In a quarter-wave line, at each end of the line the wave is 90 degrees out of phase at the quarter-wave frequency. That is why it is used as a resonant loading method -- phase shift at the terminus of up to 90 degrees sums coherently with the driver cone.
In a quarter-wave stub, the wave must travel in, and then travel back out, doubling its effective length, and making it more like a half-wave line whose terminus is the same as its inlet. I guess it effectively works to cancel out the capacitive effects of the enclosure at that frequency, so there is not a measured spike in the impedance at that frequency, and that is why they were used by Hegeman to produce a nearly-flat impedance curve in the low end. I imagine that by halving the length of the pipe, one could make it so that the stub actually reinforces the sealed-box loading of the driver at a certain frequency, though I think it would have the effect of limiting the cone's excursion at that frequency, instead of allowing the cone to move more freely.
In a quarter-wave stub, the wave must travel in, and then travel back out, doubling its effective length, and making it more like a half-wave line whose terminus is the same as its inlet. I guess it effectively works to cancel out the capacitive effects of the enclosure at that frequency, so there is not a measured spike in the impedance at that frequency, and that is why they were used by Hegeman to produce a nearly-flat impedance curve in the low end. I imagine that by halving the length of the pipe, one could make it so that the stub actually reinforces the sealed-box loading of the driver at a certain frequency, though I think it would have the effect of limiting the cone's excursion at that frequency, instead of allowing the cone to move more freely.
The R-J speaker keeps popping up. Eliptical opening may be a good one for FR and coaxial (my Eliptoflex have cane grill cloth which keeps me being able to judge due to reflections) IIRC my Contrabombarde although good in a small room could not do better than a bandpass box in a larger room - small floor-firng vents on Contrabombarde chuffed. I've got 4-18" boxes which were done R-J style - 2 played well and those were 3" coil low Q lower mass Eminence. The 4" coil woofer had more "cardboardy" signature but became tuneful in a 6.2 CF K-coupler with 1.2" gap.
one more thing - don't see well, have no printer and didn't grasp that patent - how does it work? - theres only so much magic
one more thing - don't see well, have no printer and didn't grasp that patent - how does it work? - theres only so much magic
It seems like the real question as far as the little TBI subwoofers are concerned is, how did they get the transmission line to be so small? That's going to be the main consideration as far as our own designs are concerned. Could it have something to do with the cylindrical expansion of the wave in the transmission line? Or would it have more to do with the way the woofer is compression-loaded to the line?
The goal of a traditional, full sized, closed ended tl, as far as I am aware, is to completely eat the back wave to perfectly simulate IB performance, thus negating the usual raising of driver fs when coupled to a normal box. I think that is where the similarities between the Plummer box and a real closed ended tl end. I think the Plummer box uses a different method completely to damp the driver to very low frequencies WRT driver fs. The closed ended tl uses quarter wave theory and length to kill the back wave. I think the Plummer box does it in a manner similar to a ripole, using a small air mass, and in the case of the Plummer box, foam that reacts differently at different frequencies to damp the back wave. If it really were a tl inside the Plummer box it would only be a few inches long, and even if you double the length to go from throat to mouth and back and you still barely have 1 foot of length. I don't think there is any quarter wave action going on in there within the passband of the box's output.
My first exposure to high-fidelity speakers was with my parents' old Fried Model "Q" two-way speakers. Their designer, Irving Fried, used something he called a "Line Tunnel" to produce sonic characteristics similar to a transmission line, but without the complex internal construction. The "Line Tunnel" looked like a large slotted port, but it was stuffed with a rectangular block of open-cell foam. People have told me that this was pretty much an aperiodic sealed enclosure. The idea of the ETL might be to produce a similarly resistive loading to the woofer, instead of a reactive one like a vented or sealed chamber would create.
Just A Guy, I think you're referring to the idea behind an open-ended TL. Transmission lines can be designed so that the terminus contributes output, but a heavily-stuffed, open-ended transmission line is intended to simulate the infinite baffle, with nearly no terminus output.
What I want to know is, by what factor is it possible to slow the speed of sound through the use of a very dense damping material, like compacted open-cell foam? Any TL experts care to comment?
Just A Guy, I think you're referring to the idea behind an open-ended TL. Transmission lines can be designed so that the terminus contributes output, but a heavily-stuffed, open-ended transmission line is intended to simulate the infinite baffle, with nearly no terminus output.
What I want to know is, by what factor is it possible to slow the speed of sound through the use of a very dense damping material, like compacted open-cell foam? Any TL experts care to comment?
That's exactly the purpose of damping material -- to slow down the speed of sound in the line, causing the line to appear longer to the driver. It's an adiabatic process in which the stuffing material creates turbulence in the air movement, converting air motion to heat. It releases this heat energy more slowly, causing the apparent volume of the enclosure to be greater, thereby increasing the apparent length of the transmission line. The stuffing doesn't heat up as the speaker operates. Instead, all of the stored energy is released.
MJK on this topic....
Hi Moray,
The topic of damping in a TL slowing down the speed of air has come up. I was (but I may be mistaken) under the impression that current thought on this was that the damping does not slow down the speed of sound at all.
MJK:
Your impression is correct. In a stuffed TL, the speed of sound is barely slowed at all and not at all at low frequency. The lowest speed of sound value used in my MathCad worksheets is 320 m/sec for a heavily stuffed pipe compared to a value of 344 m/sec in free space (air).
Hope that helps,
Martin
Hi Moray,
The topic of damping in a TL slowing down the speed of air has come up. I was (but I may be mistaken) under the impression that current thought on this was that the damping does not slow down the speed of sound at all.
MJK:
Your impression is correct. In a stuffed TL, the speed of sound is barely slowed at all and not at all at low frequency. The lowest speed of sound value used in my MathCad worksheets is 320 m/sec for a heavily stuffed pipe compared to a value of 344 m/sec in free space (air).
Hope that helps,
Martin
Well, at the very least this has prompted me to consider the effects of a quarter-wave stub on a woofer in an enclosure, or on a woofer coupled directly to the stub. Once I have some serious time to finish learning Akabak, it will be interesting to see if drivers with weak motors can be made to work in small sealed enclosures by decreasing the acoustic impedance on the back side of the cone at the quarter-wavelength frequency of a quarter-wave stub, incorporated into the enclosure.
As far as the TBI enclosure is concerned, perhaps the main effect extending the bass response is some sort of compression loading on one side of the driver.
As far as the TBI enclosure is concerned, perhaps the main effect extending the bass response is some sort of compression loading on one side of the driver.
"As far as the TBI enclosure is concerned, perhaps the main effect extending the bass response is some sort of compression loading on one side of the driver." - Yup.
Basically it's (kind of) like a 4th order bandpass box with a tiny tiny tiny rear chamber that thinks it is huge. This technology could probably also be used to make sealed boxes of absolutely ridiculously small proportions, so small the magnet would have to be outside the box.
Basically it's (kind of) like a 4th order bandpass box with a tiny tiny tiny rear chamber that thinks it is huge. This technology could probably also be used to make sealed boxes of absolutely ridiculously small proportions, so small the magnet would have to be outside the box.
Conscientious thinking...
well we are all on track. Nobody see's any 1/4 wave action here. That would seem to be agreed. I think that we need to look at the line sections as 1/2 wave stubs where the wave goes down the line hits the end and bounces back. We know that at the 1/2 WL frequency the stub generates maximum pressure at the ends so pressure applied to the driver at that frequency. Also the stube are intentionally only partially damped there is an open space in the stub. It would be easy to totally stuff the lines with foam but the patent is specific that the air space be there. As far as I can see this is ment to generate turbulence as are the bend (which is mentioned). So this is a lossy stub probably with a wider resonance than a fully damped or open line. Either of which could have been used but the patent is specific as to a partially damped stub. This is just out loud thinking here but I would like to think that the patent has some substance or it would be a big waste of money. Then we have an eliptoflex style reflex vent. What else do you see? It's all there in front of us. Regards Moray James.
well we are all on track. Nobody see's any 1/4 wave action here. That would seem to be agreed. I think that we need to look at the line sections as 1/2 wave stubs where the wave goes down the line hits the end and bounces back. We know that at the 1/2 WL frequency the stub generates maximum pressure at the ends so pressure applied to the driver at that frequency. Also the stube are intentionally only partially damped there is an open space in the stub. It would be easy to totally stuff the lines with foam but the patent is specific that the air space be there. As far as I can see this is ment to generate turbulence as are the bend (which is mentioned). So this is a lossy stub probably with a wider resonance than a fully damped or open line. Either of which could have been used but the patent is specific as to a partially damped stub. This is just out loud thinking here but I would like to think that the patent has some substance or it would be a big waste of money. Then we have an eliptoflex style reflex vent. What else do you see? It's all there in front of us. Regards Moray James.
If you don't already have it, get the patent for the earlier version, with the ends of the "lines" open.
This is pure speculation, but it would seem that there is little difference between the old and new box with regard to performance so the effect of closing the end of the "line" or leaving it open seems negligible.
Of course this could be an oversimplified view, as I have not studied these much besides the information from the 2 patents.
This is pure speculation, but it would seem that there is little difference between the old and new box with regard to performance so the effect of closing the end of the "line" or leaving it open seems negligible.
Of course this could be an oversimplified view, as I have not studied these much besides the information from the 2 patents.
What does it mean?
when you update a patent it is done to protect new insights of the technology. As product development progresses and new discoveries are unearthed you want to make sure that the best core features are protected in your patent. So as I see it the fact that they chose to change to closed stubs as opposed to open ones is probably significant or there would be no need to spend the extra money and change the patent. That's just my take on it. YOMV. Regards Moray James.
when you update a patent it is done to protect new insights of the technology. As product development progresses and new discoveries are unearthed you want to make sure that the best core features are protected in your patent. So as I see it the fact that they chose to change to closed stubs as opposed to open ones is probably significant or there would be no need to spend the extra money and change the patent. That's just my take on it. YOMV. Regards Moray James.
Moray, you also have to consider what is happening in the life of the patent holder. I think the new patent was a business move more than anything else. Consider this - 2 patents within a few months. Both of them are so close in detail that there is only one significant change. In between the two patents the inventor parts ways with previous business partners and starts a new business with new partners and it is assumed this is not making all parties happy. As always I could be wrong.
T
Is this it? EDITED
The 90 degree turn at the end of the duct behind or in front of the speaker represents a velocity minimum for some wavelength(s).
The inventor blocks the duct with a thickness of open cell (reticulated) polyurethane foam at some distance between speaker and the right hand turn.
The foam is positioned at a velocity maximum for some wavelength(s) and much of its (their) energy will be absorbed by the foam when reflected from the 90 degree turn.
The foam will be almost transparent to other longer wavelengths because it's not thick enough. Some proportion of their energy will be reradiated back towards the speaker and rest continues along the duct after the right hand turn....
A series of different thicknesses of foam spaced appropriately along the duct would eliminate chosen wavelengths from radiating back to the speaker cone. Or continuing further along the duct....
The inventor wants to cherry pick some wavelengths to reradiate back to the speaker at usable energy levels (whatever they might be) with usable timing (whatever that might be).
If the inventor wants to apply a given wavelength(s) he has to tap it from the duct while it is at or near a pressure maximum. (Tom Danley taps into his duct (horn) at or near pressure maximum).
For his sub application the inventor wants to use the pressure wave to steady the speaker cone when the motor can't do it. To steady it, pressure must be applied both sides of the speaker nearly simultaneously so he brings a tailored low wavelength "tapped duct" to the front of the speaker.
From looking at patent that was posted further up, I think the hard part is the geometry of the cabinet.
......
I've use the 30 pores per inch reticulated polyurethane foam to cancel out the HOMS on some old elliptical horns and it works like a hot damn for high frequencies. Low frequencies will go through it like grass through a goose. It's kinda expensive.
A cheaper alternative for experimenting with a small speaker, a duct, and a ratshack meter or panny mike, would be the polyurethane foam used for camera and instrument cases and it can be bought on ebay. It has more PPI so buy skinny stuff so you can double up thicknesses when establishing baseline.
Is this it? EDITED
The 90 degree turn at the end of the duct behind or in front of the speaker represents a velocity minimum for some wavelength(s).
The inventor blocks the duct with a thickness of open cell (reticulated) polyurethane foam at some distance between speaker and the right hand turn.
The foam is positioned at a velocity maximum for some wavelength(s) and much of its (their) energy will be absorbed by the foam when reflected from the 90 degree turn.
The foam will be almost transparent to other longer wavelengths because it's not thick enough. Some proportion of their energy will be reradiated back towards the speaker and rest continues along the duct after the right hand turn....
A series of different thicknesses of foam spaced appropriately along the duct would eliminate chosen wavelengths from radiating back to the speaker cone. Or continuing further along the duct....
The inventor wants to cherry pick some wavelengths to reradiate back to the speaker at usable energy levels (whatever they might be) with usable timing (whatever that might be).
If the inventor wants to apply a given wavelength(s) he has to tap it from the duct while it is at or near a pressure maximum. (Tom Danley taps into his duct (horn) at or near pressure maximum).
For his sub application the inventor wants to use the pressure wave to steady the speaker cone when the motor can't do it. To steady it, pressure must be applied both sides of the speaker nearly simultaneously so he brings a tailored low wavelength "tapped duct" to the front of the speaker.
From looking at patent that was posted further up, I think the hard part is the geometry of the cabinet.
......
I've use the 30 pores per inch reticulated polyurethane foam to cancel out the HOMS on some old elliptical horns and it works like a hot damn for high frequencies. Low frequencies will go through it like grass through a goose. It's kinda expensive.
A cheaper alternative for experimenting with a small speaker, a duct, and a ratshack meter or panny mike, would be the polyurethane foam used for camera and instrument cases and it can be bought on ebay. It has more PPI so buy skinny stuff so you can double up thicknesses when establishing baseline.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.