A few questions about my subwoofer

Discussion in 'Speakers & Subwoofers' started by Vaughan Odendaal, Jan 27, 2006.

1. Vaughan Odendaal Second Unit

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Hi guys,

I just would like to know a few things. I have an SVS PB10 subwoofer and I believe the tuning frequency is approximately 17.5hz according to several people who I have conversed with.

How do you calculate this tuning frequency ? I am very interested to know. Thanks.

--Sincerely,

2. ColinM Cinematographer

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The T/S parameters of the driver, the volume of the box, and the length of the port contribute to the final tuning.

3. Vaughan Odendaal Second Unit

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Do you or does anyone have the specs (port length, volume of box, etc ) on the PB10 so that I can understand how the tuning frequency got to 17.5 hz ?

I am very interested to know. Thanks.

--Sincerely,

4. ColinM Cinematographer

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You have one, right? And a measuring tape?

If you do, you can measure the box dims and the port length. You can also measure the surface area of the driver. With that data, you should be able to extrapolate some TS information in WinISD.

5. Jack Gilvey Producer

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Tuning/Fb is determined by length and diameter of vent and the volume of the box and is independent of the driver's specs. Flaring of the port can change Fb a bit.
An easy way to determine Fb is to run some low level test frequencies ( at identifiable 1 Hz increments) through the sub while feeling the motion of the driver. The frequency at which the driver moves the least is your Fb, as that's where the port's damping effect is strongest.

6. Edward J M Cinematographer

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I emailed him the close mic woofer sweep of the PB10-ISD. Fb is 17.31 Hz using 1/24 octave sweep resolution.

I also gave him a few links to Audioholics articles which deal with calculating Fb.

http://www.audioholics.com/productre...00reviewp4.php

http://www.audioholics.com/techtips/...orabsorber.php

He was also interested in how a larger enclosure volume helps with efficiency at low frequencies, so I sent him a primer on Hoffman's Iron Law. This explains why the PB10-ISD has such high output at 20 Hz with only 300 watt amp and virtually no EQ compensation; the enclosure is "right-sized".

7. Vaughan Odendaal Second Unit

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I read through Hoffmans Law. It gives me answers but it doesn't explain them for me. I still don't know why bigger enclosures give deeper bass.

I would like to understand the "how" or "why" behind these answers.

--Sincerely,

8. Vaughan Odendaal Second Unit

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Does it have to do with the compliance of air in the enclosure ? Can someone explain this for me please ?

--Sincerely,

9. Jack Gilvey Producer

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A larger box is more efficient as the air is less restrictive to the woofer's movement, so there's more deep bass for a given input power. One can achieve a desired frequency reponse in many sizes of enclosure, but a smaller Vb will take more power.
A large box is a very good design decision from a subwoofer performance standpoint. If a designer accepts a "large" box, like the PB10's, then he doesn't need to jump through very expensive hoops to acheive a performance goal.

If you really want to see the relationship, download WinISD Pro and play with it.

http://www.linearteam.dk/default.aspx?pageid=winisdpro

10. Vaughan Odendaal Second Unit

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Thanks, Jack. I appreciate the explanation.

--Sincerely,

11. Vaughan Odendaal Second Unit

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Jack, so if I'm understanding this, a bigger enclosure has less resistance on the driver compared to a smaller enclosure. This means more efficiency. Correct ?

More efficiency means deeper bass ? How does this work ? If you can please explain the mechanism that allows for this I would appreciate it a lot.

And I'm sorry if I seem like a dolt or if I'm slow or anything. I don't mean to be. Thanks in advance.

BTW, are there any books you could recommend to me that would explain these topics in detail without going heavy into math ? I'm all for theory.

--Sincerely,

12. Jack Gilvey Producer

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What little I've gathered I picked up online reading posts from folks who've forgotten more than I'll ever know. You could check out "The Loudspeaker Design Cookbook" (Vance Dickason)in the library, it's got lots of good stuff in it.

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