Pulse jet active sound attentuation

I made a device once to allow me to talk to an individual in a crowd from a
distance. I used an array of small speakers and DSPs to produce delays for
each one so that it acts like a phase array RADAR and the central beam can be
directed electronically with no motion of the array. The most difficult part is
that the signal has to be spectrally broken down and delays calculted for each
speaker AND each frequency.

This is aproblem in reinforcement. The same applies to candellation. Look up
"reciprocity callibration" for an idea of how complicated it is.

As for summing, there is nothing mysterious. If you put two engines near each
other you will sum in some places and cancel in others, but the location varies
with frequency so finding silence is probematic though there may be a "sweat
spot" if everything works out.

There is also likely to be sum and difference frequuncies and (and more if the
rsponse is nonlinear). The difference or beat frequency is the difference
between two frequencies, so if the engines are not perfectly synchonized you
will get a beat. If one is at 101 and one at 100, you get a nice loud 1 Hz beat
as any of yo uknow from synchonizing a twin in a pane or boat.

-- Charlie Springer

Hi Richard, thanks for joining the discussion, read below...

Richard Lamb wrote in message ...
SHOCK waves, not sine waves.

Any periodic signal (or sound) can be thought of as an infinite sum of
the harmonics of the fundamental. Even complicated waveforms.

and more harmonics than a '60's rock band!

Yup, lots of harmonics, each one containing its portion of the sound
power. The higher the harmonic, the less power it generally contains.

AND, to make it work, they ALL have to be exactly
(there's the E word)
and perfectly
(the P word too)

(I _could add Absolutely and score 3 for 3!)

180 out of phase?

Its really a matter of degrees (pun intended), the closer to 180 you
get, the better the cancelation, but its not an all or nothing kind of
thing. For example, a 170/190 degree phase shift cancels 98%.

There is really only one problem with your cancellation idea. To work in all
directions the sounds have to come form the same point in space. If they are
not the same, all you can do is local cancellation or an interference pattern
with some areas silent and some twice as loud.

Just take two points and start drawing circles around each. The intersections
will form lines of interference.

The PDE's are harder than pulse jets since the detonation wave is supersonic.
You exceed what you might call the ellasticity of the air and I don't know if
linear theory (like Fourier) will apply. There is probably a dramatic
discontinuity and Fourier requires at least piece wise continuity. But I have
not looked at shock physics in a long time. Sombody on RAM would know, like
Marry Shafer.

-- Charlie Springer

Read below...
(Regnirps) wrote in message ...
There is really only one problem with your cancellation idea. To work in all
directions the sounds have to come form the same point in space. If they are
not the same, all you can do is local cancellation or an interference pattern
with some areas silent and some twice as loud.

The sounds would have to come from the same point in space with the
degree of coincidence being compared to the wavelength of the harmonic
you intend to cancel. In the case of the 660lb thrust Argus motor on
the V-1 with its 47Hz pulse rate, that would be a wavelength of about
24 ft, so with 2 of them 1 ft apart, you'd have a worst case of error
of 15 degrees from ideal. Stacked in an over under arrangement like
that rocket Long-EZ at Oshkosh you'd have 0 degree error from ideal in
a circle drawn in the plane of the wings, and that 15 degree error
from ideal directly above and below.

The harmonics would suffer worse due to their shorter wave lengths.

And yes I'd agree with you, there will be harmonics that are
reinforced in some aspects.

Just take two points and start drawing circles around each. The intersections
will form lines of interference.

On .1" graph paper, make the points .1" apart and draw circles 2.4" in
radius.

The PDE's are harder than pulse jets since the detonation wave is supersonic.
You exceed what you might call the ellasticity of the air and I don't know if
linear theory (like Fourier) will apply. There is probably a dramatic
discontinuity and Fourier requires at least piece wise continuity. But I have
not looked at shock physics in a long time. Sombody on RAM would know, like
Marry Shafer.

I've wondered about air being non-linear in certain circumstances.
But out at a distance the air molecules would have to move as normal.

I had the opportunity to play with some signals in Matlab. I captured
the sound from one of the pulse jet videos from the New Zealand cruise
missle guy. One of the problems is that the signal was so loud when
recorded, it clipped/saturated which of course brings the harmonics up
even more. But proceeded anyay. Applied a time delay (roughly 180
degrees) and summed the signals. Compared the spectra from the 2
sounds. The fundamental was supressed about 10dB. I didn't even
halve the signal before adding (like 2 full size engines). Their is a
wierd 1/2 fundamental I'm seeing. It looks like the engine is firing
in pulse pairs from spectra and time domain inspection.

Regards