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Old 24th November 2003, 09:09 PM   #1
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Default Passive Rumble filter?

Is it acceptable to place a HUGE cap. in line to "rumble filter" a smaller vented box? IF not, is there any other way that I can limit input below Fb to protect against overexcursion? How do commercial manufacturers protect their smaller ported designs?
Thanks,
-fortyquid
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Old 25th November 2003, 12:33 AM   #2
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In order to keep excusrion *constant* as frequency decreases, a second-order filter (12dB/octave) is needed. This involves a large capacitor and a large inductor. Prohibitively large, methinks. Ideally, this is a line-level pursuit, but in case you have to go passive, you can probably get away with just the capacitor (a first-order, or 6dB/octave rolloff) and probably tune it to Fs.

IIRC, ported designs have a 24dB/octave acoustic rolloff below Fs so unless your resonance is really low, you'll probably be just fine. Sealed boxes would have much more trouble with the power/bass energy. Are you actually having problems with overexcursion?
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Old 28th November 2003, 12:15 PM   #3
sreten is offline sreten  United Kingdom
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Default It has been done before !

The original Wharfedale Diamonds had a series capacitor with a
reflex alignment giving an overall 5th order roll-off.

KEF used series capacitors for sealed boxes giving 3rd order roll-off. Without the the capacitor(s) - several used in parallel - Q was
1.0 to 1.1.

I'm sure that for this technique to work reasonably well the initial
alignment should be somewhat underdamped, i.e. boomy before
the capacitor is added, otherwise you'll throw out the bass with
the low bass.

So your small box should be tuned higher than normal.

Manafactures 'protect' small reflex speakers with highly
progressive suspensions, - they get stiff quickly as excursion
increases - doesn't do anything for the bass quality at all.

/sreten.
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Old 28th November 2003, 12:29 PM   #4
sreten is offline sreten  United Kingdom
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Default P.S. NappyLady >>>

I'm afraid your somewhat mistaken regarding low bass cone
excursion in reflex and sealed boxes.

The reflex rolls off at 24dB/oct because the port output below port
resonance is out of phase with the main driver - but the driver is
still moving.

Below the port frequency reflex cone movement is only limited by
the stiffness of the suspension (Vas). In a sealed box the box
volume is much smaller (usually) than Vas, the driver appears to
be much stiffer and sub bass frequencies are rejected much more
effectively than the reflex case.

/sreten.
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Old 29th November 2003, 08:47 PM   #5
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Hmm, can I get away without HP? I was under the impressiom that FR was irrelevant to cone excursion; i.e., that a 20Hz tone would send a small driver right out of the box whether the FR curve indicated that it was actually playing the note or not.
These little 5" drivers actually took lot of abuse (24Db of Eq @30Hz, a full octave below Fb!) in a ported allignment; when I switched to sealed, however, they over-excurded frequently- which makes no sense to me: I thought that sealed allignments were better about staying within the excursion limitations.

So anyway, am I to understand the following:
-A loudspeaker won't distort on a note its not playing (according to FR).
-A loudspeaker will distort at a given frequency according to the magnitude of the signal it is playing.
-If I have a 24Db/Oct rolloff in FR corrosponding to a similar rollof in power handling, I shouldn't have overexcursion problems

Is that how it is? Or am I missing something?
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Old 30th November 2003, 12:25 AM   #6
sreten is offline sreten  United Kingdom
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A ported alignment causes a sharp null in excursion around
the port frequency and generally adds quite a few dB bass
output around the port frequency without any additional
driver excursion.

A sealed box comes poorly second to a reflex alignment in
maximum 'normal music program' powerhandling, nearly all
reflex small boxes are aligned to to a ' maximum power
handling' response.

Did some-one mention real bass ?

I'll not bore you with the details that seem to be irrelevant to
your experience, if you want more volume get bigger speakers.

It seems to me low bass power handling is not your problem,
so 'sub-bass' filtering is not going to help much.

/sreten.
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