@rjj45 said:
New post by Martin J King and some (of course) very serious technical modeling. One of the take aways is that he expects to use acoustic foam in all his future builds. http://www.quarter-wave.com/Project13/Project13.html
Good read. Thanks for sharing. It is a very long, in-depth paper. I'm only up to page 25 of 74 so far! Interesting how his actual NF port measurements compare to the modelled predictions, above and below 500Hz. The modelling software breaks down above 500Hz due to complex, non-axial line reflections.
Good read. Thanks for sharing. It is a very long, in-depth paper. I'm only up to page 25 of 74 so far! Interesting how his actual NF port measurements compare to the modelled predictions, above and below 500Hz. The modelling software breaks down above 500Hz due to complex, non-axial line reflections.
I don't have the technical ability to analyze some of his findings!
If the "bumps" above 500 Hz are non-axial (side to side) - this points to the efficacy of lining most or all of the line with foam, as PMC does. And 500Hz and up is where acoustic foam is most effective at absorption.
I finished the paper and noticed that MJK also mentioned that acoustic foam was used almost exclusively by commerical TL manufacturers because it was consistent and repeatable. In my testing I noticed that when I properly "teasing out" the poly fill to completely fill the line cross section, that this had a huge impact on the measurements. And when I switched over to Ultratouch denim, I could not "tease out" the same weight of material to fill the line. When we were talking about measuring and developing a "fudge factor" to determine line stuffing density in another thread, I think @jhollander mentioned something about this. That it would very difficult to compare different materials because of this. Perhaps using some type of acoustic foam would be a way to get around this problem. We just have to figure out a good source of acoustic foam.
It is also interesting to compare impedance curves using different stuffing materials. Here is poly-fill verse ultratouch denim for some of the tests above:
I haven't done an update on this thread for a while, so I thought I'd do a final status report. I was trying to prototype a dual chamber wave cannon, custom tuned on each end to smooth out the peaks and dips in the 100 to 400Hz region. But no matter what I tried, I could not get my test cabinet to work. I think I know why it did not work, but I need to do some more testing and build a completely different cabinet shape to verify the problem.
My takeaways:
1) The longest dimension in a TL (or any cabinet) is extremely important. This length determines the frequency at which the major line peaks and dips occur. You can dampen these peaks and dips by moving the driver about 1/3 down the line in a ML-TL, but this does NOT work in a wave cannon, because both ends are open. You don't have a trap on one end like a ML-TL has.
2) To get a wave cannon to work, I would need to build a new cabinet with the driver located on an adjustable panel located near the center of the line. Each terminus would be tuned to roughly the same frequency. Adjusting the position of the woofer slightly off center should cause the nasty peaks and dips in the 100 to 400Hz region to cancel out, leaving only the main tuning frequency. I would then be able to use a small amount of damping material in the line to dampen out the remaining resonances in the 500 to 1000Hz region.
3) I would need to build another cabinet to test this. Not sure if it would work. And the line would have to be very long, almost double the length of a ML-TL for the same tuning frequencies. What do you think? Does this make sense?
I too am leading up to do several TL experiments.
I am not an expert, but I think you are wrong about the "double length" thing.
At each end of an open line, there will be an impedance mismatch and a reflection.
At least, that's what comes to mind.....
But Chahly - Stahkist don't want speakers that look good, Stahkist wants speakers that sound good!
OK, not double the length, but it would still need to be substantially longer than a convensional ML-TL. The closed trap end of a ML-TL can be designed to be substantially shorter and is only about 1/4 to 1/3 down the line. But with both ends open as in a wave cannon, the lines would have to be roughly equal with respect to the driver position, with an offset designed to create an impedance mismatch at roughly the same low frequency. Otherwise the peaks and dips in the 100 to 400Hz region will not cancel out. In order to do this, both the upper and lower line segments would need to be approximately the same length.
I don't think you are designing a dual chamber TL. The lines both sum at the driver and terminate outside the cabinet. It's like a TL with a center dividing brace, or a ported enclosure with two ports.
I think you are correct. It is not a dual chamber TL. But I think @Wolf is allowing this type of arrangement for the upcoming competition.
Technically, I think it is a type of "wave cannon" type TL with the driver buried deep inside the box. Since both the front and back side of the driver are enclosed, it tends to replicate a 6th order bandpass alignment. Hornresp can model this as a 6th order bandpass by making the lengths of the two bandpass chambers extremely long. The Leonard Audio Transmission Line (LA-TL) program can also model this by placing line sections on the front and rear of the drivers. The problem is that neither of these two programs can model the stuffing properly. Hornresp has no provisions for including stuffing in a 6th order bandpass alignment. The LA-TL program can include stuffing, but the stuffing model in this program is not accurate.
Bill when I went back and reviewed the original concept where the front and rear wave are separated it occurred to me that the model is similar to a tapped horn. That can be modelled in horn response. Note that @Wolf nixed the tapped horn idea when I asked.
Maybe that is what I should do. I should start modeling my "wave cannon" as a tapped horn. Does hornresp allow you to add damping material to both the front and rear chambers of a tapped horn?
@4thtry said:
Maybe that is what I should do. I should start modeling my "wave cannon" as a tapped horn. Does hornresp allow you to add damping material to both the front and rear chambers of a tapped horn?
YEP!
But Chahly - Stahkist don't want speakers that look good, Stahkist wants speakers that sound good!
Comments
Good read. Thanks for sharing. It is a very long, in-depth paper. I'm only up to page 25 of 74 so far! Interesting how his actual NF port measurements compare to the modelled predictions, above and below 500Hz. The modelling software breaks down above 500Hz due to complex, non-axial line reflections.
I don't have the technical ability to analyze some of his findings!
If the "bumps" above 500 Hz are non-axial (side to side) - this points to the efficacy of lining most or all of the line with foam, as PMC does. And 500Hz and up is where acoustic foam is most effective at absorption.
https://pmc-speakers.com/technology/atl
I finished the paper and noticed that MJK also mentioned that acoustic foam was used almost exclusively by commerical TL manufacturers because it was consistent and repeatable. In my testing I noticed that when I properly "teasing out" the poly fill to completely fill the line cross section, that this had a huge impact on the measurements. And when I switched over to Ultratouch denim, I could not "tease out" the same weight of material to fill the line. When we were talking about measuring and developing a "fudge factor" to determine line stuffing density in another thread, I think @jhollander mentioned something about this. That it would very difficult to compare different materials because of this. Perhaps using some type of acoustic foam would be a way to get around this problem. We just have to figure out a good source of acoustic foam.
It is also interesting to compare impedance curves using different stuffing materials. Here is poly-fill verse ultratouch denim for some of the tests above:
All stuffing shifted Fs lower. Hmm, let me check Hornresp models.
Yep, of course that's right. Moderate stuffing lowers F4 -3-4 Hz
I haven't done an update on this thread for a while, so I thought I'd do a final status report. I was trying to prototype a dual chamber wave cannon, custom tuned on each end to smooth out the peaks and dips in the 100 to 400Hz region. But no matter what I tried, I could not get my test cabinet to work. I think I know why it did not work, but I need to do some more testing and build a completely different cabinet shape to verify the problem.
My takeaways:
1) The longest dimension in a TL (or any cabinet) is extremely important. This length determines the frequency at which the major line peaks and dips occur. You can dampen these peaks and dips by moving the driver about 1/3 down the line in a ML-TL, but this does NOT work in a wave cannon, because both ends are open. You don't have a trap on one end like a ML-TL has.
2) To get a wave cannon to work, I would need to build a new cabinet with the driver located on an adjustable panel located near the center of the line. Each terminus would be tuned to roughly the same frequency. Adjusting the position of the woofer slightly off center should cause the nasty peaks and dips in the 100 to 400Hz region to cancel out, leaving only the main tuning frequency. I would then be able to use a small amount of damping material in the line to dampen out the remaining resonances in the 500 to 1000Hz region.
3) I would need to build another cabinet to test this. Not sure if it would work. And the line would have to be very long, almost double the length of a ML-TL for the same tuning frequencies. What do you think? Does this make sense?
I too am leading up to do several TL experiments.
I am not an expert, but I think you are wrong about the "double length" thing.
At each end of an open line, there will be an impedance mismatch and a reflection.
At least, that's what comes to mind.....
OK, not double the length, but it would still need to be substantially longer than a convensional ML-TL. The closed trap end of a ML-TL can be designed to be substantially shorter and is only about 1/4 to 1/3 down the line. But with both ends open as in a wave cannon, the lines would have to be roughly equal with respect to the driver position, with an offset designed to create an impedance mismatch at roughly the same low frequency. Otherwise the peaks and dips in the 100 to 400Hz region will not cancel out. In order to do this, both the upper and lower line segments would need to be approximately the same length.
ok, see that. Interesting experiment. I wonder if Hornresp could model this.
I don't think you are designing a dual chamber TL. The lines both sum at the driver and terminate outside the cabinet. It's like a TL with a center dividing brace, or a ported enclosure with two ports.
I think you are correct. It is not a dual chamber TL. But I think @Wolf is allowing this type of arrangement for the upcoming competition.
Technically, I think it is a type of "wave cannon" type TL with the driver buried deep inside the box. Since both the front and back side of the driver are enclosed, it tends to replicate a 6th order bandpass alignment. Hornresp can model this as a 6th order bandpass by making the lengths of the two bandpass chambers extremely long. The Leonard Audio Transmission Line (LA-TL) program can also model this by placing line sections on the front and rear of the drivers. The problem is that neither of these two programs can model the stuffing properly. Hornresp has no provisions for including stuffing in a 6th order bandpass alignment. The LA-TL program can include stuffing, but the stuffing model in this program is not accurate.
Bill when I went back and reviewed the original concept where the front and rear wave are separated it occurred to me that the model is similar to a tapped horn. That can be modelled in horn response. Note that @Wolf nixed the tapped horn idea when I asked.
Maybe that is what I should do. I should start modeling my "wave cannon" as a tapped horn. Does hornresp allow you to add damping material to both the front and rear chambers of a tapped horn?
YEP!
Bill, I forgot that HR does compound horns which is what you were building. I did not check the insulation.