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Name: Drew
Status: student
Grade: 9-12
Country: Canada
Date: Spring 2012

The question that has been puzzling me for quite some time is why do objects make sound? When I say that, I do not mean "Why do cows go moo?" I mean why does something like closing a door make a different sound than putting a glass down on a table, or why does a slap sound different than a punch, or why does knocking on a wall sound different than knocking on a window. Does it have something to do with the molecules in the objects or the size/shape?

Hi Drew,

Sound is a vibration, and the sound depends on the mechanical properties of the object or objects colliding. When you impact an object, it deforms, and during the process of deforming, it vibrates, which causes the air around it to vibrate as well. The object is actually moving back and forth (so slightly you cannot see it in many cases) -- it can either be a reversible deformation (called elastic deformation) or you can actually permanent change the object (called plastic deformation). In a complex object like a hand, there are both kinds of deformations happening. When the deformation occurs, the object moves and bumps up against the air next to it, and this chain reaction of bumps is what makes the sound propagate (move) outward. If the object vibrates more quickly, you get a higher pitch sound. (that is why string held taut makes a higher pitch sound than a string held more loosely -- the more taught string vibrates more quickly). With everyday sounds, the material vibration is not as simple a single string though -- you get many different kinds of vibration all added up on top of each other to make what you hear. You might have a mix of various frequencies and volumes (amplitudes) that together make the characteristic sound of a door closing or a trumpet or clapping hands.

Hope this helps, Burr Zimmerman

Hi Drew,

Yes, I do understand... very good question!

The frequencies are determined by a vibration of a mass. Resonance is when a frequency is allow to bounce about, usually within a box. That mass vibrating causes pressure waves in the air and we detect those by hearing. A soothing, pleasant richness quality is created by resonating.

Please consider picking up a guitar and plucking just one string. The sound is the vibration of the string AND the resonance from the hollow guitar box.

Pluck the string, try placing your palm on the box and pressing, opening and closing the resonance cavity... experiment with the resonance. Try plucking a string and pushing on the string up and down the frets(the long wood). You will notice dramatic changes in the tones. Now change the string from metal to nylon cord and repeat the experiment.

Most everyday sounds are mixes of frequencies, resonance and duration of the pressure wave. Cows go "Moo" because of their vocal cords vibrating and shape of the jowls. Glass breaks with a characteristic high pitch of the material. A slap is very fast and creates a high amplitude pressure wave... a punch is slower, there is recoiling of tissue - it is lower in intensity and frequency.

So it is the material, size, shape, "pluck" speed, resonance... it is a science all by itself.

Now, as for why a cow moos, or a face is slapped.... ? P. E. Hughes, Ph.D. Milford, NH


First of all, sound are pressure waves in the air. They are not the same as radio or light (electro-magnetic) waves.

We hear different sounds we hear different frequencies from 20 Hz to 20,000Hz.

Common sounds, like cows mooing, glasses being put on tables, closing a door, are different from one another because each different sound consists of a different mix of frequencies. The mix of audio frequencies depends on the items that make the object, like glass, bovine vocal cords, or wood slamming on wood with metal door catches.

Sincere regards, Mike Stewart


Sound is your ear interpreting vibrations traveling through the air, or whatever material that makes contact with your eardrum. Each of these traveling vibrations called sound waves has a certain frequency. This is the pitch of the sound. Each wave has a certain strength. This is the volume. Each wave also has a certain pattern. When we see waves drawn, we usually see the simplest: sine waves. So long as the pattern repeats over and over, any shape is possible. The shape determines what the wave sounds like, called the quality. A trombone and piano can both play a C-note. Both will have the same pitch. However, a trombone wave does not have the same shape as a piano wave. Your ear detects that shape as how the object sounds.

Dr. Ken Mellendorf Physics Instructor Illinois Central College

The sound an object makes depends upon many factors: material, size, shape (as you point out). All of these, and other, factors determine the sound an object makes when struck. Since most objects have these and other factors, as well as complex shapes striking the object produces a characteristic sound. Some devices, e.g. a tuning fork is designed to produce a single sound frequency. The oboe in a symphony orchestra produces a pitch of the “A” above middle “C”, designated as “A 440” i.e. 440 cycles / sec = 440 Hertz. That is, unless a piano is part of the instrumentation of the orchestra. In that event the orchestra must be tuned to the piano, not the oboe, because pianos are tuned differently than strings and woodwinds.

If you listen to a parade, the first instrument you will hear is the bass drum. This is because the low frequency of the bass drum is not absorbed by the air. On the other hand, when the band gets close the loudest (and highest) pitch will be the flute. The different sounds are the result of the size, shape, and materials of construction. br>
Vince Calder

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