How do I create a frequency detector that can detect low "hum" sound? (quite low frequency sound probably made by a air-con or similar type of machine)
Please feel free to ask me questions if my question is unclear.
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2\$\begingroup\$ How are you wanting to do the detection? Are you looking at a microphone in free-air, a direct electrical connection to some noise generating device, what? And how do you want to represent the detected frequency? Do you want to know that the frequency is there, or do you want to measure what the actual frequency is? \$\endgroup\$– MajenkoCommented Oct 3, 2011 at 8:51
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\$\begingroup\$ @Majenko, I was looking to build a detector to detect a low "hum" sound (to me, it is consider noise) but I believe it is a low frequency (you can still correct me - maybe you are the expert in this field). The objective would be to detect that sound and find where it is exactly located at. \$\endgroup\$– Larry MorriesCommented Oct 3, 2011 at 9:14
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3\$\begingroup\$ The problem with low hums is they have a tendancy to transmit through the fabric of the building. I had a pair of hard drives in a server that were resonating with each other in a sort of feedback loop and they created a low frequency wom-wom-wom noise (almost dub-step). Spent ages working out where it came from - it was louder the other end of the building compared to where the server was. Eventually found it only when I noticed the sound was gone when the server was off. New hard drives cured that one. \$\endgroup\$– MajenkoCommented Oct 3, 2011 at 10:26
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1\$\begingroup\$ In addition to the transmission through the fabric of the building, there is the boundary effect. "A boundary creates a 6dB lift at low frequencies, and this isn't limited to the back wall of a studio -- it's just where most people seem to notice it first." So you can't build a useful directional finder for low frequencies. \$\endgroup\$– MartinCommented Oct 3, 2011 at 12:13
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\$\begingroup\$ @Majenko, Interesting observation you have. I was thinking of building such device so that I know where is the source of that hum sound. Right now, even I shut down all my electrical devices, I still hear it especially during night time. I just want to know where is that noise by using a device. \$\endgroup\$– Larry MorriesCommented Oct 4, 2011 at 1:01
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2 Answers
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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:
- Go to your home's electrical distribution box
- Switch off all power to your home
- 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!
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\$\begingroup\$ +1 Very comprehensive and good collection of observations. \$\endgroup\$ Commented Nov 25, 2011 at 1:35
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Here is one method: Use a high quality condenser microphone, and first start by recording the hum that you are trying to identify. Use a spectrum analysis program such as "Spectrogram" to visualize the exact frequencies that you are trying to identify.
Once you have identified the frequencies in question, you can then work to create a directional reflector or enclosure that could allow some directionality sensing to your microphone, or frequency tuned resonance. (Think "slide whistle" tuned to the target frequency)
Another method would be to tune a "graphic equalizer" to "notch" the target frequency, then use the microphone to search around, assuming it is not an omnidirectional mic, but rather a directional mic, such as a Cardioid, etc... So, basically a directional mic, a graphic EQ, amp and headphones.
You can also connect live microphone inputs into "Spectrogram" software, thus allowing you to "see frequencies" as you pan your directional mic around a room, for example.
