# How come vibrations?

We all know that sound sensation is produced only when sound waves reach upto us. We all know that sound waves are disturbances propagating in air, Vibration is necessary for the generation of sound, but it always forces me to ponder that how was it known or deduced that vibration is necessary for any form of sound wave? Hope someone explains me this.

• – DJBunk Jan 9 '13 at 19:48

It looks like you're asking for some historical view of how the idea of vibrating air was formed. I honestly don't know, but I can tell that the idea of things acting at a distance has been rejected through history (even Newton thought that his gravitation theory had a flaw at supposing the force acted at a distance, potentials partially solved that), so I guess that they just thought that something has to go from the point where the sound is produced to you ears. As air is the only thing there, it must be air.

The vibration comes from classical mechanics applied to air: when you move the air, in any way, you create a difference of pressure,just by inspection of a simple air model (little balls that bounce with each other) you can conclude that must propagate. Even Newton, appliying classical mechanics, tried to calculate the speed of sound according to their knowledge of how air was formed, he made a mistake because they didn't know enough thermodynamics, and didn't include a correction made by assuming that epansions contractions can be treated as adiabatic (they're too fast for heat to be trasnfered). Just by aplying that correction (which is a constant $\gamma$) to Newton's calculations gives you the real measured speed.

• Well , what I meant is that ... lets say a drum surface when struck produce sound. Its pretty capricious to conclude that the vibration of the drum surface( which is transmitted to the air) is what required for the sound sensation to be produced. How can one conclude in general , from water surfaces, that all the surfaces must be behaving in the same way during such events.? – danny gotze Jan 10 '13 at 17:24

Sound doesn't require vibration, sound is vibration. If you take something and make it vibrate, say by plucking a guitar string, you see it vibrate and you hear it vibrate.

The vibrating string makes the air vibrate, and that air vibration travels outward like ripples in a pond. The part that hits your ear is what you hear. You can see why if you pluck a really low note on a piano wire. You can not only hear it, you can see it.

Physical sound is a mechanical wave caused by the motion (vibration) of air molecules.

The source of a sound wave does not necessarily need to persist in 'vibrating' itself (like a violin string) to initiate a sound wave. A single very sudden (i.e. step function) displacement of an object in air could generate a sound wave. Lightning is the source of thunder, but we don't think of lightning as vibrating. An airplane exceeding the speed of sound creates a sonic boom, but (I don't believe) this sound is because the airplane is vibrating.

Tinnitus is the perception of sound within the human ear in the absence of corresponding external sound. People with this condition perceive that they hear sound, but there is no physical vibration of anything.

how was it known or deduced that vibration is necessary for any form of sound wave?

It is likely that theories involving wave propagation have been around for thousands of years.

The speculation that sound is a wave phenomenon grew out of observations of water waves. The rudimentary notion of a wave is an oscillatory disturbance that moves away from some source and transports no discernible amount of matter over large distances of propagation. The possibility that sound exhibits analogous behavior was emphasized, for example, by the Greek philosopher Chrysippus (c. 240 B.C.), by the Roman architect and engineer Vetruvius (c. 25 B.C.), and by the Roman philosopher Boethius (A.D. 480-524). The wave interpretation was also consistent with Aristotle's (384-322 B.C.) statement to the effect that air motion is generated by a source, "thrusting forward in like manner the adjoining air, to that the sound travels unaltered in quality as far as the disturbance of the air manages to reach."

Excerpts from Chapter 1 of Acoustics: An Introduction to Its Physical Principles and Applications by Allan D. Pierce (published by the Acoustical Society of America)