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Name: Chris W.
Status: student
Age: 17
Location: N/A 
Country: N/A
Date: 12/9/2004


Question:
How can I create a simple sound cancellation device?


Replies:
Hi Chris,

If you mean a device to show destructive interference of sound waves, two small speakers connected to an audio oscillator (which produces sine waves) should work rather nicely. Place the speakers on a table some distance apart (one wavelength apart should work rather nicely) and stand in front of the speakers, maybe five speaker separations in front of the speakers. The main problem will be reflection of the sound waves from the walls and other parts of the room in which you are working. You should, however, be easily able to hear the difference in loudness between the places where the sounds from the two speakers interfere constructively (waves in phase) and where the sounds interfere destructively (waves out of phase) as you move parallel to the separation of the speakers.

If you are talking about noise cancelling headphones (I have the Bose, which works wonderfully), you have a more difficult problem. You need a pair of headphones, a microphone near them, and an amplifier which amplifies the background noise picked up by the microphone and makes the diaphragms of the headphones move in such a direction as to produce sound waves with a displacement at your eardrum in the opposite direction to that which would be caused by the background noise. Note that this does not work well with sound waves of a wavelength less than the distance between your ears.

Best, Dick Plano


Chris-

In most situations sound-cancellation devices cannot really work. Wave-fronts need to have the same shape in 3-D space, as well as exactly opposite amplitude, to cancel out everywhere. In most situations, un-controlled sound-waves are going every which way, and your cancelling speaker only makes waves that go spherically outwards from the point it's located at.

There are about 3 situations of "reduced spatial dimensionality" where they can do some good.

#1- you are only trying to cancel sounds coming from only one point, and you can have a microphone and speaker right at that point,

#2- you are only trying to cancel sounds arriving at one point (i.e, your ear), and you can have a microphone and speaker right at that point,

#3- your sound-space is in a narrow tube, so canceling sound-waves at one point along its length would cause sound coming from any place on one side of that point to be diminished at all points on the other side .

"One point" can be a small volume, of width less than about 1/4 wavelength at the highest frequency you hope to cancel out. This is part of why even commercially built noise-cancelling headphones (now available on the market for under $100) generally only work well up to about 1000 cycles/second. They represent "reduced situation #2" above, a separate instance for each ear. You can try to replicate them if you wish. I have not tried it yet, but as far as I know, you might obtain similar performance.

I would get headphones that with strong exclusion of outside sounds. They generally have padded rims surrounding the ear and look a bit like protective ear-muffs. To the outside of each ear-piece I would glue 2 cheap small electric microphones, one angled 45-degrees forwards, the other 45 degrees rearwards. Then for each ear I would build a ~5-transistor circuit (op-amps OK, too), which does these things:

- amplifies and combines the two microphones equally,

- rolls off (low-pass filtering) starting at 100Hz to 1KHz, (a simple, gradual filter, about 2 or 3 different R-C's, not a sharp one ) Choosing a lower cutoff frequency is more likely to succeed smoothly, but worth less to you in perceptible effect.

- has a gain=1.0 inverting stage with selector switch so you can try both including or bypassing it,

- has an audio-tapered volume control

- has a modest amount of gain, not huge

- drive the headphone speaker with only a modest amount of power, say 10mW power, (ordinary op-amp is enough power)

Run an AM radio on an all-news station to test it. Or sit next to an airport. Then you try adjusting the gain and polarity and hope it works at one setting, for low frequencies. Too much gain, it oscillates loudly in your ear, making limited power a safety requirement. Too little gain, no change occurs to the ambient sounds you hear leaking in through the headphones. At the null point in between, you should be able to hear a modest reduction of weak bass sounds and voice-tones (especially male) in your environment. If there is no "null point" in between high and low gain, try inverting the signal with that switch. If neither position works, something fancier is going wrong. It might be parasitic feedback.

Audio feedback from the headphones through the glue into the microphones is a distinctly possible problem. As is inductive coupling from the speaker coil into the microphone's electrical wires. You might have to improvise to suppress them. For audio feedback, glue the microphones to metal (quarters or heavier), glued in turn to light, soft foam, glued to the headphones. And make sure the padding around the ears seals sounds well. For electrical feedback, use twisted-pair wires for the microphones, and try some shielding. Having a good sense of local electrical ground around the outside of the headphone shell, as part of your microphone circuits, is a skill you might need.

It is an art, to tell which kind of feedback is the problem. Magnetic feedback oscillations will tend to happen at the highest frequencies your filter allows, because voltage induced in wires is proportional to the speed of field change. Audio feedback can have favorite frequencies anywhere in the filter's pass band, but it usually can be influenced by your hand, say a finger lightly touching a microphone or firmly pressing on the headphone shell, or a palm not touching but cupped over the microphone.

If you do not mind being geeky and uncomfortable, forget the sound-isolating headphones. Instead buy protective ear-muffs and wear tiny ear-buds underneath them in your ears. That does better than the usual 10-15dB noise-cancelling of headphones, even with no sound-cancelling electronics. Adding the electronic canceller, this parts-set may have less feedback problems and more initial sound isolation, and cost less for parts. The quieting effect would probably be deeper and easier to achieve. It also opens the possibility of having the sound-sensing microphone _inside_ the headset's sound-blocking shell. That way your amplifier does not have to try to match the leakage-vs-frequency curve of the sound-blocking shell, which can be difficult to do well with a simple analog filter circuit.

It is a significant electronic hobby project, but that's about as simple as noise-cancellation can get this year. Much easier to read up and buy something manufactured.

later-

I just did some web-surfing about this. (Google ["noise cancelling" project]) There is a project or two out there, at least one with plans and builder feedback:

http://www.headwize.com/projects/noise_prj.htm

It uses about 5 op-amps. Sure enough: they work, but not too well, the microphones are best mounted on foam, and it is easier to buy than build.

Some vendors are touting highly sound-isolated headphones, looking like big ear-protector muffs, as competitors to noise-cancelling headphones, which I find plausible. This set of alternatives makes choosing a single purchase difficult. I notice that most of the noise-cancelling headphones fail to specify their passive sound isolation in dB. It would be helpful if they did so, because the noise-quieting is the sum of isolation dB and cancellation dB. There is always the trade-off between the comfort of a light, open headset, and the quieting of a big snug ear-shell.

Some vendors explain that their products work best on "constant" sounds like a buzz, and don't work on changing sounds like a voice. This implies they are using an adaptive digital chip, in addition to the amplifiers and filters I described. This chip may be considered to memorize a wave form and play back its opposite, although what it really does is gradually evolve a set of time-delayed echoes that results in the smallest possible sum of input and echoes. Because this evolution is slow, it never catches up with changing sounds like talking voices. Often, these are the sounds you would like to continue hearing, so the device is potentially useful, and becoming common in ham radio as a "DSP" (digital signal processor).

Electronic hobbyists usually find this kind of thing easier to buy than build, too. The search above found several college student projects revolving around this. These look informative:

http://www-ece.rice.edu/~klwang/elec434/elec434.htm

http://amp.ece.cmu.edu/people/ratmadja/report.htm

Sony sells noise-cancelling ear-buds: (model MDR-NC11) which only have 10dB cancellation by themselves, but which would probably make an exceptionally quiet headset if you can find 29dB ear-muffs big enough or padded right to fit snugly over the tails of the ear-buds. I wonder where on the earbud the microphone is? Ear-buds are an especially interesting component for noise-cancelling, because the noise-sensing microphone and the noise-cancelling speaker are less than an inch apart so their sounds can have very small phase differences. The closeness of the destination point (your ear canal and eardrum) is probably also an advantage.

Jim Swenson



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