Pulse jet noise

[Stub]

I am interested in pulse jets and hope to make my own but the noise is a bit off putting.
Can harmonics be used to cancel out some of the noise.
I beleive that Lotus cars produced a device that cancelled out the noise of engine and running gear. I think it was a device fitted to the stereo to produce a harmonic wave to cancel out the noises.
Any help would be appreciated. Cheers.

[Anders Troberg]

Actually, the concept is very simple, you just invert the signal and it should cancel out the noise. It is used ear muffs and is very effective. A slight variation on the same theme is used in aviation helmet microphones, where there are two mics, one facing the pilot and one facing outwards. The signal from the outward mic is subtracted from the inward mic. Assuming both mics pick up ambient noise, it will be cancelled out, while only the inward mic picks up the pilot and so that signal will remain.

In this case, however, we have a significantly more difficult problem. First of all, at pulse jet generates a hell of a lot of noise, and any device that wants to cancel it out needs to be able to match it. The only reasonable device in this case is another pulse jet, synchronised but out of phase with the first. Even so, it will definately not be a stealth engine, it will still be noisy by most engine standards.

Also, this method is most effective in a small, controlled environment, like the inside of an ear muff. Once the room gets larger, the waves will start to bounce around, causing unpredictible interference patterns where the waves may cancel out in some places and actually become louder in other.

So, it is possible, but it will probably be more difficult to tune the setup than an old piano.

[JetSet]

Theoretically, it is possible to make things quiter, but I'm not sure it is something you would necessarily implement on your first engine. It will take a bit of experimenting before you get it right, which means lots of noise :) Not only lots of noise, but potentially double the noise

The technology you mentioned is also used in planes, basically they record the sound made by the engines, then instantly invert the sound waves and play it out of the speakers. The peak (top) of the engine sound wave will then be cancelled out by the trough (bottom) of the upside down engine sound wave. The problem 'in the field' is that the speakers will only send sound in once direction, whereas the pulsejet will be noisy everywhere. Your best bet would be to have 2 engines, running out of phase. Then there would be the problem of 'tuning' them.


There is a thread floating around here somewhere with lots of sound reduction ideas, however I think your best bet to not annoy the neighbours would be to build a soundproof room, and run it inside Or cheaper still - buy them some earmuffs


[EDIT]
Anders beat me

[Viv]

I am interested in pulse jets and hope to make my own but the noise is a bit off putting.
Can harmonics be used to cancel out some of the noise.
I beleive that Lotus cars produced a device that cancelled out the noise of engine and running gear. I think it was a device fitted to the stereo to produce a harmonic wave to cancel out the noises.
Any help would be appreciated. Cheers.

An interesting topic that has been discussed in the past, the Lotus system is an active noise cancelation system, this works by processing the noise signal in real time and then feeding back an identical but phase shifted noise signal via loudspeakers.

The first problem is that pulse jet combustion cycles are chaotic, that means no two pulses are the same, next is the complexity of the signals harmonic content, this leads to a high load on the real time processing requirments.

The above is acheavable through the application of enough money:-)

Once you have a negative noise signal to play back you run in to the amount of noise needed to do the job as a problem, pulse jets run from about 110 Dba upto 150 Dba noise level, your negative signal must have the same amplitude but with a 180 degre phase shift.

That will require about 3.5 kilowatts across a frequency range from 10Hz to 5Khz, add to that a speaker stack to handle the power and frequency range.

In the above you must also take account of any distortion in the reproduction of your noise canceling signal, any differance between whats supplied by the engine and your signal will appear as a hetrodyne component, look up single sideband radio signals for an explanation on that.

In summery active noise cancelation of a pulse jet is not a practicle proposition for medium to large engines and may not even be practicle for small or micro engines.

A simple experiment would be to use Boiss noise canceling headphones as your processor, this is a huge advantage as they have developed very effective algorithms and processing in a relatively cheap package, then feed the headphone speaker output in to an amplifier and speaker.

For the amatuer this is probably the easiest way to test active noise cancelation.

But keep in mind that this works by trying to predict what the noise signal will be in the future and then trying to generate a signal to match it and cancell it out, with chaotic noise this is a problem!

Viv

[Viv]

Theoretically, it is possible to make things quiter, but I'm not sure it is something you would necessarily implement on your first engine. It will take a bit of experimenting before you get it right, which means lots of noise :) Not only lots of noise, but potentially double the noise

The technology you mentioned is also used in planes, basically they record the sound made by the engines, then instantly invert the sound waves and play it out of the speakers. The peak (top) of the engine sound wave will then be cancelled out by the trough (bottom) of the upside down engine sound wave. The problem 'in the field' is that the speakers will only send sound in once direction, whereas the pulsejet will be noisy everywhere. Your best bet would be to have 2 engines, running out of phase. Then there would be the problem of 'tuning' them.


There is a thread floating around here somewhere with lots of sound reduction ideas, however I think your best bet to not annoy the neighbours would be to build a soundproof room, and run it inside Or cheaper still - buy them some earmuffs


[EDIT]
Anders beat me

It takes time to process the signal and feed it back so simply recording and inverting the signal is not enough, you have to look at the signal then predict what it will be at a future point in the signals time line then allowing for delays in the proccesing/amplifier/speakers produce a signal phase shifted 180 degres to cancell it.

Two engines running out of phase for noise cancelation is just a pet theory! the same problems apply to this setup as the active systems, the basic problem is chaos combustion and building two identical engines.

People just say there will be no noise, this is not true, there will be noise that is the differance between the two engines, identical out of phase signals will cancel but identical phase signals will add together! the complexity of the sound output from this setup is dependant on the phase addition and subtraction of the signal and also the amplitude addition and subtraction of the signal.

So you need two identical engines running exactly the same combustion cycles with perfectly matched but 180 degree phase shifted outputs.

Sorry that won't happen, you will get a lower amplitude noise level but you will still have noise.

Again I mention single sideband radio theory as a primer on whats happening here.

Viv

[JetSet]

Thanks for the info on that.

With the 2 engines, I have heard someone running 2 lockwoods, who say they both get in phase, like pendulums next to each other, creating a huge increase in noise. I had assumed there would be a way of getting the out of phase.

A single combustion chamber, with two, slightly different length exhausts to make it slightly quiter, was another idea - did I read that somewhere? Or did I dream it, and it will never work? hehe.

[Mike Everman]

Once you have a negative noise signal to play back you run in to the amount of noise needed to do the job as a problem, pulse jets run from about 110 Dba upto 150 Dba noise level, your negative signal must have the same amplitude but with a 180 degre phase shift.

That will require about 3.5 kilowatts across a frequency range from 10Hz to 5Khz, add to that a speaker stack to handle the power and frequency range.


Viv

Well put, Viv. That's the death knell for active suppression right there. As I recall, previous work with anti-phased pairs showed a reduction in noise, and a reduction in thrust, and the conclusion was that the same reduction in noise could be had by throttling down to nearly the same reduced thrust level.
TANSTAAFL

I'm a big fan of pulsejet noise cancellation and anti-gravity too, but broadcasting an omni-directional cancellation wave can only hurt the very pressure waves that make thrust for you, unfortunately. Still, I dream.

[Viv]

Once you have a negative noise signal to play back you run in to the amount of noise needed to do the job as a problem, pulse jets run from about 110 Dba upto 150 Dba noise level, your negative signal must have the same amplitude but with a 180 degre phase shift.

That will require about 3.5 kilowatts across a frequency range from 10Hz to 5Khz, add to that a speaker stack to handle the power and frequency range.


Viv

Well put, Viv. That's the death knell for active suppression right there. As I recall, previous work with anti-phased pairs showed a reduction in noise, and a reduction in thrust, and the conclusion was that the same reduction in noise could be had by throttling down to nearly the same reduced thrust level.
TANSTAAFL

I'm a big fan of pulsejet noise cancellation and anti-gravity too, but broadcasting an omni-directional cancellation wave can only hurt the very pressure waves that make thrust for you, unfortunately. Still, I dream.

Yes its deffinatly been an interesting problem to solve I can say!

Viv:-)

[Bruno Ogorelec]

hurt the very pressure waves that make thrust for you, unfortunately

Now, wait a minute here, buddy. Whoever said it was pressure waves that make thrust? The fact that you observe them while your engine creates thrust does not necessarily mean that they play a useful role in the process.

What is important in thrust is the momentum of the gas ejected at the back. I can see no contribution that pressure pulses might be making. Pulses are pulses. As they propagate, they mostly leave the medium where it was. There's very little conversion into kinetic energy.

Pulsation is undoubtedly useful in the combustion chamber. It increases combustion efficiency. It also gives you self-aspiration. Further downstream, it is useful in a thrust augmenter, if you are using one. A pulsating ejector is a more effective pumping device than a steady flow ejector. But, that's it.

In fact, to my eye, pressure pulses are potentially a waste of energy. They dissipate the energy of combustion in all directions rather than entraining the mass flow.

Are you sure that -- everything else being equal -- a pulsating jet would generate greater thrust than a steady jet? You may be right, but I'll be damned if I see the mechanism that would account for this. And I can see mechanisms working in the opposite direction.

If I were investing my money, I'd be investing it in keeping the pulses where they demonstrably do good -- inside the engine -- and try hard to prevent them from getting out.

[Bruno Ogorelec]

To add to the previous post -- one of the reasons a pulsejet will generate greater thrust in flight than stationary is the same effect that some jet engine applications use – the ejector nozzle pinch. Air flowing around the engine at great speed creates a pressure ‘tube’ that entrains the exiting hot gas flow a bit further, preventing the formation of a broad ‘plume’. In a pulsejet this is probably much more effective than in a steady flow engine precisely because it restrains the propagation of pulses perpendicular to the thrust line. I am telling you, in the generation of thrust, pulsation is the enemy as soon as it lifts a foot to get out.

[Stub]

Could there be such a thing as omni directional speakers?

[luc]

Hi Bruno,

one of the reasons a pulsejet will generate greater thrust in flight than stationary ...

I find this first sentence very contravercial when it is well know that pulse jets, especially the valved ones, deteriorate their thrust as forward speed increase.

Well, maybe my statement applys only to the valved pulse jets, but they do generate more thrust when static, for forward speed and dynamic air inlet RAM air deteriorate the valve's performance.

As for the rest, I will leave that to my acoustic expert.

Cya,

Luc

[Bruno Ogorelec]

it is well know that pulse jets, especially the valved ones, deteriorate their thrust as forward speed increase.
...
Well, maybe my statement applys only to the valved pulse jets

Yes, Luc, I think so. Lockwoods tested by Lockwood generated almost twice the stationary thrust at a couple of hundred mph (if I'm not mistaken).

I guess we should look up the data for the Logan under the same circumstances (and maybe the Gluhareff, too) because the picture is more clear -- there is no confusion with the boost generated by ram pressure.

[Bruno Ogorelec]

Are you sure that -- everything else being equal -- a pulsating jet would generate greater thrust than a steady jet?

What I really wanted to ask was this:

"Are you sure that -- mass flow being equal -- a pulsating jet would generate greater thrust than a steady jet?"

[Viv]

Are you sure that -- everything else being equal -- a pulsating jet would generate greater thrust than a steady jet?

What I really wanted to ask was this:

"Are you sure that -- mass flow being equal -- a pulsating jet would generate greater thrust than a steady jet?"

Trust you to find the crappy question in all this Bruno:-) I feel like playing too;-)

As the pulsating jet flow causes a series of vortex rings and wings do the same is there not an argument the the next jet pulse pushes against the last vortex formed from the previous pulse:-)

The two attached pictures of vortex formation due too a pulse leaving a tube end illistrate this point.

Viv (just a visitor to the land of wishfull physics:-)