Electronic – Specific frequency signal detector

Low frequencies and directionality don't mix very well:

From http://en.wikipedia.org/wiki/Sound_localization#Evaluation_for_low_frequencies

For frequencies below 800 Hz, the dimensions of the head (ear distance 21.5 cm, corresponding to an interaural time delay of 625 µs), are smaller than the half wavelength of the sound waves. So the auditory system can determine phase delays between both ears without confusion. Interaural level differences are very low in this frequency range, especially below about 200 Hz, so a precise evaluation of the input direction is nearly impossible on the basis of level differences alone. As the frequency drops below 80 Hz it becomes difficult or impossible to use either time difference or level difference to determine a sound's lateral source, because the phase difference between the ears becomes too small for a directional evaluation.

Now here is what I would do (I'll be making a few assumptions). Rather than building a intricate device to determine where the vibrations are coming from do the following when you hear the vibrations:

1. Go to your home's electrical distribution box
2. Switch off all power to your home
3. Go back to where you heard the hum

If the noise is gone you now know it is electrical in origin (just to be sure). Even though you said you switched off everything you did not say exactly how so make sure it is EVERYTHING by doing the above. If you still hear it, go outside, walk around, where do you hear it? By narrowing the parameters down of where you hear the noise will help you to easier locate the source.

If this does not help I would try to use one of these android applications: https://market.android.com/details?id=radonsoft.net.rta&hl=en

https://market.android.com/details?id=com.DanielBach.FrequenSee&feature=also_installed

You should be able to follow the low frequencies depending on the quality and sensitivity of the microphone inside the device. I suggest testing this with your PC first before trying it on the real problem: http://onlinetonegenerator.com/

Here are some devices I have found to cause low frequency vibrations:

• Refrigirators
• CRT Screens
• Florescent lightbulbs
• Pumps
• Day-night switches on streetlights
• Water flowing through an loosehanging pipe (yes, believe it or not)
• Computers (duh)

Best of luck finding that buzz!

It sounds like you want to drive the LED roughly in response to the amplitude of the sum of the two audio signals. Making a signal that is proportional to the amplitude of another is called AM demodulation. There are various options for doing that. Such circuits have been around for a long time since they are fundamental to a AM radio receiver. Yours will run a much lower frequencies, but the concept is the same.

In your case, you also probably want to threshold the signal a little. In other words, instead of the brightness being proportional to volume, the brightness will be 0 below some volume. LEDs are diodes, so have a threshold voltage built in to first approximation. For example, a common green LED drops about 2.1 V when fully lit. But, the current, and therefore the brightness falls off rapidly as the voltage gets lower. At 1.5 V it will be so dim you won't notice it on in normal office illumination.

So what I'd do for experimenting is to start by AM detecting the sum of the two audio signals. Using a opamp amplifier with two pots, allow for adjusting both the gain and offset of this signal. Then connect that to the LED with a resistor in series. The resistor should be sized so that the LED current never exceeds its maximum rating even with the largest possible voltage coming out of the amplifier.