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
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
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.
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.
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.rcavictim said:
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.
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.
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.
BassAwdyO said:
The stuffing does not affect the speed of sound in the line. What does affect the length of the line is the line geometry. Tapering such that the line is smaller at the terminus moves the fundemental resonance down vrs an untapered line. Tapering such that the terminus end is larger increases the lines fundemental resonance.
The purpose of the stuffing is to act as a low pass filter on the output of the terminus -- ie you want to stop the higher resonance modes from exiting the terminus. Some argue that in a "true" transmission line there should be almost no output from the terminus -- i call this an aperiodic TL.
How much damping & where it is all affect the low pass function (as does the line geometry). How you use it depends on your goals -- fortunately there are now tools to model it.
dave
Jonathan Bright said:
A classic TL follows a set of rules-of thumb. This was before modern modeling tools showed us that, in general, a classic TL was more often than not, less than optimum.
Offsetting the driver from the end of the line is just another tool in the TL designers kit to help optimise the low-pass filtering of the output of the terminus. If offset just right one of the undesirable line modes can be almost totally suppressed. Given that the hardest such mode is the 1st one above the fundemental this is usually the target. Where this is in any line is dependent on its geometry, and very little change in the line can move the optimum point for the driver in the line. The MJK tables give a good idea of where the best place is in a tapered TL.
And saying that offsetting the driver means a line can no longer be classic is rubbish. Very few classic lines didn't have at least a bit of offset -- it was most often just not optimum.
dave
I think it would be useless to brand a certain type of design varient. The ultimate goal is to achieve a good design, and whatever knowledge does that is what works. The speaker enclosure just started out with the concept of preventing the back waves from cancelling the front waves. I like to use the cabinet to also create slight loading on the driver just enough to break it away from the low end resonance so it will provide less back EMF at that frequency, while also allowing currents to go through the coil so that the driver remains controlled.
The TL type enclosure with suffing material provides for two things to acomplish this, 1) provide sufficient volume to keep the resonance frequency low, and 2) use damping material to reduce higher frequency box resonance and also provide friction in the line just enough damp the natural frequency of the driver.
I'm sure there are other ideas around. That's the beauty of this, almost everyone has a preference, and we really don't know who is right unless the data support theory.
The TL type enclosure with suffing material provides for two things to acomplish this, 1) provide sufficient volume to keep the resonance frequency low, and 2) use damping material to reduce higher frequency box resonance and also provide friction in the line just enough damp the natural frequency of the driver.
I'm sure there are other ideas around. That's the beauty of this, almost everyone has a preference, and we really don't know who is right unless the data support theory.
planet10 said:
I didn't get this out of MJK's papers either. It seems to me that they _do_ behave the same, and the same basic equations apply. A bass reflex box is still a pipe closed at one end and open at the other.
Indeed, your statement about one morphing into the other backs this up. If they were that different, there would be a sharp dividing point between them, not a gradual change from one to another.
They have different characteristics, but I don't see that you can claim they don't behave the same.
There's normally a difference in philosophy. With a bass reflex design, you want resonance, and you try to exploit it. With a t-line, you try to reduce resonance.
skrivis said:
I'd say the latter... i've been following MJKs work since before it was published (i was the original publisher). This includes his papers, forum interaction and direct mail....
all the things i've said are from these interactions.
All your agrguments seem to go against the flow of what is published... if i get more time i'll address your specific issues.
For instance, just looking at the MJK tables, one can see how the line taper directly affects the line resonance.... Voigt pipes are longer than traditional tapered lines for the same fundemental.
dave
The stuffing really has a minimal effect on the speed of sound inside a stuffed TL. I think the lowest value I use is about 320 m/sec for a 1.0 lb/ft^3 fiber density. Compared to the free air value of about 344 m/sec, this is less than a 10% reduction. I personally believe that this reduction in the speed of sound is due to heat transfer that occurs between the air and the fibers as a sound wave passes through the tangle, similar to the apparent increase in closed box volume that occurs when stuffing is added. I don't believe it is due to moving fibers which was commonly accepted TL theory for many years. In the past, people have claimed a 50% reduction in the speed of sound for a stuffed TL resulting in a much shorter required length. I believe the shorter length is due to geometry and has nothing to do with fiber stuffing.
The shape of the line is everything! Look at my alignment tables and see what length is required to achieve the same tuning frequency for different area ratios. I believe that the shape of a TL is one of the most important properties of the design.
You re getting tangled up in labels. I do apply labels to my designs that try and express the physics behind each. But there are no sharp dividing lines and arguing over labels is a waste of time.
Classic BR design theory assumes that the air in the enclosure is a spring and sees uniform pressure, it behaves as a lumped parameter. When you upgrade to one dimensional standing wave theory it will also will simulate a classic BR but extends to include TL's and hybrid designs (ML TL's). The way you model the air is important and there are differences between ML TL designs and classic BR designs. The next logical step is to extend the wave equation into 3D, but I will save the discussion of those MathCad worksheets for another time.
You are misunderstanding. Dave's (planet10) summary is exactly what I have been saying for years. I first came out of the closet with my TL theory and MathCad worksheets on 10/99 in a post to the old BassList. Almost immediately Dave and I entered into long discussions of TL theory and he was the first one to read and accept my early documents. Through e-mail discussions and forum postings many people (Dave, GM, and Bob Brines are probably the biggest contributors) have provided constructive feedback and we have all learned together. Dave's summary above is exactly what I am thinking and have tried to express. Maybe the different points are scattered in too many places on my site to allow a new reader to identify them up quickly. Sometimes you have to think about what has been written, I may not have explained things in the clearest manner. I am still learning!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.