50Hz Noise Cancellation  - Page 1
Forum section: electronics / projects

author: mackrc1
author_rank: Contributor
timestamp: August 08, 2015, 09:46:37 pm
content: Hello,My fish tank filter makes a loud hum which I am pretty sure is 50Hz mains hum (It's really not that loud but I have sensitive ears), I was wondering if it would be possible to design a device to produce a signal 180 degrees out of phase and cancel out the hum.I am going of what little I know in this area but this should work, no? I believe this is how those fancy Bose noise cancelling headphones work.For audio conduction I was thinking of using a surface transducer.If this is a viable solution how would I go about generating the audio signal and how much power would I need?ThanksEwan

author: singapol
author_rank: Frequent Contributor
timestamp: August 09, 2015, 03:01:49 am
content: I'm afraid it's too much work! It's part electrical and mechanical since it's a pump with reciprocating elements.Headphones are low voltage and low current circuits...you are dealing with house AC 220-240Vac voltage.It cankill...A simpler solution is buying a better made more quiet pump.Cheap pumps are like that or locate pump away from the tank.You can put it in a well padded for noise box making sure the air intake opening is not blocked and the ventilation/heat issue. Hope this helps.

author: rs20
author_rank: Super Contributor
timestamp: August 09, 2015, 03:41:49 am
content: Bose-style noise cancellation which can deal with a whole variety of different noise sources is a fairly involved process -- even if the maths is straightforward, I feel like there's a lot of DSP processing to be done I suspect.However, it'd be a good idea to record the sound with a microphone, record/view on a computer (or view on an oscilloscope) and see if it truly is a pure 50 Hz tone. If it is, then it might be quite straightforward -- making a circuit that locks on to a pure 50 Hz tone and cancels it out just a 50 Hz is much, much easier than the full-spectrum case.You could even do it with an Arduino -- a while back, I made a circuit that would lock on to 50 Hz hum in an incoming signal, and output the same signal with that 50 Hz hum subtracted away using an ATMega328 + some analog stuff. Now in that case the subtraction was happening electrically, rather than in the air, but the concept should be similar.So yeah, low-voltage isolated AC power adapter (so that you can sample the mains AC safely), divide that down so the Arduino ADC can sample it, and then the Arduino can track the phase of the AC and output another "sinewave" (PWM of some sort) with whatever gain and phase offset you want. Filter out the high frequency noise, feed that into a speaker/subwoofer (how much power, well, however much power it would take to match the source of the noise in the first place), and then just fiddle with the gain and phase settings until it sounds like it cancels out. Bonus points for having a microphone listening in and having it tune the gain and phase itself automatically to achieve maximum cancellation, but that's not strictly necessary is you want to keep things simple.Quote from: singapol on August 09, 2015, 03:01:49 amNo, the only "mechanical" that the OP is dealing with is (maybe) microphones and speakers. The OP will also be using a low voltage circuit. No, the OP does not need to deal with house AC power.All that aside, of course a quieter pump is an easier solution, but less fun!

author: AndyC_772
author_rank: Super Contributor
timestamp: August 09, 2015, 07:07:42 am
content: To cancel noise effectively, you need to measure it very close to the listener. If your mic isn't near the listener's ears, your cancelling system will produce a quiet spot in the wrong place, and elsewhere it can be louder.Noise cancelling headphones have mics close to the wearer's ears, of course. Similar systems in cars have them in the head rests for the same reason.

author: tombi
author_rank: Regular Contributor
timestamp: August 09, 2015, 07:43:26 am
content: To avoid the nodes and anti-nodes where the cancelling sound and the hum sound combine, could you put the pump, the microphone the noise cancelling speaker in a box?I was thinking that this would reduce the reverberation induced in the box and make it quieter on the outside.Not sure how shaky the physics of what I just said is...

author: SeanB
author_rank: Super Contributor
timestamp: August 09, 2015, 07:48:07 am
content: You will be better off making a box for it, with a layer of acoustic tile on the inside, and some ventilation paths for heat and air, using a baffle with acoustic tiling to damp the direct sound transmission path. Put the pump inside the box on some stiff springs to absorb vibration, and use a loop of tubing on the air output to absorb vibration.Simplest initial try is to isolate the pump mounting feet using some springs, and put the loop of tubing.

author: rs20
author_rank: Super Contributor
timestamp: August 09, 2015, 07:55:17 am
content: Quote from: AndyC_772 on August 09, 2015, 07:07:42 amThe wavelength of 50Hz sound in air is 6.8 metres... in any case, putting the speaker close to the noise source solves this problem, and there doesn't necessarily have to be a microphone at all!Don't forget, the OP is describing a single-known-hum-source-location, single-known-hum-frequency cancellation system. Don't start naming the difficulties of multi-noise-location and wideband cancellations systems, they're not necessarily relevant here! (If the OP has stated the problem precisely correctly.)

author: mackrc1
author_rank: Contributor
timestamp: August 09, 2015, 11:42:06 am
content: First off let me say I could put it in a box, I could get a quieter pump but thats no fun! The only reason I am doing this is because I saw an opportunity to learn something new!Quote from: rs20 on August 09, 2015, 07:55:17 am