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Old 26th January 2008, 04:54 AM   #1
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Default wall as a loudspeaker

I am working on trying to suppress road noise in my new townhouse. I want to use the wall as a loud speaker and was wondering if anyone has experience in doing that. If you have a source for that kind of wall use please let me know. Thanks Jim. I'm starting out using NE5532's as a phase inverting amplifier driving a power amp, I'll let you all know how it works out and what kind of circuit I end up with.
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Old 26th January 2008, 11:22 AM   #2
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Jim, you seem to be alluding to active noise control. As I understand it this is normally done at the source of unwanted sounds, which in the case of moving vehicles represents quite a challenge! The basic idea is to generate an antiphase, but equal level sound to produce a cancellation null on the desired axis. It only works where the wavelengths are long, meaning low frequencies. Some people have used a "wall of speakers" to absorb room modes, which may have an application in your situation.

I am 65 years old and have been developing an intolerance of things such as traffic noises, airconditioners, refrigerators etc that excite room modes. We had the misfortune to have a psychopath as a neighbour, and among many aggrevations inflicted on us was a 26kw airconditioner right on the boundary. Needless to say, I can hear it running in every room in our house. Perhaps I completely missunderstand what you are trying to achieve. Good luck! Keith
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Old 26th January 2008, 11:06 PM   #3
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Thanks Kieth, you are correct in your assumption that I'm attempting sound suppression of road noise. I realize that I will not be able to eliminate the noise completely but I hope to be able to suppress it to a level I can live with. My initial approach will utilize a pair of external microphones feeding three NE5532 dual op amp IC's.The first will be configured to act as a pre-amp feeding the second one set up in a phase inverting no gain configuration and the third IC as an amplifier for the inverted signal. I'll use a 100k potentiometer and a 10k ohm resistor as a voltage divider into the summing junction of the IC to control the output power. I'll feed the output to a spare sony power amp that I have and see how things work. I have some spare speakers but am still looking for a way to use the wall itself as a speaker. Anyone have any ideas on that? If you do please let me know. By the way sorry about your lousy neighbors. Jim
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Old 27th January 2008, 08:10 AM   #4
poynton is offline poynton  United Kingdom
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Old 27th January 2008, 08:17 AM   #5
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Jim, it is in the nature of human beings to live in expectation of things improving or being amenable to improvement! Whilst not wishing to pour cold water on your ideas, my experience tells me that any gains you are going to achieve are likely to be minimal and "qualified" ie a modest reduction at one place in the room. The signal inversion aspect is trivial and does not call for an inverting stage in the electronics. Simply connect the transducers (microphone or speakers) in the apropriate sense/phase/polarity. Choose a term you are comfortable with as these words have generated an unbelievable amount of debate!

Let's say you have a microphone in a weatherproof place at the front of your house and an array of speakers on the wall furthest from the street. You sit in front of the speakers. The traffic noise travels from the street and leaks through the building to arrive where you are. The same noise also arrives at your ears via the speakers, the difference being that we may have a different acoustic path length from source to your ears. Let's say you are mid way between the microphone and the speakers. Theoretically the shorter path length from source to mic, compared with source to listener is compensated for by the speaker-listener path length. With appropriate signal phase we could get a null at this mid position. One problem is that the sound leaking into the building could be arriving by different routes (arriving at different times) thus degrading the ammount of null that can be achieved. Without doubt, the gremlins that are likely to conspire against you lie in the domain of acoustics rather than electronics.

The speed of sound in air is 344metres/second. The wavelength of 100Hz is thus 3.44metres, or around 11 feet odd. At this frequency only, as you move over a distance of a half wavelength (51/2 feet) you will go from the in phase to antiphase conditions between your unwanted noise and your "cancellation" version of it.

Perhaps you could tell us a bit more about your actual situation. Are you next to a freeway or is it slow moving traffic? Is it tyre noise or exhaust noise that bothers you? Do you live at ground level, upstairs or both? The reason I am asking is I am wondering if what you are hearing is being made worse by modal excitation of your room(s) Fortunately we no longer have the neighbour described in my first post, but the miss sited airconditioner remains. Keith
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Old 28th January 2008, 08:35 PM   #6
Serge66 is offline Serge66  Switzerland
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Jim,
What is on the other side of the wall?
I have read articles that people have used a wall as an "infinite baffle". The bass response of a speaker would be excellent since the phase cancellation is inexistent.
If you have another room on the other side, that's good.
Cheers,

Serge
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Old 29th January 2008, 04:21 AM   #7
Glens is offline Glens  Australia
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Default Wall as loudspealer

Jim,
Have you considered using passive means to deal with the noise in your townhouse? Depending on the construction method (masonry, studframe + blueboard etc), the noise may be most effectively reduced by changing the resonant frequency of the structure or decoupling the inner wall surface (most applicable to stud frame construction). If your walls are studframe, the cheapest method may simply be to add mass to the parts of the walls that participate in transmitting sound energy to the interior. What sort of insulation is present in the wall cavity if any? Is the problem worst on the ground floor or the second floor? Are there significant window openings facing the road, and if so, do these transmit more sound than the walls?
Hope this helps.
Glens
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Old 12th February 2008, 05:17 PM   #8
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Hi Jim,
i fear the challange are hardly beareable. Possibly for low frequencies, but unsolvable phase problems arise by increasing frequencies.

In Theory the whole problem would be solvable by a wave field synthesis loudspeaker wall in principlei if you know all conditions. I work on such topic, you can see by my website what amount of problems would arise. It would to be consider a very complex spatial sound field structure.
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