Pulse jet active sound attentuation

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?

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.

"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.

"Rich S." wrote:

"Jay" wrote in message
. com...
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

Mark Hickey wrote:
"Rich S." wrote:


"Jay" wrote in message
.com...

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

"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


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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/

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

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.

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.

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.

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