Pulse jet active sound attentuation

[Jay]

In an earlier post I read somewhere that the pulse jet is a device
that converts fuel into noise. Someone else had suggested firing 2
engines 180 degrees out of phase with each other to cancel the sound.
If the 2 engines were stacked one over the other, then you might
actually be able to cancel a lot of the sound at least in a circle
around the aircraft in the plane of the earths surface. Below the
aircraft it would be just as loud but twice the frequency, but
standing on the ramp 50 yards from the aircraft, it might actually be
made to cancel somewhat. What do you guys think?

[Bob Kuykendall]

Earlier, (Jay) wrote:

> ...What do you guys think?

Pulse jet engines are commercially available and reasonably
inexpensive. I think you should try it!

Bob K.

[Rich S.]

"Jay" > wrote in message
om...
> In an earlier post I read somewhere that the pulse jet is a device
> that converts fuel into noise. Someone else had suggested firing 2
> engines 180 degrees out of phase with each other to cancel the sound.
> If the 2 engines were stacked one over the other, then you might
> actually be able to cancel a lot of the sound at least in a circle
> around the aircraft in the plane of the earths surface. Below the
> aircraft it would be just as loud but twice the frequency, but
> standing on the ramp 50 yards from the aircraft, it might actually be
> made to cancel somewhat. What do you guys think?

Makes my teeth hurt just thinking about it. Have you ever tried to
sychronize two piston engines?

Rich "That's just a T-37 warming up." S.

[Mark Hickey]

"Rich S." > wrote:

>"Jay" > wrote in message
om...
>> In an earlier post I read somewhere that the pulse jet is a device
>> that converts fuel into noise. Someone else had suggested firing 2
>> engines 180 degrees out of phase with each other to cancel the sound.
>> If the 2 engines were stacked one over the other, then you might
>> actually be able to cancel a lot of the sound at least in a circle
>> around the aircraft in the plane of the earths surface. Below the
>> aircraft it would be just as loud but twice the frequency, but
>> standing on the ramp 50 yards from the aircraft, it might actually be
>> made to cancel somewhat. What do you guys think?
>
>Makes my teeth hurt just thinking about it. Have you ever tried to
>sychronize two piston engines?

Besides, to really cancel, one would have to be pointing forward, and
one back (not the most efficient configuration).

I suppose it would be possible to use active electronic noise
cancellation, but it would take a HUGE transducer to make it work (and
a bazillion watts to drive it).

Besides, lookit all the Harley riders. As far as I can tell, loud
must be desirable.

Mark "can't even hear my BMW when I'm riding with a HD" Hickey

[Ernest Christley]

Mark Hickey wrote:
> "Rich S." > wrote:
>
>
>>"Jay" > wrote in message
om...
>>
>>>In an earlier post I read somewhere that the pulse jet is a device
>>>that converts fuel into noise. Someone else had suggested firing 2
>>>engines 180 degrees out of phase with each other to cancel the sound.
>>>If the 2 engines were stacked one over the other, then you might
>>>actually be able to cancel a lot of the sound at least in a circle
>>>around the aircraft in the plane of the earths surface. Below the
>>>aircraft it would be just as loud but twice the frequency, but
>>>standing on the ramp 50 yards from the aircraft, it might actually be
>>>made to cancel somewhat. What do you guys think?
>>
>>Makes my teeth hurt just thinking about it. Have you ever tried to
>>sychronize two piston engines?
>
>
> Besides, to really cancel, one would have to be pointing forward, and
> one back (not the most efficient configuration).
>

And it would only work at one temperature and one power settting.

--
http://www.ernest.isa-geek.org/
"Ignorance is mankinds normal state,
alleviated by information and experience."
Veeduber

[Morgans]

"Jay" > wrote in message
om...
> In an earlier post I read somewhere that the pulse jet is a device
> that converts fuel into noise. Someone else had suggested firing 2
> engines 180 degrees out of phase with each other to cancel the sound.
> If the 2 engines were stacked one over the other, then you might
> actually be able to cancel a lot of the sound at least in a circle
> around the aircraft in the plane of the earths surface. Below the
> aircraft it would be just as loud but twice the frequency, but
> standing on the ramp 50 yards from the aircraft, it might actually be
> made to cancel somewhat. What do you guys think?

That noise is a lot of sine wave belching out, with all kinds of harmonics.
In order to cancel, ALL of the waves would have to be the same frequency and
overtones, exactly out of phase. Sorry, not going to happen.
--
Jim in NC


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.608 / Virus Database: 388 - Release Date: 3/3/2004

[Bruce Simpson]

On 12 Mar 2004 09:21:58 -0800, (Jay) wrote:

>In an earlier post I read somewhere that the pulse jet is a device
>that converts fuel into noise. Someone else had suggested firing 2
>engines 180 degrees out of phase with each other to cancel the sound.
>If the 2 engines were stacked one over the other, then you might
>actually be able to cancel a lot of the sound at least in a circle
>around the aircraft in the plane of the earths surface. Below the
>aircraft it would be just as loud but twice the frequency, but
>standing on the ramp 50 yards from the aircraft, it might actually be
>made to cancel somewhat. What do you guys think?

That's not going to work.

The best that can be done is to run multiple pulsejets feeding a
single duct and try to keep the engines operating out of phase.

Doing this would result in the the pulses from each engine being
integrated into a wafeform that had a lower amplitude.

However, it si extremely difficult to keep pulsejet engines operating
at a fixed phase differential to each other when they're clustered
like this -- the prefer to synchronize and operate in phase.

The other problem is that smaller pulsejets are less efficient than
larger ones. You'll burn a lot more fuel to get a given amount of
thrust if you cluster several small engines together to do it.

--
you can contact me via http://aardvark.co.nz/contact/

[Regnirps]

Active noise cancellation works either over a large area if the source is a
point, or over a very small area (like a small space near your ear) if the
source is larger.

-- Charlie Springer

[Jay]

I like the idea of the single duct because it works to provide a
solution that provides reduced sound power in all aspects, rather than
only in the plane of the earths surface. I'm not sure what sharing a
duct would do to the resonance, especially being that its firing
opposite to the prefered phase of the resonating system.

Synchronization Problem- Aren't these motors fired with a spark? It
would seem you could force any phase you wanted. Of course the
frequency being dictated by the natural frequency of the pipe.

Smaller vs larger- I'm not talking about a cluster, I'm just talking
about firing 2. I'd be interested to know what the effiecy difference
is between a pair of 400lb thrust pulse jets and a single 800lb jet.

RE: Harmonics, Jim- I think you're right, but most of the noise power
is contained in the fundamental. I'd aim to cancel that. The
remainder might be tollerable.

RE: Try it, Bob K.- Unfortunately I live in an apartment, not a ranch
in Idahoe. I don't think my neighbors would appreciate me firing up a
pair of pulse jets slightly off frequency, beating against each other,
on the balcony at 2am. One of my buddies runs his private machine
shop in his apartment though. Carpet is covered in oil.

Regards

Bruce Simpson > wrote in message >...

> That's not going to work.
>
> The best that can be done is to run multiple pulsejets feeding a
> single duct and try to keep the engines operating out of phase.
>
> Doing this would result in the the pulses from each engine being
> integrated into a wafeform that had a lower amplitude.
>
> However, it si extremely difficult to keep pulsejet engines operating
> at a fixed phase differential to each other when they're clustered
> like this -- the prefer to synchronize and operate in phase.
>
> The other problem is that smaller pulsejets are less efficient than
> larger ones. You'll burn a lot more fuel to get a given amount of
> thrust if you cluster several small engines together to do it.

[dann mann]

There are some pretty big pulse jets for sale on Ebay right now. Snag
them and give it a try.

[BRO]

Noise cancellation doesn't work by timing a second pulse of sound, it works
by inverting the first one, Have a look at some interference patterns.

What you would need is a frequency based resonator to use reflected sound
front to cancell out the air movement causing the noise.


"Jay" > wrote in message
om...
> In an earlier post I read somewhere that the pulse jet is a device
> that converts fuel into noise. Someone else had suggested firing 2
> engines 180 degrees out of phase with each other to cancel the sound.
> If the 2 engines were stacked one over the other, then you might
> actually be able to cancel a lot of the sound at least in a circle
> around the aircraft in the plane of the earths surface. Below the
> aircraft it would be just as loud but twice the frequency, but
> standing on the ramp 50 yards from the aircraft, it might actually be
> made to cancel somewhat. What do you guys think?

[Thad Beier]

Jay wrote:
> In an earlier post I read somewhere that the pulse jet is a device
> that converts fuel into noise. Someone else had suggested firing 2
> engines 180 degrees out of phase with each other to cancel the sound.
> If the 2 engines were stacked one over the other, then you might
> actually be able to cancel a lot of the sound at least in a circle
> around the aircraft in the plane of the earths surface. Below the
> aircraft it would be just as loud but twice the frequency, but
> standing on the ramp 50 yards from the aircraft, it might actually be
> made to cancel somewhat. What do you guys think?

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

Interestingly, there is a PDE-powered LongEZ in the current
issue of Aviation Week. It's completely astonishing.

PDEs are Pulse Detonation Engines. Basically you get a long
pipe, fill it with an explosive mixture of fuel and air, and
then detonate it at one end. You get compression ratios
of about 40:1 (for good efficiency) and extremely rapid burning,
for low NOx production. But, they're really loud. Ridiculously
loud. Someone described the sound of it as "somebody ripping
the sky apart"

Anyway, the LongEZ powerplant is basically a regular flat
airplane engine with really long cylinders open at the bottom.
It uses the normal valves and camshafts to let the fuel-air
mixture into the cylinder (although both valves are used as
intake values, as the exhaust is out the bottom. The plane
has four pipes projecting back about four feet. There's no
description in the magazine article about how they're driving
the camshafts, I would guess it would be an electric motor
and batteries, and these will be very short flights.

The article in Aviation Week shows a number of other
PDE engines. There's an amazing engine that is basically
a 'revolver', sort of a rotary or Gatling Gun PDE. The tubes
sequentially rotate into firing position.

The article goes on to say that there could be large (like 15-20%)
increases in gas turbine efficiency if you could use a PDE to
replace the compressor and combustor with a PDE. Aviation
engineers would sell their grandmothers back several generations
for a 5% increase in efficiency...20% is shocking.

There are, of course, practical problems in the way, but the promise
is there.

thad

[Bill Daniels]

"Thad Beier" > wrote in message
...
> Jay wrote:
> > In an earlier post I read somewhere that the pulse jet is a device
> > that converts fuel into noise. Someone else had suggested firing 2
> > engines 180 degrees out of phase with each other to cancel the sound.
> > If the 2 engines were stacked one over the other, then you might
> > actually be able to cancel a lot of the sound at least in a circle
> > around the aircraft in the plane of the earths surface. Below the
> > aircraft it would be just as loud but twice the frequency, but
> > standing on the ramp 50 yards from the aircraft, it might actually be
> > made to cancel somewhat. What do you guys think?
>
> The pulsejet sounds is not a sine wave, or even close, so
> two 180-out-of-phase signals will not cancel out.
>
> Interestingly, there is a PDE-powered LongEZ in the current
> issue of Aviation Week. It's completely astonishing.
>
> PDEs are Pulse Detonation Engines. Basically you get a long
> pipe, fill it with an explosive mixture of fuel and air, and
> then detonate it at one end. You get compression ratios
> of about 40:1 (for good efficiency) and extremely rapid burning,
> for low NOx production. But, they're really loud. Ridiculously
> loud. Someone described the sound of it as "somebody ripping
> the sky apart"
>
> Anyway, the LongEZ powerplant is basically a regular flat
> airplane engine with really long cylinders open at the bottom.
> It uses the normal valves and camshafts to let the fuel-air
> mixture into the cylinder (although both valves are used as
> intake values, as the exhaust is out the bottom. The plane
> has four pipes projecting back about four feet. There's no
> description in the magazine article about how they're driving
> the camshafts, I would guess it would be an electric motor
> and batteries, and these will be very short flights.
>
> The article in Aviation Week shows a number of other
> PDE engines. There's an amazing engine that is basically
> a 'revolver', sort of a rotary or Gatling Gun PDE. The tubes
> sequentially rotate into firing position.
>
> The article goes on to say that there could be large (like 15-20%)
> increases in gas turbine efficiency if you could use a PDE to
> replace the compressor and combustor with a PDE. Aviation
> engineers would sell their grandmothers back several generations
> for a 5% increase in efficiency...20% is shocking.
>
> There are, of course, practical problems in the way, but the promise
> is there.
>
> thad

Reminds me of a story about some bored roughnecks in West Texas. Seems they
rigged up a length of 3" drill pipe so they could fill it with just the
right mixture of oxygen and acetylene and then set it off with a spark plug.
It was said the muzzle flash and report resembled a 5" Navy gun. I'd bet
that had 40:1 compression.

Bill D

[Jay]

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)

[Alan Baker]

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."

[Jay]

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/superposition/superposition.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

[Alan Baker]

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/superposition/superposition.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."

[DJFawcett26]

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.

[Richard Lamb]

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)

[AL]

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)

[Jay]

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%.

[Regnirps]

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

[Blueskies]

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.

[Mark Hickey]

"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

[Pete Schaefer]

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.

[Jay]

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

[Regnirps]

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

[Pete Schaefer]

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]

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

[Rich S.]

"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.

[Pete Schaefer]

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]

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.

[Richard Lamb]

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?