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Old 2nd January 2003, 01:53 PM   #1
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Default Bass reflex rollof questions

1/ I notice when using WinISD for example, if I make the box bigger and bigger the response gets peakier before it rolls off. Does that mean the transient response in the peakier region is worse because of the higher Q?

2/ You can have a fairly flat response or you can extend the response downward somewhat if you are prepared to tolerate a few rises and dips in the response. Do these response shapes correspond with those of active filters e.g. butterworth, chebyshev etc?
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Old 2nd January 2003, 02:14 PM   #2
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I cannot answer the second question because I am not an engineer familiar with different filter arrangements.

The answer to the first question is yes.

When it comes to bass reflex, remember this guideline. Transient response gets smoother when you start with low Qts woofers-beneath .38-and put them in boxes that are smaller than the speaker's Vas. The boxes will be tuned above the speaker's Fs.

So a speaker with a Qts of .25 going into a box that is one third of the speaker's Vas will have better transient response than a speaker with a Qts of .38 going into a box equal to it's Vas and tuned to the speaker's Fs.

A speaker with Qts above .38 fits well into a box larger than the Vas and can tune well into a box larger than the Vas. It will have the worst transient response of all.

For any given speaker, you can make the box larger or smaller within a scertain range. The smaller you make the box, the better the transient response.

It's been my experience that speakers with a Q of .5 or below sound good in a properly made and tuned box. That is my subjective opinion that some might disagree with.

Basically, the steeper the rolloff, the worse the transient response. The low Qts speakers I spoke of will yield the more gradual slope.

Even low Qts woofers in a box one third or even one quarter the Vas will yield transient response worse than a a sealed box with a Qtc of 1.

One more thing. It has been proven conclusively that the lower a frequency gets, the less the ear can distinguish distortion, phase, etc. What is acceptable at 30 Hz and even 60 Hz might not be acceptable at 200 Hz. So midranges are rarely ported.

Remember the extra output the bass reflex puts out. At the tuning frequency, the cone is only moving 1/4 distance it would if it were in a sealed box putting out the same SPL. That is a lot of output to forego in order to get a better transient response. Especially when it is in a range where the ear cannot distinguish so well.
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Old 2nd January 2003, 03:05 PM   #3
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Graham,

The answer to the second question is, absolutely. The vented box speaker can be designed to replicate a forth order filter alignment whether it be Butterworth, Chebyshev, or even the quasi 4th order QB3 alignment. The response of these systems will fall off a about 24db/oct in the stop-band and display all of the response characteristics of the classic electronic filters of the like alignment.

One approach to obtain the wider bandwidth of the vented box system and retain some of the transient capabilities of a closed box system is to use the QB3 alignment. This alignment starts to roll off at a higher frequency but maintains a shallower initial cutoff slope, as well as better cone damping. Then use the room gain to augment the lowest octave of bass. Alternatively, electronic bass boost circuits used before the amp have also been implemented with varying degrees of success.

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