Here's a question;

I believe that the Marrs cabinet is a tuned, ported, closed-back cabinet. Technically, I would think that if you mount the speaker to the front or rear side of the baffle board, it would still be "front-loaded". If you mount the driver to the front of the baffle board, it will increase the internal volume a bit of the cabinet and depending on which speaker you use, that could prove to be better or worse for the low frequency output of the system. I would guess that the Marrs cabinet is tuned for a rear-mounted driver with common characteristics. You may want to contact Marrs and ask which 15" the cabinet is tuned for.

I found the Marrs cabinets to be really minimal, and there are a few things you can do to improve them. The backs are really flimsy thin plywood. I replaced them with stiffer 1/2" plywood. This helped the sound, but is still light and minimal. Even good home hifi speaker cabinets typically have 3/4" sides and backs. But I don't see much point in putting a 3/4" back on, since the sides and front are not that thick.

The ports on the front of the closed-back model do not have tuned tubes in them. So not only is it not tuned, but there is some wrap-around sound between the front and back of the speaker cancelling out some of the volume. I got some inexpensive plastic tubes from a speaker parts supplier (on the internet, forget which one) and glued them into the ports. One tube in each cabinet is an adjustable one for tuning. I haven't taken the time to do critical tuning, but even so, these noticeably improved the low end volume.

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The ports on the front of the closed-back model do not have tuned tubes in them. So not only is it not tuned, but there is some wrap-around sound between the front and back of the speaker cancelling out some of the volume.

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Check out this tech paper from JBL: http://www.jblpro.com/pub/manuals/enclgde.pdf
If you don't find what you are looking for there it also has a great bibliography that will be most helpful if you really want to understand how speakers and cabinets interact.

Particularly infomative in this regard is Abraham B. Cohen's "Hi-Fi Loudspeakers and Enclosures," (Hayden Book Co., 072 1)

Hope this helps...
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Dave Grafe - email: dg@pdxaudio.com
Production
Pickin', etc.

1978 ShoBud Pro I E9, 1960 Les Paul (SG) Deluxe, 1963 Precision Bass, 1954 Gibson LGO, 1897 Washburn Hawaiian Steel Conversion

[This message was edited by Dave Grafe on 05 January 2005 at 11:19 AM.]

Putting tubes in "port" holes is an interesting idea, I'd like to try messing with it on all 4 holes...

Still, I think "front" mounted speaker on the baffle "projects" better( it may appear to sound brighter), I may try on one cabinet with Deltalite in it.

When I compare D-130 and Deltalite, D-130 has that, mid-scooped sound it really sounds authentic with D-10, especially on sweet "high". While the Deltalite seems to me a very well balanced, even sounding speaker( with D-10, of course), therefore, it appears to be lacking "high" sparkles, but actually, there're lots of "Highs" coming out of this speaker, but it's so even( compare to JBL), it appears to be "Darker" or "Mid-heavy". But!! It's only 8 lbs, and for that reason alone, he stays.

If you randomly put int a tube for a tubes sake,
it will likely throw off the tuning of the cab, so it's sound will be less smooth across the frequency range.

Also changing the placement of the spoeaker from front to back will as noted above cnahnge the volume of the cabinet, and thus it's calcuulated resonance curve.
Another way to make for an unblanced cab.

I've been thinking building some of these cabs myself, but stiffer, hopefully still light in weight. What difference in sound, if all else is equal (same size), would this give?

With regard to the port tubes, which diameter and length are recommended. I've seen plastic port tubes for sale on Parts Express; are they good?

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the back panel of the cab is very flexible and moves a lot with the lower frequencies

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...Is it correct to think that this 'eats' sound?

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D. Doggett you are right on track.

The issues regarding porting/tuned ducts/cabinet size are all closely related to the speaker's size and resonance. Essentially, a port allows a cabinet to act as sort of an acoustical capacitor, storing energy and releasing it when it reaches a certain frequency-specific level that the designer deems advantageous. Precisely tuning ducted ports is a pretty complex process and much more difficult than tuning simple ports, but luck DOES happen and the science is quite accessible (see the above mentioned JBL site and bibliograpy). The easiest method involves monitoring the speaker's impedance as the input frequency is swept across the spectrum and adjusting port size to flatten the curve as much as possible - with ducted ports this means having a good supply of tubing and slicing bit by bit until you slice too much off and then making a new one that is the size you just trashed. Use standard PVC pipe, don't go paying extra for somebody else to paint it black and package it for you as "acoustical ducting" or some such.

To address the original question, moving the speaker to the front of the baffle increases cavity volume and should allow you to get improved low end response but MAY require some adjustment to the port, assuming that the port was in fact accurately "tuned" in the first place. Be aware that a cabinet tuned for any given speaker won't necesssarily be properly tuned for a different model. Also be sure to provide some sort of protection for the speaker cone as it is now much more exposed.

Much more than that you'll have to read some books and learn yourself.

[Edited 'cause there's just too many darned words for me to spell correctly all at once....]

[This message was edited by Dave Grafe on 07 January 2005 at 07:33 PM.]

With some effort I have been able to acquire the Theile/Small parameters of most popular MI speakers. What I can offer is that the Mars cabs look good on paper with the BW 1501-4. It is my opinion that the thin back should be stiffened by gluing 1X2 or similar braces on it to move any sympathetic resonances to higher frequencies that don't interfere with the music.

Speaker cabs for lead instruments should not reinforce frequencies below 70-80 Hz, so this means that the cab can be smaller. With proper porting, the trapped air inside can be tuned to the correct resonance frequency to match the speaker being used.

The JBL and EVM speakers don't need large volume to produce good results. Usually a 1.5-2 cf cab is sufficient with 2 3" plastic ports 1-1/2" long. Longer ports lower the resonance of the air in the cab. Also, if too long tend to create undesirable response peaks in the 100-200 Hz range (which you can eq out with mid-sweep).

The JBL requires a 14-1/16" cutout hole for front-loading. A JBL D, K or E series will perform differently from the BW in the same cab but will still sound good with some eq.

The Delta-lite requires an 11 cubic foot cabinet, properly ported, to provide flat response. I have experimented with sealed boxes, ported and open-back with the Delta-lite and just don't really enjoy the tone as much as the BW, EV or JBL. The Delta-lite has plenty of brightness even though the high-end response rolls off sharply above 3500 Hz. The BW, JBL and EV go up to above 5000 Hz.

The smaller Delta-lite 10" and 12" speakers, on paper, look much better than the 15", so a pair of 12" wired in parallel in a ported cab might be the ideal.

Something I enjoy is to find an existing cab and see how it would work with different speakers, properly ported.

Anyone wishing to mess with cab design feel free to contact me. I have many hard-to-find parameters for popular speakers and am willing to offer the freeware and/or opinions about cabs.

Ideally you want a 32' square baffle..
this is impractical in all but large theater instalations.
(This is why most studio monitors in larger rooms ar wall sofit mounted,
and often the room is designed with specific speakers in mind.)

So the next step is balancing a smaller baffle and cabinet size,
with ways of harnising the inverse function of the speaker movment with the forward movement, Thus creating more apparent volume from the cab
without increasing baffle area.
Hense MANY differing philosophies of cabinet construction.

With a front ported cab the tube is there to adjust the phase of the rearward movement of the diaphram so it emerges delayed,
but in phase sync with the forward movement.

The issue of tuning to a resonance point below the speakers normal cut off point "In relation to its baffle size",
is another tricky part,

Too much and it then leaves nodes above the fundamental
and that throws the cab off on upper freqencies.
Too little and there is less low end.

Hence my comments about smoothness.
Each space as it's resonance, but you need to
strike a balance between size, baffle face area,
inverse energy transfer and its phase sync,
and cabinet resonances and frequency responce
of its practical range.

If a cab is tuned for it's size, then changing the driver size
won't have that much effect if the baffle area and driver placement remains the same.
If they are putting another baffle in front this might change things... If it is just a different whole in the same baffle not much difference cabinet wise.

But his might be interesting, I don't know how Duane is doing this.

If you are FRONT mounting the 15" normally,
but for the 12" are adding another baffle
with a smaller whole in front of the 15" baffle,
then with the 12" REAR mounted it would be nearly the same internal area of the cab, yet be easy to change speaker type with one construction design.

For a bass player, smoother responce is MUCH more important than the cabs lowest frequncey responce. I can boost those with the amp, but to play and have to watch every note tp be in proper relation with the last is unacceptable.

Yes we must do that between strings, but you should only have to worry about tuning the EQ to the room, not the cab.

The old Fender open backs had just enough baffling to get some womp under the twang, andthat is one reasons we like them, and as they were often placed near a wall, they had a variable phase return.

How many have moved their Twin a bit forward and back fromthe wall till it sounded sweet?
That is just like the tube in a cabinet, but the tube is a more controled version.

[This message was edited by David L. Donald on 09 January 2005 at 07:25 AM.]

David, If you want to try different tube lengths, the best idea is

Get / rent an an old dual trace scope
2 identical mics
a sine tone generator

The drill is put a tone through the amp, not loud but there,
and then mic both the speaker and the tube port mouth
with BOTH mics on the exact same plane relative to each other.

Then feed the mics into each side of the scope.
As you trim back the port and reinstall it, eventually you will see the signal phase meet relative to front and back signals.

At that point you have your best choice of port tuning.

Another issue of tube tuning a port is the tube dimensions.
The width of the tube should be calculated to be an inverse or at least sympathetic resonance to the cabinets resonance. Since it in effect IS another cabinet area.
You are trying to squeeze the same freqencies out this smaller area, and have them be compatible with the sound from the main sound source. Some have made them inverse resonances to help smooth the signal, others have made them additive to just give volume.
In any case you are getting more volume, because both sides of the cone travel are ported frontwards.

So the ouptput hole and tube width need to be relative to the cab resonance freqencies, and then the length adjusted for phase. And there is some effect between the internal back presure from the tube being smaller and the output of the cab in total.

Bass below 200hz or so is very non-directional in character, so some speakers, especially home speaker have been ported out the rear.

All these esoteric variables are why there are some many speaker manufacturors, there are an amazing amount of variables.

I think the MASTER of all this must be Dr. Amer Boze. He is noted for porting both front an back faces in convoluted patterns so both come out in exact phase, and create a MUCH larger total wave than the size normally would alow.
A true genious.

The old radio transmiter wave guides are similar in function, thought for much higher freqencies.
They were tuned for width height and length relative to their intended frequency band width.

And we all thought it was just putting a driver in a box..HA, as Ricky would say.

I really wish Dr. Boze would build a bass guitar amp.. or a steel cabinet...

[This message was edited by David L. Donald on 11 January 2005 at 02:18 AM.]

I spent about 3 hours listeneing to Ken Forsythe of EAW expound on this stuff about 20 years back... my head swam, but he just reels it on out.

Ok as I understand it.
Think of the tube as a cabinet.
And then in your case ALSO
think of it is the volume of all 4 tubes as 1 cabinet.

And then use you cabinet calculator to figure their size to resonance.

I would guess a ratio of 1.5 to 1=1.5 to 1 etc.would be good.
We use that in designing studio control room ratios for the same reasons. So the inherant ringing of the recording room is nulified by it's inverse in the monitoring room.

But of course now your "cabinet" is ROUND so some use of pi and such will be needed to calculate the tubes volume.

If this was easy everybody would do it

Of course when you trim the length of the tube it's volume changes too...

The sound will be vibrating across the width of the tube as it exits, and so this will color the sound and add it's own resonances.

One way might be to look at the harmonics produced by the speaker and the tube output also as you trim for phase.

This may all be beyond the means of the average Joe.
But getting the phase of the tubes output in sync as described above is possible, and really worth while. IMHO.

If you want to know why,
put to cabs angled toward each other, and put the same signal out of phase into each...
There is some cancelation.
This same cancelation will be happening with your tubes and the speaker if they are out of phase to it.

So it is a balance of tube output Phase and tube subcabinet Volume
relative to the original cabinet volume and speakers phase.

Guys have built computer programs to model all this stuff,
and other guys work years just juggling these variables...
Bonne Chance mon amie!

[This message was edited by David L. Donald on 11 January 2005 at 06:42 AM.]