How does a flute produce its sound?

Question

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.

Answer 1

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.

Answer 2

Some Musical instruments consists of tubular structures like pipes which are open at both ends . Normally air column is contained in it which are in contact with the atmospheric air available One can take the flute and some organ pipes . When a disturbance is introduced in such air columns say by bringing a vibrating tuning fork say of frequency n , the air column starts vibrating due to forced vibrations but if the frequency of the external disturbance matches the natural frequency of vibration of the air column then a stationary wave is formed- the phenomena is that - The incident wave gets superposed on the reflected wave from the open end and nodes/antinodes get formed-

The fundamental frequency is such that both the end pointa maintains maximum amplitude and the mid point of the tube has stationary (antinode). The wavelength supporting this frequency is such that L=(wavelength/2)

Air is free to undergo its back-and-forth longitudinal motion at the open end of an air column; and as such, the standing wave patterns will depict anti-nodes at the open ends of air columns. So the frequency of the air column n= V/2L , where V is the speed of sound. In such air columns with both end open the next harmonic produced should be such that a full wavelength of stationary wave after interference of incident and reflected wave can be formed and the condition comes out the frequency of next harmonic n(1) = V/L where L is the length of the tube(flute) -this is like n(1)=2.n where n is fundamental frequency.

In other words: if we move ahead and find the modes of vibration of open ended organ pipe or flute further harmonics can be produced keeping in the mind that the open ends can serve only as anti nodes of vibration in the stationary waves of higher frequencies.

One might have seen flutes having some holes and the player is closing or opening the holes to produce sound of various frequencies. Actually by opening a side hole the player varies the length of the vibrating air column and changes the frequency of fundamental as well as its harmonics to produce sounds of low or high frequencies and its permutation and combination leads to "music".

Answer 3

Recorder, whistles, flutes, etc. all belong to a family of instruments that use the same means of producing sound: a jet of air, a knife edge and a tube. The recorder illustrate this 'jet-edge system' very clearly. Inside the beak of a recorder mouthpiece there is a slot-shaped tube called the windway. Air blown into one end of the windway is shaped, concentrated and directed across a short gap so that it hits a sharp edge. When the air hits that edge, it doesn't simply split in two. For a brief moment, most of the air goes over the edge, and then for a moment it goes under. This switching to and fro is what produces the sound wave. This wave then reflects off the open end of the pipe, and a standing wave is produced in the air column corresponding to one of the modes.

The flute works on the same principle as the recorder. There is a windway to shape and direct an air- jet, a gap and then an edge to 'split' the air. The windway, however, is not built into the instrument itself; flutists form these between their lips. The edge to which the breath is directed is the edge of the embouchure hole.