Speaker sensitivity for multiple drivers - AVS Forum
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post #1 of 18 Old 06-06-2013, 08:47 AM - Thread Starter
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I am trying to understand the effect of adding more speakers (subs) to an enclosure is to the sensitivity. The signal would obviously be cohesive.

Here is where I am out, I think I am right?

Doubling the number of speakers gains 6 dB [ 20 log (2) ]
Doubling the speakers halves the wattage to each speaker which looses 3 dB [ 10 log (.5) ]

The net effect to the sensitivity seems to be an increase of 3 dB / 1 W.

This means that 4 x 4ohm speakers can be wired at 4ohm for a 6dB sensitivity increase.

Am I right?
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post #2 of 18 Old 06-06-2013, 09:04 AM
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Quote:
Originally Posted by NathanJ View Post

This means that 4 x 4ohm speakers can be wired at 4ohm for a 6dB sensitivity increase.
Am I right?
Correct, and if you wired them all parallel for a 1 ohm load the increase would be 12dB. That assumes your amp could handle a 1 ohm load.

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post #3 of 18 Old 06-06-2013, 09:43 AM - Thread Starter
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Thanks Bill. That is what I thought. Because I am a bit new to this and I am trying to full grasp it, I am keeping the speakers and the amps separate. That is why I am figuring the speakers 1 W sensitivity and resistance. At that point I can look at the amp and see the wattage gained from dropping resistance. Like you said though, that would be theoretically be 4x the power, 6dB more because of the additional wattage.
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post #4 of 18 Old 06-06-2013, 10:00 AM
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"Am I right?"

yes.

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post #5 of 18 Old 06-06-2013, 11:18 AM
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Quote:
Originally Posted by NathanJ View Post

I am trying to understand the effect of adding more speakers (subs) to an enclosure is to the sensitivity. The signal would obviously be cohesive.

Here is where I am out, I think I am right?

Doubling the number of speakers gains 6 dB [ 20 log (2) ]
Doubling the speakers halves the wattage to each speaker which looses 3 dB [ 10 log (.5) ]

The net effect to the sensitivity seems to be an increase of 3 dB / 1 W.

This means that 4 x 4ohm speakers can be wired at 4ohm for a 6dB sensitivity increase.

Am I right?
You ONLY get 6dB when you double the drivers IF they are close enough together acoustically (less than 1/4 wavelength center to center). If they are further apart than that-you will get some freq specific cancellations. You will get some power addition.

So generally only lower freq devices will be able to couple together well-due to the longer wavelengths.

So more drivers will make it louder-but it will not sound as good.

With loudspeakers-fewer drivers will always sound better. So use as few as possible.

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post #6 of 18 Old 06-06-2013, 12:33 PM
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Quote:
Originally Posted by NathanJ View Post

...Doubling the number of speakers gains 6 dB [ 20 log (2) ]
No. Radiation in space obeys inverse square law, but radiatiing area is simple addition. 2 cones in close proximity are +3dB
Quote:
Doubling the speakers halves the wattage to each speaker which looses 3 dB [ 10 log (.5) ]...
Also no. Load varies, depending if you use parallel (+3dB) or series (-3dB) wiring, the only two options available.
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This means that 4 x 4ohm speakers can be wired at 4ohm for a 6dB sensitivity increase.

Am I right?
Ironically, yes.

MTMs are the simple example.
- two 8-Ohm drivers in parallel are 4 Ohm load, draw twice the power (+3dB) and have twice the area (+3dB) so the total voltage sensitivity is +6dB.
- two 4-ohm drivers in series are an 8 Ohm load, draws half the power (-3dB) and still have twice the area (+3dB), so there is no change (0dB) in total voltage sensitivity.

In the 4-driver case, I assume series/parallel wiring to achieve a 4 Ohm load. Two 4-Ohm drivers in parallel are a 2 Ohm load, and a +6dB sensitivity. Two of the 2-ohm loads in series are back to 4 Ohms, but as we saw above, with no net add in sensitivity.

Example of 8-ohm drivers in parallel: Aviatrix

Example of 4-Ohm drivers in series: TriTrix

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post #7 of 18 Old 06-06-2013, 01:25 PM - Thread Starter
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I think I am going in the right direction, so here is what I am trying to decide. I have 4 SI 18s that are the dual 4ohm versions. I have only used 2 of them so far, in a dual opposed box powered by a single EP4000. I am thinking that I may get 2 more and make 2 towers of 3 subs. I was trying to decide if it made sense to grab 2 more, and if I would need a Clone or if another EP4000 would work.

I made a spreadsheet that shows the different variations of what I can do. For the wattage figures I used the RMS figures from the specs.


I like the option with 2 EP4000s and 3 subs each. Looking at the clone option, I'm not even sure if it can do that 6 ohm stereo load.

Does this look sensible?
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post #8 of 18 Old 06-07-2013, 11:08 AM - Thread Starter
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Slow day here, so I was looking at my spreadsheet again and I changed it a little.

The last option, option 9, should be roughly what popalock has going on...

I know that my wattage figures are a bit off, but they should be close enough for comparison. They were figured from looking at the advertised stereo figures and matching a trend line that closely followed the data. The bridged values are calculated from the stereo figures.

I am currently running option 1 and I was considering stepping up to option 5 which would be purchasing 2 more subs and 1 more amp. I would gain around 4 dB, but my power per sub would be closer to the recommended 600w.

I wasn't sure if I needed another amp or not, looking at this spreadsheet I see that if I don't add another amp but add my 2 more subs, I lose 1 dB.
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post #9 of 18 Old 06-07-2013, 12:01 PM
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So my stacking of my speakers which are designed for this in a line array gets me 3 dBs or 6 dBs of sensitivity? They go from 8 ohm speakers to dual stacked 4 ohm speaker and twice the power available.

AVR-Yamaha A830
amps-5 Adcom 555 in 850 watt monoblock mode
sub amp-Sanway FP14K
LCR-Dual stacked BFM DR-250's
Surrounds- Dual stacked BFM W10's
subs-12 SI 18's ported 6hz.
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post #10 of 18 Old 06-07-2013, 12:08 PM - Thread Starter
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I screwed up on the last column by adding the dB wrong. Because these are cohesive signals they add 6db and not 3db. This makes my analysis wrong from before.
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post #11 of 18 Old 06-07-2013, 12:14 PM - Thread Starter
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Originally Posted by MKtheater View Post

So my stacking of my speakers which are designed for this in a line array gets me 3 dBs or 6 dBs of sensitivity? They go from 8 ohm speakers to dual stacked 4 ohm speaker and twice the power available.

I believe your sensitivity goes up by 3dB and you gain up to 3dB for the amplifier also. You probably won't get the full 3dB from the amp though, maybe 1.5 or 2.
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post #12 of 18 Old 06-07-2013, 12:20 PM
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So my stacking of my speakers which are designed for this in a line array gets me 3 dBs or 6 dBs of sensitivity? They go from 8 ohm speakers to dual stacked 4 ohm speaker and twice the power available.
6dB, sort of. The 6dB rule is pretty linear with direct radiators up to around 8 drivers or so. But with each doubling of driver count the upper frequency f3 is reduced by a factor of 0.7, so eventually the 3dB that you get from increased radiation efficiency no longer occurs, and all additional gains are purely power related. Otherwise efficiency would eventually exceed 100%, whereas the actual limit with direct radiators is closer to 25%.
In your case with horns you're already much closer to their roughly 50% maximum efficiency limit to begin with. You'll get your 6dB at those frequencies where the efficiency limit isn't already being approached, but less than that where it is. The net result is flattening of response.

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post #13 of 18 Old 06-07-2013, 12:42 PM
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That exactly is what happened! I bet I gained near 6 dBs on the bottom(80hz) and 3-4 dBs on top. My response was flat with a slight dip at 2.2khz. So maybe my sensitivity is 105-106 dBs at 80hz and 109 above 200hz. Either way the THD is under 1% during reference level playback except at that dip which raises to 3%.

AVR-Yamaha A830
amps-5 Adcom 555 in 850 watt monoblock mode
sub amp-Sanway FP14K
LCR-Dual stacked BFM DR-250's
Surrounds- Dual stacked BFM W10's
subs-12 SI 18's ported 6hz.
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post #14 of 18 Old 06-07-2013, 01:10 PM
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a good first attempt to quantify things.

many additional things that you need to consider.

1. sensitivity depends on the enclosure in the bass as much if not more than the sensitivity spec.

for example, doubling the enclosure size might get you +3db of sensitivity in the low end.

this is why infinite baffle subs require very little power. same thing with horns, but they are more complicated.

2. co-located subs provide more spl then subs located around the room, so the additional spl gain depends in part on
where the subs are placed.

3. amplifier power is all over the map. it declines with frequency.
it declines with time. some amps drop power after 20ms or 50ms, while others last longer at spec power.
it declines with impedance (almost no amps actually double in power as impedance is halved as theory suggests).
some manufacturers specs are closer to actual rms power while others are more "optimistic".

4. drivers will compress with additional power. doubling drivers is almost always better than doubling power
even though both in theory provide +3db.

hope some of that helps.

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post #15 of 18 Old 06-07-2013, 01:16 PM
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here is an example.

dayton 18"ho in 2 cubic feet (blue), 4 c.f. (red), 8 c.f. (yellow), and 16 c.f. (green)

100 watts all sealed.

all have the same upper end sensitivity.

however, there is almost a 10x difference in sensitivity at 10hz.

blue would require 10 times the power for the same spl as the green at 10hz,
but with 10 times the power, now you are into power compession in the driver.

this is one way that it gets a little more complicated.

again, hope this helps.

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post #16 of 18 Old 06-07-2013, 01:47 PM - Thread Starter
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Thanks LTD, I was originally trying to decide if I needed another amp or not. Now I have decided it is probably a good idea and the price of the EP4000s have gone up again at both Amazon and Walmart, so I'll wait for a while to see if it drops below 300 again.

I haven't tried using software to model a sub yet, I really should do it.

What I am considering is building an AT screen and placing my speakers behind it like that one picture of Austins with the 8x1 subs vertical. Mine would only be 3x1 tall. Left - Sub - Center - Sub - Right, all vertical so I think I would gain nearer to 6dB than 3dB.

What is the down side of running a larger sealed box? I know you can throw more power at a small box, but I don't see that as a benefit. Does the size (volume) of the box change the sound of the sub other than the volume(dB)?

Thanks.
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post #17 of 18 Old 06-07-2013, 03:42 PM
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Quote:
Originally Posted by fbov View Post

- two 4-ohm drivers in series are an 8 Ohm load, draws half the power (-3dB) and still have twice the area (+3dB), so there is no change (0dB) in total voltage sensitivity.

Right; but you also have better thermal handling and less distortion from not being driven at 100%, and less current flowing through the breaker. So you could then re-double the amps and subs again before being limited by the power grid.

Drawing say 15amps, the same as the singular sub, but gaining 6db from having quad subs and dual amps.

If 1 amp and 1 sub is 100db@4ohm @ 1w, then in theory:

1 amp and 2 subs series is 100db@8ohm .5W
1 amp and 2 subs parallel is 106db@2ohm 2W
1 amp and 4 subs series is 100db@16ohm .25W
1 amp and 4 subs parallel is 112db@1ohm 4W
2 amps and 2 subs is 106db@4ohm 2W
2 amps and 4 subs series is 106db@8ohm 1W
2 amps and 4 subs parallel is 112db@2ohm 4W
4 amps and 4 subs series is 112db@4ohm 4W

(Assuming monoblocks)
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post #18 of 18 Old 06-07-2013, 05:02 PM
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"What is the down side of running a larger sealed box?"

there are at least two.

1. since larger enclosures provide less air spring resistance against the driver,
it is easier to push the drivers past their physical damage point (coil crashing into
back plate or tearing of surrround/spider, or other some such.)

2. they take up more space, which also tends to reduce placement options.

there may be a third related to system q/damping, but it still doesn't seem to be resolved
whether that is simply a frequency response matter or something more than that.

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