Section 4: The Nature of Sound

Sounds All sounds are caused by something that vibrates. Your vocal cords vibrate in relation to controlled bursts of air and produce sounds to give rise to speech. Sound waves are longitudinal waves that rely on moving matter (a medium) to transmit the energy.

Moving through a Medium Sounds travel at different speeds through different mediums. Sounds move fastest through solids as the molecules are closer together. Liquids transmit sound at a slower speed than in solids. Sounds travel the slowest in gases as the molecules are most spread out. As sound is a mechanical wave, it requires a medium. Sound is not transmitted in outer space as there is not enough matter.

Hearing Damage What commonly occurs in hearing damage is that hearing has been damaged due to constant exposure to loud sounds. Damage occurs in the inner ear at the hair cells in the cochlea. Hair cells can be easily damaged or destroyed by continuous exposure to excessive volume levels. These sensitive hairs do not grow back if lost and the resulting loss of hearing is permanent.

Intensity of Sound The amount of energy of sound is measured in its intensity. Intensity is a measure of amplitude and the distance from a sound source. Intensity is measured in a unit called the decibel. The decibel scale measures sound intensity levels and can be used to prevent hearing damage. Generally prolonged exposure to sound levels above 90 dB (decibels) cause permanent hearing damage. See figure 15 on page 515. Hearing damage is slow but cumulative and permanent. Watch your ipod volume level!

Frequencies of Sound Recall that frequency is a measure of how many wavelengths pass a point per second. In sound, frequency describes the pitch of a sound. A wave with a low frequency has a low pitch like a low bass note. A wave with a high frequency has a high pitch like high treble note. Humans vary in the range of frequencies they can hear, but generally our hearing range is from 20 Hz to 20,000 Hz.

Infrasound and Ultrasound Infrasound is a range of frequencies that are below human hearing (< 20 Hz). Ultrasound is a range of sound frequencies that is above 20,000 Hz. Ultrasound is what is utilized in underwater sonar as the distance of objects is measured in the time it takes for an ultrasound echo (reflection) to return. Ultrasound is used in medicine to map structures in the body (like a fetus) and can be used in some operations to break up matter (kidney stones).

The Doppler Effect Sound is a series of compressions and rarefactions as carried in a sound wave. As objects move toward you, the pitch of the sound seems to rise. This phenomenon is caused by the sound wave compression being closer together. As an object moves away from you, the pitch seems to drop lower. This is because the sound waves are becoming more spaced apart. The Doppler effect is defined as the change in pitch or wave frequency due to a moving wave source.

Doppler Effects If you are moving past a stationary sound source, you will also experience the Doppler effect. Basically either you or the sound source must move to cause the Doppler effect. The Doppler effect can be used to detect the speed of moving cars (radar guns) or to track wind movement in dangerous storms.