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Old 27th November 2002, 03:00 AM   #1
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Default Other bass boost post

I posted this on the Home Theater Forum and the PE Tech Talk boards, but I know you guys know a lot about enclosures, so take a look at the part that asks about enclosure Q.

-----

I have a question about the Parts Express subwoofer amplifier. I am planning to build a 1.2 cubic-foot sealed subwoofer and I will be needing a bit of bass boost help from the amplifier. The 300-794 allows up to 5 dB of boost but my question is:

does anyone know the slope of the EQ in the 300-794? (e.g., 12dB/oct, etc...)

Also, will a Qts of about 5 destroy a sub driver with a good 25mm of mechanical Xmax? What should I be careful of if I pursue this? At only 270w (the 300-794's measured power into 4 ohms) am I better off with a Q of about .6?

-----

Wow, I'm talking about sealed subwoofers. I feel so esoteric right now.
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Old 27th November 2002, 03:52 AM   #2
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As much as 270 Watts, you mean?
Not "only 270 Watts".
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Old 27th November 2002, 03:56 AM   #3
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Default Re: Other bass boost post

Quote:
.....Also, will a Qts of about 5 destroy a sub driver with a good 25mm of mechanical Xmax? What should I be careful of if I pursue this? At only 270w (the 300-794's measured power into 4 ohms) am I better off with a Q of about .6?
Is that supposed to say 0.5 Qts?
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Old 27th November 2002, 11:53 AM   #4
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Default oh

Right. A Qts of .5

Will that toast my driver?
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Old 27th November 2002, 03:10 PM   #5
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Why should it?

Not to be technical, but Qts, as generally written, refers to the driver when it is unenclosed-out of any box. Qtc. refers to the driver when it is put into a sealed box. Unless the volume of the box is several times the Vas of the driver, (which is rare), the driver in the sealed box will have a higher Qtc than a Qts. The smaller the box compared to the driver's Vas, the higher Fc and Qtc will be compared to Fs and Fs.

Example: A driver with a Qts of .5, an Fs of 30 and a Vas of 2 cubic feet is put into a sealed box of 2 cubic feet.

Both Qts and Fs will be raised 1.4 times. So Qtc = .7, Fc = 1.4. With the same driver.

So when you say "Qts" are you talking about the driver inside the box, or outside the box?
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Old 27th November 2002, 03:29 PM   #6
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Default I meant Qtc.

The finished enclosure has a total Q of 0.52

That is, a Qtc of 0.52
in a 1-cubic-foot enclosure
With a woofer that has 12mm one-way RMS Xmax but it clearly can do more that that.

Here's what I wound up with, and I think this design is quite a discovery:

A 1-cubic-foot sealed box with a Qtc of .52 (with a Qtc of 0.5 being considered "Transient-Perfect"). But the box on its own has an F3 of 51 Hz. However, when this enclosure's response is plotted on the same page as the frequency response of the Parts Express 300-796 plate amp (with 6dB bass boost at 30 Hz) the enclosure's response plus the assist from the bass boost (I took the two curves and averaged them) yields a tremendous frequency response: Flat from 85 Hz (+/-1dB) or so down to 30 Hz, with a total system F3 of 25 Hz, which is the same as my current ported enclosure using this driver except that my current enclosure is actually sort of an EBS alignment with a big shelf from 40Hz down to 25Hz.

This design is sort of a cross between an Assisted Resonance System and an equalized system like the Bag End subwoofers.

So now I have found a transient-perfect subwoofer with 270 watts, flat response down to 25 Hz, and an enclosure that is only 1 cubic foot using a now-discontinued 10" subwoofer driver (replaced by the new version of the Blueprint 1001) that has long excursion and a max power handling of 350w. And I'm wondering if this will be causing me to bottom my driver out at moderate levels.
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Old 27th November 2002, 04:15 PM   #7
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First, I just want to post this formula so everyone can figure out how putting a speaker into a sealed enclosure works.
Attached Images
File Type: gif formula qtc, fc.gif (3.8 KB, 400 views)
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Old 27th November 2002, 04:22 PM   #8
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Default Just LT it...

The LT circuit is a very interesting animal.

If you use a Linkwitz Transform circuit, you can get the final Q to 0.5 with virtually any sealed enclosure.
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Old 27th November 2002, 07:57 PM   #9
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Default Power restrictions

I can't use a Linkwitz transform circuit because my power is limited. And I can't get more power because my budget is limited (and how much my parents will let me get away with is also limited). But if I have this particular situation that I previously described (Which is NOT an LT, just ordinary bass boost), how can I estimate cone excursion so I don't have it flopping wildly and bottoming?
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Old 27th November 2002, 08:01 PM   #10
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Default depends on your driver

depending on the driver you have.

I doubt you will bottom out the driver before the amp starts to clip.

270 watts isn't a whole lot of power for most modern subwoofer drivers.
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