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27th February 2005, 01:43 AM  #1 
diyAudio Member
Join Date: Jan 2004
Location: Canada

Mutual coupling question
I found this in some ElectroVoice product literature:
"When two speaker systems are placed side by side, the woofer cones “mutually couple,” causing the two systems to act as one system with twice the effective cone area at verylow frequencies, giving an additional 3dB increase in maximum acoustic output. Mutual coupling will occur when the frequency is such that the centertocenter distance between the two woofer manifolds is less than about onehalf wavelength." I have two identical subwoofer cabinets, placed side by side in a corner of my listening room. They are crossed over actively at 90 Hz. I've been trying to get my head around the mutual coupling thing as it applies to these boxes. Here are the numbers: Centertocenter distance = 44.7675 cm Estimated Speed of Sound = 34500 cm per second Frequency = Speed of Sound / Wavelength For Wavelength = 89.535 cm, Frequency = 34500 / 89.535 = 385.3 Hz What I get from this is that mutual coupling only occurs above 385.3 Hz. I wouldn't call these "verylow frequencies"! What am I missing here? EDIT: I did a search here and now I'm even more confised.... I found this formula: Fo = c/(d*sqrt(n)) Where "c" is the speed of sound in the air in metres per second (normally 343 is used, but can vary slightly according to air temperature and humidity). "d" is the distance in metres between the two speakers "n" is the number of sources. Fo = 343 / ( .447675 * 1.4142 ) = 541.77 Hz Help? Thanks, dooper 
27th February 2005, 02:18 AM  #2 
diyAudio Member
Join Date: Jun 2001
Location: Eugene, OR

The mutual coupling occurs below 385Hz, not above.
P.S. The corner of the room is the worst place to put your woofers for a number of different reasons which I can't go into now due to an upcoming mutual coupling. 
27th February 2005, 02:23 AM  #3 
diyAudio Member
Join Date: Jan 2004
Location: Canada

Thanks for taking time out from your courtin' to clarify for me
Cheers, dooper 
27th February 2005, 06:00 AM  #4 
diyAudio Member

I dont see any Coupling per say. Anyone can see that two drivers will yeild 3db more for the sheer fact that they will be twice as powerful.
If it were like 6db or something then perhaps that would be something! Basically what you're asking about is the concept of acoustic phase. HERE'S FACT Descrete frequencys have descrete wavelengths. what you're trying to do is make two radiating bodies act as one.
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27th February 2005, 06:12 AM  #5 
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Join Date: Jan 2004
Location: Canada

My understanding, based on information from EV and other sources, is that when two LF drivers are situated close together, 1 + 1 = 4, and it is indeed a 6 dB increase compared to one driver with the same available power.
Free watts are good, and so are free decibels. Are you saying that a division of Telex Corporation would lie to us? Cheers, dooper 
27th February 2005, 06:29 AM  #6  
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Location: Eugene, OR

Quote:


27th February 2005, 04:38 PM  #7 
diyAudio Member

Yeah that really is something...
well I myself would want to test that! What I wonder is how well this works at extremely low frequencys. If its true you will get a boost of 6db at 20hz and below with speakers spread over 25' apart. Where does that extra 3db come from?
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27th February 2005, 05:44 PM  #8 
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Join Date: Jan 2004
Location: Canada

From the JBL Sound System Design Reference Manual
Code:
The phenomenon of mutual coupling always comes to our aid in increasing the power output of combined subwoofer units. Figure 76A shows the transmission coefficient for a direct radiator as a function of cone diameter. The solid curve is for a single unit, and the dotted curve is for two units positioned very close to each other. In addition to the double power handling capability afforded by the two units, the dotted curve shows a 3 dB increase in transmission coefficient at low frequencies. This is due basically to the tendency for the two drivers to behave as a single unit with a larger cone diameter, and hence higher efficiency. Thus, at B, we see the relative response of a single woofer (solid curve) compared to two such radiators (dashed curve). Note that the upper frequency transition point for the pair is 0.7 that of the single unit. The more such units we combine, the lower the effective cutoff frequency below which mutual coupling is operant. As an example, let us pick a large cinema with the following physical parameters: V = 14,000 m3 S = 3700 m2 T60 = 1.2 seconds R = 2500 m2 We will use the JBL 2242H LF transducer. Taking into account its power rating and its dynamic compression at full power, we note that its power output in acoustic watts will be: WA = (WE x reference efficiency)10dB/10 where WE is the transducer’s continuous power rating (watts) and dB is the transducer’s power compression at full power. Substituting the values of WE of 800 watts, reference efficiency of .004, and power compression of 3.3 dB, we get the value of 15 acoustical watts. The reverberant level in a space with a room constant of 2500 is then: LREV = 126 + 10 log 15  10 log 2500 = 104 dB SPL We can now construct the following table: Number of Units Maximum Level Power Input 1 104 dB 800 W 2 110 dB 1600 W 4 116 dB 3200 W We cannot continue this process much beyond that shown here. What happens is that the frequency below which mutual coupling takes place falls below the nominal cutoff frequency of the system, and eventually all we see is a simple 3 dB increase per doubling of elements. For multiple subwoofers outdoors, it is best to assume that levels fall off according to inverse square law. dooper 
27th February 2005, 06:25 PM  #9  
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Join Date: Jan 2004
Location: Toronto, ON, Canada

Quote:


27th February 2005, 06:27 PM  #10 
diyAudio Moderator

Don't overcomplicate it. Let's think about a situation where two drivers are acoustically close. Ignore one of them for a moment. Feed a signal voltage to the other it will push a certain amount of air. Now feed the same signal voltage to the other one. It will push the same amount of air. 1 + 1 = 2. You'll double the acoustic power for the same input signal voltage. That's 3dB.
Now at the same time, for that given signal voltage, you've doubled the current (you're running two motors). That means you've doubled the driving power. Voila! There's the other 3dB! 3 + 3 = 6.
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