# Somethings up with db's and cone area...



## Rrrrolla (Nov 13, 2008)

People keep saying (for instance) that if you add another subwoofer to a system with the same amount of power as the existing one, you get a 6db gain. I do NOT understand this logic. In my mind, this should be twice as loud, not 4 times as loud. I keep hearing that if you double cone area or power you get a 3db increase in output. How the heck do you get an increase by doubling cone area (adding another sub) without giving it any power, also, how the heck do you get any more sound from putting power into something with NO cone area. Don't they need each other to exist?Something is not right here. Can someone please explain this to me in a way that will make sense?


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## minbari (Mar 3, 2011)

ok lets take this as an example. these numbers are made-up, but the math is not.

(1) 12" sub with 1000 watts and it puts out 120db.

If you add one more 12" sub and split the power, you will get 3db more sound. double the cone area, but you have not added power. each sub will get 500 watts.

now if you add one more sub AND double the power, then you will get 6db. double the cone area and double the power. 1000 watts each, for a total of 2000 watts.

where the confusion might come in is because most people dont replace the subs coil configuration and/or amplifier. 

So if you had a 12D4 sub wired to 2ohms on a 1ohm stable 2000watt amplfiier you will get 1000watts to the sub. adding a second 12D4 will drop the ohms to 1, giving you 2000 watts and double the cone area. 6db gain.


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## trevordj (Feb 22, 2009)

Rrrrolla said:


> People keep saying (for instance) that if you add another subwoofer to a system with the same amount of power as the existing one, you get a 6db gain. I do NOT understand this logic. In my mind, this should be twice as loud, not 4 times as loud. I keep hearing that if you double cone area or power you get a 3db increase in output. How the heck do you get an increase by doubling cone area (adding another sub) without giving it any power, also, how the heck do you get any more sound from putting power into something with NO cone area. Don't they need each other to exist?Something is not right here. Can someone please explain this to me in a way that will make sense?


A *10dB* gain is often quoted as being perceived to be twice as loud, not 3dB. A 6dB gain is therefore not perceived as *4 times as loud* as you mentioned.


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## trevordj (Feb 22, 2009)

Rrrrolla said:


> How the heck do you get an increase by doubling cone area (adding another sub) without giving it any power


Because more air is displaced...



> also, how the heck do you get any more sound from putting power into something with NO cone area


I assume you mean how do get increased output by increasing the power to a particular subwoofer without changing its cone area. The answer is, assuming you are within the mechanical and thermal limits of the speaker, the increased power will lead to increased cone excursion and increased acoustic power (increased volume).


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## ecbmxer (Dec 1, 2010)

Because it is a non-linear relationship


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## Oscar (Jun 20, 2010)

It is solely because of the *mutual coupling* of frequencies when the centers of the drivers are within 1/4 wavelength of the highest frequency being played. For typical subwoofers that are crossed over @ 80-100Hz, the 1/4 wavelength ends up being between 2.8-3.5 ft. So for 80Hz LPF, so long as the centers of the drivers are within 3.5 ft of each other you get an extra 3dB without doubling the power. If you double the power instead of splitting it between two drivers, then you get another 3 dB for a total of +6dB. Minibari had it close, but it was only a re-iteration of what you had already heard, not an explanation as to _why_. This ^^^^ is why. 

(_Note as was said before, this is NOT NOT NOT 4x as loud. You need a 10dB increase to be just twice as loud, hence you would need a 20dB increase to be 4x as loud_.)

As an extreme example, if you had a 12" with 500 sqcm of piston area, and replaced the cone with one with 1000 sqcm area with the same weight as the original (and an appropriate frame/basket of course) with the same motor, you'd also get 3 dB increase. This is because everyone that has actually read books like Vance Diackson's Loudspeaker Cookbook can see that the formula for 1W/1m SPM is _directly dependent_ on *Vas*. Doubling piston area doubles Vas, which in turn doubles sensitivity. BUT, with double Vas you need to increase enclosure volume accordingly, as nothing is truly free---hoffmans iron law prevails always. All this stuff is "hidden" in these things called books.


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## trevordj (Feb 22, 2009)

Oscar said:


> Vance Diackson's Loudspeaker Cookbook... All this stuff is "hidden" in these things called books.


Quoted for truth. I think this book should be required reading. Not that I understand everything in it, but it is definitely worth a read. 

By the way... I did some google advanced searches about 6 months ago and found this book in electronic format online, for free.


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## Sine Swept (Sep 3, 2010)

I read somewhere that 120dB is 1 trillion times louder than 0dB!!


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## Orion525iT (Mar 6, 2011)

Sine Swept said:


> I read somewhere that 120dB is 1 trillion times louder than 0dB!!


No 1 trillion times louder than 0db, is still just 0db.


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## Rrrrolla (Nov 13, 2008)

Orion525iT said:


> No 1 trillion times louder than 0db, is still just 0db.


Thats not correct. 0db SPL is actually a measurement of sound level. There exists levels of sound below that, into the negatives.

Anyhoo, Still dont see how you get 3db's by doubling cone area and by adding another subwoofer and NOT powering it up. And then when you do power it up, you get another 3db's. Its a pretty simple concept that if you double pistonic DISPLACEMENT you get 3db's of gain, at least it is in my head. But I keep hearing that you get 6db's. So is it 3db's or 6db's for twice the pistonic displacement? Regardless of the method you use.


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## minbari (Mar 3, 2011)

Rrrrolla said:


> Thats not correct. 0db SPL is actually a measurement of sound level. There exists levels of sound below that, into the negatives.


ussualy when you express something in a neagtive "DB" you are referancing that to something. almost always that reference is max output. so if you have a volume setting of -30db, then it is 30 db less than maximum.

an actual 0db SPL is as low as it can go. you cant have negative SPL


> Anyhoo, Still dont see how you get 3db's by doubling cone area and by adding another subwoofer and NOT powering it up. And then when you do power it up, you get another 3db's. Its a pretty simple concept that if you double pistonic DISPLACEMENT you get 3db's of gain, at least it is in my head. But I keep hearing that you get 6db's. So is it 3db's or 6db's for twice the pistonic displacement? Regardless of the method you use.


you are absolutely right, if you dont power it, you get nothing. I dont think anyone is suggesting that you are not giving the second sub power, that is retarded. 

3db gain for double the cone area OR double the power. 

6db gain for double the cone area AND double the power.

if you only double the cone area, you will only see 3db gain. as I mentioned above. if you have a single 12" sub with 1000 watts, then add a second 12" sub and split the power between them (so they get 500 watts each) then you gain 3db. If you add a second sub and add a second amplifier so they BOTH get 1000 watts, then you gain 6db.


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## Rrrrolla (Nov 13, 2008)

minbari said:


> ussualy when you express something in a neagtive "DB" you are referancing that to something. almost always that reference is max output. so if you have a volume setting of -30db, then it is 30 db less than maximum.
> 
> an actual 0db SPL is as low as it can go. you cant have negative SPL
> 
> ...


The db scale is a relative scale, and yes, you can have negative db ANYTHING, if 0db SPL was no sound at all, then the scale would not work. 0db SPL is some amount of sound, that is a given. It is the amount of sound to which all other sounds are referenced.

Why would you get 3db if you add another sub and give both half the power? They will each now be displacing half the amount of air as compared to before. The amount of spl total should be exactly the same. Does anyone see what I'm saying here? You doubled the cone area, yes, but you are moving each cone half as far. This should not be a 3db gain should it?


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## minbari (Mar 3, 2011)

Rrrrolla said:


> The db scale is a relative scale, and yes, you can have negative db ANYTHING, if 0db SPL was no sound at all, then the scale would not work. 0db SPL is some amount of sound, that is a given. It is the amount of sound to which all other sounds are referenced.


maybe you need to look up the difference between decibels and DB SPL. There is a difference. decibel is jsut a logrithmic scale, where you reference it is how it is measured.

read the paragraph on "sound pressure level"

Sound pressure - Wikipedia, the free encyclopedia



> Why would you get 3db if you add another sub and give both half the power? They will each now be displacing half the amount of air as compared to before. The amount of spl total should be exactly the same. Does anyone see what I'm saying here? You doubled the cone area, yes, but you are moving each cone half as far. This should not be a 3db gain should it?


Has to do with the way they acoustically couple. model it if you dont beleive me.


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## Oscar (Jun 20, 2010)

Rrrrolla said:


> Thats not correct. 0db SPL is actually a measurement of sound level. There exists levels of sound below that, into the negatives.
> 
> Anyhoo, Still dont see how you get 3db's by doubling cone area and by adding another subwoofer and NOT powering it up. And then when you do power it up, you get another 3db's. Its a pretty simple concept that if you double pistonic DISPLACEMENT you get 3db's of gain, at least it is in my head. But I keep hearing that you get 6db's. So is it 3db's or 6db's for twice the pistonic displacement? Regardless of the method you use.


There is some confusion here. No one said (perhaps other than yourself) anything about adding another un-powered subwoofer driver. That would be pointless as it would act like a passive radiator. In such a weird situation OF COURSE you won't get ANY dB increase what so ever---the other driver is not getting ANY power!! 

Your original inquiry was..


> if you add another subwoofer to a system with the same amount of power as the existing one, you get a 6db gain. I do NOT understand this logic


Here you say you ARE giving the second driver the same amount of power as the 1st. So before ANYONE can answer your question: "So is it 3db's or 6db's for twice the pistonic displacement? " is.......it depends....are the pistonic displacements reproducing identical signals AND are they withing 1/4 wavelength of the highest frequency being played???? You seem to want to inter-mix physical linear displacement with dB levels---this is not the correct thing to do. It is not necessarily a linear relationship. Stick with dB levels and dB levels to keep things simple.

Without answering those questions no one can really answer you. For signals that are not in the same frequency band, not phase-coherent and far apart then you only get +3dB. But then again, we're really talking about subwoofers right? And to be more specific, we're talking about multiple subwoofers located close together right?? And to be even more specific, we're talking about said subwoofers playing identical signals right?? If so, the answer is still in post #6.


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## Rrrrolla (Nov 13, 2008)

0db is the reference, it can NOT be nothing at all. How would that even be possible? Everyday I work with decibel scales, normally we use dbm (Which is a power level referenced to 1mW). How could we measure something referenced to nothing at all LOL! 10000 times louder than nothing is still nothing. Trust me, 0db spl is not NO sound at all. Its just 10^10 times less loud than 100db. And -3db is one half of that.

The way they acoustically couple? Can you elaborate on that? So youre saying if I can make 130dB SPL with 1 ten inch subwoofer, if I use four of those same subwoofers, giving each one exactly 1/4 the power, I can have 136 dB without using no more power that before?

I can model it all day in winisd, it tells me the same thing you are saying, but I think it's wrong. It doesnt make any sense.

Now if I double the power to a single subwoofer I get 3db more cone displacement, agreed. But if I add another subwoofer and give the same amount of cone displacement as the first subwoofer, I should get 3db's.


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## Rrrrolla (Nov 13, 2008)

Oscar said:


> There is some confusion here. No one said (perhaps other than yourself) anything about adding another un-powered subwoofer driver. That would be pointless as it would act like a passive radiator. In such a weird situation OF COURSE you won't get ANY dB increase what so ever---the other driver is not getting ANY power!!
> 
> Your original inquiry was..
> 
> ...


They are the exact same subwoofers in the exact same enclosures, right beside each other, sitting in a meadow, I am measuring exactly 1meter from the center of both cones. Is it 6db's less spl if I unplug one speaker? I think it's 3dbs less.


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## minbari (Mar 3, 2011)

Rrrrolla said:


> 0db is the reference, it can NOT be nothing at all. How would that even be possible? Everyday I work with decibel scales, normally we use dbm (Which is a power level referenced to 1mW). How could we measure something referenced to nothing at all LOL! 10000 times louder than nothing is still nothing. Trust me, 0db spl is not NO sound at all. Its just 10^10 times less loud than 100db. And -3db is one half of that.


come on , this is semantics now. 

"if I have $0 in my bank acount and double it, I have $0, therefore I can never have any money?"

if 0dB SPL is effectively nothing, you can still get louder from nothing. it might be infintesimaly quiet (meaning there is some amount of nearly unmeasureable sound) but it is effectively, nothing.


> The way they acoustically couple? Can you elaborate on that? So youre saying if I can make 130dB SPL with 1 ten inch subwoofer, if I use four of those same subwoofers, giving each one exactly 1/4 the power, I can have 136 dB without using more power that before?


damn skippy


> I can model it all day in winisd, it tells me the same thing you are saying, but I think it's wrong. It doesnt make any sense.


Then all modeling SW is wrong, because they will all say the same thing. all SPL and RTA meters must be wrong too, because they will confirm it.


> Now if I double the power to a single subwoofer I get 3db more cone displacement, agreed. But if I add another subwoofer and give the same amount of cone displacement as the first subwoofer, I should get 3db's.


you get more excursion, that is why the cone displacement is higher.

sorry, but you are wrong, if you dont want to listen to the more than one person telling you this and giving you more than one way to confirm it, then I am done beating my head on the wall.


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## Oscar (Jun 20, 2010)

Rrrrolla said:


> The way they acoustically couple? Can you elaborate on that? So youre saying if I can make 130dB SPL with 1 ten inch subwoofer, if I use four of those same subwoofers, giving each one exactly 1/4 the power, I can have 136 dB without using no more power that before?


Acoustic (mutual) coupling information can be found all day on Google if you do not have access to any libraries what so ever.

Question for you: can you get the centers of the four subwoofers ALL within 1/4 wavelength of the highest frequency you're playing? 




Rrrrolla said:


> They are the exact same subwoofers in the exact same enclosures, right beside each other, sitting in a meadow, I am measuring exactly 1meter from the center of both cones. Is it 6db's less spl if I unplug one speaker? I think it's 3dbs less.


if they are both playing random noise sound that is not phase coherent, then you're right, it's 3dB less. If they are both playing the same exact signal and you unplug one, you will lose 6 dB.


So in summary:

*In-coherent summation* results on-average in a 3dB increase for doubling sound sources.

*Coherent summation* in close proximity (withing 1/4 wavelength of highest frequency) results in on average +6 dB increase for doubling sound sources.


If you wanna think it is 3dB loss in your scenario where you unplug one speaker, so be it. It ain't my loss if you do or do not believe what has already been tested by others to be true.


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## trevordj (Feb 22, 2009)

Oscar said:


> Vance Diackson's Loudspeaker Cookbook can see that the formula for 1W/1m SPM is _directly dependent_ on *Vas*. *Doubling piston area doubles Vas, which in turn doubles *sensitivity. BUT, with double Vas you need to increase enclosure volume accordingly, as nothing is truly free---hoffmans iron law prevails always. All this stuff is "hidden" in these things called books.


Your answer is right there.


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## Rrrrolla (Nov 13, 2008)

I don't see what the difference is if you double cone displacement with one woofer (by doubling opwer to it) and doubling displacement by adding another woofer to the system driven by the same amount of power. Either way you are displacing the exact same amount of air. Can anyone explain (show me a diagram or something) why one is 6db's and one is only 3 db's? I still can't wrap my head around the difference guys. I feel like I'm missing something here, but I can't for the life of me figure out what it is...


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## therapture (Jan 31, 2013)

Rrrrolla said:


> Why would you get 3db if you add another sub and give both half the power? They will each now be displacing half the amount of air as compared to before. The amount of spl total should be exactly the same. Does anyone see what I'm saying here? * You doubled the cone area, yes, but you are moving each cone half as far. * This should not be a 3db gain should it?


Cone travel vs. wattage is not a linear function. If you cut half the power, the cone may still travel 75% of it's max.

As someone noted, it's a logarithmic function.


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## therapture (Jan 31, 2013)

Rrrrolla said:


> I don't see what the difference is if you double cone displacement with one woofer (by doubling opwer to it) and doubling displacement by adding another woofer to the system driven by the same amount of power. Either way you are displacing the exact same amount of air. Can anyone explain (show me a diagram or something) why one is 6db's and one is only 3 db's? I still can't wrap my head around the difference guys. I feel like I'm missing something here, but I can't for the life of me figure out what it is...


Two subs with 1000 watts will drive the cone to 75% of their max excursion. That's "150%".

One sub with 1000 watts will drive the sub to 100% of it's maximum excursion. 

150% of something (sound, or air displacement) is louder than 100%. Get it?



You are still assuming that every watt of power moves the cone a specific amount. It's not linear. As the travel goes up, it takes more and more power to move it the same distance.


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## Rrrrolla (Nov 13, 2008)

This is the first time I have ever heard this (I thought it was a linear relationship until you get close to the excursion limits, then things get muddy). So you're saying if I push my subwoofer with 100w, then 200w, the gain is NOT 3db spl, but 150% which is around 1.8dB? If that's true, then there's another point for all modeling software to be wrong.


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## trevordj (Feb 22, 2009)

Double the subs, same power to the system, Vas increases, sensitivity of the system increases. Read books.


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## Rrrrolla (Nov 13, 2008)

I will start reading


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## therapture (Jan 31, 2013)

Rrrrolla said:


> This is the first time I have ever heard this (I thought it was a linear relationship until you get close to the excursion limits, then things get muddy). So you're saying if I push my subwoofer with 100w, then 200w, the gain is NOT 3db spl, but 150% which is around 1.8dB? If that's true, then there's another point for all modeling software to be wrong.



The numbers I used are arbitrary, but the function/idea is the same.

Problem with doubling power on a single woofer, is that the woofer needs to be able to handle it accordingly (duh) or you don't gain 3db.

In a perfect world, sub excursion would be perfectly linear, but alas, we don't live in a world like that.


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## minbari (Mar 3, 2011)

therapture said:


> Cone travel vs. wattage is not a linear function. If you cut half the power, the cone may still travel 75% of it's max.
> 
> As someone noted, it's a logarithmic function.


this is something I never touched on, but complettely true.

think of it like a spring. the farther you pull the spring, the harder it gets to pull. The spider is just like that.


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## RNBRAD (Oct 30, 2012)

That's why most woofers reach a point of minimal to no gain. Example: 12" kicker XPL's output is pretty much maxed at 100 watts. It can handle 500 though. From 100 watts input to 500 watts input the sub gains 2 decibels. Every sub made operates in the same fashion but they level off at different wattages. In some instances the 2 db is worth it (SPL comp)but for many the cost is not worth the gain.


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## Rrrrolla (Nov 13, 2008)

This makes sense to me:

The entire 6db of gain comes from the fact that each of the two speakers is effectively radiating into half space instead of a single speaker radiating into whole space. 

I dont know why I had such a hard time getting my head around this concept. Hearing it this way, it kinda makes sense to me now. This must be especially true in a car, where the whole space is already quite small. I wonder if it might affect the transfer function.


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## minbari (Mar 3, 2011)

Rrrrolla said:


> This makes sense to me:
> 
> The entire 6db of gain comes from the fact that each of the two speakers is effectively radiating into half space instead of a single speaker radiating into whole space.
> 
> I dont know why I had such a hard time getting my head around this concept. Hearing it this way, it kinda makes sense to me now. This must be especially true in a car, where the whole space is already quite small. I wonder if it might affect the transfer function.


most definately does effect transfer function. effects near field effect too


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## IDGAF (Dec 27, 2009)

It's already been fleshed out in here in theory, but I can confirm the theoretical with real world results.

I compete in SPL and I do a ton of testing. I've seen these numbers in real life. 

As for the adding an unpowered sub into the mix (not sure where that came from), you could see an output bump in the neighborhood of +3db at resonance (passive radiator).


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## Rrrrolla (Nov 13, 2008)

I undesand it now, what I cuoldnt ge tmy head around was the extra 3db coming from thin air. But it comes from the fact that each speakers's new environment that it is producing energy into is now half of what it is with a single speaker. That concept eluded me. The near and far field ideas also make sense to me now. The speakers are having an effect on each others radiatiog envirnoment, thanks for all your patience guys. Its still a pretty abstract idea, especially to think it happens in an open environment, but only close to th speaker and withing a 1/4 wave distance of any given frequency being played. Cool stuff!


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## minbari (Mar 3, 2011)

Acoustics are witchcraft!

Sent from my motorola electrify using digital farts


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## Oscar (Jun 20, 2010)

Rrrrolla said:


> I undesand it now, what I cuoldnt ge tmy head around was the extra 3db coming from thin air. But it comes from the fact that each speakers's new environment that it is producing energy into is now half of what it is with a single speaker.


Nope, your understanding is still incorrect. Adding sound sources does not change the acoustic radiation pattern. It narrows the dispersion _a bit_, but at bass frequencies you don't go from "whole space" into "half space". Inside a vehicle, you're already in half-space to begin with! Even though this a whole 'nother topic which you can research on your own, I will list some basic examples

4pi space = a speaker hanging in the air well above the ground and far from any other surfaces
2pi space= a speaker laying on the ground firing upwards to the sky
1pi space= a speaker laying on the ground, but firing towards you from against a wall in a large building, such as a concert hall.
0.5pi space= a speaker in the corner of a room, on the ground, immediatly next to two walls on both sides.

The small spaces of a vehicle interior are _so confined_, that any subwoofer is already in 0.5pi space (regardless if it is next to any walls or not).

What you need to read up on is "mutual coupling", which you haven't searched for, not even in google, because I know for a fact that a few good links come up, such as these:

Mutual coupling question - Page 3 - diyAudio

^^^ read that a few times until it sinks in ^^^^^^

This is a very advanced question, believe it or not, that you have asked, and without actually studying the science (meaning: mathematics) of acoustics, you'll just have to take our word for it. So the extra 3dB DOES come from thin air! The problem is without knowing acoustic principles such as mutual coupling, the answer is as invisible as the air.


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## Rrrrolla (Nov 13, 2008)

I did read about mutual coupling, and that's where my understanding came from. I will keep reading since I'm wrong. Thanks for the link!


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## Rrrrolla (Nov 13, 2008)

The link you posted is exactly what I read, Bill Fitzpatrick is the one that posted the idea that I re-itterated, or at least tried to in my own words. How far off the mark am I? Because it actually makes sense to me.


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## strakele (Mar 2, 2009)

I was all set to provide a simple answer and example of this, but after I sat down to think about it, it is less intuitive. The coupling answer really is the only way to explain what otherwise doesn't really make sense.

I had to make sure though, so I set up 2 speakers right next to each other in a basically empty room, Behringer mic 1m away, and played some tones. Unplugging one yielded a 6dB drop. It's real.


Also, for the dB discussion: http://www.makeitlouder.com/Decibel Level Chart.txt

There are 'sounds' less than 0dB. 0dB is basically the threshold of hearing. Less than that, as an example from the link is something like the sound of a shrimp chewing food from 100m away. Nobody could actually hear it - takes extremely powerful microphones, but it's there. There's a lot of cool stuff in that link, including the upper limit to SPL, which is 194dB. After that it stops really being 'sound waves' and becomes a shock wave.


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## Oscar (Jun 20, 2010)

Rrrrolla said:


> The link you posted is exactly what I read, Bill Fitzpatrick is the one that posted the idea that I re-itterated, or at least tried to in my own words. How far off the mark am I? Because it actually makes sense to me.


Well something is wrong here, because I just clicked that link that I posted and it took me straight to a post by user "cocolino". Is this not the post that you get when you click on the link?? Perhaps you clicked on the link and tried to make sense of the whole entire post? I can't really tell what you did from my end since I'm not there to witness what you're seeing on your computer screen.....but what I can tell you is that I linked you the exact post in a thread that I wanted you to read. Let's start with that----did you read post #22 by user "cocolino"?

edit: I _think_ I see what you did. You skipped right over that loooong post that my link takes you to, that goes over the phenomenon of mutual coupling, and instead you went to post #27 and read Bill F's incorrect idea.


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## Rrrrolla (Nov 13, 2008)

I read the entire thread, and either way you look at it, from Cocolino's or Bills perspective, it makes sense. I like Bill's way of looking at it because it is simpler, but I understand Cocolino's explaination. I did have to read it a few times, but it all came together for me when I read Bill's post. Anyways, it's good to know, because it's definitly not immediately intuitive. This has convinced me to use 2 subwoofers instead of 1. I was going to use 1 that had twice the power handling and xmax, but I am going to change it out for sure now with the pair I have sitting around.


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## Oscar (Jun 20, 2010)

Rrrrolla said:


> I read the entire thread, and either way *you* look at it, from Cocolino's or Bills perspective, it makes sense.


Not for me. The way *I* look at it, Bill is incorrect on his idea. If you [hopefully] continue to educate yourself on advanced topics like this about acoustical radiation patterns, you'll [hopefully] eventually see why BillF is wrong on this. cheers.


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## Rrrrolla (Nov 13, 2008)

Its not an easy concept thats for sure. But I have a pretty good grasp on it now. I also undestand Pi space radiation, already understood it as it pertains to RF transmission. Someone that is having a hard time understanding where ther other 3db comes from could PROBABLY read Bills thread and understand that it does not just appear for no aparent reason. Its kind of a backwards way of looking at it, as its probably easier to picture the room being half as big to each radiator with 2 radiators of the same excursion than to picture the two waves from 2 radiators affecting each other's acousic impedance in free space because they are close to each other. I'm just going to continue to picture the room space dividing in half each time you double the cone area, ie. 4 of the same suboofer in a car will yeild a gain of 12db's instead of the intuitive 6db.


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## Oscar (Jun 20, 2010)

Just because it's easier, does not make something correct.

If that were true, then answer me this....

If you have a speaker radiating in 4pi space where there are NO walls _what so ever_ that the speaker can interact with---simply hung miles above and away from anything (that_ is_ the definition of 4pi space to begin with), and add another speaker in the same kind of situation that we've been talking all along----another identical speaker playing identical content, right next to the 1st one so that it is within 1/4wavelength of the highest freq. being played, receiving the same original amount of power as the 1st one was/is. If you measured the output you _still_ get +6dB compared to just a single one. 

How can this be if there is no "_room_" what so ever to "_seem half as big to each of the radiators_". Remember, this is 4pi space (it is completely boudarly-less) where sound waves dissipate completely into the atmosphere. There is nothing around the speakers what so ever. Yet still we would see +6dB compared to one speaker.......your view about radiation space being transformed by room-boundaries fails to work in this example since there is no room/vehicle/etc. So there is only one possible explanation.....the original assumption about transforming the radiation space being the reason for the extra +3dB is _in_correct..........therefore the explanation is the same explanation as before.

In mathematics we call this Proof By Contradiction. Feel free to google if you like. 


Just trying to enlighten you....


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## gijoe (Mar 25, 2008)

I think I missed something. Is "mutual coupling" the same as constructive interference? I'm not familiar with mutual coupling with regard to acoustics, so it's either something I haven't learned about (only one college physics class on sound), or it's what I'm thinking of as interference. Help me out.


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## Oscar (Jun 20, 2010)

gijoe said:


> I think I missed something. Is "mutual coupling" the same as constructive interference? I'm not familiar with mutual coupling with regard to acoustics, so it's either something I haven't learned about (only one college physics class on sound), or it's what I'm thinking of as interference. Help me out.


It is a special case of "constructive interference", which is a more general term. 

In both constructive interference and mutual coupling, both sound sources are in-phase with each other, _but_ in mutual coupling, the sources' centers are within 1/4 wavelength distance of the highest frequency being played. Constructive interference doesn't have this special condition to be met, which is why it's a more general situation. 

So mutual coupling is _always_ constructive interference, but constructive interference _isn't always_ mutual coupling (due to the extra condition necessary).


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## rambronco (May 10, 2013)

I wrote a post to put it as much in laymens terms as possible but that was before I saw there was a second page, and after reading some of the posts on the second page I realized it has gone way beyond laymens terms 8)~~~~~~.


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## Rrrrolla (Nov 13, 2008)

Oscar said:


> Just because it's easier, does not make something correct.
> 
> If that were true, then answer me this....
> 
> ...


I'm enlightened bro! You are certainly a very intelligent man. I had no idea the concept even existed until this thread. I understand what you are saying. And I agree that everything changes depending on the radiation "field" and reflective surfaces (or lack thereof). But in a car, and at a low enough frequency, a great way to look at it is to picture the cars volume diving in half each time you double subwoofers with same frequency, power, phase... At least then its easy to picture the effect in your mind, for me it's more difficult to picture the radiation pattern in free space, there are too many variables that would change the field where the two speakers would be having an effect on each other. Seems like a simpler concept in 1/2 pi space. I'm going to leave the free field picture to your brain to play with.


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## gijoe (Mar 25, 2008)

Oscar said:


> It is a special case of "constructive interference", which is a more general term.
> 
> In both constructive interference and mutual coupling, both sound sources are in-phase with each other, _but_ in mutual coupling, the sources' centers are within 1/4 wavelength distance of the highest frequency being played. Constructive interference doesn't have this special condition to be met, which is why it's a more general situation.
> 
> So mutual coupling is _always_ constructive interference, but constructive interference _isn't always_ mutual coupling (due to the extra condition necessary).


Thanks Oscar, I appreciate the clarification!


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