[billvs]

This is a bit off-topic for this group, but it seems that there are some knowledgable folks here. So, is it possible to use head-phone style noise cancellation circuitry connected to loudspeakers to cancel a point source of 60Hz transformer hum?
 
I live across a lake from a very large transformer, and it has a very loud 60Hz hum. The folks who built it just put up a theoritically sound reducing wall, but it is ineffectual.
 
In theory, I should be able to take the guts of a sound cancelling headphone, (like http://gilmore2.chem.northwestern.edu/projects/noise_prj.htm ) connect it to an amplifier and do larger-scale sound cancellations. For ideal operation, the sound would have to come from a point source, and the mic and speaker would have to be colocated with the sound source.
 
However, the transformer in question is about 30ft x 20ft. The wavelenght of the 60Hz sound is about 12ft, right? So, the source is not a point source. Even if it were a point source, I do not think I could get the county water department (who own the transformer) to mount the gear on their site.
 
So, my plan B would be to try to do noise cancelation of one or more areas on my property. There is very little multipath, and the sound is largely from a single direction - across the lake. Would putting a mic, processor (tuned to 60Hz for simplicity) and speaker at the corner of my deck work? At the corner of my property? Based on the 12ft wavelenght, I should be able to protect an area of a few multiples of 10 ft, right?
 
My EE degreee is about 30 years old, so my brain hurts when I think about this stuff.
 
Thanks in advance
Bill VerSteeg
 

 

[razvitm]

noise cancelling headphones cancel acoustic noise, not electromagnetic interference noise... if the transformer can be heard then in theory it can be active noise cancelled... mount a microphone outside of your house and use it to capture the 60HZ hum, connect it to the "guts" you take from some active noise cancelling headphones, and mix it with the audio you are playing... it should produce an 180 degree phase shifted signal that will cancel the hum. in theory... as far as I know noone has tryed this before
 
I just realized that your speakers have to be able to reproduce low frequency signals like 60Hz in order for them to be effective at noise cancelling that frequency

 

[tzhuge]

My understanding is that in practice active noise cancellation only works for a somewhat narrow range of frequencies. Low frequencies are too omnidirectional, and high frequencies are too difficult to phase match. It works to the extent it does with headphones because both the mic and the driver are close to your ear.
 
With speakers, and a frequency like 60Hz, it's probably infeasible.

 

[jcx]

low frequencies are easier in that the region of cancellation is best in the near-field of your noise canceling radiator <<1/4 wavelength
 
the problem of "free field" cancellation is far beyond that of the noise canceling headphone's design which just has to cancel inside the earcup
 
you would need something like phased arrays both for measurement of the noise propagating your way and for the canceling speakers to be able to match the noise wavefront
 
and if the noise were just 60 Hz it probably wouldn't be too noticeable - but magnetostriction will double the fundamental frequency and can give higher harmonics for a much more annoying "hum/buzz"
 
this makes free field cancellation even harder because you need higher spatial resolution/more transducers in the arrays as the wavelengths become shorter and the noise wavefront more complicated with diffractions, reflections
 
 
wearing the noise canceling headphones is your best bet