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Old 3rd May 2004, 02:10 AM   #1
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Default very low Q woofer

I somehow got the notion that Q=0.5 was ideal for a subwoofer, and that the more you strayed from that number, the worse the transient response would be. Now I discover speakergod Linkwitz putting a Peerless XLS 12" in a 50 liter sealed box.

http://www.woodartistry.com/linkwitzlab/thor-intro.htm

What's up with that? I calculate a Q of 0.28. Am I missing something?

It looks like I am never going to build my first subwoofer, because I am in a perpetual state of confusion. How does a person ever learn this stuff????
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Old 3rd May 2004, 02:30 AM   #2
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The equalization circuit effectively raises the Q to a more useful level and extends the low frequency response.
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Old 3rd May 2004, 01:05 PM   #3
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If WinISD is to believed, then a Qts of 0.5 is the perfect number for a bass reflex box at least.
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Old 3rd May 2004, 02:53 PM   #4
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Quote:
Originally posted by Bill Fitzpatrick
The equalization circuit effectively raises the Q to a more useful level and extends the low frequency response.
The EQ flattens out the frequency response curve, but how could it affect the resonance properties?
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Old 3rd May 2004, 03:09 PM   #5
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Quote:
Originally posted by Dave Jones


The EQ flattens out the frequency response curve, but how could it affect the resonance properties?
A Linkwitz transform changes a sealed sub from its previous f and Q to a new target f and Q.

The Linkwitz T ransformed sub has the same properties (f, Q, group delay etc.) as as any box and speaker combination that would produce the same f and Q.
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Old 3rd May 2004, 03:27 PM   #6
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Default Speaker Qt

Generally, drivers with a Qts of .2 up to .5 will work well in a reflex enclosure.
Sealed boxes work best in the higher end of this range, say from .35 up to .55 Qts.
Open or infinite baffle speakers are .6 and above.

When you see a driver Qts in the mid to high .30's, it will usually work well in either enclosure.

If you think about Qts as a reflection of motor strength vs mechanical properties like compliance, the higher the number, the less damping of the cone; lower number means stronger motor and greater cone damping.

Loosely speaking, a sealed box has more cone movement than a reflex box, and requires slightly less cone control, or higher Qts.
Although, as a driver passes resonance (Fs) in a reflex box, all kinds of movement can occur, and the particular Q of the sealed alignment can alter cone movement, too.

Select a driver with a Qts of .45, for example, and winISD will default to a sealed enclosure, if you click "next" and don't choose a box type. The program is picking the enclosure best suited to the TS parameters for you.

The small bar graph in the "box" tab is a visual indicator of the flexibility of the driver in different alignments, with various color bands showing up.

Tim
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Old 3rd May 2004, 03:59 PM   #7
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Okay, I think I understand how EQ affects resonance. What we are interested in (I guess) is the ratio of stored energy to energy going into the equalizer (not the speaker). Right? In other words, the Q of interest is that of the system that comprises the speaker, box, and equalizer.

So now I'm thinking about just getting a 1 cu ft box and slapping a Peerless XLS 10" in there with a plate amp that has a 6db boost at 30Hz. With the room gain in my small office, that should do quite well, I think, maybe, perhaps, unless I am mistaken.

Here's a graph of the speaker in a nominal 26 liter box and no EQ. Whatcha think?
Attached Images
File Type: png xls-10-sealed-26.png (2.8 KB, 897 views)
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Old 3rd May 2004, 05:21 PM   #8
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Default Suitable woofer

Just look at the properties you need for the ideal woofer for the task, for a minute.

That 1 cu. ft. enclosure size does not give you any low frequency extension to speak of, with the woofer starting to roll off at 300 Hz, and being well down at 30 Hz. With a low Qts woofer in a sealed box, your chances for success are limited. Use a woofer near .4 Qts or higher for a sealed enclosure.

Sealed box:
Room gain in a perfectly sealed room is 12 dB/octave - so 9 dB is more like it in the real world. If you match the slope of the roll off to 9 dB gain, you will see very low extension. Right now your slope is more like a reflex enclosure and too steep for room lift to help.

Your box size is not permitting this, indicating too much woofer for the box, or a poor choice for a small sealed sub.

Tim
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Old 3rd May 2004, 09:47 PM   #9
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Default Re: Suitable woofer

Quote:
Originally posted by Tim Moorman

Room gain in a perfectly sealed room is 12 dB/octave - so 9 dB is more like it in the real world. If you match the slope of the roll off to 9 dB gain, you will see very low extension.
Tim

9dB/Octave seems like too large a number for room gain for the 20Hz to 80Hz range. Where did your number come from?

Adire Audio uses a room gain of about 4 dB from 80Hz to 20Hz in their subwoofer design white papers. This is only about 2dB/octave.

They show the overall room gain from 20Hz to 300Hz to be about 12dB overall but the gain in the 20Hz to 80Hz range is somewhat shallow.

It is not clear what size room they are using for a "typical" room in their model. Presumably something like a 4m x 4m room. Something like a car would have a higher gain that starts at a higher frequency.
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Old 3rd May 2004, 11:42 PM   #10
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Default Remember the impedance curve!

Some arguments in favour of a low Qts woofer in a small sealed box:

1) In most cases of a sealed woofer, the impedance peaks at a comfortably high value somewhere around 30Hz~70Hz, right where electrical equalization is most needed. This means that the woofer can be equalized to perform similarly to a ported or high Qts woofer, but with a cooler amp and smaller box.

2) A low Qts means that the back-pressure behind the speaker is blocked more effectively than with a high Qts woofer. This is because the stronger magnet allows the amplifier to have better control over the voice coil, so it can resist external forces more effectively. It may appear that a high Qts woofer would have an advantage because the mechanical resonance produces a useful bass boost, however: the 30~70Hz resonance is not the only resonance that is produced. Inevitably there are also internal box colourations, and they may be audible especially if a gentle crossover slope is desired. A low Qts speaker will block these colourations much more effectively, and the differences in low-frequency response show just how important the improvement could be at higher frequencies.

3) A sealed air spring is much more linear than a speaker's spider suspension, and the air suspension will be the dominant factor in a small sealed box especially if the VAS of the speaker is high. This will result in lower excursion-related distortion.

CM
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