Just for the sake of asking... (about Bass)

[planet10]

Originally posted by kelticwizard
Qts: .76

This is an ideal Q for one of Rick Shultz's alphaTL -- just published in audioXpress.

dave

[kelticwizard]

Dave:

I didn't know that Qts for Transmission Line was so different from the vented box. Perhaps BAM would like to try his hand at the Transmission Line, after all.

[kelticwizard]

Okay, guys, you are going to have to bear with me.

Subwoofer Simulator and the rest of the freeware normally does not model pipe resonances.

I am downloading Unibox. I am learning Unibox. Unibox might or might not model pipe resonances.

I am also thinking of downloading Martin King's Transmission Line program. The problem is finding a free version of MathCAD-he doesn't use Excel.

I wonder, does Martin L. King's program contain a way of modelling a chamber on top of the Transmission Line? For that is what a Daline is-a chamber on top of a Transmission Line.

I shall try Martin L. King's site and see what happens.

[planet10]

Originally posted by kelticwizard
I am also thinking of downloading Martin King's Transmission Line program. The problem is finding a free version of MathCAD-he doesn't use Excel.

I wonder, does Martin L. King's program contain a way of modelling a chamber on top of the Transmission Line? For that is what a Daline is-a chamber on top of a Transmission Line.

Mathcad explorer (12 MB download thou)

http://sliunix.lanecc.edu/~jamies/mcexp802.exe

Martins model handles a daline with no problem. You need to use the sevtions version of the worksheet.

Martin also has the 1st of his alignment tables (each table has to restrict the lines geometry to work), which can get you in the ballpark real quickly (daline isn't covered in these yet)

dave

[GM]

The top chamber is 0.54 cu ft. The Line has three sections of 21.875", 21.75" and 20.5" for a total line length of 64". The width and depth of the line is 3.5" by 6". Normally, a line of this size and length would tune a 0.54 cu ft box to 39 Hz or so.

Hmm, just looking at the dwg. and knowing that BH5 reduces the cab Vb some, I get a net Vb = ~.229ft^3. The vent's effective size would be a constant ~2.5" x 6", and its base length as measured down the middle comes to 73". This wouldn't be its total acoustic length though since the inside height of the cab must be factored in. This adds ~12" to the length, or 85". Then there's the end correction, or ~0.618*((2.5*6)/pi)^0.5 = ~1.35". ~(13560/4)/86.35 = ~39.25Hz, so let's just say it's tuned ~ to Fs.

It has two 0.5mH inductors in series, so figure ~0.7ohms of series resistance using a large gauge, upping Qts to ~0.332 using the 5NV4211DB's published specs.

I have not seen a response curve for the Daline. However, Focal gives the -3 dB down point as 40 Hz, and Focal is a quality manufacturer.

Plugging all this into BoxPlot for a basic alignment yields a T/S max flat 0.28ft^3 Vb/48.6Hz Fb. Shrinking it to 0.229ft^3/40.4Hz Fb, the acoustic response is a nice smooth/well damped roll off (no underdamped dip in the FR or peaking at Fb) beginning around 125Hz, with an F3 in the high 40s. Factor in the pipe effects and based on experience a F3 shift to 40hz with a bit more midbass output seems reasonable. If you're curious you can model it in MJK's excellent MLTQWT spreadsheet for a more accurate sim. BH5 really damps well, so I suggest using 1.0lb/ft^3 as an approximation and the driver's position should be listed as L*0.0853 for this design, which is the driver's distance from the top of the cab divided by the pipe's physical length (~7.25/85).

Anyway, looks like whoever designed this did themselves proud with this one, and assuming you accept my analysis, should give you enough insight to adapt it to other drivers.

HTH,

GM

[GM]

Well, phooey! Decided to input this into MLTQWT and noticed I'd used 0.3 instead of 0.23 for Qes, skewing everything, so ignore my previous post.

The top chamber is 0.54 cu ft. The Line has three sections of 21.875", 21.75" and 20.5" for a total line length of 64". The width and depth of the line is 3.5" by 6". Normally, a line of this size and length would tune a 0.54 cu ft box to 39 Hz or so.

Hmm, just looking at the dwg. and knowing that BH5 reduces the cab Vb some, I get a net Vb = ~.229ft^3. The vent's effective size would be a constant ~2.5" x 6" (~2.1851" radius), and its base length as measured down the middle comes to 73". This wouldn't be its total acoustic length though since the inside height of the cab must be factored in. This adds ~11" to the length, or 84". Then there's the end correction, or ~0.618*((2.5*6)/pi)^0.5 = ~1.35". ~(13560/4)/85.35 = ~39.71Hz, so let's just say it's tuned ~ to Fs.

It has two 0.5mH inductors in series, so figure ~0.7ohms of series resistance using a large gauge, upping Qts to ~0.26 using the 5NV4211DB's published specs.

I have not seen a response curve for the Daline. However, Focal gives the -3 dB down point as 40 Hz, and Focal is a quality manufacturer.

Plugging all this into BoxPlot for a basic alignment yields a T/S max flat 0.13ft^3 Vb/61.2Hz Fb. Expanding it to 0.229ft^3/40.4Hz Fb, the acoustic response is a nice smooth/well damped roll off (no underdamped dip in the FR or peaking at Fb) beginning around 225Hz, with an F3 of 50hz. Factor in the pipe effects and based on experience a F3 shift to 40hz with a bit more midbass output seems reasonable. If you're curious you can model it in MJK's excellent MLTQWT spreadsheet for a more accurate sim. BH5 really damps well, so I suggest using 1.0lb/ft^3 as an approximation and the driver's position should be listed as L*0.5682 for this design, which is the driver's distance from the top of the cab (minus 1" for the BH5) divided by the chamber's net length, or ~6.25"/11".

Anyway, looks like whoever designed this did themselves proud with this one, and assuming you accept my analysis, should give you enough insight to adapt it to other drivers.

HTH,

GM

Addendum: plugging the numbers into the MLTQWT spreadsheet shows a typical resonant pipe that would be less so with damping in the vent pipe per the spec, F3 = ~33Hz, Fb = ~28Hz. It appears that what I considered a worst case scenario for how much the BH5 affects the dims turned out to be not conservative enough if the published specs and my assumption as to how much series resistance there is ~accurate.

Anyway, eschewing all this and calculating a Daline my way using the 0.26Qts and Fb = Fs, taking the max flat 0.13Vb and increasing it 1.414x I get a rear chamber of ~0.1837ft^3, or a ~golden ratio 11.03"h x 6.82"w x 4.21"d.. Maintaining the same driver vertical spacing, the woofer is 7.25"/11.03" = ~0.6573*L.

Using a pipe area = ~Sd equates to 13.423in^2/6.82" w = ~1.97" d.. Actual pipe length = ~13560"/4/40.4Hz = ~83.91"- 11.03" - (13.423/pi)^0.5 = ~70.81".

This sims F3 = ~35Hz, Fb = ~32Hz. Works for me. To get an F3 = 40Hz required the pipe to be shortened to 55", or a ~50hz 1/4WL, showing how much the Vb damps the pipe action.

[kelticwizard]

Originally posted by GM

Hmm, just looking at the dwg. and knowing that BH5 reduces the cab Vb some, I get a net Vb = ~.229ft^3. The vent's effective size would be a constant ~2.5" x 6" (~2.1851" radius),

When looking at another drawing not done as cleanly as the above, I arrive with a Vb of .229 Ft³ as well. However, it turned out that the black, shaded blocks were made not of wood, but of Black Hole 5, a substance with which I am not familiar.

I looked up Black Hole 5, and it turned out to be an expensive "viscoelastic" substance similar to Rubatex and other pipe coverings, only more sophisticated, in some way. I decided to treat Black Hole 5 according to author David Weems' rule for damping materials such as polyfill, wool, etc. That is, squeeze the material in your fingers, and see how far it compresses. The compressed volume is what you consider the actual thickness. Well, I have some Rubatex here, I squeezed it, (very soft), and I had no trouble compressing 90% or more. I assume Black Hole 5 is similar. So I considered the actual thickness of Black Hole 5 essentially negligible, as I would polyfill.

Regardless of whether Black Hole 5 is like Rubatex or not, it is soft damping material and flexible. In your analysis, you are saying that two inches of Black Hole 5 on the walls is the same as 2 inches of wood. Without having used the product, I would tend to doubt that, although if that has been your experience with it, I wouldn't go against it. Similarly, using the same "Black Hole 5 = wood" principle, you reduced the line cross section from 3.5" x 5" to 2.5" x 5". Again, I cannot say with authority that is not the case, but I would like to know from someone who has used it that this soft, flexible rubber like product is to be treated the same as hard wood for the purpose of computing volume.

It seems to me that, on the Line/port, we are dealing with low frequencies, which have a penetrating power, and are much more likely to "see" a difference between a soft rubbery product and solid wood than higher frequencies will.

Originally posted by GM:
and its base length as measured down the middle comes to 73".

I measured the lengthof the Line in the following manner: When the Line came to a boundary and was about to do a 180º turn, I did not go all the way to the wall. Rather, I went halfway. For a 2.5" opening, I cut 1.25" off the length. Essentially, this is measuring down the middle. I have not built a Transmission Line, but certain books I have read on the matter said that is the way to go. Hence, my measurement will be shorter than yours. Again, if it has been your experience that to measure all the way to the end of the boundary, I won't oppose it. I just went by guidelines.

[GM]

I don't see how Weems remark is valid WRT something like BH5, and I can only squeeze the foam portion enough to matter, so his 'test' for apparent density is way too 'flexible' to be of any use IMO. The BH5 I've used is 1.38" thick, and not knowing for sure how much to include as 'transparent' (I've only used it to damp metal panels in my convertible), took a 'leap of faith' that since the bends in the pipe were 2.5" in cross section that the 3.5" width with BH5 emulated this, so used 1" as an acoustic filler for all intent and purposes of calculating Vb. For sure, necking down the bends to create a series of shorter 1/4WL resonators would at best be a design of dubious tonal quality IMO, further 'proof' in my mind of the validity of my assumption.

One could argue that the rear chamber 'breathes' around Fb due to the sheer quantity used, making for a ~constant Vb regardless of pressure, but if it does, then the roll off would be more, further lowering the F3, and combined with the modest pressures the driver can generate, dismissed this as irrelevant for reverse engineering purposes. Same for the pipe since the BH5 'feels' the dominant pressure in shear, leaving the foam the major 'drag' on pipe action, with the balance of the BH5 damping the cab's construction.

Anyway, none of this was meant to imply that BH5 = wood.

WRT measuring a ~constant cross sectional area pipe, right, you measure down the middle, but I believe you forgot to include the bends in your calc..

GM