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two sub effects on SPL?


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14 replies to this topic

#1 of 15 Frank Mowry

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Posted April 04 2003 - 08:38 PM

I'm looking at making my first sonosub and had a question regarding the effects of second sub (exactly the same) added to the mix. Is there a formula that computes the added SPL achieved while adding another identical sub? I played around with WinISD and notice if a select 1 vs 2 drivers in a vented box tuned to 25 Hz it appears that I get about a 3 dB increase vs the single across the board.

My dilemma is wanting to use "what I have sitting in the parts bin". I've got the tools, connectors, MDF, etc. Just need to pick up a plate amp and the sonotube.

I've currently got 2 Phoenix Gold XMAX 10s. I know using a car sub won't provide the best performance but despite them being pretty nice subs for small car encosures it doesn't seem that anybody even considers tens for their cars anymore. I can't even sell both for the price of a Shiva. Haven't put them up on ebay yet but similar sales seem to indicate they go for 50-60 bucks a piece. That's why I figure I'll at least start the project to see how it works. These subs are in the WinISD database. I'm getting the standard PE 250w amp to drive on.

In WinISD I played with these numbers which seem doable:
Number of drivers : 1
Box type : Vented
Box size : 85.9 l
Tuning frequency : 25.00 Hz
Vent : 1 vent(s)
14.74 in length for each
4.02 in round

Tube would be only about 23 inches tall at 18 inch diameter. Call it Stubby. SPL would be a rather anemic 88 db max at 30 hz dropping to 86 across the board. Is this a total waste of time? Adding the second driver with the same parameters yields the 3 dB increase across the board. It also yields the cost of another amp which could be put towards a better driver. The second driver in the same tube isn't practical for me from a design standpoint but replicating the first sub would be. Does 2 drivers in 1 enclosure yield the same increase as 2 drivers in separate enclosures (given the same total volume)?

Feel free to tell me if this is a waste of time (aside from gaining experience).

Thanks,
Frank

#2 of 15 Travis Cain

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Posted April 05 2003 - 07:28 AM

I'm no expert, but since none of the experts have chimed in...

From my understanding, it works like this:

Say for instance you have 1 driver with x number of watts in a 50L box and it gives you a max of 100db.

Now, if you build another identical box seperate and do NOT increase the power applied, you won't gain a single db. If you double the power applied since your power handling doubles due to having 2 motors, you gain 3db.

But, if you were to take 2 of those drivers and put them together in a 100L box and still do not increase the power, you will gain 3db from acoustic coupling. But once again, if you double the wattage you gain another 3db for a total of 6db.

So, it is by far the most economical to put them both in the same box to get the free 3db from acoustic coupling.

Something else to consider is when you have 2 seperate subs in the same room, you have to deal with them cancelling each other out at some freq's in different parts of the room. Since I have never used 2 subs in the same room before, I know very little about it. I'm sure there are lots of folks here that can fill you in on that aspect, should you decide on seperate boxes..

#3 of 15 Frank Mowry

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Posted April 05 2003 - 10:48 AM

Thanks for the info. I'd still like to hear others chime in for my curiousity, but it is probably a moot point as it looks like I've got a deal to trade for an Adire Shiva.

Frank

#4 of 15 Joel X

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Posted April 05 2003 - 10:54 AM

This doesn't quite feel right.

Now, if you build another identical box seperate and do NOT increase the power applied, you won't gain a single db. If you double the power applied since your power handling doubles due to having 2 motors, you gain 3db.


I suppose I don't fully understand what it takes to acoustic-couple...

I would think that increasing the diaphragm would have some effect on efficiency. So adding another speaker even without more power would add spl's. I think.

#5 of 15 MichaelAngelo

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Posted April 05 2003 - 11:42 AM

Acoustic coupling occurs outside the box. Two boxes can be placed together to achieve the same effect as having the drivers in the same box. The back sides of the drivers don't contribute to coupling. It's the face of the drivers being in close proximity to each other that produces the coupling effect. Theres a formula, I couldn't find it, that determines the coupling of 2 drivers, based on distance apart. But whether they're in the same box, or separate boxes, doesn't matter, as long as the cones are as close as poss.

Kicker states that's one "benefit" of their square drivers- better acoustic coupling, because they can be placed close together with a minimum of wasted space.

#6 of 15 Ryan Schnacke

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Posted April 05 2003 - 04:00 PM

Once I've got my 2 sonosubs level-matched and in phase I get +6dB by running both subs rather than just one even though they're 12 feet apart. Yeah, 12 feet! At the time I was running 80Hz tones to get everything (both subs plus mains) in phase at the crossover point.

I've also tried level matching a sub with the main speakers at the crossover point. I get the same +6dB gain by running speakers + sub vs just the sub or just the speakers. This worked at 2 friend's houses where, in both cases, the sub was across the room from the mains. But you've got to have the phase adjusted correctly to get this. In fact, this is a great indicator of whether you're sub is phase matched with your mains.

#7 of 15 Frank Mowry

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Posted April 06 2003 - 11:35 AM

Well now that my trade fell through it looks like I'll go ahead with the original project and start with a single sonotube for the 10 inch Phoenix Gold Xmax.

Thanks for the info guys.

BTW, if anyone wants to trade me a decent Home sub for my two car Xmaxes, drop me an email or PM. Lemme know what you have. Pics available on request. Phoenix Gold recommends a .3-.6 ft^3 box. I had mine in a ~.35. Pretty nice bass for limited space. I'm in Baltimore, MD.

Frank

#8 of 15 Travis Cain

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Posted April 06 2003 - 03:54 PM

Sorry for the misinformation. I recall reading a similar thread before, and I apparently misunderstood it regarding acoustic coupling.

#9 of 15 David Lorenzo

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Posted April 06 2003 - 04:36 PM

You got almost everything right Travis. Just the fact that acoustic coupling occurs outside of the enclosure was throwing you off a little.

Considering how big the wave lengths are of sub woofer frequencies, it makes sense that coupling would occur even if the subs were located across the room from each other.

#10 of 15 Bill Fagal

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Posted April 06 2003 - 11:58 PM

Let Ryan's experience be your guide--two equivalent subs driven by two equivalent amps (wired identically) will give you 6dB more than a single at a given amp gain.

Some rules of thumb:

A) Two drivers will yield 3dB more than one. Four drivers will yield 3dB more than two. (Assumes a single amplifier, equal total impedences, and equal power levels.) This is a result of the doubling of the cone radiating area which yields a better acoustic impedence match (i.e. moves air more efficiently.)

B) If you cut load (speaker) impedence in half, an amplifier will pass double the current at a given voltage setting (which equals a 3dB gain, all else being equal). Double the impedence, and the inverse is true.

Therefore:

2 drivers wired in series to a single amp will yield a gain of 0dB over a single driver (+3dB for doubling cone area, -3dB for doubling impedence).

2 drivers wired in parallel to a single amp will yield a gain of 6dB over a single driver (+3dB for doubling cone area, +3dB for halving impedence).

2 drivers wired to two amplifiers will yield a gain of 6dB over a single driver & amp (+3dB for doubling area, +3dB for doubling amp power).

#11 of 15 Richard Greene

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Posted April 07 2003 - 01:26 PM

If you enjoy your music at 85dB, for an example, no matter how many subwoofers you use, you'll adjust the volume so it's about 85dB. That means adding one or more subwoofers will have no effect on the SPL you prefer.

But maximum possible bass output will increase 6dB by adding a second identical powered sub stacked on top of,
or sitting right next to the first sub. These locations
do not really "couple" the subwoofers -- they just prevent destructive interference that always happens when subwoofers are more than 1/4 wavelength apart (a good
Rule-of-Thumb)

100Hz. has an 11.3 foot wavelength, if anyone is interested ... and 10Hz. would have a 113 foot wavelength.


Place the two subs on opposite sides (left & right) of a rectangular room and they will excite the
side-wall-to-side-wall axial room mode out of phase = cancellation at that narrow band of frequencies
(a band of frequencies typically 5 to 10Hz. wide with a center frequency in Hz. roughly equal to
565/width of room in feet).

The sharp null (bass output typically down -6 to -12db at those room mode frequencies) will result in maximum average bass output up only about 3dB from adding a second subwoofer located far from the first subwoofer ...
compared with +6dB when the subs are together.
Another way of looking at this is to say that two
separated subwoofers require roughly double the amplifier power (+3dB) to develop the same SPL as two subs stacked or located side by sides. That's a significant difference.

#12 of 15 Ryan Schnacke

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Posted April 07 2003 - 02:28 PM

Richard has a good point here. I can only get +6dB in a limited range of frequencies since the subs can't be in phase at all freqs when they're halfway across the room from each other. My average gain is indeed closer to +3dB when all sub freqs are considered. And in fact, I found that the phase setting that gave the best in-room response was NOT the phase setting that gave me +6dB at 80Hz. Go figure Posted Image

#13 of 15 Dan Wesnor

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Posted April 07 2003 - 02:55 PM

If the drivers are wired in parallel, you gain 6dB in efficiency (output/power). If they are wired in series, you gain no efficiency. Either way, you gain 6dB in maximum output, which should be your real concern. Also, you will lower the amount of distortion. I find that more cubic inches of displacement usually leads to a more "effortless" sound.

This applies only if they are placed near each other - same box it best. If they are placed across the room, all bets are off. You may actually lose output due to intereference.

I used to run 2 subs (different boxes, different amps), and got no change in volume from it. Nada, zilch, nil. I could switch off either sub and the sound meter didn't change. But the frequency response did get worse with 2 subs - lesson learned.

#14 of 15 Bill Fagal

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Posted April 08 2003 - 01:34 AM

As I hear it, getting smoother response from one sub than from two is the exception rather than the rule. Of course, each room is unique like a beautiful snowflake...

This is an interesting read. True, its findings are only applicable to that set of room dimensions, but it's still an education.

#15 of 15 Frank Mowry

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Posted April 08 2003 - 03:02 AM

Interesting stuff. I've checked out all the sonotube links and saw a few people with dual sonotubes in opposite corners from where the screen sits. Maybe the article Bill Fagal referenced would interest them. It does look nice but apparently it's not providing a performance bump unless maybe they're running them out of phase or something.

Seeing as how they seemed to recommend two subs at midpoints on opposite walls (at least given their room dimensions), I wish they would have addresss two subs at midpoints on adjacent walls. That would be the optimal configuration for me aside from two next to each other or in the same box (tube).

Frank