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Old 26th June 2008, 01:43 AM   #1
MCPete is offline MCPete  United States
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Default acoustically suspended or not

Parts Express offers a "Woofer Selection Guide" that states a recommended box internal volume Vb for a closed-box alignment of their woofers for sale. Among 8 inchers, some are recommended to be acoustically suspended (alpha or Vas/ Vb equal to or greater than 3) while others are not.

They say that the alignments of their selection guide are derived from the (software program) BassBox. Besides being too cheap right now to buy BassBox, I think that there must be a simple way to look at the specs of a woofer and be able to decide if power handling will be seriously compromised if a given woofer isn't acoustically suspended.

For two woofers of the same size, certainly relative VAS should say something about the matter. The woofer with the smaller VAS has less compliance of its spider and surround and thus would be the more likely candidate to be able to live without acoustic suspension. This generality however really doesn't get me anywhere in making a decision.

What is needed, probably, is an equation taking VAS, Xmax and sensitivity into account. Probably those who created BassBox know this equation.

Does anybody know of this equation (if it exists) or perhaps can think of a few simple calculations that might let me look at a spec sheet for a woofer and be able to tell which side of the alpha = 3 divide the woofer stands on?
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Old 26th June 2008, 01:57 AM   #2
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A good paramater to look at is Qts. If it is below roughly .4, your resonant frequency in box goes way up. You get very little bass response in a sealed enclosure.

www.carstereo.com/help/Articles.cfm?id=30

Here is a link to a box calculator, you can punch in some numbers from different drivers and you should get the idea.

I believe there is a simple formula for determining if a driver is better in a vented or sealed box, but I don't remember it offhand.

JJ
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Old 26th June 2008, 02:07 AM   #3
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All is revealed in Unibox - heck of a box modeler.
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Old 26th June 2008, 03:02 AM   #4
BHTX is offline BHTX  United States
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Default Re: acoustically suspended or not

Quote:
Originally posted by MCPete
They say that the alignments of their selection guide are derived from the (software program) BassBox. Besides being too cheap right now to buy BassBox, I think that there must be a simple way to look at the specs of a woofer and be able to decide if power handling will be seriously compromised if a given woofer isn't acoustically suspended.
EBP (Efficiency Bandwidth Product)

EBP = Fs/Qes

However.. Qts, Mms and/or sensitivity, and maybe a quick glance at Fs usually tells me enough.

Also, I think after modeling so many hundreds of woofers over time, you start to know what to expect. Well, I guess it doesn't take THAT long. But after a while, you'll see what I mean if you continue to model woofers constantly. I often find myself guessing in my head real quick at the F3 and box size for .707 Qtc while entering parameters. I'm usually pretty close, and often spot on. I definitely couldn't do this just a couple years ago.
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Old 27th June 2008, 02:26 AM   #5
MCPete is offline MCPete  United States
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Default still puzzled

Thanks for all of the replies so far. I guess so far nobody reading my post knows exactly how to go about solving my problem.

Modelling software programs certainly are convenient. I wouldn't be across the board opposed to using one. But I think there is a danger of letting the computer do all of the work, ending up with a good speaker system, and being at a loss to explain why the design is good. So as much as possible, I like to get to know the math behind the solutions.

EBP or efficiency bandwidth product I believe is used in deciding the greater suitability of a woofer for a closed-box or vented alignment.

Qts by itself doesn't indicate what the box resonance frequency or Fc will be. If free air resonance frequency or Fs is sufficiently low, then even if alpha must be 3 or greater, then Fc won't be excessively high. Also, Fc = sqrt(alpha +1)Fs, so rising Fc is at a slower rate than alpha.
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Old 27th June 2008, 11:10 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

Vas, Fs and Qts are all inextricably linked, and given these parameters
the box size / bass extension pretty much depends on the wanted
efficiency, the higher this is, the bigger box or less bass extension.
(cone mass for driver size is a good efficiency pointer)

Often ignored (completely) is that Vb<Vas linearises the suspension
at low frequencies, Vas non-linearity is a major cause of distortion
below ~ 100 Hz. You cannot guarantee high quality bass with
just box modelling from small signal parameters.

/sreten.
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Old 27th June 2008, 05:56 PM   #7
Ron E is offline Ron E  United States
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Default Re: acoustically suspended or not

Quote:
Originally posted by MCPete
Parts Express offers a "Woofer Selection Guide" that states a recommended box internal volume Vb for a closed-box alignment of their woofers for sale.

I think that there must be a simple way to look at the specs of a woofer and be able to decide if power handling will be seriously compromised if a given woofer isn't acoustically suspended.

What is needed, probably, is an equation taking VAS, Xmax and sensitivity into account. Probably those who created BassBox know this equation.

Does anybody know of this equation (if it exists) or perhaps can think of a few simple calculations that might let me look at a spec sheet for a woofer and be able to tell which side of the alpha = 3 divide the woofer stands on?
Fc/Fs = Qtc/Qts = sqrt (Vas/Vb+1)

What this tells you is that For Vas/Vb = 3, Fc/Fs = Qtc/Qts = 2

So if you want to design a speaker with Qtc= 0.7 and acoustic suspension, you need a woofer with Qts less than or equal to 0.35

Qtc is the response shape factor.

When designing a sealed woofer, you specify wither Qtc or Vb and the rest falls into place. While a formula could be derived for calculating excursion vs power, and is quite simple actually, there is no need for it.

At DC:
i = V / Re - here V is the voltage input (power = V^2/Re)
F = Bl * i (current)
F = k*X - you can calculate k from Cms. ( k = 1/Cms - watch your units!) see the wikipedia Thiele small article....
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Old 27th June 2008, 09:39 PM   #8
Ron E is offline Ron E  United States
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Default Re: Re: acoustically suspended or not

Quote:
Originally posted by Ron E
at DC:
i = V / Re - here V is the voltage input (power = V^2/Re)
F = Bl * i (current)
F = k*X - you can calculate k from Cms. ( k = 1/Cms - watch your units!) see the wikipedia Thiele small article....
continued.....

Filling in the blanks for you here:
Bl*i/k = X, where X is excursion
X is in meters if you use Cms in [m/N], Bl in Teslas and current in amperes.

This is relevant at frequencies of DC up nearly to resonance, depending on Qtc.
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Old 27th June 2008, 11:38 PM   #9
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Default Re: still puzzled

Quote:
Originally posted by MCPete
Thanks for all of the replies so far. I guess so far nobody reading my post knows exactly how to go about solving my problem.

Modelling software programs certainly are convenient. I wouldn't be across the board opposed to using one. But I think there is a danger of letting the computer do all of the work, ending up with a good speaker system, and being at a loss to explain why the design is good. So as much as possible, I like to get to know the math behind the solutions.
It's nice to know and understand the math behind it but it's not really necessary - the modeling software will play with the numbers. All you need to do is learn how to use it properly.

You don't need to understand how your operating system works on your computer to use it; you don't need to understand how your car converts the gasoline it guzzles into forward motion to drive to the beer store.

Quote:
Originally posted by sreten

Often ignored (completely) is that Vb<Vas linearises the suspension
at low frequencies, Vas non-linearity is a major cause of distortion
below ~ 100 Hz. You cannot guarantee high quality bass with
just box modelling from small signal parameters.

For most drivers with a higher Qts (suitable for closed box), the Vb will be less than VAS. This is normal...

Am I seeing this right?
Seems to me that acoustic suspension can be tried with any driver with a large VAS. Large VAS equals a loose suspension and would greatly benefit from the air spring support of the smaller box.
F3 will suffer a bit, but this would be a small price to pay for a smaller box and less bass distortion.
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Old 28th June 2008, 02:27 AM   #10
MCPete is offline MCPete  United States
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Default I can't communicate?!

I like the discussion so far, but it's off the mark from what I am essentially driving at. As I strayed myself in my second post I'm not going to complain.

Basically what I'm saying is that it is very likely equation(s) are behind BassBox specifying one driver to be acoustically suspended and another one not, and I would like to see them. Also because I am cheap, I want to be able to know whether or not a woofer must be acoustically suspended by looking at the spec sheet for the woofer and not having to rely on a computer program. I think that probably most of us with more interest than usual in speaker systems want to understand what makes them "tick" (this is probably a really outdated expression nowadays). The computer programs can be useful to check the correctness of our understanding, but I think that they can be counterproductive if relied on too much.

You know very possibly also there are some criteria for specifying alpha one way or the other for a particular woofer that aren't etched in stone. If you don't know what those criteria are, then you can't be sure that you can agree with the final assessment.
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