Sound waves are compressional waves that are produced by a vibrating object. The nearby air particles vibrate with the same frequency as the frequency of the vibrations of the object. Sound waves can only travel through matter, where the energy is transferred by the collisions between the particles in the material the wave is traveling in. Sound waves travel faster though solids and liquids.
Table 1: Speed of Sound in Different Materials
|
Material
|
Speed (m/s)
|
|
Air (20°C)
|
343
|
|
Glass
|
5,640
|
|
Steel
|
5,940
|
|
Water (25°C)
|
1,493
|
|
Seawater (25°C)
|
1,533
|
|
Rubber
|
1,600
|
|
Diamond
|
12,000
|
|
Iron
|
5,130
|
The amount of energy that a wave carries past a certain area each second is the intensity of the sound. One standing close to an object releasing sounds waves would hear a more intense sound than one standing further away. The intensity of sound waves is related to the amplitude. Sound with greater amplitude has a greater intensity. Intensity is measured in decibels (dB) and it ranges from 0dB to 150 db. Regular conversation has an intensity of about 50 dB and any sound with an intensity of 120 dB or higher is painful to people.
The frequency of sound waves is determined by the frequency of the vibrations that produce the sound and is measured in hertz (Hz), the number of vibrations per second. People are usually able to hear sounds with frequencies between about 20 Hz and 20,000 Hz. Pitch is the human perception of the frequency of sound. Sounds with low frequencies have low pitch, and sounds with high frequencies have high pitch. Humans hear sound when a sound wave reaches the ear and causes structures in the ear to vibrate.
Generally people start to lose their hearing as they grow older, a condition known as presbycusis, where structures in the inner and middle ear change as people age. Some people are born with hearing problems, but others can develop it during their lifetime. Prolonged exposure to sounds about 85 dB can damage your hearing.
This is a common problem within our population. According to the most recent statistics for the United States, about 8.0 million had difficulty hearing what was said in a normal conversation with another person (even when wearing a hearing aid).
Hearing aids are electronic devices that bring amplified sound to the ear. A hearing aid usually consists of a microphone, amplifier, and receiver. It receives sounds through a microphone, which converts the sound waves to electrical signals and sends them to the amplifier. The amplifier increases the power of the signals and sends them to the ear.
In this lesson, students will apply their knowledge of sounds waves and sound transmittance to investigate the transmittance of sound through various materials and design either a hearing aid or a ear protector.
