Double 136a's with single 2118?

[tagerbaek]

Hello Lansingheritage,

I started building speakers just 3 years ago, so I'm still a rookie struggling with the basics. Here's two interrelated questions:

Q1. I've got four 136a's and a pair of 2118J, all in good shape. I plan to do a 3- or 4-way, active x-over. In your experience, will a single 2118's cut it in terms of power handling, output, etc, the double 136a's having a efficiency of 93+3=96dB (see Q2) vs the 97dB of the 2118, or should I lose them and get 2123's?

Q2. About dual drivers, there seems to be general agreement that if one uses two drivers in parallel, eg two 136a's, each with a sensitivity of 93dBspl/2.83Vrms/1m, one will get +6dB more output, eg 99dBspl, than with a single driver (with the same rms voltage drive). Fine, the 3dB are because you deliver twice the power to the drivers, but I cannot grasp why each driver apparently becomes twice as efficient (ie, the remaining 3 dB) just because it's now working in tandem. Taking the argument ad absurdum and doubling the number of drivers - and therefore also the efficiency - over and over, one eventually arrives at a situation where the drivers would apparently be delivering more acoustic power than they are receiving in the form of electric power. That was a long question, I hope someone knows a shorter answer!

Thanks in advance, Thomas

[edgewound]

The quick answer is because you are doubling the cone area with two drivers... that gives you the second additional 3dB of output.

The first additional 3dB of output is by halving the impedance or doubling the load to 4 ohms from 8 ohms.

[macaroonie]

The efficiency of the bass drivers is irrelevant --- since you plan to go fully electronic the overall system efficiency is that of the least efficient diver.
As an example lets say your tweet was a ribbon with effficiency of 86dB therefore to even begin to sound correct everything else will have to come down to that level either by padding or by gain reduction at the amp. stage.

[toddalin]

The quick answer is because you are doubling the cone area with two drivers... that gives you the second additional 3dB of output.

The first additional 3dB of output is by halving the impedance or doubling the load to 4 ohms from 8 ohms.

If you think about it, I don't think that is quite right.

First, lets assuming that the amp can deliver the current so that the voltage doesn't sag and the power does truely double when halving the impedience.

Now assume that at 1 watt the driver would have a peak to peak excursion of 0.1" producing 90 dB.

Add the second driver and the power goes to 2 watts. But each driver is still receiving just 1 watt so still has a p-t-p excursion of 0.1". You are now moving exactly twice the air (and that's what is actually making the sound regardless of the watts involved), so the sound should twice the power, or 3 dB louder so now 93 dB.

Now lets say that the amp has a max of 100 watts RMS (then it blows up) and each speaker can hold 100 watts (then they blow up). So, the amp would run out of power before you ever got the additional 3 dB would would only be possible at maximum cone excursion for both drivers.

I don't think it works like that.

What I've been able to deduce is that when you add the second speaker and halve the impedience you pick up 3 dB by virtue of doubling the power and moving area.

The other 3 dB comes from the "acoustic coupling" of the speakers. Assuming the speakers are in phase, each speaker produces as much sound as the single speaker did. But the acoustic peaks of the sound waves are additive and this is where the other 3 dB come from.

This is the case at any and all volume levels.

I've never read this anywhere, but having a grasp of acoustics, this is really the only way I can reconcile it in my mind. :hmm:

[edgewound]

Todd,

Here's the deal....theoretically, anyway. He wanted the short answer.

If you have a 130A(8 ohm) 1 watt/ 1 meter...you have 97dB sensitivity.

If you have (2) 130B(16 ohm) 1 watt/ 1 meter...now you have 100dB sensitivity...twice the radiating cone area....coupling if you will.

Now...(2) 130A @ 97dB 1 watt/ 1 meter each....now pulling 2 watts with double the cone area and double the power...

You now have 103dB sensitivity...twice the speaker, twice the power.

To get there with one 130A, you would need 4 watts or four times the power for a 6dB increase in SPL to make up for half the cone radiating area.

[toddalin]

Todd,

Here's the deal....theoretically, anyway. He wanted the short answer.

If you have a 130A(8 ohm) 1 watt/ 1 meter...you have 97dB sensitivity.

If you have (2) 130B(16 ohm) 1 watt/ 1 meter...now you have 100dB sensitivity...twice the radiating cone area....coupling if you will.

Now...(2) 130A @ 97dB 1 watt/ 1 meter each....now pulling 2 watts with double the cone area and double the power...

You now have 103dB sensitivity...twice the speaker, twice the power.

To get there with one 130A, you would need 4 watts or four times the power for a 6dB increase in SPL to make up for half the cone radiating area.

Nope, still don't see it.

When you have both at 1 watt/1 meter, you've doubled the power (each gets 1 watt) so you can't double it again or you're double counting..., or each is only receiving 1/2 watt and moving half as far.

[edgewound]

Nope, still don't see it.

When you have both at 1 watt/1 meter, you've doubled the power (each gets 1 watt) so you can't double it again or you're double counting..., or each is only receiving 1/2 watt and moving half as far.

Ok....nevermind.

[Allanvh5150]

Here is a much easier way to look at it. The sesitivity of JBL's are generally measured with one watt input. If you have a speaker with a 97db sesitivity driven by 1 watt, adding a second speaker using the same test, i.e. 1 watt input, the sensitivity will increase to 100db. If the power input doubles, the figure will increase to 103db, double the power and add another 3db. The sensitivity figure is a baseline figure, the actual SPL figure when driven by, say, a one hundred watt amplifier will be much higher. So in general, if you add another speaker and your amplifier can run to twice the power with half the load, most amps cant do this, the output will increase by 3db. Make sense?:)

[pos]

Regarding mutual coupling and 3/6 db gains I think I just recently understood the whole thing, and it is quite simple in fact.
I will try to expose my point of view, but I hope the language barrier will not blur the message too much.

When you double the amplitude of the signal you get a 6db boost. You can try that on your soundforge or anything like this. That is also the reason why 16bit gives you a 96db dynamic range: 16*6=96.

So if you double the voltage, you double the amplitude of the electric signal, and so you double the amplitude of the cone motion, and you get 6db.

BUT to double the voltage you need to multiply the power by four!
this is given by these equations:
P = UI
and U = ZI

so P = U^2/Z

there is a power of two on the voltage!

So here is the deal:
If you want a 6db boost you have two choices:
- multiply the power by 4 => the cone amplitude will double => +6db
- take 2 amplifiers with the same power (ie mutliply the power by 2) and feed to different speakers => each cone will move the same, but the total cone area has double => the amplitude has double => +6db (with a cotion, see below)

these two things will give you your 6 db, but one case will need twice the power of the other

now there is a draw back, which is stressed by the mutual coupling "theorem" :

If you want two signal to sum perfectly and get your 6db boost, these two signals need to be in phase. If they are not in phase you will get less than your 6db (you can also try that with soundforge, ading sinusoid with a small time shift).
That is the reason why woofers have to be close together to get the "mutual coupling" thingy : The more their distance increases and the less "in phase" the summed signal will be on average (in fact if you stand exactly between the two speakers you will always get your 6db boost, up to a frequency were the distance between your hears starts to matter).
So the more you increase the distance, the longer the wavelength for which the summing will be good.
If you exeed a certain distance you only get a quadratic summation, with an average of 45° phase shift, and you only get a 3db boost...

So, to sum that up:

- IF you double your cone area and you are able to feed each cone with the same power (ie in practice if you have an amplifier that can put twice the power with half the load, OR if you use two amplifiers)

- AND if you seat directly in the middle of the two woofers OR if these woofers are close enough together for the highest frequency (ie the smallest wavelength) they have to reproduce

- THEN you will get a 6 db boost

If the first statement is not true you will not double the amplitude of your signal, so will get less than 6db of boost.
If the second statement is not true you will have a quadratic summation, and only get a 3db boost on average (and up to 6db for the lower frequencies)


The second statement is difficult to maintain, and that is the reason why we always tend to say that doubling the number of speakers and amplifiers gives us a 3db boost, but that is only due to the quadratic nature of the summation you get most of the time...

So when you double the number of speakers and amps (cone area and power) the theoritical gain is always 6db, but the partical gain is only 3db most of the time due to phase issues. "mutual coupling" is a special case where you can enter the theorical territory and get your 6db gain for real, because phase issues can be ignored.

I hope I made my point, sorry for my bad english

[Allanvh5150]

If the speakers are doubled, you get a 3db boost. If the power is doubled, you get a 3db boost. If the amplitude is doubled, you get a 6 db boost. Go back to the original example, 1W/m gives 97db so 2W/m gives 100db. If you put 1W/m on 2 speakers, i.e. .5W on each, it drops the sensitivity of the driver to 94db. The 1W/m rule always has to stay constant for measuring purposes. In the real world if you have 500 watts in to a 2225 for example, to get a 3db increase you would need to put in another 500 watts. This normally means another speaker and amplifier. Doubling the whole system still only yeilds a 3db increase. Look at the specs for the JBL 4430 at 93db and the 4435 at 96db.:)

[pos]

Doubling the whole system still only yeilds a 3db increase.
In pratcice that is often true, due to quadratic summation.
But you will get 6db on axis for most of the frequency range, and 6db everywhere for the bass frequencies (the closer the drivers, the higher the limit frequency).

In phase summation => +6db
quadratic summation => +3db

this is also why some filters are disigned with a -3db gain at the cutoff frequency, whereas others give you -6db. You choose the filter that gives you a flat summation depending on your specifiaction (on axis or out of axis, frequency range, driver distance, etc.)

and this is also why drivers are directive: the larger the cone the higer the directivity, because when you move off axis signals from the left and the right side of the cone will be delayed, and that phase shift will cause high frequencies to sum in a quadratic manner (the larger the cone, the lower that frequency)


Look at the specs for the JBL 4430 at 93db and the 4435 at 96db.:)
That is something else, entierly. The 2234H is 96db sensible by itself. The second 2234 is an helper woofer, and the rolloff and tuning is calculated to get a flat curve with the help of mutual coupling in the bass.
The 4355 is a better exemple of mutual coupling. Here you get 96db with two 93db drivers, but in the bass (maybe under 150hz) you really get 99db and that is the reason for the low tuning of the cabinet.

The two designs have a similar frequency response and sensitivity at the end, but the 4435 design is much clever because you have virtually no phase issue (quadratic summation is not something desirable).

[tagerbaek]

Thanks for the answers, but, no, I still don't get it.

Exemplifying again: take a 90dB/1W driver, feed it 2W, you get 93 dB. Take two drivers, feed them 1W each (coherent) and you get 96dB for the same 2W total power input.
That is strange. It is also absurd, in its extension, because if you keep doubling up, then the cluster of drivers would eventually output more acoustic power than they are receiving from the amplifier. Could it be that the efficiencies of multiple drivers in fact do not add up linearly, but merely appear to do so as long as the efficiencies are far below 100 %?

Thomas

[Mr. Widget]

That is strange. It is also absurd, in its extension, because if you keep doubling up, then the cluster of drivers would eventually output more acoustic power than they are receiving from the amplifier.No, because in this example you also doubled the electrical input. If both woofers are getting a full watt, you are supplying twice the initial power of 1 watt for 1 woofer.

As was also pointed out above, since you are planning an active system it is mostly an academic exercise.

Back to your original question, the 2118 can work. It will be the limiting factor as far as maximum SPL.


Widget

[pos]

Exemplifying again: take a 90dB/1W driver, feed it 2W, you get 93 dB. Take two drivers, feed them 1W each (coherent) and you get 96dB for the same 2W total power input.
Yes, you get 96db IF the drivers are close enough for an inphase summing of the considered frequencies.


That is strange. It is also absurd, in its extension, because if you keep doubling up, then the cluster of drivers would eventually output more acoustic power than they are receiving from the amplifier.It can appear to be strange, but it is not: when you double the power you do not double the amplitude (voltage), because P = U^2/Z.
You need to multiply the power by 4 to multiply the amplitude by 2.
You can see that if you have a speaker simulation software that shows driver excurtion: simulate 1W input and look at the excurtion at a given frequency, that simulate it with 4W and you will see that the excurtion only doubles.

So by adding more speakers and amps you gain more than just adding amps.

But, again, this is only true for in phase signal summing

[toddalin]

POS is basically saying the same thing that I am and there is an easy way to demonstrate it.

You pick up 3 dB by having the two speakers of equal volume level adding their respective "tones." You pick up an additional 3 dB if the tones are the same and in phase because of the way waves (be they sound or water, or ???) add together.

When two wave crests/troughs come together simultaneously, they are additive and this is acounting for the other 3 dB.

OK, the test:

Take two woofers in small enclosures so they can be placed very close or moved apart.

Connect a stereo amp, one channel to each woofer. Play a pure sine wave through each channel (one at a time) and calibrate the sound level using a real sound level meter to some level (e.g., 90 dBL).

(If the mono amp driving two woofers can truly double its power into half the load, then the use of stereo amps of equal power is warranted for this test. But a true mono amp driving both speakers would be better because there could be small phase/delay/frequency issues between the two channels that would alter the wave form and test slightly.)

Now play them simultaneously and observe the new volume level. Is it 3, 6, or some other dB level louder???

Move the speakers apart. Does the addition value change? Bet is does as the wave crests/troughs become less additive.

Now run the speakers out of phase. If is was just a matter of power and cone movement and not the physics of wave addition, the volume level should be no different than if they are in phase. But I bet it is considerably different as the crests/troughs are now canceling each other out, at least to some extent.

Now for the coup de gras.

Use two signal generators, one on each channel. Set one at 60 Hz and the other at 100 Hz, each at 90 dBL. Run them together and observe the volume level. Now sweep the frequency of one to the other and observe the volume level when the wave become additive. I would bet that it gets louder at that point. But the waves must be exactly in phase so as to be purely additive. (I don't know how this would be accomplished with separate signal generators.)

[pos]

Yes toddalin, I agree with you.

But not on that former quote:

Add the second driver and the power goes to 2 watts. But each driver is still receiving just 1 watt so still has a p-t-p excursion of 0.1". You are now moving exactly twice the air (and that's what is actually making the sound regardless of the watts involved), so the sound should twice the power, or 3 dB louder so now 93 dB.

If you move twice the air you get 6db on axis, and the power (acoustical or electrical) is multiplied by 4.

To double the pressure (moved air) you can double the excursion of one driver (ie 4 times as much electrical power) OR add a second driver and feed it with the same power. In this second case you have the same excurtion but twice the area.
These two things lead to the same result, the only problem being that when doubling the cone area you also increase the directivity (ie you lower the frequency for which the summation will be quadratic off axis). That is the whole point of the "mutual coupling" formula

[tagerbaek]

Hello again,

thx for the replies, yesterday our 4yo kid woke us up in the middle of night, so I did some more thinking:

1. Using two drivers definitely adds 3dB efficiency compared to a single driver. Efficiency means dBspl per Watt and is not to be confused with sensitivity, which is dBspl at some given input voltage drive, typically 2.83Vrms.
It is not so strange, per se, that two drivers have a combined efficiency that is double that of single driver, considering that the combination amounts to a single driver having twice the motor force (BL product) and twice the piston area SD.

2. The reason that the efficiency does not rise indefinitely when adding more drivers has nothing to do with amplifier output, coherent addition, wavelengths, etc.
It is because the work/second (ie Watts) that the cone performs in order to move the air in front of it does not stay constant when adding more drivers.
If a single driver has to move the air, the pressure is relatively low. This means that when you're feeding the driver 101 Watts, for example, 1 Watt goes towards moving the cone, say 1mm. The other 100 W goes into heating up the VC. Using two drivers, because the partial gas pressures from the drivers add up, each driver now uses 2 Watts to move the cone 1mm, because the work performed is proportional to the pressure, while still losing 100 W in ohmic VC losses.
The point is that the difference between 101 W and 102 W is so small that we ignore it.
However, as the number of drivers increases, the work that goes into moving the cone is no longer negligible, compared to the VC losses. Taking 25 drivers, for example, the total gas pressure is now 25 times higher, and each driver must be fed 125 W (100 W for the VC and 25 W for the air load) to move the cone that same 1mm. With 100 drivers, each speaker needs 200 W total to move the cone 1mm.
An interesting point is that while the efficiency of EACH driver actually rises (from 1/101 = 0.99% with a single driver, to 25/125=20% with 25 drivers, to 100/200=50% with 100 drivers - the same effect that is seen in horn loading, where the driver works against a higher pressure) the combined system efficiency does not rise indefinitely.
These simplified examples are just to show the basic mechanism. There are, of course, lots of second order effects that I have ignored.
It could be interesting do try it out using compression drivers, which are typically 20-30% efficient, thus the diminishing "return on investment" wrt efficiency with multiple drivers should be measurable fairly quickly.

Cheers, Thomas

[Ian Mackenzie]

The benefit of twin 15 inch woofers is improved power handling and lower distortion for a given spl below say 100 hz.

Just plug in two 136 A into Bassbox and see for yourself.

If you are going active as most of us do with these drivers you will have more dynamic headroom using twin woofers in parellel. Its mean you are less likely to approach the X max of the woofers in practise.

In the case of the 2118J its a nominal 16 ohm load and has -3 db less net senitivity than the 2 136A drivers in parellel so it will be necessary to scale and proportion your amps accordingly.

I would recommend an amp rated for 200 watts rms into 8 ohms for the 2118J as this will deliver 100 watts into the 16 ohm load presented by the 2118J or possibly a bit more to ensure it can keep up with the 2 x 136As.

Crossover over the 2118J between 150-250 hertz with a minimum 12 db butterworth slope.

In this case an amplifier rated at 100-150 watts into 8 ohms that can deliver 200-300 watts watts continously into 4 ohms would be appropriate for driving the 136A's. The key is to allow a minimum of 6 db headroom so you are not clipping the amps at nominal listening levels or stressing the drivers .

The late John Eargle's Recording Handbook covers the subject very thoroughly with several examples of JBL monitor design.

I am not sure if JBL still views these forums but itwould be a good idea for you guys to buy or loan a copy.

[pos]

Sorry tagerbaek, but you are wrong.

Moved air is not linearly proportionnal to power! To double the amount of moved air you need to multiply the power by 4.
P = U^2/Z


]It is because the work/second (ie Watts) that the cone performs in order to move the air in front of it does not stay constant when adding more drivers.

This is the important point:
what the cone perfomes is moving air. The volume of air that is moved is given by the cone area multiplied by the cone excursion. It is not directly related to the power, but to the signal amplitude.

If you double the power you gain 3db. No more no less.

If you double the volume of moved air you gain 6db, and to do that you can do two things (one or the other) :

1- keep the same cone area and double the excursion. Here you need to give your woofer 4 times the power (because P = U^2/Z, if U is doubled P is multiplied by 4)

2- double the cone area and keep the same excursion. Here you need to add a second woofer, and to keep each woofer's excursion the same you need to double the power (by using the amplifier ability to drive a lower load with more power or by adding a second amplifier)

case 2 need half the power to get the same result, but it needs the two acoustical signals from the two cones to ass in phase, wich in practice is only true for "low" frequencies (where "low" is given by the mutual coupling formula) or perfectly on axis listening.


EDIT, as a side note: Of course you do at get a 6db boost each time you add a woofer: you need to *double* the number of woofers to get this boost. So from exemple going from 1 to 2, from 2 to 4, etc...

[tagerbaek]

thanks for the input Ian,

I reckoned that the single 2118 could just about keep up with the double 136a's. Max spl is a close match.

Impedances etc are not an issue, I have six Crown gradient amplifiers (for MRI), each rated 4kWrms/30kW peak, which I shall be using (a bit modified and down-scaled, though :)) to drive the units.

Intuitively, two 4in VC's vs a single 2in VC seems a bit skewed, but OTOH, the 136a's are probably displacement limited anyway before power handling becomes a issue. That's why I was asking for LH opinions. I'll go for it, play with the x-over freq, and if the 2118's do like this :barf:I'll substitute 2123's or something else.

Thomas

[tagerbaek]

pos, I think we actually might be in agreement, our differences residing in the terms in which we argue:

If you read my argument, I'm sticking to a constant displacement, ie 1mm.

PV=pressure x volume = energy, where:

the volume in question= piston area SD x displacement.

the pressure in question is the sound pressure level spl.
The dimension of spl = pressure (Pascal).

To move a cone (one cycle) at a certain pressure costs a certain amount of energy.

To cycle it continuously costs power = energy x cycles per second. This is what a loudspeaker generally does.

If you have two drivers, their partial gas pressures add up, giving double pressure in front of the drivers. This accounts for the 6dB increase in dBspl. Note that dBspl is defined as 20log(P/Pref), where Pref is 20uPa. A doubling amounts to 6dB (and not to 3dB, as the case is with power, which is 10log(P/Pref)).

Thus, with two drivers, we get +6dBspl, but at what price? The drivers certainly each deliver double power into the air, since the pressure has doubled, while the displacement and the piston area have remained the same. The pair of them thus deliver 4 times the power; power which has to be supplied by the amplifier.

This sounds like we have gained nothing: four times the acoustic power out, which the amplifier simply supplies. But the trick is that, to take my previous example, if the single driver delivers 1 Watt acoustic output, the pair of them delivers 4 Watts of acoustic power. But this is next to nothing compared to the 100W lost in each VC, regardless of whether the driver is operating alone or paired. So, for a single driver: 101W electrical input gives 1W acoustic output. For a pair, 204W input gives 4 Watts output. That's pretty much a doubling in efficiency.

That's it for me, thx for the input, I'm going skiing now. Over and out. Thomas

[pos]

Thus, with two drivers, we get +6dBspl, but at what price? The drivers certainly each deliver double power into the air, since the pressure has doubled, while the displacement and the piston area have remained the same. The pair of them thus deliver 4 times the power; power which has to be supplied by the amplifier.

This sounds like we have gained nothing: four times the acoustic power out, which the amplifier simply supplies.

No, with two drivers the amplifier only needs to supply twice the power for this 6db boost to happen.

[toddalin]

[quote=pos;203969
If you double the volume of moved air you gain 6db
[/quote]


OK, this one still bother me. It I have one backhoe digging a hole and it produces 87 dB at 50 feet, and I add a second backhoe, the level goes up to 90 dB (based on all of the literature).

But two backhoes are moving twice the air volume, so why wouldn't this also increase by 6 dB to 93 dB? :blink:

[pos]

Yes, here you get a +3db boost because these two signals are not in phase: they are not even the same. So you have a quadratic summation, as if you were summing two different pink noises together. The two signal are similar in frequency response, but they are not correlated.

Now record the sound of one backhoe digging and, later on, reproduce this sound with two speakers. Now you will have a chance to have an in phase summation at some places and frequencies (when you are exaclty in equal distance from the two speakers, and also everywhere else for the low frequencies) because now the two signals are correlated.

[toddalin]

Yes, here you get a +3db boost because these two signals are not in phase: they are not even the same. So you have a quadratic summation, as if you were summing two different pink noises together. The two signal are similar in frequency response, but they are not correlated.

Now record the sound of one backhoe digging and, later on, reproduce this sound with two speakers. Now you will have a chance to have an in phase summation at some places and frequencies (when you are exaclty in equal distance from the two speakers, and also everywhere else for the low frequencies) because now the two signals are correlated.

So you are back to where I was originally. :blink:

Simply moving twice the volume of air only produces a 3 dB change and the other 3 dB is due to the wave peaks/troughs being added. :hmm:

[Ian Mackenzie]

thanks for the input Ian,

I reckoned that the single 2118 could just about keep up with the double 136a's. Max spl is a close match.

Impedances etc are not an issue, I have six Crown gradient amplifiers (for MRI), each rated 4kWrms/30kW peak, which I shall be using (a bit modified and down-scaled, though :)) to drive the units.

Intuitively, two 4in VC's vs a single 2in VC seems a bit skewed, but OTOH, the 136a's are probably displacement limited anyway before power handling becomes a issue. That's why I was asking for LH opinions. I'll go for it, play with the x-over freq, and if the 2118's do like this :barf:I'll substitute 2123's or something else.

Thomas

Grab a pair of JBL 2202's and you have the makings of a 4350.

Are they original 136 cones or 2235 recones?

If you have plans on reconning I would look at the 2234 kits (without the mass ring) and adopt the 4435 tuning and look at the 4435 as a clone.:)

Doubling the cone area equates to 1/4 the excursion at 50 hertz as I recall.

Drew's clues in the Library will also give you an appreciate if using mutiple drivers.

[pos]

So you are back to where I was originally. :blink:

Simply moving twice the volume of air only produces a 3 dB change and the other 3 dB is due to the wave peaks/troughs being added. :hmm:

Well yes, of course. If two sources move the same volume of air in opposite directions they will even cancel each other (-inf dB !)
We are talking about adding speakers that play the same signal (correlated), and having them sum acoustically in phase. If you manage to do that (ie equal distance or low frequencies), then you get 3dB more than simply doubling the power to one woofer.

Well, I think we agree :)

[pos]

Doubling the cone area equates to 1/4 the excursion

Doubling the cone diameter.
By doubling the cone diameter you multiply the cone area by 4 (because A = Pi*r^2, the power of 2 strikes back!), so you multiply the amount of moved air by 4, so you can move the same volume of air with only 1/4 the excursion.

[Allanvh5150]

It seems I am going to have to sit down with some textbooks, unlearn what I have learned and relearn everything I know on the subject. I always thought that if you put 1W into a driver and measure the output at one metre and get a figure of say 95dB, then get another driver of the same type and feed it with 1W, take the reading again and get 98dB. If you just go ahead and connect a second speaker to the 1W source you will only have half the power on each driver so nothing will change. The thing here is when testing is done in the lab, i.e. anechoic chamber yada yada yada, the 1W source is a constant 1W. If the load is 4, 8, 16 or any other figure. To get the 3dB increase you need to add the 1W source to the other driver. In reality, adding 1 driver to and existing driver driven with 400Watts will NOT add 3dB, simply for the fact that most amplfiers will not go to twice the power with half the impedence. To get the 3dB increase you need to add another driver and another 400Watts. All the testing I have done over the past 25 years tells me this. If you have a system and want to add 3dB to it you need to add a whole other system.
Right, where are those books.......:)

[johnaec]

Here's a recent thread on this "power coupling" from ProSoundWeb: http://srforums.prosoundweb.com/index.php/t/31361/8370/

While they're mainly talking about some custom pro-sound horn subs, some of the posts are fairly informative, especially towards the end, with some theoretical examples. The whole thread isn't that long, (less than 20 posts), so you don't have to dedicate an evening to it... :)

John

[subwoof]

Well I will step away from this argument...seems that all sides have a part of the answer while no one has it all...and I do not profess to because this is NOT a linear equation and the variables are not exactly defined. Different cabinets, loading, spacing, placement, etc ALL go into play with LF cone coupling.

There seems to be a lot of confusion over doubling - in DB terms doubling the voltage is 6db, power 3db, This is indisputable.

BUT a smattering of amplifier scenarios and cabinet couplings have been presented on this thread so it is not comparing apples to oranges, litres to quarts or kumquats to koala's

Anyways, my point is that the paper cone on the 2118 is a thin, rip-prone mistake from JBL. In the multitudes of cones that have passed thru the JBL service center I helped set up in 1993, COUNTLESS 2118. 2119 and their variants have come into the shop with paper cone damage even from systems that were power controlled, within "spec" and generally well taken care of. They are just too SMALL for any decent application.

If you want to use a cone mid above a PAIR of 136A'a that I strongly suggest either the 2202 ( AKA the 4355 monitor ) or the 2012H high power 10"

The 2012's frame width is only 1/8 inch wider that the extended lip on the 2118 and you have a REAL high power cone / coil and a large magnet.

Sell the 2118's and move up.

sub

[Zilch]

It's covered in the Eargle sound reinforcement manual in the subwoofer section. Mutual coupling gives you an extra 3 dB. With each doubling of drivers, the frequency at which mutual coupling occurs also moves lower, so at some point (8 drivers, as I recall,) you're done with the free lunch.... :yes:

[rs237]

Zilch hello, the free lunch is not dependent on the number of Driver. The free lunch will end when the distance between the membrane centers larger than 1/4 wavelength. By two tightly side by side mounted 15 "drivers out of the Mutual coupling already at about 200Hz. Look here
http://www.audioheritage.org/vbulletin/showthread.php?t=20088&highlight=TAD&page=3
http://audioheritage.org/vbulletin/showthread.php?t=5896&highlight=%22acoustic+coupling%22


regards
juergen