Dyno Cell Active Noise Control

[Craighell]

OK, so here’s my goofy idea. I need to reduce noise in a dyno room. The machines will not have to be completely quiet. Just MORE quiet than they are now.

The worst part of dyno tuning is the step and hold testing where engines are held at constant rpm regardless of load. The dyno will either hold the rpm indefinitely or move from one specific rpm point to the nest holding the rpm for perhaps 1.5 to 3 seconds. Therefore I believe even the slowest responding processing unit would be able to do a reasonable job at inverting the signal since the unit would have time to sample and adjust to match the frequency.

I have mechanical silencing on the room exhausts, but the room itself still leaks noise to the outside environment. Low frequency noise is my biggest enemy as far as emanating from the cell. My thought is not to try to kill the sound in the exhaust, but to kill the sound in the room.

Here’s my idea and please tell me if I’m way off base here. Use the mic and processor from some noise canceling headphones and amplify the signal output to drive large-scale audio speakers near the exhaust outlet.

Am I insane? Has anyone attempted anything similar?

 

[GregLocock]

Probably won't work, as described. The circuitry in NC headphones is very simple compared with what you need. If you google around you'll find a circuit diagram for some headphones.

However, IF the noise is coming from the tailpipe outlet (and that is something you need to check) then technically your idea can be made to work - I have done it myself, but I had a proper ANC computer to do the maths. Basically you need to set up some remote monitoring mics, and then optimise the drive signal to the speaker to minimise the sound at the monitoring mics. See any number of papers by Nelson and Elliott at ISVR.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

Since he has physical access to the engine, I'm wondering if something more primitive could work. I.e. an oscillator physically phase- locked to the crank, running at firing frequency, driving loudspeakers.

I'm guessing you might need to adjust the waveshape of the oscillator once, then use a microphone to control the speaker gain so as to minimize the net noise.

The idea is that things like synchronous detection and ANC should be a little easier if you don't have to recover the decoding clock from the data.

On the other hand, the phase of the noise at the end of the pipe is also affected by the gas dynamics in the pipe, and compensating for that, open- loop, might be harder than just locking onto the noise you have.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

You need to adjust both phase and magnitude. You are right, syncing the thing off the crank is the easy way to generate the basic signal. You could build up a map of amplitude and phase vs rpm and throttle opening, for example, although the systems I've used don't do that, they were adaptive.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Craighell]

Hi guys,

Does someone actually make/sell such a computer/board? The reason I ask is because there are many papers but, to build such a device including programming the processor and hoping I don't goof the matchs, and building up the breadboard, then creating the amp/speaker systems sounds like I would spend more time building and tuining the card than I would testing each engine.

The noise I'm kiling by the way, is in the dyno room itself as it is a chassis dyno located inside a complete cell. The exhaust empties into the room proximal to the exhaust ducting,but not too clase as to induce any tuning effects. Thus the sound level inside the room is deafening.

The noise from the room's exhaust ducts is pretty minimal. The low frequency sound that penetrates the walls of the room and shop is the culprit I am trying to beat down. I don't think I made that clear in my earlier post.

I'm doing some digging arund to see what the easiest method would be. I like the idea of tinkering - that's the idea behind this - I just don't want to waste my time on the impossible or create a $10,000 DIY failure.


Thanks for the help. Still doing lots of Google-ing

 

[Tmoose]

As I was reading your post a young person drove by, his system thumping essentially a single note, somewhere in the distance.

I'd still want to ID the problem frequencies.
What is the wall construction?
I'd measure, or at least feel, the relative vibration of the wall surfaces at mid height and at edges and corners.

 

[MikeHalloran]

My limited experience says that building such a system, using a single board computer, commercial amplifiers and speakers, and an algorithm that you already mostly know would take about three months full time.

It might go quicker with something like LabView on a PC, but the total cost would be about the same.



Mike Halloran
Pembroke Pines, FL, USA