if an object for instance does not create successive compressions and rarefactions in air but only creates continuous compressions would we be able to hear these compressions as sound ?
Yes , because we hear the sound of a moving jet (it creates only compressions in air but the air behind and in front of the compression acts as a rarefaction)
The same must apply when we blow air out of our mouth , but we see that the sound we create is very less than that created by a flute using the same amount and conditions of air.
As per as i know in a flute we vibrate the air inside it , and in open air we also do the the same , so then what mechanisms account for the difference ?
in other words my question is that please explain how a flute works , in simple language as could be understood by middle school student who doesnt know what is meant by resonance.
Best Answer
Without using the terms resonance, harmonics etc you might explain it as follows.
Air blown through the lips with the aid of the edge of a hole at one end of the flute produces a series of pulses of higher than atmospheric pressure or compressions.
These compressions travel towards other open end of the flute.
Part of a compression escapes from the end but a part is reflected and travels back up the flute to the other end of the flute where again a part escapes and a part is reflected.
If the length of the flute is $L$ and the speed of the compression is $c$ it takes a compression a time $t = \frac {2L}{c}$ to travel from the hole by the lips, be reflected at the other end and then arrive back at the hole by the lips.
If at the same time when the compression arrives at the hole by the lips a new compression is being formed, the compression is magnified (there is a greater change in pressure).
If this process of magnification is repeated time and time again then that compression pulse and others like it produce a much larger amplitude series of compressions travelling towards the ear.
Compressions that do not take a time $t = \frac {2L}{c}$ to travel back and forth are not magnified indeed they may be destroyed if at the lip hole end an arriving compression meets a newly produced rarefaction (reduction in pressure).
Since it takes a time $t = \frac {2L}{c}$ for the compressions to travel up and down the tube this is also the time interval between compressions arriving at the ear.
The rate at which these compressions arrive is called the frequency $f = \frac 1 t = \frac{c}{2L}$ and so a note of frequency $f$ is heard. This note is called the fundamental.
Higher frequencies (harmonics) can be produced by, for example, producing a compression at the lip end when there is a compression just arriving at the other end. The frequency of this harmonic would then be twice that of the fundamental.
So a musical instrument has a source of pulses, a confined region along which the pulses can travel with only certain frequencies of pulses reinforcing one another.