Pulse jet active sound attentuation

Thad Beier wrote in message ...
The pulsejet sounds is not a sine wave, or even close, so
two 180-out-of-phase signals will not cancel out.

Wouldn't 2 identical square waves cancel if you summed them exactly
180 degrees out of phase? Fourier tells us that any periodic signal
can be completely represented by the sum of an infinite series of
harmonically related sinusoids. So I think what you really mean to
say is that the 2 sounds are not identical and thus will not
completely cancel, and I would be inclined to agree with you. The
point I was making in an earlier post was that the fundamentals and
some of the lower harmonics can be made to cancel, and these are the
harmonics that contain most of the sound power. The remaining sound
would be lower in volume but higher in pitch.

Here's some news I found from a google.

PULSED DETONATION ENGINE SHIPPED FOR VEHICLE INTEGRATION: As part of a
joint Propulsion Directorate/Air Vehicles Directorate program to
evaluate the feasibility of using
pulsed detonation engine (PDE) propulsion with manned aircraft, PR
recently shipped a complete
engine assembly to Scaled Composites in Mojave, California, for
vehicle integration. The engine
was shipped following successful testing of a prototype flight-worthy
pulsed detonation engine. It
is believed that the test demonstrated the first self-contained PDE in
operation. The prototype
engine consists of a PDE assembly and pod which contains everything
required to make a selfcontained
propulsion system. This includes an auxiliary power unit, oil system,
fuel pumps and
June 2003 Page 2
Prototype flight PDE (in background
on test stand) and pod (foreground)
which supplies air to the integrated
engine.
Royce Bradley (left) and Curt Rice (right) prepare to ship a
complete PDE to Scaled Composites. The PDE and tubes will
replace a conventional pusher prop engine on an experimental
Long-EZ aircraft. The pod below and forward of the PDE
contains the APU, alternator, pumps, etc. required to make a
self-contained propulsion system. This pod will be contained in
a bomb or ventral-tank like structure underneath the Long-EZ.
fuel injection system,
alternator, battery, throttles,
and control computers, as
well as superchargers to
enable static starts and nonself
aspirated operation.
The complete engine is
constructed from off-theshelf
components and is
designed to meet FAA
durability requirements for
experimental propulsion
systems. The shipped
engine assembly consists of
a PDE and pod integrated
together to create the
second of three flight
engines. This engine is
currently being integrated with an experimental Long-EZ airplane using
a mount designed by Burt
Rutan's company, Scaled Composites. Following fabrication of the
engine mount, a duplicate
mount will be shipped back to Wright-Patterson AFB for installation on
a ground test Long-EZ
using flight engine number three. Engine number one, which has
additional test instrumentation,
will take part in outdoor acoustic testing and be used as a spare.
Although the current joint
program between PR and VA is studying integration issues with a manned
subsonic airframe,
pulsed detonation technologies are expected to have performance
benefits in the Mach 0-4+
Integration of the Pulsed Detonation Engine with a flight test Long-EZ
aircraft at Scaled Composites. The first-stage wingtip of Burt Rutan's
X-Prize entry is overhanging the PDE powered experimental aircraft.
June 2003 Page 3
Dr. Charles MacArthur
Dr. MacArthur receives the Meritorious
Civilian Service Award from Col Mike Heil,
PR Director
regime and for hybrid/combined-cycle applications. The current program
addresses structural,
acoustic, and durability concerns while maturing this potentially
revolutionary propulsion
technology. (F. Schauer, AFRL/PRTS, (937) 255-1554)

In article ,
(Jay) wrote:

Thad Beier wrote in message
...
The pulsejet sounds is not a sine wave, or even close, so
two 180-out-of-phase signals will not cancel out.

Wouldn't 2 identical square waves cancel if you summed them exactly
180 degrees out of phase? Fourier tells us that any periodic signal
can be completely represented by the sum of an infinite series of
harmonically related sinusoids. So I think what you really mean to
say is that the 2 sounds are not identical and thus will not
completely cancel, and I would be inclined to agree with you. The
point I was making in an earlier post was that the fundamentals and
some of the lower harmonics can be made to cancel, and these are the
harmonics that contain most of the sound power. The remaining sound
would be lower in volume but higher in pitch.

No. You're assuming that the compression portion of the wave is equal in
shape to the rarefaction portion of the wave, and thus can be cancelled
by a rarefaction from another engine in close proximity.

But this is highly unlikely to be the case.


Here's some news I found from a google.

PULSED DETONATION ENGINE SHIPPED FOR VEHICLE INTEGRATION: As part of a
joint Propulsion Directorate/Air Vehicles Directorate program to
evaluate the feasibility of using
pulsed detonation engine (PDE) propulsion with manned aircraft, PR
recently shipped a complete
engine assembly to Scaled Composites in Mojave, California, for
vehicle integration. The engine
was shipped following successful testing of a prototype flight-worthy
pulsed detonation engine. It
is believed that the test demonstrated the first self-contained PDE in
operation. The prototype
engine consists of a PDE assembly and pod which contains everything
required to make a selfcontained
propulsion system. This includes an auxiliary power unit, oil system,
fuel pumps and
June 2003 Page 2
Prototype flight PDE (in background
on test stand) and pod (foreground)
which supplies air to the integrated
engine.
Royce Bradley (left) and Curt Rice (right) prepare to ship a
complete PDE to Scaled Composites. The PDE and tubes will
replace a conventional pusher prop engine on an experimental
Long-EZ aircraft. The pod below and forward of the PDE
contains the APU, alternator, pumps, etc. required to make a
self-contained propulsion system. This pod will be contained in
a bomb or ventral-tank like structure underneath the Long-EZ.
fuel injection system,
alternator, battery, throttles,
and control computers, as
well as superchargers to
enable static starts and nonself
aspirated operation.
The complete engine is
constructed from off-theshelf
components and is
designed to meet FAA
durability requirements for
experimental propulsion
systems. The shipped
engine assembly consists of
a PDE and pod integrated
together to create the
second of three flight
engines. This engine is
currently being integrated with an experimental Long-EZ airplane using
a mount designed by Burt
Rutan's company, Scaled Composites. Following fabrication of the
engine mount, a duplicate
mount will be shipped back to Wright-Patterson AFB for installation on
a ground test Long-EZ
using flight engine number three. Engine number one, which has
additional test instrumentation,
will take part in outdoor acoustic testing and be used as a spare.
Although the current joint
program between PR and VA is studying integration issues with a manned
subsonic airframe,
pulsed detonation technologies are expected to have performance
benefits in the Mach 0-4+
Integration of the Pulsed Detonation Engine with a flight test Long-EZ
aircraft at Scaled Composites. The first-stage wingtip of Burt Rutan's
X-Prize entry is overhanging the PDE powered experimental aircraft.
June 2003 Page 3
Dr. Charles MacArthur
Dr. MacArthur receives the Meritorious
Civilian Service Award from Col Mike Heil,
PR Director
regime and for hybrid/combined-cycle applications. The current program
addresses structural,
acoustic, and durability concerns while maturing this potentially
revolutionary propulsion
technology. (F. Schauer, AFRL/PRTS, (937) 255-1554)

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

Alan Baker wrote in message ...
No. You're assuming that the compression portion of the wave is equal in
shape to the rarefaction portion of the wave, and thus can be cancelled
by a rarefaction from another engine in close proximity.

But this is highly unlikely to be the case.

Isn't this the case for a sinusoid? If I excite an air mass with a
speaker being driven by a sinewave which moves the paper cone in a
sinusoid fashion, and then I have a microphone whose diaphragm is made
to move in the same manner because of the excitation of the air mass,
and I read a sinewave, then wasn't the air mass moving in a sinewave.
Now I do this for with a bunch of speakers, all playing tones but
which are multiples of the lowest frequency, wouldn't this also be the
case? The combined sound would be that of a pulse jet ripping apart
the air. "Super position does hold."

This web page has some great intuitive animation:
http://www.gmi.edu/~drussell/Demos/s...rposition.html

One weakness of my above explanation is that air does not respond in
an entirely linear fashion, so maybe superposition doesn't entirely
hold. This is what they're depending on with the "throw your voice"
technology where they're "mixing" 2 ultra-sonic (and directable)
sounds and having them produce a sum (inaudible) and difference
(audible) sound at the intersection.

Regards

In article ,
(Jay) wrote:

Alan Baker wrote in message
...
No. You're assuming that the compression portion of the wave is equal in
shape to the rarefaction portion of the wave, and thus can be cancelled
by a rarefaction from another engine in close proximity.

But this is highly unlikely to be the case.

Isn't this the case for a sinusoid? If I excite an air mass with a
speaker being driven by a sinewave which moves the paper cone in a
sinusoid fashion, and then I have a microphone whose diaphragm is made
to move in the same manner because of the excitation of the air mass,
and I read a sinewave, then wasn't the air mass moving in a sinewave.
Now I do this for with a bunch of speakers, all playing tones but
which are multiples of the lowest frequency, wouldn't this also be the
case? The combined sound would be that of a pulse jet ripping apart
the air. "Super position does hold."

This web page has some great intuitive animation:
http://www.gmi.edu/~drussell/Demos/s...rposition.html

One weakness of my above explanation is that air does not respond in
an entirely linear fashion, so maybe superposition doesn't entirely
hold. This is what they're depending on with the "throw your voice"
technology where they're "mixing" 2 ultra-sonic (and directable)
sounds and having them produce a sum (inaudible) and difference
(audible) sound at the intersection.


The weakness in your argument is assuming that a pulse jet produces
anything close to a sine wave.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

LOL - personally, I suspect if you use two pulse jets to cancel, all that will
happen is you will have a pulse jet twice as loud! But then again, I am not a
noise expert ..... well sort of I guess, I know how to make it.

LOL also...

But that twice as loud assumes that the sounds will 'sum'. If the argument that the sounds cannot subtract, then they
should be just as hard to sum....

--
Dan D.



..
"DJFawcett26" wrote in message ...
LOL - personally, I suspect if you use two pulse jets to cancel, all that will
happen is you will have a pulse jet twice as loud! But then again, I am not a
noise expert ..... well sort of I guess, I know how to make it.

"Blueskies" wrote:

LOL also...

But that twice as loud assumes that the sounds will 'sum'. If the argument that the sounds cannot subtract, then they
should be just as hard to sum....

Think "double-barrel shotgun".

Mark Hickey

I don't seem to be following the thread here...

SHOCK waves, not sine waves.

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

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?



Richard (one eyebrow WAY up)

Submarines use a device (the name escapes me just now) to do exactly
what you guys are talking about.
AL

180 out of phase?



Richard (one eyebrow WAY up)

You can do it when you're only producing real low fundamental frequencies
and can control the phase. In a pulse jet or any kind of rocket motor, it's
a ton of high-frequency broad-band noise. You might be able to control one
or maybe two frequencies (assuming they are correllated), but forget about
the rest of the junk.

"AL" wrote in message
...
Submarines use a device (the name escapes me just now) to do exactly
what you guys are talking about.