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Author Topic: well, efficiency of multiple boxes raises up to....???  (Read 7574 times)

Ivan Beaver

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #10 on: January 20, 2006, 09:05:10 pm »

With Tom Danley's previous company, we did a concert using 4 bdeaps per side in a cloverleaf configuration).  This was the biggest usage of these cabinets to that date.  We measured the sensitivity with 1 watt into all the cabinets (1/4W per cabinet) at more than 100% effieciency. How could that be?

Tom said that this was due to the directivity that we got by using 4 together and the large frontal area of the array.  This of course was at the topend of the response.  As freq got lower the sensitivity lowered. The ratio of the wavelength to the size of the array got lower.

Tom- I hope I have not misquoted you here

Part of your the "issue" with your question depends on where the measurement is taken.  If it is taken fairly close, as you add cabinets, you have to actually put them somewhere.  As the number of cabinets grows, so does the distance of the further cabinets away from the mic.  This will account for some "losses" in the adding of cabinets (vs the calculated numbers).  But more cabinets will give you a greater directivity at a lower freq.

Tom would be the best source for a good answer-.
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Tamas Tako

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #11 on: January 21, 2006, 06:48:53 pm »

Hi,

I would like to write some addition to my original post:

I think the maximum efficiency one can reach is 50% in the case of loudspeaker boxes/ arrays.
The sensitivity of a box or an array can be more than 109 dB (50% efficiency in halfspace) due to the increased directivity. As Ivan wrote for 4x BDEAPs they measured around 115-116 dB 1W1M sensitivity (it was measured at 10m distance and then calculated to 1m reference using the inverse square law.

I think measuring at 10 or 20m distance is a good method to get reliable data.

So back to my original question:
If I can get 116dB sensitivity from 4x BDEAPs, can I get 3 dB more from 8 BDEAPs? (totaly 6 dB more max SPL?)
Let' say 10 meter away from the array.

Can one get the same sensitivity (efficiency and directivity) from Vented cabinets? What do you think, how many 2x18" vented cabinets one would need to stack  to reach the 50% efficiency? And what amount of directivity this stack would have?

Another question is if some design is optimized for a number of 8 cabinets meaning that they are 50% efficient, it means that the radiation impedance is 2x the Rdc of the drivers.
Now what if I stack more than 8 together? In this case the radiating impedance changes, so that the 50% theoretical maximum is no more reached...

Is it right?

If yes, What is the amount of boxes where the maximum SPL will be the same if we say both the horn loaded boxes and the vented cabs have the same power ratings?
I just would like to get a clear sight, that doubling the number of boxes not always add 3dB of sensitivity...
So after there is a lot of enclosures there, if you double the number, you will not get 6 dB more max. output, right?

After all it helps maybe a bit to decide what type of subwoofer one need for their purposes depending on the goal, how many boxes he want to use, how mutch SPL he wants to get, how mutch directivity he neds and how mutch amplifierpower he wants to add.

Thanks,

Tamas

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Ivan Beaver

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #12 on: January 22, 2006, 03:17:05 pm »

Now this is just a guess, but I would think?? that doubling the number of cabinets would continue to increase the max SPL at the same 3-6dB rate(assuming added amplification).  The limits become the physical size of the array (and the inverse square distance losses I mentioned earlier) and the saturation/linearity of air.  At some point the distance between the cabinets in the array (at various freq) will start to cause cancellations-resulting in less output, but this would be due to the physical size and not the added SPL. You cannot put all of them is single point in space.

And of course cost.
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Tamas Tako

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #13 on: January 22, 2006, 07:08:33 pm »

Hi Ivan,

Yes, you are right about that physical dimensions can couse the interference at the point of the listener or measurement microphone. That is right. Then Let's define nmy original question that the lid
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Ivan Beaver

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #14 on: January 22, 2006, 08:44:09 pm »

I don't know if the cluster will reach a maximum efficiency.  I believe that is more reserved for a single cabinet spec.  I will talk to Tom and see what he says.
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Walt de Jong

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #15 on: January 23, 2006, 08:41:14 am »

Hello,

If 100W electrical power goes in, the maximum acoustical output is limited to 100W (remember the laws of thermodynamics)

A cluster will therfore be limited in its efficiency. But due to the increased directivity it may look like you are more than 100% efficient.

Best regards,

Walt
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Joe Jones

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #16 on: January 23, 2006, 09:12:21 am »

The maximum efficiency of an ensemble of direct radiators cannot exceed approximately 25%, and that of horns approxinately 50%("An Efficiency Constant Comparison between Low-Frequency Horns and Direct Radiators", D.B.Keele, AES Convention May 1976, AES preprint 1127.  

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Tamas Tako

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #17 on: January 24, 2006, 11:12:52 am »

Joe Jones wrote on Mon, 23 January 2006 15:12

The maximum efficiency of an ensemble of direct radiators cannot exceed approximately 25%, and that of horns approxinately 50%("An Efficiency Constant Comparison between Low-Frequency Horns and Direct Radiators", D.B.Keele, AES Convention May 1976, AES preprint 1127.  



Thanks Joe,

I was asking for something like that...

Thanks again,

Tamas
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Tom Danley

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #18 on: January 24, 2006, 12:42:28 pm »

Hi Tamas, all

Sorry for the delay, I’ve been busy here.
A bunch of questions, Joe answered an important one and I will expand a bit.
Direct radiators exhibit an increase in efficiency when placed close together (less than about 1 /4 wl) because each feels the pressure of the other, “mutual radiation pressure”.
An alternate explanation is that when a radiator is acoustically small, it operates on the sloping portion of the radiation resistance curve. When one adds a second radiator, close enough to actually acoustically couple (that 1 /4 wl thing), then it is as if one moves up the resistance curve because the area has doubled.
This stops at a point where the drivers are more than about 1/4 wl apart and not the 1 /2 wl spacing most manufacturers curiously say is needed for line source driver spacing.
That spacing produces directivity but not mutual coupling.
Alternately, one can say that when the total coupled radiator area is large enough to reach the flat portion of the radiating curve, then a further increase in area causes no further increase in acoustic loading.

Also, when one has a large number of radiators, one also has the mechanical losses associated with all the drivers. This loss is in parallel with the acoustic radiation so a low Qm driver will have a lower ultimate efficiency in an array.
A thumb rule (as defined by Don Keele) of 25 % for an upper limit of efficiency for an array of direct radiators seems about right.  Don is also one of the under appreciated horn folks that helped make what we have today.

For horns, 50% efficiency is sort of a “holy grail” and very hard to get in practice.
One can make horns which are greater than 50% efficient but this has a tiny impact in the sensitivity. It normally takes a group of practical sized horn boxes to get to 50%.

An efficient speaker radiates more than an average amount of the input Wattage as acoustic power. Efficiency can be defined as the difference in the output compared to the power lost in all the non-radiating resistances.
So far the discussion has been about mechanisms involved in delivering more power into the acoustic load for a given input power.

Part two is “where does it go” or directivity.
Once on has an array of coupled woofers or horns that is larger than that where the radiation resistance curve flattens, then the additional size causes directivity over an omni directional source.
Here, as the frequency climbs, the fixed physical size source is acoustically  larger and so has more directivity.
For a horn, it becomes the wall angle which eventually defines the radiation angle, for the array of direct radiators, the issue is more complicated.
As the frequency rises, the spacing between drivers becomes excessive (greater than 1 / 3 wl) mutual radiation stops and they revert to an array of point sources which have self interference.
The full range horns I design have a straight walls for a constant directivty and have all the other frequency ranges join together where the dimension is less than 1 /4 wl so the add coherently.

In theory an measurement  “self interference” condition is “not good” but on the other hand can be exploited sometimes.  
A Line source exhibits a frequency /size dependant, reduced SPL fall off with distance because it has “self interference”.
Compared to the typically horrible Concert speakers they replaced, the line sources were a big improvement to be sure but they are far from ideal acoustically for the same reason.

Compare a perfect line source and a perfect point source and one sees a huge difference in the time response for one thing. Anywhere the line source exhibits non-inverse square law fall off, it has comparably poor time response due to the extended source nature.
Oh well, until the marketing folks decide its time for another change and steer the masses in another direction, we will have line sources everywhere you look, even when it’s a poor choice.

Anyway, you can hear this time smear in a large array of woofers , feed in an impulse like a kick drum, on axis it sounds fine because all the path lengths are nearly the same.
Do the same thing from off to the side and it sounds slow, woofy and not distinct as a result of the combination of different path lengths and directivity.
For clarity, one only wants ONE time between the input signal and when the sound hits your ears, not multiple arrivals from each source.

So far as the Bdeaps, I designed those back at Servodrive to fit next to a corner, it was actually Mike my partner who had the idea to stack them in a clover leaf and then discovered anomalous sensitivity.
When I measured them stacked that way, I found the 10 meter 1Watt level was 97.5 dB, which is 117.5 dB 1W 1M equivalent.
The arrangement produces about 10dB of forward directivity over a horn of equal sensitivity but no directivity.
Two sets would have more directivity BUT revert to self interfering point sources when the center to center spacing of the outlets becomes greater than 1 / 3 wl.
In other words, just like with line source drivers or woofer arrays, unilateral / coherent addition stops when the center to center spacing of the sources is about 1 / 3 wl or greater.  The larger the pile , the lower the coherent upper limit is.

The TH115 and DTS-20 box is configured so that one can build an array with them also.
If one lays a pair of 115’s down (flat or in a V), with the mouths touching and then stacks more on top, one can make an array of considerable size.
A stack of 2 by 2 is already about 50% efficient, a larger stack is not likely to increase the voltage sensitivity much but the vertical directivity will become narrower and there will be a resulting rise of apparent sensitivity in front.  A system the size of the 4 Bdeaps has about 10dB of forward gain in that frequency range so you could base a guess on  what 115’s would do on that I suppose.  At some point we will do another round of outdoor measurements on a “pile”.

Anyway, I guess if one boils it down, one has a few points.
Mutual radiation provides that advantage up to the point that the size of the array reaches the flat part of the radiation curve / or where Qm dominates.
An upper frequency limit also exists based on the center to center spacing of individual sources being less than 1 / 3 wl apart (really 1 /4 wl).
In the range that both conditions are met, a coherent wavefront can be launched with directivity, controlled by the shape of the array and time relationships etc.
Man, this ended up being long, sorry.
Hope that helps,

Tom Danley













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RYAN LOUDMUSIC JENKINS

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Re: well, efficiency of multiple boxes raises up to....???
« Reply #19 on: January 24, 2006, 01:27:09 pm »

I read all of your posts Tom's as they are very enlightening.  Unfortunately you sometimes explains things so well that you lose me.

Am I reading this correctly that for direct radiator there is no addition coupling where the drivers are more they when the center are more than 1/4 wave length appart?  Does that meen that for boxes with a LPF set for 90hz that the divers need to be within 3.12 feet of each other before there is "self interference" in the high fequency range or am I reading your post wrong?

Thanks in advance for any more enlightenment.

Ryan Jenkins
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Ryan Jenkins
"Two days until the end of when I don't know what to think.  Three days until I start the cycle all over again!"

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