THE SCIENCE OF SOUND
by Laurisa White Reyes
What is a sound wave?
First of all, we should ask the question: What is a wave?
A wave is a disturbance caused by a vibrating object. This disturbance transports energy from one place to another through a medium of interacting particles. This medium can be just about anything including water, metal and air. Sound waves travel through air. A vibrating object disturbs the particles in the air nearby. Those particles begin to vibrate and disturb their neighboring particles, which in turn disturb their neighboring particles, and so forth.
For example, imagine throwing a stone into a lake of water. When the stone hits the water ripples radiate away from the stone through the water. These ripples are waves and react to the stone the same way air reacts to vibrations. Likewise, a bell, when struck with a hammer, vibrates sending sound waves through the air.
Waves that require particles to transport energy, such as sound waves, are called mechanical waves. There are some kinds of waves that do not require particles. These kinds of waves are called electromagnetic waves and can travel in a vacuum (an environment where there is no air and no particles.) Mechanical waves, including sound waves, cannot travel through a vacuum. In other words, they cannot travel through space.
The nature of a sound wave is different from other waves. Going back to those ripples in a pond, water ripples, or waves, have distinct crests (the highest point in a wave) and troughs (the lowest point). Water waves are an example of transverse waves. Sound waves do not actually have crests and troughs. They are caused by compressions and rarefactions in air particles.
The source of sound is a vibrating object. An object vibrates when it moves left to right very fast. As the object moves to the right, it pushes against the air particles next to it and pushes them close together – compression. As the object moves to the left it leaves a space between itself and the compressed air particles – rarefaction. The air particles continue to affect other neighboring particles in the same way and so the sound wave travels in repeating patterns of compressions and rarefactions. This pattern is called a longitudinal wave.
What is pitch and frequency?
Even though sound waves do not really have crests and troughs like water waves, they are described as though they do. Wavelengths are measured from the top of one wave crest to the next, or from one compression to another. Objects that vibrate very fast create short wavelengths. There is little space between compressions. Objects that vibrate slowly create long wavelengths. The space between compressions is longer.
Frequency is the measurement of the number of crests within a certain amount of time. It is also measured by the speed at which the particles of air vibrate. This speed of vibration is referred to as Hertz (Hz).
1 Hertz = 1 vibration or compression per second.
For example, if you were to pluck a guitar string it might vibrate 500 times per second. Its movement would cause the surrounding air particles to vibrate at the same frequency and so on. The sound wave’s frequency would be 500 Hz. Referring back to the way a vibrating object causes compressions and rarefactions of air particles, the frequency may also be determined by measuring the number of compressions within a certain amount of time. The same guitar string would create 500 compressions -- again, 500 Hz.
The human ear is capable of detecting a wide range of frequencies from about 20 Hz to 20,000 Hz. Frequencies below the audible range are called infrasound and those above the audible range are called ultrasound. Some animals are capable of detecting very high frequencies of ultrasound. Bats, which rely on echolocation to navigate while flying, can hear frequencies as high as 120,000 Hz. Dolphins can hear frequencies as high as 200,000 Hz. On the opposite end of the scale, elephants are capable of hearing very low frequencies, as low as 5 Hz.
Frequencies within our audible range, that is frequencies we can hear, are recognized as pitch. Imagine sitting in front of a piano. Tap your finger against one of the keys close to the right end of the piano. The sound, or pitch, will be very high. That is because the corresponding piano string is vibrating very fast. Now tap a key near the left end of the piano. The pitch of that note is very low because the string vibrates at a much slower rate. The difference in pitch is determined by the rate at which an object vibrates, or by the Hz. The higher the Hz, the higher the pitch.
What are decibels?
Let’s refer back to that guitar string we mentioned earlier. If you were to pluck the string very hard, the sound it makes would be louder than if you plucked it softly. The string vibrates at the same frequency no matter how hard you pluck it, but by plucking it hard, the string will require more room to move from right to left. This range of movement is called amplitude. Amplitude correlates with how much energy is exerted by a vibrating object. If an object has a high amplitude, if it exerts a lot of energy and vibrates within a wider range of movement, it will transfer that energy onto its neighboring particles and so forth until it reaches a receptor, such as a human ear. The measurement of this energy level is called decibels or db. The equation used to measure decibels is a little complicated for this article, but you can find it as well as a great deal more information about sound at The Physics Classroom website:
Decibels determine the volume of sound. Sounds that are very loud have high decibels. Sounds that are very soft have low decibels. The lowest decibel a human ear can detect is 0 db and is called the Threshold of Hearing. Humans can hear very loud sounds, but sounds with too high of decibels can cause damage to the ear. A human whisper is about 20 db. Normal conversation is about 60 db. A vacuum cleaner is about 80 db. If you were sitting in front of the speakers at a concert, the sound would be about 110 db. The threshold of pain, or the intensity that would begin to do serious damage to the ear is 130 db. A military jet engine is 140 db. At 160 db, the eardrum would instantly burst.
Sound is an amazing thing! The elements of sound covered by this article are just the beginning. Other elements to explore include resonance, harmonics, the speed of sound, and the Doppler effect. All these and more can be studied in depth through a variety of resources. Check out books on sound at your local library or search for The Science of Sound on the Internet. The website mentioned earlier is the most comprehensive site I have seen on sound. -- L.R.