Damped (lossy) BR ports

[Pjotr]

Hi,

I am investigating de possibilities for a bass enclosure that I am up to building. There is already a thread about the cons and pro’s of various systems TL's vs. Closed Box or Ported

Most promising looks the open ended damped TML approach because it limits cone excursion at the low end of the pass band and the line output can nicely supports bass response. Further this system can have a low overall Q-factor for reproducing clean realistic bass. Many reading states that a TML has 2nd order response.

Last weeks I did a lot of simulations with the model of Augspurger in Pspice (published in the JAES, Vol.48, No.5, 2000 May) and the MathCad model of Martin King. I did the simulations on a Davis 25 SCA 10W] and an Audax PR 240 ZO unit. Both units look promising for clean dry bass reproduction

Although I found nice TML alignments, responses were not 2nd order at all. These were something between 3rd an 4th order with low overall Q. This is not mysterious. The column of air in a open ended TML behaves somewhat as a 2nd order mass-spring system coupled to the 2nd order mass-spring system of the speaker unit.

I also simulated the speaker units in a 60L BR enclosure. Not a regular one but with one with a LOSSY reflex port. When the loss of the BR port is high enough I got near the same responses as with a well-tuned TML. Such a BR tuning does not have the irregular ripple in the response at higher frequencies a TML have, can be build with a smaller box and is far more easy to build and to tune. It has also the advantage of reduced cone excursion at low frequencies, so more bass output is possible for a given speaker and reduced IM distortion as well.

I know there is a similar ScanSpeak trick with a BR port filled up with drinking-straws.

Does have anyone tried this damped (or lossy) BR ports? I am curious about if it can compete with a TML.

Cheers

[JoeBob]

What do you mean when you say a lossy port? What is it you used?

[kelticwizard]

I don't think he actually made one I think he just simulated the lossy port on his program. He wants to see if anyone has made one.

I haven't made one, but I do know there are a few commerical speakers that stuff their ports. In his 1971 paper on vented boxes, Thiele mentioned designs that restrict the flow of air using stuffing or by making many small vents instead of one or two large ones, (the "distributed vent").

Apparently, some manufacturers think it gives a desirable response.

If nobody comes forward with experience on lossy vents, I would suggest the following-this is a guess on my part.

Tune the box using a conventional vent. Using an online tone generator, send a tone to the loudpseaker of Fb, the tuned frequency of the box. Using any microphone and multimeter or online oscilloscope freeware program, measure the voltage of the mic at that point.

Stuff the port until the mic, at the same level, puts out one half the voltage that it did with the unstuffed port. That means the port output is 6 dB down.

In most bass reflexes, at Fb the port output is 12 dB higher than the cone output. (If you decide to measure this, be certain to account for the size difference of the cone and the port). So by stuffing the port, the port output is cut in half. This should certainly affect the impedance response, and probably the output response as well.

If I had to guess, I would think that doing this would make the response of the box, and the rolloff slope, about halfway between the closed box response and the vented box response for that volume speaker.

[roddyama]

Pjotr,

The thread you referenced is a bit slanted toward TL’s because I was looking for a box configuration for the mid-bass (>80Hz). IMO the BR is the best compromise for a home system sub-bass. All lower order alignments requires a larger box for the same f3, or a higher f3 for the same size box.

WRT the high loss port, I’m assuming you progressively raised the port Ql and checked the response. Is that correct?

I have never experimented with varying port losses, but I would think that it shouldn’t be too difficult. There used to be a product called Vario-Vent that would allow you to do physically, what you’re simulating now. Unfortunately, I was unable to find it on the web. I think it was a Dynaudio product, which is now gone.

Rodd Yamashita

[kelticwizard]

I believe that some speaker company manufactures an equivalent to a Variovent. Forgot the name momentarily.

Here's a fellow who made his own Variovent. Seemed to work well enough.
http://mywebpages.comcast.net/kidder...Subwoofer1.htm

[kelticwizard]

ScanSpeak makes them. They are called Aperiodic Vents or Flow Resistors.

However, judging from what I am reading of them, they seem to be way of making a sealed box lossy. Nobody is mentioning anything about any substantial port output or reduction in cone excursion.

Anyway, these are the vents. I am sure European dealers of ScanSpeak have them as well. This page is from Parts Express in America, which I included just to give you an idea of what these things are.
http://www.partsexpress.com/pe/pshow...=296-546&DID=7

[lohk]

The Naim Audio SBL had a kind of internal variovent between two chambers of different size in a sealed enclosure.

[kelticwizard]

Navin has an enlightening post on aperiodic enclosures in the following thread.
Subwoofer Qtc and 'tightness'

[navin]

hey guys,

i am not as knowledgeable as most of you and my observations and postings are based on practical stuff. i dont have access to measuring software, books, etc.. so i use my ears.

what little i know of lossy ports.

my first attempt was using a 12" in a 1 cu. ft box. you know what it must have sounded like it was a pioneer car sub and they even recomended a 1 cu ft sealed box but the sound was ugh. i figured it was back pressure that was not being fully absorbed with the dampng material to i cut a hole and installed a 4" dia port 12" long I think to release the pressure. that solved one problem and started another big one note bass.

so is stuffed the port thinking it was the port that was adding to the orignal bass and after stuffing the port the bass got better defined almost like a good sealed box or TL.

why dont ask me. i dont have theoretical skills or other tools. the object is to experiment.

a similar story lead to the 3 box sealed box that i use for my 2 x 12" woofer box.

usually it starts with building a box that should work (thumb rule method) since I dont measure drivers (I did a few for acedemic reasons) and most published specs are way off i have simple rules like 0.5-0.7 cu. ft for 6", 1 cu. ft-1.2 cu.ft for 8" etc...also WAF dictates box size anyway.

once the box is built power up the driver (the tweeter and other compoents are not instaled neither are the holes cut.). then i add stuffing. if stuffing done do the trick. cut a hole. add a port etc.... since i favour heavy bracing i can close parts of teh box (using wood screwed to the braces) and all sorts of fun stuff. this goes on till i like what i hear.

my argument for DIY is that you are building speakers for your ears, music and enviroment. you know that best not bose, b&w, polk, jbl, wilson, thiel etc.....

use that knowledge to build something you like to listen to. my friends find my system to laid back. i never cared.

today when the come over, after 2-3 hours of listening they like the sound as they do not suffer from fatigue. also with well recorded cds you can hear the cymbals and all the high stuff just as clearly only it does not jump out at you.

would they buy such a system? probably not? they dont want to expend that much energy. a $500 all in one from sony is their choice. some of tehm have spent upwads of $1000 on all in ones. the like the lights, the exotic eq settings and the fascia. we are still friends. they think i am a nut. i think the same. would i give my right hand for anyone of them? yes. ditto for them.

sorry for rambling.

[Pjotr]

Hi folks,

Thanks for input and thanks Navin, you are exactly pointing out what I am up to with this concept. Putting bass-speakers in small boxes does not work. I did experiment a lot with these together with active compensating. Thiele and Small pointed out that the displaced volume of the speaker unit must be kept below 5% of the box volume to keep distortion due to the non-linearity of the enclosed air of the box reasonably low. I have found that this amount is still too high for low distortion. A few tenths of a percent simply sounds better.

What I am up to is a BR enclosure that has loss or a flow resistance of the port itself. Most simulation programs take into account the total Q factor of the whole BR system. That is NOT what I am talking about, I am talking about the losses of only the port.

The air in a BR port is acting as a moving mass. This mass forms a resonating system together with the spring action of the enclosed air of the box. By giving the movement of the air mass in the port viscous damping (loss) the Q-factor of this resonating system can be lowered. In a normal BR system this Q-factor is kept as high as possible giving an overall 4th order response. This is not what I want.

I know of “Variovents”, they are readily available from my DIY loudspeaker shop here. These units are mounted at the outside of the cabinet. As a result the pressure in the port is modulated to a great extend by the box pressure and hence modulating the mass of the air in the port. This makes tuning difficult. Also a variovent makes the port more resonating on its own resonant frequency defined by its length. To circumvent this I am thinking of implementing the damping by making the port of a set of small pipes to create the flow resistance.

A couple of years ago I wanted to simulate a box with an auxiliary bass resonator. However in those days there was no software available that incorporated the compliance and Rms of the ABR. So I build an electro-mechanical model in Pspice that took care of these. Attached the simulations I did with this model. A BR port has no compliance so this was left out but I kept the losses of the port: Rmp.

The box has a volume of 60L and the BR port is tuned to Fs of the LS of 28Hz. This is somewhat arbitrary because Fs and Qms of the speaker are raised by the compliance of the box. Here is where a BR box differs from a TML. A TML does not raise Fs and Qms much, at least if there is no coupling chamber used.

The simulation is done with high loss and moderate loss of the port. The black line is the overall response, the red line the response of the speaker unit and the blue line the response of the port. Cone excursion is meters_peak with 1V_peak input (1m == 1 mm or 1 millimetre). The impulse response is for the high loss port.

Note, that my goal is limiting cone excursion, so the speaker can handle higher bass levels, and low overall Q. I am not up to earth-shaking bass reproduction.