Properties of Sound Waves

Sound waves travel at about 340 m/s in the air – much slower than the speed of light. This explains why you almost always see the flash of lightning before hearing the crash of the thunder.

Sound is caused by vibrations of the front of a violin or a cello, or of the column of air inside a trumpet. In the case of a loudspeaker you can see that the cone of the loudspeaker moves in and out and changes the pressure in the air in front of it. The sound travels as longitudinal waves.

longitudinal-waves-rarefaction

The compressions and rarefactions of sound waves result in small differences in air pressure.

Sound waves travel faster through liquids than through air. Sound travels fastest through solids. Th is is because particles are linked most strongly in solids. Note, however, that sound must have a medium through which to travel. Unlike electromagnetic waves, sound will not travel through a vacuum.

High-pitch sounds have a high frequency. Examples of high-pitch sounds include birdsong, and all the sounds that you hear from someone else’s personal player when they have set the volume too high. low-pitch sounds have a low frequency. Examples of low-pitch sounds include the horn of a large ship and the bass guitar.

The human ear can detect sounds with pitches ranging from 20 Hz to 20 000 Hz. Sound with frequencies above this range is known as ultrasound. Ultrasound is used by bats for navigation and by doctors for looking at unborn babies.

Sound waves

The ear is far more easily damaged than most people realise, and care needs to be taken both with the volume of sound and the length of time that the ear is exposed to it. The damage is cumulative, and so you don’t notice it at first. Many older rock stars have serious hearing problems, and younger ones often wear ear plugs. Loud sounds have high amplitude whereas quiet sounds have low amplitude. The unit in which we measure the loudness of sounds is the decibel (dB). Decibels are used to measure various electrical quantities as well but when they are used for sound. 0 dB is defined to be the quietest sound that can be heard. This then makes a quiet room at night about 40 dB; a noisy classroom is 60 dB; the sound 1 m from a vacuum cleaner is 80 dB; a loud disco could be 100 dB, which would be illegal in many countries; sounds of 120 dB are extremely painful; and windows break at 160dB.

The ears start to be damaged at 85 dB, a level that some personal players can reach without difficulty. Sound waves can be displayed on an oscilloscope by using a microphone. This produces a voltage-time graph on the oscilloscope.