Re: air speed measurement above mach 1



Raul Ferreira wrote:
> For someone hear something there must be some
> variation in the air pressure

Yes.

> at a certain frequency.

No. Any variation is a sound, whether or not it is
periodic.

An explosion is not periodic, but it creates a very loud
sound.

> So the wake is formed only by the plane jet noise "trapped" in a
> single bigger wave?

Yes.

> The pressure of air in the fuselage doesn't count?

No, as that air is separate from the outside air.

> Quote from that link
> "When an airplane travels through the air, it produces sound waves."
>
> This seems to me that it is refering to the sound produced by
> variations in air pressure into the fuselage.

It is not. Have you ever traveled by air? Have you ever been
near a jet aircraft? It is very loud. What sound dominates?
Do you hear people talking inside the aircraft, or do you
just hear engines?

> Or it is just from the plane jet engine?

The engines.

> I had an ideia. This summer holidays i am going to pick up my
> electrodynamic microphone and record some planenoise traveling above
> the sound speed. With an osciloscope and spectrum analyser i might
> have an ideia of the variations of air pressure of a sonic boom.

That would be interesting, especially if you have a chance
to record a "boom". Where do you have the opportunity
to observe supersonic aircraft?

When I was a child, US Air Force jets were frequently flying
over head and creating booms (there was a base nearby).
Eventually this stopped as I believe the government insisted
they could not create booms over occupied land. Then instead
of booms I would sometimes observe aircraft flying overhead
far ahead of their sound. (I'd see the plane, and clearly
hear the noise coming from a point in the sky far behind
the plane).

And now that base is closed and there are no supersonic
aircraft at all there.

- Randy

.

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