Damped (lossy) BR ports

[MJK]

The Bullock and White TL software is based on Bradbury's moving fiber model. The software does not work very well for simulating a TL. Even Bullock, in his paper based on this program, noted that the correlation between test data and his calculations was not very good.

I tried the program a few years ago and even built up a MathCad model using the same moving fiber equations. The MathCad model and Bullock's software were in good agreement. This was the original model that I used to try and correlate against my early test line measurements. It did not work.

My advise would be not to use this program. But the final choice is for the TL DIYer to make.

[Ron E]

Pjotr,

In the limit, the lossier the port is, the closer the box resembles a sealed box. You will get a small lower impedance peak, which if you then stuff the crap out of the box, you will lower the upper peak as well. ?Then you will have an impedance curve which looks like what TL fans like to see.

Will it sound like a TL? - methinks it will sound like a damped lossy sealed box.

TL's are an interesting design problem, but I have not found them to sound any better than a properly designed conventional enclosure, which would probably be 1/3 the size. They may sound different, perhaps due to the comb filtering that is often present.

Different <> better.

[kelticwizard]

Originally posted by MJK
The [Bullock and White] software does not work very well for simulating.... the correlation between test data and his calculations was not very good....

I tried the program...It did not work.

My advise would be not to use this program.

Oh, DAMN!!!

After all this, NOW he tells me!!!

[MJK]

Sorry Kelticwizard,

I did not mean to rain on your parade. Hopefully you only invested a little time and did not build anything. I messed with that program for a while also when I first got started.

Keep playing with the Mathcad, it is not that hard!

[kelticwizard]

Pjotr:

I have not yet gotten started on Martin King's program.

However, as I understand it, you want an enclosure type which gives the cone excursion relief while producing a second order rolloff.

Such an enclosure would appear to exist. I have not built one yet, though. However, it was written up in two articles in the Journal Of The Audio Engineering Society.

It is the Augmented Passive Radiator by Thomas J. Clarke.

I have a page missing from the first article. I can Email you what is left of the first article and all of the second if you want.

I discussed this enclosure at length on page 5 of the following thread. It has charts, frequency response simulations, and diagrams of the unit.

New Reference Speakers with Full/Wide-Range Driver

The first post I wrote about it is in the middle of page 5. It begins with the words: "Here is the enclosure I was talking about."

I would think this enclosure would be of real interest to you. It would seem to supply either

A) A quarter octave lower F3 compared to a normal bass reflex with the same driver and the same 24 dB/octave rolloff rate; or

B) The same F3 as the conventional bass reflex only with a 12 dB/octave rolloff instead of a 24 dB/octave rolloff rate.

Hope this helps.

[phase_accurate]

Hi kelticwizard

If you'd like to lower a driver's Qts (electrically via it's Qes) then you don't have to wire a resistor in parallel with the driver but a negative resistor in series with it !

There was once an issue of Wirleless World whose emphasis was on bass reproduction.
There was an author presenting a special tuning he had used:
He took a driver put it in a box whose volume was too small to even get a decent response as closed box, i.e. Qtc > 1, fc < 100 Hz.
He then ported the enclosure with quite a low fb (I think it was in the 30ies).
The result was a response with a hump above 100 Hz followed by a second order response down to the tuning frequency where it slowly changed into a fourth order response.
He then simply took a Linkwitz transform circuit to make it flat ......

I thought that his proposal was a bit extreme and played around with "perfect box". I haven't built such a box so far (I am more into "real" closed boxes at the moment) but in the simulations the following tuning didn't look that bad: You take the volume that would give a decent closed-box tuning (i.e. a Qtc between 0.5 and 0.8, depending on application and taste) and then tune the port about an octave lower than the fc of the closed box would be.

Regards

Charles

[nuconz]

hello,

i realize this reply is a bit untimely, but here goes.

the original prototype of the jbl aquarius 4 speaker employed a distributed port. i have one of these cabinets.

subsequent revisions of this speaker used a port tuned to about 40 hz.

the speaker employed an upward firing 8" driver (le8t) in the top of the enclosure, similar to one of the allison speakers. the top 2/3 of the cabinet was stuffed to the magnet of the woofer with batting. the lower 1/3 chamber had no stuffing at all. the distributed port consisted of 4 1" holes drilled through the cabinet.

the distributed port had a more "mellow" sound, similar to a TL.