Post by: Tamas Tako on January 17, 2006, 03:14:48 PM
Tom, Mark, Ivan and all the other people here (please forgive me if your name wasn't there..) I have a question regarding efficiency here...
If one would build a large stack from
a/ Double 18" vented boxes
The efficiency of one 2x18" vented cabinet is around 95..98 dB
Say 100dB for easier calculation...
b/ Horn loaded subs ( TH 112/115/Vortex or whatever)
The efficiency of one Horn loaded cabinet is say around 106dB
in this particular case for easier calculation.
If you have 2, 4, 8, 16, 32, 64 pcs in a stack, how the efficiency raises in both a/ and b/ cases?
I mean having two boxes instead of just one, You would get 3 dB more efficiency and 3 dB more power handling giving 6dB more max SPL.
But efficiency can not raise up to more than 100% right?
And the number of boxes where the 100% efficiency would be reach is not infinite....
So I guess:
Doubling the number of boxes not always results 6 dB max SPL gain anymore. If you have 64 instead of 32 you may only get 3 dB more SPL and this only becouse of the increased power handling and no efficiency increase...
Sure I know SPL can be increased not just by efficiency and input power, but with more directivity as well... (see BDEAPs)
At some point you neither get more directivity when you already reached the dimensions where the majority of the stack is working under quarter space conditions, however this can be true always just for a part of the stack and not for the speakers on the both ends... So you may get more and more boxes to work with increased directivity and with increased radiating impedance due to the sound (air) pressure generated by the other boxes...
So my question is, If Horn loaded boxes have or have not any advantage in this manner when they are used in big stacks?
SO If I use 64 pcs of 2x18" vented cabinets Do I get less Max SPL than using 64 pcs of Horn loaded Subs if the total input power is the same in both cases?
I mean If I only can reach an efficiency of say 120dB 1W 1m including directivity in both cases, then I get the same max SPL when feeding with the same total power, right?
Clear, that I could reach this efficiency margin with horns easier and with less cabinets. Maybe with just 16 pcs...
But what if I add there some more horns? I gues there should be a border somewhere, above I dont get more efficiency, I just can pump more watts into the speaker...
I am wondering if this all is true or not?
If anyone has to say something about this I would appriciate any explanation!
Thanks,
Tamas
Post by: Paul O'Brien on January 17, 2006, 03:38:48 PM
Sensitivity is the sound output level resulting from a given input signal and measured at a given distance from the driver, usually indicated as ??spl@1w/1m in spec sheets.
Efficiency if the amount of that input signal that is actually converted into sound, while the rest is dissipated in heat, soaked up in back emf, produces cone movement....Not usually found in any spec sheets. This is typically between 1-10% for direct radiator designs, and up to 50% for horn loaded designs.
There is a limit to the maximum efficiency you can achieve,(directly related to the box type) but the more boxes you stack together the higher the sensitivity.
For example.. it doesn't matter how many reflex boxes you stack together, you'll never reach the efficiency(amount of sound output per watt) that a stack of horns will produce from the same power. Because of the horns efficiency advantage, it will take fewer horns and less power to match the bigger reflex stack.
Post by: Tamas Tako on January 17, 2006, 04:01:52 PM
Thanks for your reply, however I am afraid, you don't really understod me.
I was not wanting to mix the two terms sensitivity and efficiency. However they ara sisters. Efficiency shows you, how many acoustic watts you will get when you put one electronic watt into the device in %.
Sensitivity shows you how many accoustic watts you will get for an 1 watt input if you know the coverage pattern.... So you see the are almost the same if you add that directivity is an important part of the difference between efficiency and sensitivity. The fact, that one of it is in dB and the other usually in % doesn't matter If you imagine that distortion can be figured in % and in dB as well....
This is why I was writting about directivity coused sensitivity gain where the efficiency remains the same at the same time...
Anyway thanks,
Tamas
Post by: Paul O'Brien on January 17, 2006, 04:30:56 PM
Post by: Tamas Tako on January 17, 2006, 04:45:05 PM
I am not sure you will get more SPL in the farfield with horns than with direct radiators when the 1m SPL is the same... I also read earlier some post that the radiator is the driver and 1m far from the horn means 3-4m from the driver and the inverse square law should be used this way. I have measured this and this is not true!
Tamas
Post by: Paul O'Brien on January 17, 2006, 05:01:19 PM
| Tamas Tako wrote on Tue, 17 January 2006 16:45 |
I am not sure you will get more SPL in the farfield with horns than with direct radiators when the 1m SPL is the same... I also read earlier some post that the radiator is the driver and 1m far from the horn means 3-4m from the driver and the inverse square law should be used this way. I have measured this and this is not true! Tamas |
OK.. from what I understand when measuring a speaker systems sensitivity, the test position is 1m from the "radiating surface" of the cabinet. That's essentially right at the cone for direct radiators, but at the mouth of a horn loaded box.
Post by: Tamas Tako on January 17, 2006, 05:08:47 PM
This is How I understand and what I have measured
Post by: Walt de Jong on January 17, 2006, 05:41:52 PM
Best regards,
Walt
Post by: Antone Atmarama Bajor on January 18, 2006, 11:35:09 AM
Horns have more of a linear Air coupling. You should compare measurements with Direct radiators at 100W 10m instead of 1W 1m.
Antone-
Post by: Michael_Elliston¶ on January 20, 2006, 12:12:23 AM
Post by: Ivan Beaver on January 20, 2006, 09:05:10 PM
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-.
Post by: Tamas Tako on January 21, 2006, 06:48:53 PM
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
Post by: Ivan Beaver on January 22, 2006, 03:17:05 PM
And of course cost.
Post by: Tamas Tako on January 22, 2006, 07:08:33 PM
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
Post by: Ivan Beaver on January 22, 2006, 08:44:09 PM
Post by: Walt de Jong on January 23, 2006, 08:41:14 AM
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
Post by: Joe Jones on January 23, 2006, 09:12:21 AM
Post by: Tamas Tako 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
Post by: Tom Danley on January 24, 2006, 12:42:28 PM
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
Post by: RYAN LOUDMUSIC JENKINS on January 24, 2006, 01:27:09 PM
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
Post by: Tamas Tako on January 26, 2006, 08:39:35 AM
It is good to know there is alway people who can answer questions which are unusual...
This all means, that a big array of Vented subs could has less efficiency (25% max vs, 50% max) and less directivity as well than Horn arrays.
This means less and again less sensitivity...
Great knowledge base here!
Just come a further question into my mind...
What are SPL levels at low frequency (20...100Hz) where the air will compress in front of the array so that the pressure wave will not travel as usual, cousing larger SPL attenuation than the 6dB/ distance doubling??
Thanks
Tamas
Post by: Tom Danley on January 27, 2006, 02:50:25 PM
Ryan, maybe it would be more correct to day that unless the sources are about 1 / 4 wl apart, they do not combine in a way that is uniform in all directions or fully “feel” the radiation pressure of the other unit.
For example, two sources a half wavelength apart radiate in a figure 8 pattern with nulls in line with the sources and forward / rear lobe. No gain in efficiency is attained but there is some forward gain from directivity.
The greater the spacing, the larger the number of lobes and nulls, conversely, once the distance is ¼ wl or less, the radiation pattern is a circle and fully mutually additive.
So, yes the 3.12 feet is how close you need the sources at 90 Hz for them to fully add in all directions.
Tamas, well so far as I’m aware, the phenomena that causes excess absorption is only present at “high” sound levels. It was called “shocking up” by my old boss Roy Whymark, an old time English acoustician.
Here is what happens (same thing as throat distortion also), sound travels at, well the speed of sound.
The speed of sound is temperature dependant also.
When one has a really loud sound, say 155dB or more, one finds the pressure side of the wave is actually a little bit warmer than the vacuum side. The result is, the pressure side, travels a little bit faster than the vacuum side. If you apply this to a sine wave, over a number of wavelengths, the top of the sine creeps forward until a saw tooth wave is created.
Here is why the reference to Shock. A shock wave is created by a pressure which is high enough to heat the air via pressure. The shock wave travels outward at a new speed governed by how hot it is.
At the tail side, one finds the rarified recovery part of the wave which makes those cool visible expanding clouds of condensed moisture. Loud Sound can be dangerous.
Anyway, the same thing which cause the shock wave to be a saw tooth cause an intense sine wave to become one. If you can get a 155dB sine wave and a reflector, one can levitate Styrofoam balls up to about 1 /4 wl in diameter. Get to 165dB and you can levitate iron, glass, live cockroaches (in out 21KHz acoustic levitation back at Intersonics). At 173 dB, one can light a cigarette with acoustic friction.
One can put your finger in a sound this loud (at 21KHz) but any gap between your fingers will be burned immediately. For reggae or dance floors, there might be a happy medium, perhaps some non-lethal sound level that would also cause clothing to rapidly fatigue, fail and fall off without the unpleasantness of catching on fire haha.
Kids, I’m just kidding
Best,
Tom
Post by: RYAN LOUDMUSIC JENKINS on January 27, 2006, 04:12:13 PM
Post by: Tim Duffin on February 02, 2006, 01:42:46 AM
By the way, I have sat in a competition car audio truck that hits 166Db at 80hz-- it is very uncomfortable even with earplugs and sounds unlistenable at any freq. above 38Hz. Presumably this is because of the sawtooth wave phenomenon which you mentioned.
T
Post by: Joe Jones on February 03, 2006, 05:16:35 PM
| Quote: |
I think that with a woofer with suitable force factor, one may be able to get the total radiating volume of an 18" woofer into a 10"x10" space. |
I think all you'd get is a new way to blow out candles. The effect would be pretty much the same as using too small a diameter port on a reflex box, and the result would be just a lot of chuffing noise.
Post by: Tim Duffin on February 04, 2006, 04:30:52 AM
By the way, if it was possible to get enough power from a dual 18 to blow out candles 10feet away, wouldn't you want that?
T
Post by: Joe Jones on February 04, 2006, 10:22:41 AM
Post by: Chris Davis on February 04, 2006, 05:04:22 PM
Post by: Antone Atmarama Bajor on February 05, 2006, 01:38:52 AM
I think that a horn is an acoustical transformer that transforms High pressure Low Velocity Energy into High Velocity Low Pressure energy so You would create the opposite. I think it will still follow the same rules of horn Bandpass behavior. But you would have to do it right, and what would the application be? Blowing up skirts and knocking hats off with Bass? The only problem is the origin of the Horn is normally a reactive radiating component. I don't think its as simple as swapping the position of the Radiator.
Even though it does seem to be what people do with Transmission line subwoofers But they have to stuff the chamber with all sorts of Damping materials to cut down on Resonance.
I may have some of that backwards but I have wondered about a reverse horn or contraction.
Antone.