Midranges: sealed or "vented" enclosures

[turboyam]

Hey guys!. I have this question. My future project is a D'Appolito
(wmtmw) with 15in woofs, 6in, mids and slot tweeter.
I want to have 3 separated "sub enclosures" (one enclosure divided in 3 sections), 2 for the woofers and one for twin mids and tweeter/ x-over.

Did you try to "vent" (decompress) the midrange enclosure? what are the results?.
I was thinking about letting those 6 inch mids "breathe" thru a hole or slot
behind the enclosure.
I may use Audax pr170 or some Fostex mids...
I think using a pair of 6inchers in a sealed "air suspension" section is bad
idea...
Any thoughts?
Thanks guys!.

[DougL]

In any vented enclosure, excursion is not well controlled below the tuning frequency of the enclosure. My call, very bad idea in general for a midrange.

Never tried it, but there doesn't seem to be an up side that I can see.
So my question to you is: Why do it?

Just My 2 cents.

Doug

[pheonix358]

Seal the mids, you can use aperiodic vents quite well!

[ScottG]

It depends on the drivers and the enclosure volume..

There are Bass-Reflex designs and Aperiodic designs.

Because it's an MTM consider comb filtering in relation to the upper freq. extension of the mid.

EX. the Faital Pro 3FE20 1 liter Bass-Reflex with an 80 Hz tuning freq. and a 1.25 inch diameter tubular vent.

It should of course have a High-pass filter at *least* a half-octave above the tuning freq. (filter dependent).

[Curmudgeon]

One possibility with the sealed enclosure is to size it to rolloff the mids at the desired xover frequency. That way you can avoid using a series highpass capacitor, which if really good is really expensive.

One possible drawback is that this limits you to a second order highpass; but for many drivers that should be ok. Dimensions tend to be small, and I use both felt on the walls and Acousta-Stuf to minimize internal reflections.

P.S. I certainly agree with Doug L about the vented enclosure's lack of cone control below fbox.

[rabbitz]

The Loudspeaker Design Cookbook has a good section on this and here's a small note:

If you cannot get an enclosure resonance from a TL or vented configuration at least 2 octaves below the crossover frequency , use a sealed enclosure. The benefits of using the TL and vented enclosure include less rear reflection (TL) and less midrange cone excursion, with less Doppler distortion (vented).

The largest problem in mid enclosures is the backwave / internal reflections and needs to be addressed by damping, larger enclosure and use of non parallel walls. This can be overcome by using an OB for the mids but can create other issues in the design.

[pheonix358]

rabbitz, I agree completely. I routinely use a tube construction (very simple and elegant solution for mids in a larger enclosure) and use an aperiodic vent that is almost the size of the tube. The operation of the aperiodic vent is controlled by the density of material used. In practice this tends to eliminate much if not all of the backwave. I still use woofers with relatively low fs, an octave (or two if I can) below the range I want from the mid.

Terry

[turboyam]

Good points guys. My thinking was that if i use a midbass driver as a
midrange, one that is suited for vented enclosure, woulden't it restrict
it's mid/high frequency response if it is mounted in a sealed sub-enclosure?.

Another question that's bugging me; those dedicated midranges with
stiff suspensions, (treated cloth), why do they make em stiff suspended?.

Seems to me that those closed back "dedicated mids" always sound bad
compared with a midbass or a full range in a large size sub-enclosure?.

Thanks guys .

[rabbitz]

Originally Posted by turboyam

My thinking was that if i use a midbass driver as a midrange, one that is suited for vented enclosure, woulden't it restrict
it's mid/high frequency response if it is mounted in a sealed sub-enclosure?.

The mid bass used as a mid would not be working significantly in it's piston range so there's no degradation in the mids when used for mid duty only. The crossover is usually a couple of octaves above where the driver would normally be producing bass in say a 2-way. The advantage of using a mid bass driver is you can crossover lower (e.g. 200Hz) where a dedicated mid may have to crossover much higher (e.g. 500Hz) due to limited excursion / power handling available.

pheonix358

Yes, the backwave is a bitch and your solution works well. I heard a P13 used as a mid in a similar arrangement as well as with the back sealed. The sealed version had so much energy coming back through the driver it ruined the speaker IMO.

Troel's has a very nice mid enclosure on this speaker.
Troel's Cyclop

[Curmudgeon]

The back wall is the most critical, and that one alone can be sloped.