Midranges: sealed or "vented" enclosures

turboyam

Member
2006-05-03 8:06 pm
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!.
 

ScottG

Member
2003-02-04 12:23 am
US
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

Member
2005-11-02 7:13 pm
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.
 
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.
 
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

Member
2006-05-03 8:06 pm
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 ;).
 
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
 
A bit of reading is in order; you want to use the sealed box size that provides you with a relatively flat frequency response down to the chosen crossover frequency. (That's where the Q that Phoenix mentioned comes in; that and the driver's transient response.) There are numerous inexpensive or free box design programs that use a driver's resonant frequency, stiffness/compliance, Qts and desired response to calculate final box size. The crossover frequency chosen should be based on the low end characteristics of the mid driver, and the high end characteristics of the woofer. Thus mid cabinet, mid driver, and woofer should not be chosen independently of each other.

If this is your first project, I'd advise not designing from scratch; there are quite a few kits out there...

This is a potentially large hobby that can fill a lifetime; if you don't have a technical background, it'll take a while to reach the point where you can design your own from scratch, based on what you've learned that you like to that point.
 

ScottG

Member
2003-02-04 12:23 am
US
Thanks, took notesss. Anyone familiar with Audax Pr170 6inch mids?.
Only one thing i don't like about them tho is that, they have a flat
foam suspension so when comes re-foam time, hummmmm...:confused:.

Again, in an MTM profile you should look for smaller diameter drivers.

Additionally, the Audax is quite efficient.. and if wired in parallel is exceedingly efficient.

What is the *tweeter* you are looking at?
 

turboyam

Member
2006-05-03 8:06 pm
Oh and i forgot, i'm considering a pair of Fostex Fw405n too.
I still have to model all of those to see what's a better match.
It won't be my first try at diy, i have done quite a few, it's just
that i want to make sure what "todays trend" of mid sub-enclosures
look like.
Thanks to all!.
 

ScottG

Member
2003-02-04 12:23 am
US
I should have mentioned that it's going to be a very efficient system.
I'm planing to use a Beyma Cp21f slot tweeter and a pair of either
Beyma 15w100r or Audax Pr380m2 :D...
I need a pair of "rock monitors" ;).
And yes, in a D'Appolito config.

Well good luck with that! (..and the only thing further I'll say is that I hope you are using an active digital crossover). ;)