Help with effective port length in ML-TQWTs

[Rudolf]

The Martin King simulation for my ML-TQWT gave port dimensions Rport=1.0"and Lport=2.75", resulting in a (wanted) terminus resonance at 35 Hz.
I built the ML-TQWT with exactly these port dimensions. On measuring the woofer and terminus sound level response I found the resonance being at 30 Hz. In the simulation this would be equivalent to Lport~4".
Is this a value I should have accounted for as end correction for the port? Or is it a inherent fuzziness of the simulation?
What port length would you recommend to get the desired 35 Hz?

Thanks for some helpful ideas.

Rudolf

[MJK]

Hi Rudolf,

Is this a value I should have accounted for as end correction for the port? Or is it a inherent fuzziness of the simulation?

There will be an actual length of a port and an effective length of a port. The length that you enter into ML TQWT as Lport is the actual length. There is an end correction applied to the internal end on the second page of the input, this is done automatically. An end correction is inherent in the open end acoustic impedance that is programmed into the worksheet. I am not sure how you calculated the Lport ~ 4 inches but this would be close to the effective length taking into account the end corrections.

I have had pretty good luck with the predictions from the MathCad worksheets correlating very closely with the near field measurements of the finished speaker. I am not sure why you got 30 Hz instead of 35 Hz but that is still not too bad. Have you rechecked the drivers T/S parameters to make sure what you input into the simulation is still valid? Did you measure and calculate the response without any stuffing? This is probably the best way to get a clean correlation between calculated and simulated responses.

Hope that helps.

[Rudolf]

Thanks MJK

for making clear that the port dimensions in your worksheet are the actual dimensions.

I haven´t been able to get the individual T/S-Parameters of my drivers since my measuring equipment isn´t properly calibrated (frequency measurements are valid of course). But I have seen T/S-Parameters for this driver from 3 independent sources, which fall into a 5% bracket for almost every single TSP. So I guess the driver is manufactured with high consistency but there is certainly still room for variation.
Measurements (and simulation) were done WITH stuffing. So this might be another source of indeterminacy.

A last factor that comes to mind: The port of my TQWT is not at the very bottom of the 42" pipe, but in the front baffle - port center 2 1/2" from the bottom of the pipe. Could this be accountable for some difference too?

By the way: This discussion is really cherry picking since I am already highly pleased with the ML-TQWTs as they are. So my couriosity is more about the ins and outs of the simulation worksheet than any audible deficit.

Thanks for your dedication and patience.

Rudolf

[MJK]

Hi Rudolf,

A last factor that comes to mind: The port of my TQWT is not at the very bottom of the 42" pipe, but in the front baffle - port center 2 1/2" from the bottom of the pipe. Could this be accountable for some difference too?

You might want to try the "Ported Box" worksheet, it will allow you to model an ML TQWT and also account for the port position. Between the stuffing and the possible variation in the T/S parameters these might explain 30 Hz vs 35 Hz. But you are already pretty close and having a little lower bass is not so bad. I understand your desire to fully understand the results, I do the same thing.

Glad you like the enclosure,

[Rudolf]

Hi MJK,

I have downloaded the Ported Box worksheet as you recommended. Just did a new simulation with my TQWT dimensions - especially accounting for the exact port position.
Result: The woofer and terminus SPL diagram is almost exactly showing what I measured. Its some terrific work you have done!

Two last questions - the first regarding the enclosure geometry definition of the worksheet:
What is the meaning of the second expression in the SO/SL calculation - the subtracted value? I only entered the values for inner width and depth in the first part and ommitted the second part. Anything wrong with that?

Second question:
If I build enclosures with typical TML dimensions (aka "pipes") and simulate them with the Ported Box worksheet - would the stuffing still follow the rules of ML-TQWTs or should it be applied like in a normal BR box?

Again, thank you for your much appreciated help.

Rudolf

[MJK]

Hi Rudolf,

Its some terrific work you have done!

Thank you very much, I appreciate the positive feedback.

What is the meaning of the second expression in the SO/SL calculation - the subtracted value?

This is a left over from the ML TQWT article. In my original design I used some corner reinforcement pieces to help me line up the sides to the front and the back. I subtracted out this small area from S0 and SL. I should take this calculation out of the distribution worksheets since it has caused some confusion for several people. If I have time I will try and do that tonight and upload a replacement worksheet.

If I build enclosures with typical TML dimensions (aka "pipes") and simulate them with the Ported Box worksheet - would the stuffing still follow the rules of ML-TQWTs or should it be applied like in a normal BR box?

Yes, the stuffing modeling is exactly the same in all of the worksheets.

Sounds like you are making progress and having some fun,

[GM]

>Result: The woofer and terminus SPL diagram is almost exactly showing what I measured. Its some terrific work you have done!
====
This doesn't jive with my experience WRT how the vent location in a pipe affects the FR/Fb so I would appreciate it if you will post the driver specs and your cab dims.

TIA,

GM

[MJK]

Hi Rudolf,

What is the meaning of the second expression in the SO/SL calculation - the subtracted value?

I just checked the latest version of the ML TQWT worksheet on my site and found that I had removed this confusing calculation in the latest revision. I think the revision date is 12/20/03 on the worksheet. I did a little cleaning up and fixed a minor bug which should not inpact your results. You might consider redownloading the TL worksheets so that you have the latest versions.

[Rudolf]

I think the revision date is 12/20/03 on the worksheet. ... You might consider redownloading the TL worksheets so that you have the latest versions.

Hi MJK,
I tried out the download.
Result look at picture below (I did a little composing ;-) though).
I have no cue from what end of the line the discrepant outcome results.

Rudolf

[MJK]

Hi Rudolf,

Missed that worksheet. It is fixed now.

Thanks for pointing that out,