally over the open end of the glass tube. The tuning fork forces the air column in the tube to vibrate. Starting from a small length of the air column increases its length. For a certain length of the air column the sound of the tuning fork becomes loudest. At this position the air column resonates with the tuning fork frequency. In other words the tuning fork and the air column vibrate with the same frequency.

This phenomenon, in which there is remarkable increase in the amplitude and hence the loudness of the sound when the frequency of air column becomes equal to that of the tuning fork, is called resonance. When a tuning fork vibrates near the edge of the tube, it starts sending the compressional waves. These waves, when strike the water surface, are reflected. These reflected waves and incoming waves produce stationary waves in a state of resonance.

There will be an antinode at the open end of the tube and a node at the other end. Since the distance between a node and the next antinode is equal to one quarter of wavelength, hence, we can write the following equation for the stationary waves, which appears in the vibrating air column.

Where  is wavelength of the stationary waves and is equal to four time of the measure of length of the air column in the first position of the resonance.

 Do you know? Autovehicles, aeroplanes, heticopters, trains, heavy machinery, loud speakers are the major sources of noise pollutions.

 Interesting information Trees and different appliances are also used to reduce the noise.

13.6 Audible Frequency Range

We know that sound is produced by a vibrating body. A simple pendulum also vibrates but it does not produce any sound. The reason is that its vibrations are very slow. A human ear can her a sound only if its frequency lies between 20Hz and 20,000Hz. Sounds of frequency beyond 20,000Hz are inaudible because the eardrum cannot vibrate so rapidly. The audible frequency range differs a little for different persons. The above mentioned audible frequency range is only an average. It also decreases with age. Young children can hear sounds of 20,000Hz frequency, but old people cannot hear sounds even above 15,000Hz frequency.

 For your information Sounds rated from 90dB to 120dB are annoying and affect nervous system. Sounds of more than 120 dB cause hearing loss and headache.