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Old 5th February 2006, 03:31 PM   #1
bcycle is offline bcycle  United States
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Default x over calculations

Hi,

I'm building a subwoofer and have a plate amp (DAYTON SA240 240W SUBWOOFER AMPLIFIER). There's not a lot of documentation on this thing.

One review says "A built-in “rumble filter” attenuates the lowest audio frequencies to help conserve amplifier headroom and protect drivers. The “rumble filter” is actually a 12 dB/octave high-pass filter that is centered at 20 Hz, the F3 is 18.2 Hz. Since most source material does not contain information below 20 Hz, usually we are not missing anything. But, in cases where information is available this low, we generally need to protect our drivers from this potentially damaging information. Since excursion demands increase with decreasing frequencies, even the beefiest sub can start to run into overexcursion problems. This is particularly a concern in vented subwoofer systems where overexcursion occurs rapidly below the tuning frequency."

I'm having trouble determining whether this thing has any sort of a high-pass equalization filter that I can use in my box calculations. If it does, does anyone know how exactly would I figure this into my WinISD Calculations.

Thanks!

Greg
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Old 6th February 2006, 12:49 AM   #2
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If you're using the WinISD beta, your first step is to download the WinISD Pro alpha version.

In WinISD Pro, you go to the "Effects/EQ" tab, click Add, and select a highpass filter, order = 2, cutoff = 18.2. I would assume a Butterworth filter unless there's evidence to the contrary.

You'll see that you'll lose a couple of dB above 20 Hz, but the cone excursion will be greatly reduced below 20 Hz. That will keep the woofer from bottoming out when your movie soundtrack goes bananas at 10 Hz.

Take a look at this thread on another board. Plots from the movie "The Incredibles" show some strong output in the 5 Hz range. Good reason to have a filter on a vented woofer!
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Old 6th February 2006, 01:41 PM   #3
bcycle is offline bcycle  United States
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Thanks lousymusician.

Wow, 5 Hz! Is it safe to assume then that the Rumble Filter in the amplifier description is the same as a high pass filter? That it's already part of the amp's circuitry? There's no setting on the amp that I can see.,,

Quote:
Originally posted by lousymusician
If you're using the WinISD beta, your first step is to download the WinISD Pro alpha version.

In WinISD Pro, you go to the "Effects/EQ" tab, click Add, and select a highpass filter, order = 2, cutoff = 18.2. I would assume a Butterworth filter unless there's evidence to the contrary.

You'll see that you'll lose a couple of dB above 20 Hz, but the cone excursion will be greatly reduced below 20 Hz. That will keep the woofer from bottoming out when your movie soundtrack goes bananas at 10 Hz.

Take a look at this thread on another board. Plots from the movie "The Incredibles" show some strong output in the 5 Hz range. Good reason to have a filter on a vented woofer!
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Old 7th February 2006, 03:27 AM   #4
simon5 is offline simon5  Canada
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Yes highpass filter at 20 Hz = rumble filter.

A highpass block the information below the tuning point. Only the "highs" pass.
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Old 7th February 2006, 04:20 AM   #5
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what is the point of modeling the system in winisd pro? does adding the passive crossover mimicking the "rumble filter" have any effect on box design? why does it need to be considered?
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Old 7th February 2006, 05:33 AM   #6
simon5 is offline simon5  Canada
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Quote:
Originally posted by jaygeorge1979
what is the point of modeling the system in winisd pro?
So we can see what it will closely look like in real life.

Quote:
does adding the passive crossover mimicking the "rumble filter" have any effect on box design?
Yes.

Quote:
why does it need to be considered?
A rumble filter will drop the low end faster than without one. At the rumble filter frequency, you've lost 3 dB.

It's not absolutely necessary to consider the rumble filter, but it's a good idea. Some could even use it to tweak the design. You could use the rumble filter to tame a 3 dB peak at the tuning frequency caused by a too large box for example.
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Old 7th February 2006, 03:20 PM   #7
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i see...thank you simon

when this rumble filter is included, does it change the volume of the box needed to be built?
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Old 7th February 2006, 08:56 PM   #8
simon5 is offline simon5  Canada
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Like I said, you could build a larger box, but it's not absolutely necessary.
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Old 7th February 2006, 10:21 PM   #9
bcycle is offline bcycle  United States
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Question Re: x over calculations

Quote:
Originally posted by bcycle
The “rumble filter” is actually a 12 dB/octave high-pass filter that is centered at 20 Hz, the F3 is 18.2 Hz.
Hence my struggle. The above statement is from a review of the amp that I bought. The software I'm using (Bassbox) has fields for inputting an Fx and a Qx parameter. I'm not exactly sure which values are entered in these fields but when I select the software to "suggest" a box size using the "Vented Box with Active HP filter" it yields a dramatically smaller box ( 2.381 cubic feet) with 19.67 F3! Bassbox calculates a Qx of 16.78 and an Fx of 16.78.

Given the amp description should design this with the "Vented Box with Active HP filter" setting?

I've attached some pdfs showing my calculations. Some of them seem a little wacky, especially the group delay. (I'm still not savvy with all the graphs as yet.

As always, I appreciate the help!

Greg
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File Type: pdf combined.pdf (90.7 KB, 15 views)
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Old 8th February 2006, 05:37 AM   #10
simon5 is offline simon5  Canada
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Look at the excursion, you'll see some problems there, you set the Q too high.

Usually, 2nd order highpass is Q = 0.707
Try this with a Fx of 18.3 Hz ( or 20 Hz? )
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