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[AllenDeneau]

I'm trying to learn more of the "science" of sound and have a few questions regarding subwoofers and acheiving the optimal performance...

Sorry, it's long but it all deals with subwoofers and their performance when coupled...

First, coupling: I understand the ideas and basic principles of subs coupling but I want to explore the science further...

Boxes producing frequencies below 100Hz coupled together gain 3dB and if touching a boundary such as a wall then it's 6dB etc... I also understand how it's frequency dependant but, what I'm unsure of is at what point, physically, do the subs begin to couple, or as they are moved apart, do they un-couple?

Do the boxes actually have to be touching each other or are they ok as long as they're within 1/4 wavelength apart at their crossover point? If you have subs crossed over at say 90Hz and the subs are within 3' of each other on the same plane, are they effectively coupling? If you move them closer will the coupling strentghen to the full 3dB or does it matter?

With that, say you have 2 subs stacked on each other and the top sub has 1" rubber feet, is that causing any effect or change in coupling? Or is there any benefit to them touching on a larger side vs a smaller side? Say they're 2' wide and 3' tall, if you stand them end to end on the 2' sides, is there any difference than if you stack them sideways on their 3' sides?

What more can you teach me with regard to sub deployment and coupling?

Ok, next subject regarding subs and their performance with respect to different power AND coupling.

For example I have Peavey SP118 subs powered by PLX 3402s at 8 ohms. The power rating for my subs is 350/700/1400 and the 3402 is 700watts at 8 ohms. There's also a 4 ohm version of my sub..

What I'm trying to learn is the the math for determining output gain for different power ratings of cabs.

So my subs are 700 watts at 8 ohms and the 4 ohms subs will then run my amps at 1100 watts which per cabinet is just about a 2dB difference. That part I get, where I'm not sure is when you start determining the output of the higher power rating when coupled....

So if I had 2 8 ohm cabinets coupled together vs 2 4 ohm cabinets how do you arrive at the correct values for proposed output? Do you figure the difference of each cabinet first then add those together then add the 3dB for coupling?

What's got me really confused is the manuals say the power rating, tuning, and output are identical between the 8 ohm and 4 ohm version, is that typical?

Here's part of the reason I'm asking. I'm trying to find more of my identical subs but haven't been able to but I have found a couple pairs with the 4 ohm drivers. Also, after blowing a pair the dealer put the wrong baskets in my subs and I'm going to be reloading them to the correct baskets. I've been thinking that I'd reload them to the 4 ohm versions of the correct baskets to match the other subs I may buy.

So ultimately that would give me 8 SP118 subs at 4 ohms, which would give me a 2dB gain per sub which isn't noticable BUT what about when coupled?

So I want to learn the math for future reference AND looking for input as to why I should or shouldn't reload to the 4 ohm drivers...

Does this question make sense? HELP....

[Bob Leonard]

Allen,
Your speakers output is not based on the impedence of the voice coil. Output is SPL at the speakers long term power handling capablity regardless of impedence. According to the Peavey site the SP 118 are rated as shown below;

Power Handling:
600 Watts continuous
1,200 Watts program
2,400 Watts peak
Sound Pressure Level, 1 Watt, 1 Meter in Anechoic Environment:
98.0 dB (2.83 V input)
Maximum Sound Pressure Level
(1 meter):
126.0 dB SPL continuous
132.0 dB SPL peak

Combining two (2) of these subs should allow for a gain of up to 3db SPL. However, max long term is 126db and driving them harder for long periods will result in failures.

Keep in mind that 126db long term is pretty good output for ANY single 18" sub. It is combining subs that will increase output and for that purpose alone you should try to match the impedence of all of the drivers. And IMO any speaker will sound better when being driven by an amp that is NOT bridged, keeping in mind that +/- 2-3db won't amount to a piss hole in the snow.

[Elliot Thompson]

I'm trying to learn more of the "science" of sound and have a few questions regarding subwoofers and acheiving the optimal performance...

Does this question make sense? HELP....

Hi.

In order to achieve optimum performance you will require power that will offer an addition +3 dB to your loudspeakers.

1000 watts: 0 dB

1000 watts – 2000 watts: +3 dB

2000 watts – 4000 watts: + 3 dB  (+6 dB)

4000 watts – 8000 watts: + 3 dB  (+9 dB)

8000 watts – 16000 watts: + 3 dB (+ 12 dB)

This means if you were using eight, 8-ohm Peavey SP 118, each cabinet would have 2000 watts at its disposal.

To achieve + 3 dB from your loudspeakers the same principal applies.

1 sub: 0 dB

2 subs: +3 dB

4 subs: + 6 dB

8 subs: + 9 dB

You will achieve an additional +3 dB once the Peavey SP 118s are coupled together

1 sub: + 0 dB

2 subs: +3 dB compared to 1 sub

4 subs: + 3 dB compared to 2 subs

8 subs: + 3 dB compared to 4 subs

Since one Peavey SP 118 offers 126 dB maximum, continuously @ 1- metre, half space, eight cabinets should be around 144 dB maximum, continuously @ 1-metre, half space providing each loudspeaker is getting a little over 1000 watts.

Best Regards,

Elliot.

[AllenDeneau]

Thanks guys, it's becoming a bit more clear however I'm still a bit unsure.. Sorry I know my responses are long but I am thirsty for the knowledge I've been missing for so long.

Bob, my SP118's are the older versions matching the SP2G mains. Here's their info:
-3dB = 48Hz
350/700/1400 power
99dB = sensitivity
122dB continuous SPL
128dB max SPL

Until I can afford to upgrade this is what I have to work with. I'll be adding a couple or 4 more subs to the system as it's still more affordable than buying all new right now.

So, my boxes are all 8 ohm drivers driven by a side of a PLX3402 each at 700 watts. I most usually couple all 4 subs in a cube, 2x2 array, when possible as I've found it to give me more punch further back in the room. So I know I've gained SPL by having 4 instead of 2 AND they're coupled which should add a bump in output as well..

Bob, can you explain a bit more about your comment stating you think a sub driven in non-bridge mode sounds better? I don't know I've ever really listened for a difference but I'd love to know what ould cause that effect...

I also think, for sake of ease, it'd be better to have all subs matched in impedance as well. Not to mention through multiples I will get a bump, even if slightly, in SPL by pushing 1100 watts each instead of 700 watts I believe...

I, sadly, do understand that 2-3dB isn't a big deal but as the cabinet quantities increase that little bump of 2-3dB will eventually start to add up, I'd think. Kind of like taking a penny and doubling it each day, eventually (30 days) you'll have $5.3 million Got a penny for me? LOL!

Elliot, so I think I understand your lists but let me be sure. I get that each time you double the power you gain +3dB.. Also when going from 1 sub to 2 gets +3dB, then from 2 to 4 gets another +3dB.

So let's say none of these are coupled, they're all over 10' from each other, never are but just to be sure they aren't coupled, I want to make sure I have this first part understood 100%. If this is true, then the below would be the resulting SPL gain correct?

1 of my subs = 122/128dB SPL
adding a 2nd sub, not coupled = 125/131dB SPL (+3dB)
adding the 2nd pair = 128/134dB SPL? (+ another 3 dB)
if I added a 3rd pair = 131/137dB?

OK so I'll assume I'm correct in my understanding of SPL gain through doubling of cabinets...

So having 4, not coupled, I should have seen a +6dB gain in SPL giving me @ 128dB SPL... Now let's say I take 4 of these subs at 8 ohms each, each is still getting the 700 watts, and cube them in a 2x2 array... They'd be on the floor and not coupled to any other boundary other than themselves. Would that give me an additional +3dB (131/137dB)for coupling them, regardless of quantities or; because I'm coupling 4, instead of 2, I'd see a +6dB SPL gain giving me a 134/140dB SPL, in theory...

So in less specific terms you calculate the SPL gain through quantities of subs first, then add +3dB per pair you couple?

How would the acoustical gain change if they were set by pairs, spread apart more than 15', so 2 on one side of a stage and 2 on the other other side of the stage?

So if the above is true, and I understand what you're saying, is the benefit of coupling infinite? So if I continue to double the number of subs I'll continue to add +3dB in SPL output?

Now here's hopefully an easier question, how close to they have to be to actually couple? Do they physically have to touch? Does a set of 1" rubber feet between each box make a difference?

I'm thinking I remember reading that as long as the boxes are within a 1/4 wavelength of each other at their crossover freq. they'll essentially be coupled, is that correct?

OK, I think I've asked enough for this round, thanks guys. Looking forward to your answers to see if I'm understanding...

[Elliot Thompson]

Here is a quick reply before I go to work.

Once you spread the cabinets around the room everything changes in terms of gain.
The whole idea of calculating the SPL once you couple the subs is having only one boundary (the floor) the subs are sitting on in which the sound is propagating from one source (direction).

Having the speakers scattered about will create room gain, which cannot be calculated for you are do not have all the sound coming from a point of source (one direction). Under those conditions, it must be measured with an SPL meter. An SPL will calculate the overall sound pressure level in the venue at a given distance.

Once you pair two subs together you gain the coupling effect of two subs together and not the other subs on the other side of the room. Having subs from different sources excites the venue more than having the sound propagating from one source and creates more room gain.

Fixed dance clubs follow the above method (Scattering loudspeakers throughout the venue) to have an even amount of SPL around the dance floor without boring your eardrums with a lot of SPL from one direction. 

You should also download the PDF entitled “Boundary Cancellation - How to Eliminate Low Frequency Cancellation with Proper Woofer Placement.”

http://www.peavey.com/support/technotes/

Best Regards,

[Douglas R. Allen]

Have 2 like subs.
Put 1 of the subs down in a field with the other sub not around and put 1 watt into the sub. Say you get 98dbs at 1 meter.
Bring the other sub over :edit: and set it down beside the other sub touching and put 100 watts into it. This will give you 100 watts into each sub or 200 watts total ( In this example the subs have the same phase response )
You will now have 104 dbs on your meter.

You gain 3dbs by doubling the power. 3 dbs for twice the surface area. 6 dbs in total when you double your subs in this maner.

As you continue to double your subs/power this will continue ( if stacked correctly ) until you reach the wave length that the subwoofer system is reproducing. If I remember right.... :-/

Of course this is very general but easy to try for yourself.
Measure for yourself and you'll see you get more than 3 dbs when you double subs and power when they are properly coupled.

Douglas R. Allen

[Lee Buckalew]

You gain 3dbs by doubling the power. 3 dbs for twice the surface area. 6 dbs in total when you double your subs in this maner.

As you continue to double your subs/power this will continue ( if stacked correctly ) until you reach the wave length that the subwoofer system is reproducing. If I remember right.... :-/

Of course this is very general but easy to try for yourself.
Measure for yourself and you'll see you get more than 3 dbs when you double subs and power when they are properly coupled.

Douglas R. Allen

In my experience, in real world layouts you won't ever see a 6dB/SPL doubling of subs over there entire operating range since the phase response can't be exactly the same with drivers in physically different locations (even side by side).  With really good subs that are very consistent from driver to driver and box to box you may get a 4dB/SPL increase.  That tends to be a reasonable figure for estimating.

Lee

[Ivan Beaver]

I, sadly, do understand that 2-3dB isn't a big deal but as the cabinet quantities increase that little bump of 2-3dB will eventually start to add up, I'd think. Kind of like taking a penny and doubling it each day, eventually (30 days) you'll have $5.3 million Got a penny for me? LOL!

Actually you have it wrong.

Think of it in this way.  You start to work a day for $1 .  But the deal is that you have to work twice as many days as before to get a $1 raise.

So you work 2 days for$2.  Then 4 days for $3.  Then 8 days for $4 and 16 days for $5.  That is NOT $5 per day-but $5 total.

When you think of doubling the power to get 3 dB-it is easy at first.  But when you get to 10,000 watts-you have to go to 20,000 watts.  When you get to  300,000 watts, you have to go to 600,000 watts, to get the same 3dB as you would from 1 watt to 2 watts.

[Brad Weber]

Do the boxes actually have to be touching each other or are they ok as long as they're within 1/4 wavelength apart at their crossover point? If you have subs crossed over at say 90Hz and the subs are within 3' of each other on the same plane, are they effectively coupling? If you move them closer will the coupling strentghen to the full 3dB or does it matter?

I think many people believe that there is no coupling at greater distances and then at 1/4 wavelength you suddenly get a 3dB jump due to coupling.  But it does not work that way, the interaction varies and 1/4 wavelength is simply the point at which some nominal mutual coupling begins to occur for two sources while the theoretical additional 3dB from full coupling assumes not only both sources being the same level but also both sources literally occupying the same space.
 
In your example, at 90Hz 3' is just under 1/4 wavelength so you might get some coupling, but less than 3dB.  If you moved the sources closer together you would both increase the coupling at lower frequencies and increase the frequency below which some coupling occurred.
 

You gain 3dbs by doubling the power. 3 dbs for twice the surface area. 6 dbs in total when you double your subs in this maner

Since you are addressing acoustic summation of the output of the speakers then what matters is the acoustic output of each source and the physical relationship of those acoustic sources, neither the power to the speaker nor the surface area are relevant except in terms of how they affect the output or physical relationship.
 
Two identical sources (i.e. reproducing the same content at the same level) with random coherence will result in a 3dB increase.  If those same two sources are co-located so as to be phase coherent with one another it will be a 6dB increase.  This throws many people as in acoustics you are often addressing sources with different content or with no specific phase relationship so the outputs sum directly.  It is only when you start dealing with sources reproducing the same sound and with the direct sounds arriving with some relevant relative phase relationship between them that summation and coupling start to be a factor.

[Jeffhtg (Jeff Kenney)]

Actually you have it wrong.

Think of it in this way.  You start to work a day for $1 .  But the deal is that you have to work twice as many days as before to get a $1 raise.

So you work 2 days for$2.  Then 4 days for $3.  Then 8 days for $4 and 16 days for $5.  That is NOT $5 per day-but $5 total.

When you think of doubling the power to get 3 dB-it is easy at first.  But when you get to 10,000 watts-you have to go to 20,000 watts.  When you get to  300,000 watts, you have to go to 600,000 watts, to get the same 3dB as you would from 1 watt to 2 watts.

/lurkoff

$1 paycheck = 1 day
$4 paycheck = 2 days ($1/day is what you asked me to work + $1 day raise)
$6 paycheck = 3 days
$12 paycheck = 4 days

I'm calling my lawyer.. Ivan's trying to rip me off!
/lurkon

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