s this setup OK? (especially for MJK and GM)

[MJK]

I really think that the enclosure suggested by GM together with a BSC could be way better than the CT 142, those two humps don't look too good to me, no matter what the reviews may say.

I would definitely agree with the above statement. GM's designs are always well though out and technically solid. I think that you could build GM's design with confidence that the performance would be very good.

 

[mr_push_pull]

Well, thank you MJK and GM. Your answers have been very helpful, as always.

 

[Dave Jones]

Greg,

The Focal label is a leftover from the sample problem run in the downloaded worksheet. People don't seem to change the title, only the T/S parameters, when they run a simulation for their drivers. By not updating the title it stops others from double checking the driver's specs they are using, somebody might actually have measured T/S results for the particular driver being sim'ed.

Martin, on at least one of the worksheets, the comment that contains the driver's name is locked so one cannot change it.

 

[mr_push_pull]

The box using the parameters above (L = 39.5", zd = 14.63", zp = 37", So/SL = 63"^2, density = 0.2, rp = 1.25", Lp = 4.125") has finally been built (it took A LOT for the drivers to arrive). Audible result: boomy, rumbling bass. And I tested it without the adjusted port (5.8"). I can't even think about what shortening the port will do to the sound. MJK and GM, can you comment on this a bit please?

 

[MJK]

mr_push_pull,

Have you measured the T/S parameters of the actual drivers to see if they match the values you used in your simulations? Rereading the thread, it appears that you did a lot of design work with the manufacturer's specs. If the actual and the spec T/S parameters are dramatically different it might explain some of the problems you are hearing.

 

[mr_push_pull]

mr_push_pull,

Have you measured the T/S parameters of the actual drivers to see if they match the values you used in your simulations? Rereading the thread, it appears that you did a lot of design work with the manufacturer's specs. If the actual and the spec T/S parameters are dramatically different it might explain some of the problems you are hearing.

No, I haven't measured the drivers. I will do this as soon as I get home just to avoid this subject coming up again. I'm sure that the deviations will be negligible.

 

[GM]

Audible result: boomy, rumbling bass.

Assuming you mean exactly this (no language barrier), then either the speaker would have to be incredibly underdamped (not likely) or it's a room modes and/or speaker placement issue. Stick some thick foam, cotton socks, drinking straws, or similar in the vent to make it aperiodic and report back.

GM

 

[mr_push_pull]

Assuming you mean exactly this (no language barrier), then either the speaker would have to be incredibly underdamped (not likely) or it's a room modes and/or speaker placement issue. Stick some thick foam, cotton socks, drinking straws, or similar in the vent to make it aperiodic and report back.

GM

Just to make sure, this is dictionary.com's definition for rumbling: "(to make) a deep, long, rolling sound." This is what I hear. I tried clogging the port with a lot of damping material (I don't think air was able to pass through that), and the boominess was gone, but the bass disappeared almost completely.

 

[GM]

Right, so the speaker is exciting room modes (eigenmodes) too much. You can either unstuff the vent and try different speaker layouts and/or a longer vents and/or finding the right amount of vent stuffing to get the best balance between LF output/extension and a 'clean' /'tight' reproduction.

Understand though that the room dominates the LF response, so normally the more you can control the room's response, the better. These sound like they are near/in the corners, maximally exciting the modes, in which case tuning the cabs much lower should suffice.

GM

 

[Greg B]

What is the amplifier? I've had that issue before with bass reflex speakers and low DF tube amps. An old Knight 'spud' tube amp hooked up to some radioshack 1354 in BR cabinets was practically ridiculous, though the speakers worked fine with everything else. The amp itself sounded surprisingly good with sealed boxes or TLs.

It could be the room modes as well. Multiple problems is normal in DIY.

GB

 

[mr_push_pull]

I guess I owe everybody apologizes. I measured the drivers and these are the results:

Published params:
Qes = 0.51
Qms = 2.16
Qts = 0.41
Fs = 36

Measured T/S params:
Qes = 0.65
Qms = 2.5
Qts = 0.52
Fs = 39.5

These sound like they are near/in the corners, maximally exciting the modes, in which case tuning the cabs much lower should suffice.GM

They are placed near the corners indeed. I'll wait some more untill I'll have the time and patience to retest them thoroughly, and come back with reports.

What is the amplifier? I've had that issue before with bass reflex speakers and low DF tube amps. An old Knight 'spud' tube amp hooked up to some radioshack 1354 in BR cabinets was practically ridiculous, though the speakers worked fine with everything else. The amp itself sounded surprisingly good with sealed boxes or TLs.

It could be the room modes as well. Multiple problems is normal in DIY.

GB

The amplifier is a piece of crap by everybody's dictionary I guess, but it is SS. With the strongly varying impedance I have (no impedance EQ), valve amps are out of the question (not that I owned one).
Multiplem problems, eh? I have:
- deviations of T/S specs
- bad room
- poor impedance curve combined with bad amp
Should I go on?

But still I have some hope

 

[MJK]

The T/S parameters don't look that bad. Try updating the MathCad simulation and see if an adjustment to the port length is needed. The enclosure design will probably still work.

I would not be too worried about the amp, it is probably the least of the problems.

The room placement is an interesting issue that I had not thought of, it was a good point made by GM.

Sounds to me like some simple small adjustments will move you towards better performance. Eventually you will be limited by the quality of the driver and any correction circuits you need to apply.

 

[mr_push_pull]

Try updating the MathCad simulation and see if an adjustment to the port length is needed. The enclosure design will probably still work.

I reran the sims with the correct params and the results were not bad, a little less bass but no bump in the response that could explain the boominess. What I'm also worried about is cone excursion, which at this moment is two low (distorsion limit reached at 15W).

 

[MJK]

What I'm also worried about is cone excursion, which at this moment is two low (distorsion limit reached at 15W).

How loud are you planning on playing these speakers? If you are looking at reproducing acoustic music in a small to medium room, at reasonable volume levels, then I would not be too concerned about this parameter. But if you want to rattle the windows with some of the popular junk on the radio today, then maybe a different driver would have been better. I have not had any objectional distortion issues with drivers that have 1 mm Xmax values even when pushed to fairly loud levels. I am sure I could create issues at higher volumes but I would not be enjoying the music anymore and would be leaving the room quickly.

 

[GM]

What are the measured Vas, Re, Le? You can't ~accurately 'see' the difference in a sim without these. WRT running out of Xmax at 15W, it should be closer to 30W so I have to wonder if you're using some form of EQ such as a 'loudness' or boost ckt. built into the amp like is common in old SS or a typical plate amp. For sure with a low efficiency/power handling driver, short of compression horn loading there won't be any quality reproduction at anywhere near 'live' levels.

GM

 

[rdf]

Earlier in the year MJK wrote:
The worksheets were written assuming that one dimension was so long that it dominated the low frequency resposne enough to ignore the other two dimensions. Between 10 Hz and 100 Hz this is easily accomplished and the T/S modeling is accurate.

But then I plotted the responses out to 1000 Hz so that the harmonics of the fundamental could be seen and placement of the driver to suppress the harmonics could be studied. At some point above 100 Hz the model startes to become less accurate for a number of reasons one being standing waves in the remaining two dimensions of the enclosure. Hopefully stuffing will attenuate these out of plane waves and the plotted results will be reasonable.

That was immensely helpful, thanks. Two quick questions:

- Are the back and side wall elements still present in your BR sheets?
- Are they applicable to modelling MLTLs?

Thx in advance.

 

[MJK]

Hi rdf,

Are the back and side wall elements still present in your BR sheets?

Not sure I completely understand your question. All of the MathCad worksheets downloaded from my site are 1 dimensional in the long direction of the enclosure. The only difference between the Ported Box and the ML TQWT worksheets is a stub at the bottom that allows the position of the port to be changed in the Ported Box worksheet.

I do have 3 dimensional versions of the Closed and Ported Box worksheets but they have a math bug that I need to go back and fix. I hope to get back to these this winter.

Are they applicable to modelling MLTLs?

The best worksheet for a modeling ML TL (and ML TQWT also) enclosures is the Ported Box worksheet. The added degree of freedom of port location can be important in the design. In the Ported Box worksheet, I have assumed the entire enclosure is filled with stuffing. To eliminate the stuffing in the bottom of the enclosure multiply the Density entered for the last couple of sections by zero in the detailed input on second page of the worksheet.

Hope that answers your questions,

 

[rdf]

Thanks much! Answers it perfectly. The confusion was due to my misread of the context in which the statement

The worksheets were written assuming that one dimension was so long...

was made. I thought this simplifying assumption was applied only to the MLTL worksheets. Thanks again for the clarification.

 

[mr_push_pull]

How loud are you planning on playing these speakers? If you are looking at reproducing acoustic music in a small to medium room, at reasonable volume levels, then I would not be too concerned about this parameter. But if you want to rattle the windows with some of the popular junk on the radio today, then maybe a different driver would have been better. I have not had any objectional distortion issues with drivers that have 1 mm Xmax values even when pushed to fairly loud levels. I am sure I could create issues at higher volumes but I would not be enjoying the music anymore and would be leaving the room quickly.

Thanks for assuming such musical tastes. They weren't build with acoustical music in mind but I never listen to any radio junk. I think there are quite a lot of musical genres apart from acoustical jazz and classical and radio junk.