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Old 23rd December 2006, 12:52 AM   #1
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Default decibel difference for # of subs

Mtx says 6 db difference in doubling the subs/ X max.

I always heard it was 3 db.

which one is it????

http://www.mtxaudio.com/Flash2/volume_displacement.cfm
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Old 23rd December 2006, 05:09 AM   #2
hifiger is offline hifiger  Canada
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If you have one driver which can produce 87db at 1 watt 1 meter, and add one more for a total of two watts and two drivers, you will get a 6db increase in volume. If you do not double the power as well as the number of drivers/Sd (more xmax does not increase sensitivity, it usually will decrease it if anything.) then you will only get a 3db increase in Spl. If you double the power with one driver to two watts, then you would get a 3db increase in Spl as well. I belive this is for quarter space loading. I am not shure how these rules change as the loading situation changes..

Remember that these rules are only completely accurate when power compression is not a factor. Power compression is essensially when the voice coil of the speaker get hot enough that it becomes more resistant to the flow of electricity. This causes the gains through powering a driver with more power, to curve downwards.

Hope my wandering ramble helps....
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Old 23rd December 2006, 05:12 AM   #3
hifiger is offline hifiger  Canada
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I just read your link to the mtx website.......
whooo.
That almost makes me sad, how they inacurately compare a sub to an engine...


If you want to use a calculator, dont use that one, it does not include box size and input power into the formula. Just because the driver can physically sustain a certain frequency (at the usual measurment frequency of 1000hz) does not mean that the voice coil can take the power. (depending on other factors such as pressure recistance in small boxes, bandpass boxes, and horns just to name a few....

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Old 23rd December 2006, 04:38 PM   #4
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yeah that was werid i know....

It shows a JL audio W7 12" to be louder than an Mtx 9500 12". I know a guy who has a w7 and he said he heard a 9500 and the 9500 is noticable louder, although i dont know what boxes they had.....
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Old 2nd January 2007, 08:58 AM   #5
Pulse-R is offline Pulse-R  Australia
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It seems that a lot of people don't understand that sensitivity (or efficiency) of a loudspeaker will determine it's output for a given power input.

very simplistic, as different drivers work better in different enclosures at different frequencies, but I have yet to be proved wrong.
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Old 2nd January 2007, 09:25 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
that +6db rule, as said earlier, is +3db due to double the power input and +3db due to double the cone area.

BUT,

your +3db due to doubling the cone area ONLY works when the distance across the drivers is less than one quarter the wavelength of the highest frequency you want to reproduce.


If the distance across the drivers is 24inches (two 10ich drivers mounted just 6inches apart) then the highest frequency that can benefit from the full +3db due to doubled cone area is 144Hz.
The effect reduces to +0db at 288Hz (half wavelength) for this distance.

Bigger drivers and/or bigger separation distances reinforce each other at ever lower frequencies.

However, if you take beaming (lobing) into account you can design a directional speaker that has high sensitivity over a narrow audience angle and less sensitivity off to the sides. Those tall narrow arrays (often seen/heard in churches) mounted above head height use this principle to project a beam of sound out over the audience without deafening the nearby listeners.
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Old 2nd January 2007, 12:42 PM   #7
forr is offline forr  France
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Default Trying to avoid confusion

For the same excursion, if the drivers are set close to each others,
doubling the emitting area (doubling the number of speakers)
increases the total acoustic power four folds, +6 dB. This is due to what is called mutual radiation.

If the distance across the two drivers is greater less than one quarter of the wavelength, the total acoustic power decreases but the on axis pressure is still 6 dB higher.


If connected in parallel to an amp with low output impedance, doubling the number of speakers doubles the current in the circuit, so the electric power dissipated is doubled, and the efficiency (= percentage of acoustic power to the electric power) is doubled.

However the voltage has not changed. So, the "sensitivity" (= acoustic level in dB SPL at 1 m for a given voltage) is +6 dB more.

Passing from efficiency to sensitivity involves the notion of nominal impedance and this is where much (sometimes cheating) confusion relies.
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Old 2nd January 2007, 07:55 PM   #8
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ok then what if you double the xmax? (same driver size and same watts.)

Would that also be a +3db increase?
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Old 2nd January 2007, 07:59 PM   #9
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Quote:
Originally posted by Ihrcke04
ok then what if you double the xmax? (same driver size and same watts.)

Would that also be a +3db increase?
+6dB at frequencies where you are excursion limited.
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Old 5th January 2007, 06:31 AM   #10
Pulse-R is offline Pulse-R  Australia
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this assumes a free-space, or half-space radiation also - as more enclosed spaces will resist the increase due to pressure effects.
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