Dual Mic to cancel ambient noise - Page 1
Forum section: electronics / projects

author: atwoz
author_rank: Regular Contributor
timestamp: September 17, 2014, 03:34:29 pm
content: Hello,I want to use an electret microphone to detect if an underwater pump is on or off. I have no way of tapping the cable or anything like that. The only thing I can do is place a device in one of the tank's walls.The problem is that there will be random ambient noise (in many frequencies). People talking, walking, music, etc. I need a way to distinguish between the pump's noise and the ambient noise, to see if it is running or not.I was thinking of using 2 microphones to do this. One directly attached to the tank (mic1) and another rotated 180 degrees facing the other way (mic2). What I want to do is feed both mic outputs to some low pass filters and amplifiers, then feed the lowpass filtereted mic2 to an allpass filter to add a little delay (to take into account the separation between the mics) and then feed both filtered mic1 and mic2 inputs into a differential amplifier to get some sort of ambient noise cancellation. The outer mic2.Am I just lost or could this work? Any ideas, experiences, comments?

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 17, 2014, 03:44:12 pm
content: It sounds very doubtful to me. You haven't explained how you distinguish between the noise of the motor vs. any other noise, whether conducted through the air or through the tank.Note that you don't have to "tap into" the cable to tell whether current is flowing. It is rather common to use completely isolated Hall effect magnetic sensors adjacent to the wire.

author: lapm
author_rank: Frequent Contributor
timestamp: September 17, 2014, 05:29:50 pm
content: If this is big water tank that always holds water then yous smallest problem is people talking. More noise conducts throw ground alone. Cars, forklifts, etc.. make massive noise when driving by.Personally i would try any other method then microphone for this. Exactly why you cant measure pump wires? all you are interested if there is ampers going throw, not really how many amperes goes throw. If this is some sort of industrial installation tell them install extra contact in electric cabin to allow easy external monitorin throw low voltage sens line.

author: SirNick
author_rank: Frequent Contributor
timestamp: September 17, 2014, 06:40:31 pm
content: Sounds like the option of last resort to me. How about a flow meter? Optical or hall-effect sensor? (Not just for current sensing, but rotational, if applicable.)

author: nctnico
author_rank: Super Contributor
timestamp: September 17, 2014, 07:18:48 pm
content: I'd use a flow meter. That way you know the pump is working for sure. Magnetic flow meters are very robust because they contain no mechanical parts:http://www.flowmeters.com/magnetic-technology

author: Marco
author_rank: Super Contributor
timestamp: September 17, 2014, 08:23:02 pm
content: Does a bomb have to go off when the pump turns on or something? Otherwise it just seems a very strange set of requirements.

author: Sebastian
author_rank: Regular Contributor
timestamp: September 17, 2014, 08:29:30 pm
content: You said you can't tap the cable... could you put a coil around it?

author: DavidDLC
author_rank: Frequent Contributor
timestamp: September 17, 2014, 08:41:38 pm
content: Quote from: atwoz on September 17, 2014, 03:34:29 pmThe only thing he can do is to put devices on the walls.What about a combination of microphones and vibration detection, is the pump strong enough to cause vibration ?That can be another posibilityDavid.

author: atwoz
author_rank: Regular Contributor
timestamp: September 17, 2014, 09:16:04 pm
content: I don't have access to the cables of the pump nor the pipes. My only option is the walls of the tank. I was also thinking of placing a very sensitive accelerometer to detect any vibration in the tank, but I think I could get false positives.I will try building the a prototype to see what happens..

author: DavidDLC
author_rank: Frequent Contributor
timestamp: September 17, 2014, 09:18:19 pm
content: Quote from: atwoz on September 17, 2014, 09:16:04 pmThat is why I mentioned a combination of microphones and other devices.David.

author: nctnico
author_rank: Super Contributor
timestamp: September 17, 2014, 09:46:26 pm
content: Quote from: atwoz on September 17, 2014, 09:16:04 pmThis project sounds iffy to me. A tank with water has a big mass so it will dampen most of the vibrations of a pump. Various water levels will also change the resonating frequency of the tank. What might be a solution is a water level meter. If the water level decreases it means the pump is on.

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 17, 2014, 10:19:34 pm
content: Dunno about "magnetic", but there are ultrasonic methods of measuring flow that just clamp on the outside of the tubing/pipe.They used that method last week to measure the blood flow in my carotid arteries (which was normal, BTW).

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 06:44:02 pm
content: Bandpass filter the output from the microphone to pick up the peak of the pump noise. Make the passband broad enough to account for variation in pump speed.

author: Things
author_rank: Regular Contributor
timestamp: September 24, 2014, 10:57:22 pm
content: How close can you get to the pump? Possibly you could use a combination of a microphone + EMI detection to detect if it has turned on (assuming it's a large pump, like a pool pump).

author: atwoz
author_rank: Regular Contributor
timestamp: September 29, 2014, 03:44:42 am
content: Like 40cm from the pump. Detecting EMI sounds interesting. Do you happen to know of a reference design?