Re: air speed measurement above mach 1



Raul Ferreira wrote:
> For instance, the sonic boom, it happens when the plane reaches sound
> speed. I was told that it happens because the pressure of the air
> against the fuselage is "converted" into sound ( correct me if i
> wrong).

Pressure disturbances don't need to be "converted" into
sound. Pressure disturbances and sound are the same thing.

The sonic boom is a shock wave, a very large pulse of
pressure which then propagates.

> This sound has a frequency or it is just a sudden change in
> pressure?

It is a sharp pulse which contains a very wide range of
frequencies, the same as a thunderclap. Have you ever heard
a nearby lightning strike? (Far away, the pulse is not so
sharp any more and you hear only the low frequencies).

> After sonic boom, the plane generates loud noise sounds,

The engine noises are loud before and after the sonic boom.

> so there are
> vibrations everywhere. What kind of sound does a fighter pilot hear
> in the plane above Mach 1 if he could hear it?

It gets very quiet, because he no longer hears the loud
sound of the engines propagating from wing to cockpit. He
can still feel and hear vibrations propagating through the
metal of the aircraft, but these are much quieter than the
jet engine sounds you hear (through the air) below Mach 1.

> I suppose that it is
> something so loud that he has to use some protective device.

No, quite the opposite. The sonic boom is created at the
wings and never gets to the pilot, as he is moving faster
than sound.

> By the way, the fighter pilot hears the sonic boom has we (on the
> ground) do? Does he feel any kind of reaction on the plane at Mach1?

I don't know what he feels except acceleration. But he
definitely does not hear the boom. He is ahead of the boom.
It will never catch up with him.

> When i wrote sound speed barrier i think i mean sound speech barrier.
> Do you agree?

What do you mean by "sound speech barrier"?

- Randy

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