Well I you're way above my level but I'll just throw this out there. Have you taken into account the effects of room modes? My guess is that the real problem is the interaction of reflections, however, modes could yeild similar results.
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Array Tips?
Hello everyone,
This is my first post on the AVS forum, so thanks for having me. I'm currently doing a master's research thesis on using a speaker array to control room acoustics via the use of out-of-phase cancellation. My question is relatively vague, but it obviously requires alot of background info first so...
I've worked with everything from a 2 speaker array to a 12 speaker array and many different sizes inbetween. Essentially, I'm pumping a low frequency sine wave (I've used 200, 320, 500, 800 and 1kHz sine tones) through a single source speaker towards the audience section in a shoebox sized room (aprox 18 meters long, 7 meters wide, 5 meters tall). My array then is placed inbetween the the source and the audience (pointed towards the audience) in a way to account for time delays etc. All speakers have been calibrated accurately for volume and time delay based on distance from the source so that the sine wave output by each speaker in the array is exactly 180 degrees out of phase with the wave coming from the source once it passes over it.
What I get are patches of extreme cancellation (source outputting 100db and areas of cancellation of less than 40 db) randomly around the room. Each frequency yields different areas of the same extreme amount of cancellation, but most areas are spherical with a diameter of around 40cm. There seems to be no rhyme or reason to the location and size of these cancellation areas.
Now the room is not anechoic so I know there are plenty of reflections reducing the effeciency of my system; however, shouldn't I start to see some regularity in the wavefront output by the array? I've put as much insulation (duvet covers) all over the walls to try to dampen the room as much as possible. I know they're not the best insulation, but that's why I'm not using any tones below 200hz. Without getting into too much detail, I can assure you that the distances between each array speaker and each other are well under the wavelengths of the test tones and their volumes are calibrated in accordance with the decay per distance metered by the source playing by itself.
I'm hoping to see a larger area of cancellation maybe 1-2 meters directly in front of the array, but so far I have only seen random circles of cancellation all over the room. Has anyone else dealt with array cancellation much? I of course have a thesis advisor here who helps out as well, but I just wanted to throw this out there for second opinions. Any help in achieving more widespread (location-wise) cancellation would be of great help. Thanks a bunch everyone!
-Tommy
This is my first post on the AVS forum, so thanks for having me. I'm currently doing a master's research thesis on using a speaker array to control room acoustics via the use of out-of-phase cancellation. My question is relatively vague, but it obviously requires alot of background info first so...
I've worked with everything from a 2 speaker array to a 12 speaker array and many different sizes inbetween. Essentially, I'm pumping a low frequency sine wave (I've used 200, 320, 500, 800 and 1kHz sine tones) through a single source speaker towards the audience section in a shoebox sized room (aprox 18 meters long, 7 meters wide, 5 meters tall). My array then is placed inbetween the the source and the audience (pointed towards the audience) in a way to account for time delays etc. All speakers have been calibrated accurately for volume and time delay based on distance from the source so that the sine wave output by each speaker in the array is exactly 180 degrees out of phase with the wave coming from the source once it passes over it.
What I get are patches of extreme cancellation (source outputting 100db and areas of cancellation of less than 40 db) randomly around the room. Each frequency yields different areas of the same extreme amount of cancellation, but most areas are spherical with a diameter of around 40cm. There seems to be no rhyme or reason to the location and size of these cancellation areas.
Now the room is not anechoic so I know there are plenty of reflections reducing the effeciency of my system; however, shouldn't I start to see some regularity in the wavefront output by the array? I've put as much insulation (duvet covers) all over the walls to try to dampen the room as much as possible. I know they're not the best insulation, but that's why I'm not using any tones below 200hz. Without getting into too much detail, I can assure you that the distances between each array speaker and each other are well under the wavelengths of the test tones and their volumes are calibrated in accordance with the decay per distance metered by the source playing by itself.
I'm hoping to see a larger area of cancellation maybe 1-2 meters directly in front of the array, but so far I have only seen random circles of cancellation all over the room. Has anyone else dealt with array cancellation much? I of course have a thesis advisor here who helps out as well, but I just wanted to throw this out there for second opinions. Any help in achieving more widespread (location-wise) cancellation would be of great help. Thanks a bunch everyone!
-Tommy
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I'm assuming you've taken into account that the wavelength at 1KHz is only about a foot, so, for instance, if the speakers are on 1' centers, you will get nodes and anti-nodes at 3" (quarter wavelength) intervals. At the lower frequencies, room modes will really get you. I used to think that having a subwoofer right next to your listening position would give the flattest response. Some measurements convinced me that was absolutely wrong. If you get more than a foot or so away from the speaker, the room modes give huge irregularities in frequency response. If it is possible, you might take the whole array outdoors to get a baseline on what it does in an anechoic chamber. You still get ground reflections, but this should be easy to calculate, much easier than having 6 surfaces to deal with.
It sounds like a very interesting project. It's nice to hear of someone doing some original research on a problem that needs fixing. In my opinion, the speakers and their interaction with the room is the only true problem facing true high fidelity reproduction in the home. I think speaker dispersion has a much bigger effect on the reproduced sound than even frequency response. Reproducing a symphony in a standard shoebox living room also seems impossible. The 'second venue' effect with near field echos that aren't present in the hall where the performance occured gives you the feeling you're in a small space.
I think that's why you can almost instantly tell live music from recorded music. The sound from a real instrument is not truly a point source. A violin sounds completely different from above than it does in front of the performer. These different radiating modes are perceived through reflections by the listener and give clues that no single speaker is going to be able to produce.
Multi speaker phased arrays like you're attempting to create are the future of audio, so please stay interested. People with interests like you are what is sorely lacking in today's audio industry.
Bob Smith
It sounds like a very interesting project. It's nice to hear of someone doing some original research on a problem that needs fixing. In my opinion, the speakers and their interaction with the room is the only true problem facing true high fidelity reproduction in the home. I think speaker dispersion has a much bigger effect on the reproduced sound than even frequency response. Reproducing a symphony in a standard shoebox living room also seems impossible. The 'second venue' effect with near field echos that aren't present in the hall where the performance occured gives you the feeling you're in a small space.
I think that's why you can almost instantly tell live music from recorded music. The sound from a real instrument is not truly a point source. A violin sounds completely different from above than it does in front of the performer. These different radiating modes are perceived through reflections by the listener and give clues that no single speaker is going to be able to produce.
Multi speaker phased arrays like you're attempting to create are the future of audio, so please stay interested. People with interests like you are what is sorely lacking in today's audio industry.
Bob Smith
Thanks for the input guys and the kind words Bob. I also agree that arrays have the ability to help ease some acoustics headaches that we currently have. However, I'm not as convinced as I used to be because these tests that I'm running are just one big headache with very little to show for it. I'm sure I'd get better results in an anechoic room. Trying it outdoors is a thought, but that introduces outside noise (my campus is in a busy city centre). I'd love to try it in a grass fieldn in the middle of nowhere or something.
I know that room reflections were going to be a problem, but I at least hoped that I would get a broader array of at least mild cancellation in front of the array, but nothing of the sort was observed. I'm going in for my last 12 speaker test on Thursday and let you know what I get, but I'm not holding out much hope for making the whole room silent or anything. Cheers
I know that room reflections were going to be a problem, but I at least hoped that I would get a broader array of at least mild cancellation in front of the array, but nothing of the sort was observed. I'm going in for my last 12 speaker test on Thursday and let you know what I get, but I'm not holding out much hope for making the whole room silent or anything. Cheers
Re-reading the post I realized you were trying to cancel an acoustic emitter in a room. This is probably impossible. If you think about it, the original source and the 'canceller' are both sources of acoustic energy. It is going to be impossible to 'cancel' acoustic energy except for the very narrow places you mention you are measuring. The total acoustic power into the room is equal to the power from the inital source plus what your canceller is placing in the room. At a frequency of 1 KHz, this is going to happen every 3" or so in the space you're measuring. Absorption woujld require the equivalent of an anechoic chamber, where the acoustic energy is converted to heat by the absorbing material in the chamber. This is why even large chambers are not specified to very low frequencies, it would take absorbers equivalent in size to wavelengths close to those you are absorbing. In the case of a 20 Hz tone, this would be about 50', which is quite impractical.
Quote:
Check
Meyersound.com
They have done this for years with subs.
| Originally Posted by TommyBoyGomes Thanks for the input guys and the kind words Bob. I also agree that arrays have the ability to help ease some acoustics headaches that we currently have. However, I'm not as convinced as I used to be because these tests that I'm running are just one big headache with very little to show for it. I'm sure I'd get better results in an anechoic room. Trying it outdoors is a thought, but that introduces outside noise (my campus is in a busy city centre). I'd love to try it in a grass fieldn in the middle of nowhere or something. I know that room reflections were going to be a problem, but I at least hoped that I would get a broader array of at least mild cancellation in front of the array, but nothing of the sort was observed. I'm going in for my last 12 speaker test on Thursday and let you know what I get, but I'm not holding out much hope for making the whole room silent or anything. Cheers |
Check
Meyersound.com
They have done this for years with subs.
1 - 6 of 6 Posts
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