15" Drivers, Part 2: Mathcad sims versus SPL graphs

[cuibono]

Having been looking at the following threads, and planning my own OB system (visaton B200 + 15" bass driver), I decided to do some sims, at MJKs suggestion, to compare them with the manufactures datasheets.

These are what inspired me:

New MJK Baffle Article

15" Bass Driver for OB (40-200Hz)

Visaton B200+Eminence Beta 15=OB!

Currently, Vix is having problems integrating his Beta 15 with a B200, and I've been looking into using a Dayton IB15 in its place. MJK's recent OB design article suggests that the Alpha 15 would be a good bet.

To me, the problem seems to be a discrepancy between the simulated results and the measured results. I've run the simulations, and have made some pictures to compare the simulation and SPL graphs. These follow:

[cuibono]

Those last three graph are for the IB15, Alpha 15 and Gamma15. The following two are for the Acoustic Elegance Lambda Dipole15 and the Knight 15, which are also considerations for OB use. All simulations were done in MJKs single driver OB simulator in Mathcad, changing only the T/S parameters. The baffle is 40" tall by 24" wide.

[cuibono]

Now, these are the data sheets for the drivers. Unfortunately, the Knight and Lambda drivers don't seem to have SPL graphs published, so we can't really consider them beyond the sims.

[cuibono]

Now, I've made a little chart of what I think the salient points are - comparing 100, 50 and 25 Hz output levels of each of the drivers, both simulated and measured SPL.

[cuibono]

Now, the thing that really stands out in the simulated response is how well the Alpha 15 does, which has already been noted by MJK (although it hasn't been compared to non-Eminence drivers as of yet). The Alpha 15 gets about 6dB more output compared to any other driver at 50 and 100Hz. But the confounding thing is how the trend is reversed for the measured SPL levels - at the Alpha is lowest in output at 2 out of three frequencies. I wish the Lambda and Knight has fr graphs...

The first thing to question is if the SPL graphs were measured the same, they may not have been. I don't know. But the IB15 looks like a clear winner if you consider just the SPL graph - its quite flat to 20Hz, and displays no midrange peaks, which the Alpha has quite bad (it seems Vix was experiencing some problems because of this with his Betas).

So the question is, which do you trust, the simulated responses (I bet MJK does), or the measured SPL graphs? Also, how do you reconcile the very flat IB15 graph versus the more sloppy Alpha 15's?

And the biggest question, in the end, which is going to be a better driver (the speakers I'm putting together will be biamped with a computer XO/time alignment, so things like efficiency and fr are not as important - my goal with the bass is quality, not quantity).

[ttan98]

Originally posted by cuibono
Now, the thing that really stands out in the simulated response is how well the Alpha 15 does, which has already been noted by MJK (although it hasn't been compared to non-Eminence drivers as of yet). The Alpha 15 gets about 6dB more output compared to any other driver at 50 and 100Hz. But the confounding thing is how the trend is reversed for the measured SPL levels - at the Alpha is lowest in output at 2 out of three frequencies. I wish the Lambda and Knight has fr graphs...

The first thing to question is if the SPL graphs were measured the same, they may not have been. I don't know. But the IB15 looks like a clear winner if you consider just the SPL graph - its quite flat to 20Hz, and displays no midrange peaks, which the Alpha has quite bad (it seems Vix was experiencing some problems because of this with his Betas).

So the question is, which do you trust, the simulated responses (I bet MJK does), or the measured SPL graphs? Also, how do you reconcile the very flat IB15 graph versus the more sloppy Alpha 15's?

And the biggest question, in the end, which is going to be a better driver (the speakers I'm putting together will be biamped with a computer XO/time alignment, so things like efficiency and fr are not as important - my goal with the bass is quality, not quantity).

Stimulation is one aspect of speaker design, the other is the measured response. Eg. I bought local generic woofers with reasonable Qts=0.55, the response curve on open baffle is not to expectation, ie it rolls off rather rapidly, hence I redesign and make it mounted on a U frame the response curve improves dramatically. The woofer sounds quite good even though the woofer is very cheap. I can get about 87dB at 50 Hz.

I would opt for the Dayton from the response curve view point. Each woofer has certain sound characteristics, Alpha may not have a better response curve however the tonal quality you may find is better than Dayton. POSSIBLY the Dayton with better response may sound drier. An example is Martin bought JBL driver which is more expensive than Alpha, possibly better response curve, he found the Alpha to sound better, hence recommend it ever since.

Hope this maybe of some assistance. Cheers.....

[Scottmoose]

Well, I don't think Dayton or Eminence, when they measured their drivers, were using your proposed baffle to do it on. So of course the simulated results and the IB measurements will be different. That's hardly a discrepancy. You wouldn't expect the drivers to give the same SPL plot as the advertised graph if you stuffed them into a small BR box, right? Well, the same principle doesn't vanish because they're mounted on an OB.

Generally your comparative table shows a small drop-off in SPLs in the LF, which is exactly what you'd expect from a driver mounted on a baffle where the acoustic short-circuit eventually starts causing cancellation. This is before room effects etc. are brought into the equation too, though baffles are somewhat less affected if positioned right as they activate fewer room resonant modes. Clearly, the worksheets are doing their usual superb job, but remember that you have to learn how to interpret the data they give you too. To give one example, if you see a BR giving a ruler-flat anechoic predicted and measured response, you can be certain it's no longer going to be ruler-flat in-room due to room-modes etc.

If you want simple, I'd generally be inclined to go for a high Q driver like the Alpha 15 as you'll be able to use a narrower baffle. The alternative approach, which is more complicated and harder to get right, is to use several drivers with high motor-power & some form of EQ on these or the WR driver they're supporting to balance out the SPLs. Both work, so YMMV.

What you're seeing, & the reason the Alpha is doing well in LF SPLS compared to the other units, is it's high Q. If you haven't read Martin's white paper on the subject, I suggest you look into it: http://www.quarter-wave.com/General/OB_Design.pdf

[peterbrorsson]

Originally posted by Scottmoose
Well, I don't think Dayton or Eminence, when they measured their drivers, were using your proposed baffle to do it on. So of course the simulated results and the IB measurements will be different. That's hardly a discrepancy. You wouldn't expect the drivers to give the same SPL plot as the advertised graph if you stuffed them into a small BR box, right? Well, the same principle doesn't vanish because they're mounted on an OB.

Generally your comparative table shows a small drop-off in SPLs in the LF, which is exactly what you'd expect from a driver mounted on a baffle where the acoustic short-circuit eventually starts causing cancellation. This is before room effects etc. are brought into the equation too, though baffles are somewhat less affected if positioned right as they activate fewer room resonant modes. Clearly, the worksheets are doing their usual superb job, but remember that you have to learn how to interpret the data they give you too. To give one example, if you see a BR giving a ruler-flat anechoic predicted and measured response, you can be certain it's no longer going to be ruler-flat in-room due to room-modes etc.

If you want simple, I'd generally be inclined to go for a high Q driver like the Alpha 15 as you'll be able to use a narrower baffle. The alternative approach, which is more complicated and harder to get right, is to use several drivers with high motor-power & some form of EQ on these or the WR driver they're supporting to balance out the SPLs. Both work, so YMMV.

Hi Scott and cuibono,
The thing is, as I see it (no doubt un-qualified), that Dayton and Gamma hold up against the sim quite nice, Alpha not. Anomalies in the T/S parameters?

Peter

[Scottmoose]

I doubt it. Eminence are usually pretty (very) accurate with their specs. It's a low Q unit (underdamped) so it should generally keep the LF up better on an OB than lower Q drivers. Martin's paper which I linked to above discusses this issue, with several examples, including a couple of these units.

So... a demon -stration.

This is the Gamma 15 in a sealed cabinet (easiest to quickly knock togethewr -ignore the red plot, we're not interested in that; it's the broken blue dotted line that's important.

[Scottmoose]

And this is the Alpha 15, same box (not that that matters). Note the blue plot which is the driver's IB response derived from the T/S parameters.

What do we see? The higher Q unit has the peak at Fc & lower mass-corner characteristic of all such comparatively under-damped drivers. This trait is mostly hidden by the smoothing Eminence apply to their published graph, and also by the way they measure them (that's purely an observation, not a critcism -there are many ways of doing both). Using these two graphs, I imagine you can see why the Alpha15 has the superior LF gain over the Gamma15, and the latter, better damped unit, with a higher mass-corner, will require a substantially larger / wider baffle to prop up it's lower frequencies.

Hope that helps
Scott