How To Read Martin King's Step Response Graph?

[kelticwizard]

Below is the step response from Richard Small for both
Closed Boxes of various Qtc and for Vented Boxes of various alignments. For the Vented Boxes, I wrote in the alignment details. a=Vas/Vb, the usual designation

[kelticwizard]

However, Martin King's MathCAD sheets give a step response that looks very different. It seems that both Closed Box and Vented Box give step responses which look almost identical in MJK sheets.

Below is the MJK step response for a Closed Box and Vented Box with the equivalent Small graph next to it. The box in question is 25 liters, tuned to 42 Hz in the Vented version.

Closed, the Qtc=0.54, close enough for comparison to Small's 0.50 graph. Vented, the box is a Classic Bass Reflex, where Fs=F3, Vas=Vb, and Qts=0.383.

Here are the Thiele-Small parameters for the driver, derived from BoxModel so they all work together:

Fs=42
Vas=25 L
Qms=3.5
Qes=0.43
Qts=0.383
BL=4.49 TM
Sd=83 sq cm
Re=5.7
Mms=5.8 g
Le=0.7 mH

[kelticwizard]

Does anyone know why the MJK step response looks almost identical for these two very different boxes, while the Small graphs show such immense differences?

[MJK]

Did you set the stuffing density to zero? The internal damping from the stuffing might be a contributing factor.

[kelticwizard]

The stuffing density in the Ported Box, (made with the Ported worksheet), was set at 0.1 lb/cu ft. The stuffing density in the Closed Box, (made wth the Closed worksheet), was mistakenly left at it's default, 0.25 lb/cu ft.

To rectify this, I just did a sim in the Closed worksheet with the stuffing density set to 0.1 lb/cu ft, same as the Ported. It does not seem to make much difference, but here it is..

The box in question is supposed be 18.4 in tall, by 12 in wide by 7 in deep. Hence the cross section area of 84 sq in. The midline of the driver was 6 in from the top, in both cases.

[kelticwizard]

And here is a GIF of the User Input stage and box info. This is for the Ported sim-the Closed Box is the same, (at least the second Closed Box sim, which has a stuffing density of 0.1 lb/cu ft, just like the Ported).

For completeness, I just thought I would throw in the fact that this is a pretty old computer, IBM 225 MHz Pentium II, 256 meg RAM, 20 gig HD. Win 98 SE. Norton 2005, script blocking disabled. I use the free demo version of MathCAD 8. I do not know if this has any effect on this computer's ability to simulate MathCAD sheets correctly, or if there are errors because of any of this.

[MJK]

Your computer and set-up is fine so I don't see any problems. I think I know the answer to your questions, I'll rerun the simulation when I get home tonight and post the results. Good question.

[kelticwizard]

I just realized you wanted me to set the stuffing density at 0.0 lb/cu ft. Here are the Ported and Closed Box sims set for zero stuffing density.

The Ported sim does have a teensy, almost invisible hump peaking around 0.012 sec or so, but compared to the huge difference in the Small graph, it does not seem to illustrate much difference between the Ported and Closed Box boxes.

[kelticwizard]

Martin:

I just saw your response after I posted the above post.

Thanks for answering. I look forward to your answer tonight, (or whenever you can if tonight turns out to be inconvenient).

[MJK]

Okay, here goes an attempt at adding a picture.