Will adding foam lining change the apparent cross sectional area of a tapered transmission line? I am trying to improve 25 year old speaker not build new. It is a "classic" transmission line with a 10 inch driver at one end and a 6 foot line that tapers from 10" x 13.5" to 2.4" x 13.5 inches. One half inch foam would obviously drastically change the cross sectional area if one just calculates the area using the narrowing caused by the foam. But is that the correct way to calculate this? The goal is the improve the cabinet as the folds in the line use less than 1/2 inch MDF. So far bracing has greatly improved the knuckle rap test. The taper goes quickly down to 6" x 13.5" after 6 inches line length and then tapers fairly uniformly there after. I am thinking of using 1/2 inch foam on only one side of the line for the first two feet of the line which is about 3.6 inches at it's narrowest. The crossover to ESL panels is at 172Hz.
Adding absorbent material lining wont change crossection of TL. It only increases acoustic resistance, assuming proper material is used. But effect will be smaller than stuffing the whole crossection of the pipe.
Pictures?
Forget the foam.
Lines like that will be guessed, not designed, so often the best damping is to push towards anechoic.
Fill the volume with very well tested aciustastuff or equivalent. Probbaly take a day to tese enuff well enuff to fill that line.
dave
Forget the foam.
Lines like that will be guessed, not designed, so often the best damping is to push towards anechoic.
Fill the volume with very well tested aciustastuff or equivalent. Probbaly take a day to tese enuff well enuff to fill that line.
dave
Oops. I probably should have said all but the last 11 inches are filled with the originally installed Acousta-Stuf or some such but I didn't do a very good job filling as it is a little lumpy. With the bracing it sounds much better but seems to measure worse. But in fairness there is more than one variable as I added a 3/4 inch BB plywood baffle to the existing 3/4 inch MDF and decoupled the woofer with a foam gasket. I also replaced the woofer surround as it disintegrated when I removed the woofer. I am going to remove the fill and weigh. According to Hornresp the 2.4 cubic foot volume needs a minimum of 2.15 lbs to remove wiggles in impedance etc. I am pretty sure there much less in it now. The impetus for the foam is to apply over a very thin panel that vibrates quite a bit. The designer used an accordion like wood panel for the curve. I don't know what that is type of woodworking is called.
I almost always stuff the first 2/3 of a tapered line with poly fiber (Acousta Stuf) at a density of 0.75 lb/ft3, but I always use a taper ratio of 10:1 or higher in order for the taper to help reduce the magnitudes of the naturally occurring response dips/peaks from the odd harmonics of the line's 1/4-wave length resonant frequency. This higher taper ratio and a lower stuffing density creates a smoother response and a bit lower bass response. As best as I can calculate your line has an effective length of ~87" due to its two taper ratios, meaning its 1/4-wavelength resonant frequency will be on the order of 39 Hz, which would be appropriate for a driver with an Fs of 39 Hz or thereabouts and a Qts of 0.40 or thereabouts.
Paul
Paul
I disassembled and weighed the damping material at 0.66lbs (300g) per speaker. Speaker volume is about 2.4 cubic feet. Hornresp modeling and also a Stereophile test show impedance ripples so I think it should have more damping. I don't know how much and I don't want to try various amounts in multiple tests as these speakers are nearly 100Lbs each and with only one opening stuffing is not easy. What audible differences would one likely hear with varying stuffing densities?
The primary reason you're seeing multiple impedance ripples is because the line has a small taper ratio (~4:1) and it takes a ratio of 10:1 or so to smooth them out acceptably. Also, the stuffing density the line seems to have is small also at 0.3 lb/ft3 or less, which doesn't help. I believe you said only the last 11" of the line were not stuffed. In a 10:1 tapered TL one doesn't need to stuff beyond about the first 60% of so of the line's length, and then at 0.75 lb/ft3 or so. Any stuffing beyond that length or a stuffing density of 1 lb/ft3 or higher begins to greatly and negatively affect the bass response reach. Unfortunately you're kind of stuck and in sort of a situation where you "can't have your cake and eat it too"; it's just a poor design as you've described it. Planet 10 is welcome to chime in. He might not agree with me and be able to offer more uplifting thoughts.
Paul
Paul