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Topic: Noise Cancellation of Loud Laboratory Equipment (Read 3900 times) previous topic - next topic
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Noise Cancellation of Loud Laboratory Equipment

Hi All,

A basic is it possible type question.

I run a microscpy imaging facility that has some loud equipment in it. The main culprit is a chiller unit that keeps a high end laser cool. It creates a constant low hum from the fan and pump in the chiller unit.

So i want to know if i can record the sound, invert the wave form and play it back into the room. I assume if i get the timing of the play back right it should reduce the sound.

Is this a viable soloution, or is there another way i could do this?

Thanks

Noise Cancellation of Loud Laboratory Equipment

Reply #1 –
I believe that Lotus cars produced a prototype real-time noise cancellation system for one of their cars (some time ago). That technology can only have moved on.

The danger that you would have is that there is only a tiny chance that you will achieve cancellation using your proposed method - you have a 50/50 chance of actually increasing the noise.

Noise Cancellation of Loud Laboratory Equipment

Reply #2 –
Quote from: Cammyron on
Hi All,

A basic is it possible type question.

I run a microscpy imaging facility that has some loud equipment in it. The main culprit is a chiller unit that keeps a high end laser cool. It creates a constant low hum from the fan and pump in the chiller unit.

So i want to know if i can record the sound, invert the wave form and play it back into the room. I assume if i get the timing of the play back right it should reduce the sound.

Is this a viable solution, or is there another way i could do this?

Thanks


This is called Active Noise Control. Typically recordings are not used as you would be unable to account for changes in the sound source. Instead, a microphone (or vibration sensor) picks up the sound wave and plays the inverted sound back. This is most effective when the microphone and loudspeaker is located very close to the sound source, or very close to the listener. If it is close to the sound source it can reduce the level of the sound in the entire room, if it is located away from the sound source, then it will only be effective in some parts of the room and will actually increase the level of noise in the room in other places.
It can be  tricky to do because you must make sure that you don't get feedback.
It is most effective at low frequencies where long wave lengths make it easier to generate the inverse sound wave.


By far the bet form of noise control is reducing the noise at the source. It this case that would be making sure the fan and pump is well maintained, and conditions are such that it doesn't have to work too hard. Also, reducing vibration will help. Large vibrating surface only have to move a small amount to make a lot of sound. (this is why acoustic guitars and such like have large bodies, to better convert the string vibration into airborne sound). For example, if a small motor is suspended on a string, it will make a quite buzzing sound. If you bolt the same motor to a wall or table, then it gets much louder, becuae the whole wall or table is vibrating.

There is lots more to say in terms of noise control, but I have to go.






Noise Cancellation of Loud Laboratory Equipment

Reply #3 –
Quote from: Cammyron on
Hi All,

A basic is it possible type question.

I run a microscpy imaging facility that has some loud equipment in it. The main culprit is a chiller unit that keeps a high end laser cool. It creates a constant low hum from the fan and pump in the chiller unit.

So i want to know if i can record the sound, invert the wave form and play it back into the room. I assume if i get the timing of the play back right it should reduce the sound.

Is this a viable soloution, or is there another way i could do this?


The problem is that the noisy equipment creates a non-uniform sound field in the room, and you would need to create an exact inverse sound field at every point in the room to cancel it.

The next stage of insight would have you co-locating your noise reduction apparatus with the source. Of course your noise canceller would have to exactly duplicate the sound field of the noise source, but since they are co-located, that is at least a thinkable problem.

The next stage of insight would find you investigating whether conventional noise reduction (baffles and absorbers) would be a simpler and more economical solution.

This is where most such engineering projects end up! ;-)

Noise Cancellation of Loud Laboratory Equipment

Reply #4 –
Quote from: Cammyron on
...is there another way i could do this?

Although it's almost a technically insurmountable problem to find a global solution for everybody in the lab in question, have you considered using a pair of noise-cancelling headphones per individual?

As the problem isn't so much that the sound exists as that you can hear it (as ridiculous as that may sound, bear with me), it's the most effective method of noise-cancellation from an individual viewpoint as it places the fix as close to the true location of the problem (ie, the eardrum) as is technically practical so is likely to provide the best possible level of attenuation.

Cheers, Slipstreem. 

Noise Cancellation of Loud Laboratory Equipment

Reply #5 –
Quote
It can be tricky to do because you must make sure that you don't get feedback.
It is most effective at low frequencies where long wave lengths make it easier to generate the inverse sound wave.


I agree that this will be tricky, but if you have access to a "PA system", it might be worth trying some experiments... And it might be interesting and fun!  You're probably going to need a fair amount of power, and I'm thinking that the type of PA system used by small musical groups would be a good start.  (I wouldn't use a "hi-fi" system, because there is a chance of blowing-out a tweeter, or something like that.)

A sound level meter would be helpful too.  (I've seem them for around $30 USD.)  That would help you to get equal levels from the noise source and speaker.  (You could use a long cable to get the speaker away from the noise source to measure & adjust the volume).

Once you've got equal volumes from both sources (and both sources near each other), you should be able to tell if the sounds are adding or subtracting.  (Some amplifiers invert, and some don't.)  If they are adding, just reverse the speaker connections to reverse the phase.

Quote
you have a 50/50 chance of actually increasing the noise.
Actually, with recorded sound, you are going to get timing/phase drift...  The original noise and the recording will drift in-and-out of phase, and the sound will be alternately added & subtracted.  I have no idea how fast they will cycle, but if they cycle quckly it will just "sound louder".  It might be worth trying as a starting point...  If the phase-cycle is slow enough, you can get an idea of how well this might work.    With a recording, you don't have to worry about feedback, and you can adjust/measure the speaker volume with the equipment turned off.


My "gut feeling" is that you can get some improvement, but that the remaining sound will still be annoying.  I've heard of industrial noise cancellation systems, and if you've got a budget, you might look into it.  Again, my "gut feeling" is that these may bring the noise in a factory from "dangerous" to "safe" (but still annoying) levels.  I'll bet these are expensive, and it might be less expensive to find another solution...  Reducing/blocking the noise at the source, or maybe even moving the equipment or the people!

The low-frequency hum should be easier to cancel than higher frequencies.  The longer wavelengths make it easier to generate and propagate a complementary wave.  (But, low frequencies require more power.)    Higher frequencies are more likely to be reflected/delayed/phase shifted and will be more difficult.  OOhhh...  crossover networks in 2-way or 3-way speaker systems can introduce phase shifts too.  So, you might get cancellation from the woofer, but addition from the tweeter, etc.

P.S.
Where I work, we have a compressor in the "back room" that comes on intermittently.  Somebody built a wood box around it to reduce the noise.  I can still hear it from another room, but I think the box helps.



Noise Cancellation of Loud Laboratory Equipment

Reply #6 –
Thanks for all the rplies and ideas.

So basically it won't be possible to record the sound and play it back. The idea of having aset up noise cancelling headphones for each user could be a good idea, although wouldn't giving them a pair of earmuffs do the same job?

As to boxing up the unit to cut down noise, i don't think that is really possible. Part of it is a chiller unit that needs air flow to work. If i box it up then the box will need fans on it to keep the air moving inside (adding to the noise).

It might be easier to see ifi can get the whole unit moved into the wall cavity or roof space.

Thanks again

Noise Cancellation of Loud Laboratory Equipment

Reply #7 –
Quote
It might be easier to see if i can get the whole unit moved into the wall cavity or roof space.
  That sounds like a great idea!

Too bad we won't be getting results from your noise cancellation experiments!   

Quote
The idea of having aset up noise cancelling headphones for each user could be a good idea, although wouldn't giving them a pair of earmuffs do the same job?
  Maybe you can get away with that with "factory workers", and hearing protection (ear plugs or over-the-ear "earmuffs") are the standard for noisy environments.  But, "lab workers" (technicians, engineers, or scientists?)  generally expect a more pleasant working environment.    I don't like either solution.  When people are using hearing protection or are listening to headpbones, it makes communication difficult and creates a less friendly environment. 

With headphones, at least, the employees might feel like they are getting something (music). But, a full day with headphones (or hearing protection) can get uncomfortable.