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

I guess you're talking about interference patterns and such. All that stuff
is irrelevant. The noise coming from a pulse jet will only have a couple of
periodic components. The rest is a bunch of quasi-random, aperiodic noise
resulting from some extreme turbulence. While turbulence phenomena are
chaotic in nature and not truly random, you might as well treat them as
random for this case. You never get a periodic interference pattern setting
up.

"Regnirps" wrote in message
...
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

The spectrum of the sound from the engine does indeed have several
strong harmonics that rise up from a random noise floor. I think
you'd be able to cancel the periodic components of the noise leaving
the random part behind. If I had to guess, you'd get rid of the buzz
and be left with the roar. So with cancelation it might be similar to
a jet turbine in sound.

Regards

p.s. As far as synchronization, if I understand correctly, most of the
simple forms of these motors are started with a spark, but then carry
on self sustained operation at the natural frequency of the pipe. You
could however use a spark to ignite the charge slightly early and thus
force synchronization.


"Pete Schaefer" wrote in message news:%gc6c.32308$J05.219351@attbi_s01...
I guess you're talking about interference patterns and such. All that stuff
is irrelevant. The noise coming from a pulse jet will only have a couple of
periodic components. The rest is a bunch of quasi-random, aperiodic noise
resulting from some extreme turbulence. While turbulence phenomena are
chaotic in nature and not truly random, you might as well treat them as
random for this case. You never get a periodic interference pattern setting
up.

"Regnirps" wrote in message
...
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

"Jay" wrote in message
m...
The spectrum of the sound from the engine does indeed have several
strong harmonics that rise up from a random noise floor. I think
you'd be able to cancel the periodic components of the noise leaving
the random part behind. If I had to guess, you'd get rid of the buzz
and be left with the roar. So with cancelation it might be similar to
a jet turbine in sound.

Regards

p.s. As far as synchronization, if I understand correctly, most of the
simple forms of these motors are started with a spark, but then carry
on self sustained operation at the natural frequency of the pipe. You
could however use a spark to ignite the charge slightly early and thus
force synchronization.

Okay, already! That's enough, "I suppose" and "I think".

Build something and report back next Thursday. Documentation to follow
within five days. Oral boards will convene at their convenience no less than
two weeks after the thesis is finalized and submitted.

Warning - the penalty for failure is severe!

Miss Twyla Geeter.

OK. We'll stand by for the check to fund this boondogle.

"Rich S." wrote in message
...
Build something and report back next Thursday. Documentation to follow
within five days. Oral boards will convene at their convenience no less
than
two weeks after the thesis is finalized and submitted.

Yup, that jives with what I've seen. If you could cancel out the fundamental
and a couple of harmonics for a given direction, it's still going to be
freakin' loud.

"Jay" wrote in message
m...
The spectrum of the sound from the engine does indeed have several
strong harmonics that rise up from a random noise floor. I think
you'd be able to cancel the periodic components of the noise leaving
the random part behind. If I had to guess, you'd get rid of the buzz
and be left with the roar. So with cancelation it might be similar to
a jet turbine in sound.

Pete Schaefer wrote:

Yup, that jives with what I've seen. If you could cancel out the fundamental
and a couple of harmonics for a given direction, it's still going to be
freakin' loud.

EH? WHADYA SAY?