Damping TL cabinets?

[graks]

Have searches and read about damping TL cabinets. After what i have read there seems to be various methods... I am now finishing building the Seas Thor speakers, and i am a little unsure about the damping/stuffing...

[BassAwdyO]

I think line stuffing is pretty important.

But I have my impressions on why it works. I dont think the speed of sound is nessicarily the only factor effected by using stuffing, but in a transmission line the stuffing adds apparent mass to the air inside the line lowering the resonance frequency of the line and also damping higher frequencys.


Thus it increases apparent line length, while it smoothes the impedance curve and lowers group delay

[Jonathan Bright]

gracks Hi!
As usual there is a lot of good science and also nonsense written in this area. I have Bailey's original paper in front of me and he was a great fan of "long fibre wool". In my country this was also known as "roller lapping" in the craft shops. This had a cult following amongst many.
Others such as IMF used glass fibre I think. Robert Fris in his "Daline" specified a terylene ( I can't spell so that might be phonetic) thread which I think was the soft white fibre that is used in water filters from tropical fish aquariums. Most agree that the fibre should not be allowed to pack down or get compressed. This has led to people crossing the insides of their t/lines with various ways of supporting the material.

Now the science, approximately! Bailey's experince led to the following statement. The packing density should be "about one pound for every 2 to 3 cubic feet." You'll have to convert that to metric! Roughly, 1 kilo is about 2.2 pounds and there are 28.3 ltrs in a cubic foot, but check those.

I hope that is helpful. There are some extreme views on this area with people measuring things to three decimal places! That just creates unnecessary anxiety. Bailey was the modern "father" of this type of enclosure and has some claim to know what he was talking about. The thing is to keep the fibre pretty uniform throughout the line as its main purpose is to do as the previous post suggested. It is not to just dampen out reflections between the walls.

I have Colloms book on High Peformance Loudspeakers and he is pretty convinced that the t/line enclosure is a form of bass reflex enclosure. The early lines (late 1960's) got very good reviews and one reason is that the nature of the cabinet's construction meant that they were invariably much better braced that the average infinite baffle or bass reflex. The partitions inside the box tended to break up flexing of the side panels and as the lines often tapered there was a lack of parallel surfaces. All this made for what we now know to be "good" practise in cabinet construction and reduces delayed resonance. I suspect that it was this as much as any inherant advantage in the principle of T/lines that helped give good bass.

[soongsc]

You probably want to visit this site for some info.
http://www.quarter-wave.com/

[Jonathan Bright]

graks. I put "Seas Thor" into Google and got several responses. I notice one kit specifies "Dacron" for the line filling. I think that's pretty widely available. I think people use it to fill pillows don't they?

[dnsey]

My IMF TLS50s used 'eggbox' profiled foam - very much like the anechoic wall treatment.

[rcavictim]

As I understand it, in a TL you only have to put stuffing in the first half of the line, the driver end. This is to damp out any standing waves that can occur lengthwise in the line at frequencies higher (shorter wavelength) than the fundamental created by the actual line length. Such higher resonances will really color an enclosure and make for very uneven frequency response/amplitude effects. I stuffed mine this way while monitoring with an oscillator and O-scope in the design of my Ultor-XTM speakers shown in my avatar and the curative effect of the damping is remarkable in this regard. One should not overdo the density though or acoustic efficiency will suffer. I'm sure there are other effects as well such as frequency lowering, etc. as described by others.

[graks]

On the Seas website they say: "400 g (14 oz) of the damping material should be placed in the front channel; 250 g (9 oz)
in the rear." But is this placed on the transmission line only, or is it on all walls? How high up on the rear channel should i have damping material?

[bwbass]

Originally posted by rcavictim
As I understand it, in a TL you only have to put stuffing in the first half of the line, the driver end. This is to damp out any standing waves that can occur lengthwise in the line at frequencies higher (shorter wavelength) than the fundamental created by the actual line length. Such higher resonances will really color an enclosure and make for very uneven frequency response/amplitude effects. I stuffed mine this way while monitoring with an oscillator and O-scope in the design of my Ultor-XTM speakers shown in my avatar and the curative effect of the damping is remarkable in this regard. One should not overdo the density though or acoustic efficiency will suffer. I'm sure there are other effects as well such as frequency lowering, etc. as described by others.

AFAIK, fully stuffing the length of a classically designed TL gives you response very close to that of infinite baffle, IE very smooth and extended but with reduced effieiency and a fairly high rolloff frequency.

Stuffing about the first half, as you suggest, suppresses the most obnoxious of the standing waves and harmonics while retaining some low-end gain similar to a bass reflex box.

Putting the driver around 1/5 of the line length away from the closed end is a another good way to smooth out the response without having to stuff the line so much that you lose low end gain.

[Jonathan Bright]

graks, With t/lines the damping usually fills the whole channel and is not just for the walls. You want to get the enclosed space filled with the damping material. Damping material has a different function in t/lines than it does in other sorts of enclosures. Try and spread the material out evenly through the whole line. With your question about the rear channel I would think that it should be filled completely. That is, you should see material at the opening at the end of the line. The damping add mass to the column of air and makes it heavier. That does a number of things as previous posts have suggested but it is essential for the correct working of the line.

bwbass. I know what you are saying about mounting drives along a line. I think someone suggested 1/3 of the way along was a good idea but they then really cease to be classic t/lines.