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Acoustic suspension theory-- bass driver loading


DaveD

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I want to make sure I am correct in my understanding of acoustic suspension theory as it relates to bass driver throw and the loading effect of the trapped air upon the movement of the cone.

Lets take two 8 inch drivers with long throw. The first one was designed to be used in a specific cabinet with a known volume of air inside and its throw was determined by the designer to be the best for that particular cabinet and volume of air. Its throw need not be much longer that the amount which can be used in that cabinet. Its in-room response (utilizing the benefit of room boundary reinforcment) reaches 40 hertz at the -3dB level referenced to 1K hertz at 10% distortion.

The second 8 inch driver was designed by the same engineer to operate in a much larger volume cabinet and it has a significantly longer throw than the first one. In its cabinet, it reaches lower frequencies than the first driver can in its own cabinet. It will play 28 hertz at -3dB with 10% distortion if located in the same position as the the first driver in its smaller cabinet. In the upper reaches, it is designed to respond so that it can cross over at the same frequency as the first, shorter throw driver, which is 140 hertz.

Now, we take the second driver and put it into the first, smaller, cabinet. Can it reach any lower than the first driver can due to its much longer throw, or is its throw not usable because of the volume of air inside the cabinet? My understanding is that it cannot play lower. Is that right?

Will the second, longer throw driver in the first, smaller cabinet reach the same 40 hertz at -3dB output as the first driver but at a lower distortion level than 10% due to its longer throw? I think that the answer to this one is yes. Am I right?

Are there any sonic disadvantages to using the second, longer throw driver in the smaller cabinet?

This is the jumping off point for any discussion that corrects my errors or adds the other variables and considerations which I have not included because I don't know what they are.

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Dave, I'll dip my toe into the water here and offer some theoretical answers to your questions which have been derived from loudspeaker software.

I used BassBox Pro to generate 2 scenarios. A standard 10 inch AR speaker was chosen. It came preloaded with the software. I don't think the change in driver size from your 8 inch to the 10 inch I used makes much difference. The driver was AR #1210074-2. It has a PP cone and foam surround. The Fs is 30.8 Hz, Re nom. = 4 ohms and Qts is about .49, very suitable for an acoustic suspension design.

The link below shows a jpeg image scanned from a BassBox printout of response curves. The red lines are for scenario 1 described below and the black, scenario 2.

Scenario 1) X max was set at .25 inches and box volume was .75 cu. ft. and heavily stuffed. The resultant Qtc was close to 1 (0.949) yielding a slight response hump around 100 hz. Box F3 was 63.

Scenario 2) X max was increased to .75 inches and box volume increased to 3 cu. ft. All other parameters remained the same which may not be practical for the driver. Qtc was lowered to .629 and box F3 dropped about 10 hz. Note, howewver, the much larger cone excursions for scenario 2 in the second graph from the top.

This exercise was somewhat elementary and brief. However, it does indicate scenario 2 will most likely yield a bit more low end.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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Can you explain what you mean by "long throw?"

Typically, this term refers to how far a driver's cone can move before it bottoms out. The way this is specified, generally speaking, is known as Xmax. However, the "throw" of a woofer is not directly related to its relative performance in a sealed or vented system.

One can have a very stiff, low-compliance woofer that has a high Xmax. One can similarly build a high-compliance, floppy woofer with little usable excursion. The best kind of box for a given woofer is determined by the driver's resonant frequency, its compliance and its "Q" (or "damping"). The throw of the woofer is a secondary factor, which effects how loud the driver will play on short-term bass peaks.

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>Can you explain what you mean by "long throw?"

>

>Typically, this term refers to how far a driver's cone can

>move before it bottoms out. The way this is specified,

>generally speaking, is known as Xmax. However, the

>"throw" of a woofer is not directly related to its

>relative performance in a sealed or vented system.

>

>One can have a very stiff, low-compliance woofer that has a

>high Xmax. One can similarly build a high-compliance, floppy

>woofer with little usable excursion. The best kind of box

>for a given woofer is determined by the driver's resonant

>frequency, its compliance and its "Q" (or

>"damping"). The throw of the woofer is a secondary

>factor, which effects how loud the driver will play on

>short-term bass peaks.

I agree Ken regarding your Xmax comments. The BassBox Pro results were only affected by the change in box volume, not the change in Xmax.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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>Can you explain what you mean by "long throw?"

>

>Typically, this term refers to how far a driver's cone can

>move before it bottoms out. The way this is specified,

>generally speaking, is known as Xmax. However, the

>"throw" of a woofer is not directly related to its

>relative performance in a sealed or vented system.

>

>One can have a very stiff, low-compliance woofer that has a

>high Xmax. One can similarly build a high-compliance, floppy

>woofer with little usable excursion. The best kind of box

>for a given woofer is determined by the driver's resonant

>frequency, its compliance and its "Q" (or

>"damping"). The throw of the woofer is a secondary

>factor, which effects how loud the driver will play on

>short-term bass peaks.

Thanks to Carl and it is great to see Ken participating here as he has done in the past. See? That's what I meant about getting into all of the things that I don't understand. Let's take a different approach-- I was trying to be generic and now I will be very specific.

The actual drivers I have in mind are the original 8 inch bass driver that Henry Kloss designed for his Ensemble system, and the similar but longer throw one that he designed for his Powered Subwoofer II, a much larger cabinet. This second one is used with an amplifier that is equalized in order to offset the natural rolloff of the speaker's low end response in order to extend the low frequency reach of the subwoofer.

I don't know anything about the relative compliance or Q properties of these drivers, only that the first one was made to match the Ensemble cabinet and the second one was made to match the PSW II cabinet and amplifier (and that it has a heat sink in place of the dust cap and a different kind of basket). Now, the reason for my question is that this second driver was also used in middle and late versions of the USA Ensemble, all the way through until the end of the Chinese production of the New Ensemble. I wonder if it was a cost saving compromise to use this driver in the Ensemble cabinet, or if Henry could have designed it to serve double duty without degrading the bass performance of the Ensemble versions in which it is used.

I have a pair of the originals and a pair of 1987-88 production Chinese ones with the second driver. I can only subjectively compare their sound in one room, and this is hardly a good test since the room has such a strong effect on the bass. Since I want to buy another pair on eBay when I get the chance, I wonder which one to prefer.

Yes, I like the Ensemble, and am prepared to endure criticism for my choice.

Would rather be listening to records...

Dave

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>I have a pair of the originals and a pair of 1987-88

>production Chinese ones with the second driver. I can only

>subjectively compare their sound in one room, and this is

>hardly a good test since the room has such a strong effect on

>the bass. Since I want to buy another pair on eBay when I get

>the chance, I wonder which one to prefer.

>Dave

Yes, well, that should be 1997-98 production Chinese ones. From one decade to the next I can easily lose track.

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Dave, at least for me, your specific case does not have enough driver performance detail to render an opinion on which one to buy. Without the T/S parameters (which could be sifnificantly different) between these two drivers, I'm afaid we've reached an impass.

I suggest you buy both and run a test to see which one you like, and, in turn, sell the one you don't like back thru e-bay.

Nothing ventured, nothing gained as they say.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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