Best Of

And one day VSAT will test a set like that too right @xschop? Let me know if you make more! Remember, I still got a set of RTA 11TLs, a set of SDA-SRS2's to finish up.

Polk Audio SDA Compact System Speakers Pair Left Right
Currently listed at shopgoodwill.com starting bid $49.95
https://shopgoodwill.com/item/263067527
local pickup only in San Diego

DarqueKnight wrote: »

"In recalling the efforts of DIY speaker builders I have studied, they tended to concentrate on stiffening the basket and reducing its vibrations, rather than reducing reflections from the inside of it."

"The foam, like any damping material, has more effect on vibration abatement, which reduces overall coloration due to basket vibration. Ideally, the only thing that should be moving is the cone/voice coil/surround assembly. Taking out the vibration from the cabinet, basket, and baffle allows the driver to operate more cleanly and efficiently because it isn't "fighting" with the basket."

I got this idea largely from reading the Vandersteen Midrange Driver Patent, where he addresses the undesireable effect of internal basket and magnet motor reflections that are time delayed and out of phase with the desired signal, coming back and impinging upon the rear of the cone where some fraction of them are again reflected back and another fraction are absorbed, and the remaining fraction is actually transmitted through the cone to the listener in the form of undesirable distortion.

He doesn't go into mechanical vibrations attenuation of the basket at all in his patented midrange driver design. As far as my foam mod goes, I am sure that is at play, but to a much lesser extent. When I did only my left speaker I had to raise that speaker's signal by 0.4dB to bring it back to being balanced with the right speaker with whitenoise. Both were balanced as closely as possible with the equipment I had, to within 0.1dB prior, with whitenoise. I believe that was due to the attenuation of those backwave reflections Vandersteen talks about in his patent. This is the same thing KEF is doing with their new META speaker.

Gardenstater wrote: »

Really looking forward to SPL vs Freq. and Harmonic Distortion vs Freq., especially the latter because wouldn't that be where out of phase reflections of the back wave impacting and transmitting through the cone would most likely have a noticeable effect?

In recalling the efforts of DIY speaker builders I have studied, they tended to concentrate on stiffening the basket and reducing its vibrations, rather than reducing reflections from the inside of it.

I agree that frequency response and harmonic distortion tests would have revealed more useful information that the T/S parameters. However, I was scared off by the vice grip of the Armacell. I might be braver in the future, with a smaller size SDA loudspeaker.

Gardenstater wrote: »

How does some foam on the basket affect the free air resonance of Impedance vs. Freq. (Fs)? I'm still learning.

The foam, like any damping material, has more effect on vibration abatement, which reduces overall coloration due to basket vibration. Ideally, the only thing that should be moving is the cone/voice coil/surround assembly. Taking out the vibration from the cabinet, basket, and baffle allows the driver to operate more cleanly and efficiently because it isn't "fighting" with the basket.

Gardenstater wrote: »

I think that there is an error in the Qts calculation for the unmodded driver though, which if corrected, makes the avg. for unmodded vs modded pretty insignificantly different. Qts = (Qes x Qms)/(Qes + Qms).

Thank you for catching that. The number in my handwritten notes is "0.3138" and I wrote it incorrectly when transferring to the Excel spreadsheet.

I don't know the design details of SDA drivers well enough to know what numerical amount of difference in the T/S parameters is considered significant/insignificant. I do know that when it comes to loudspeakers, small changes, like a few degrees of change of toe in, or a change in tweeter dome material, can have significant audible effect. SDAs are a non-conventional speaker design. Something that might be numerically insignificant in a conventional speaker might be a big deal with SDAs. I would love to be able to discuss my measurements with a Polk engineer...but those days are long gone.

Most intriguing were the measurements of the mod-reversed driver being closer to the modded driver than the stock driver. However, this was a small sample size of one. It's possible this driver is an outlier.

Gardenstater wrote: »

I think the Fs of 36.34 in Meas. 1 of unmodded driver is a bit of an outlier and maybe could be thrown out, also, bringing the avgs. closer for unmodded vs modded?

I don't know that that measurement is an outlier. A driver is always going to react a little bit differently to successive electrical test signals. Also, I did not use lab grade test equipment with close to identical test signals for repeated trials. That is why I did five frequency sweeps and averaged them.

@DarqueKnight Meticulous as always!! Some were afraid that the foam would fall off and end up on the floors of their speakers and some were afraid they couldn't remove if they didn't like it. Your input will help those people.

Really looking forward to SPL vs Freq. and Harmonic Distortion vs Freq., especially the latter because wouldn't that be where out of phase reflections of the back wave impacting and transmitting through the cone would most likely have a noticeable effect?

How does some foam on the basket affect the free air resonance of Impedance vs. Freq. (Fs)? I'm still learning. I think that there is an error in the Qts calculation for the unmodded driver though, which if corrected, makes the avg. for unmodded vs modded pretty insignificantly different. Qts = (Qes x Qms)/(Qes + Qms). I think the Fs of 36.34 in Meas. 1 of unmodded driver is a bit of an outlier and maybe could be thrown out, also, bringing the avgs. closer for unmodded vs modded?

Thanks for attempting to shed more light on this for us, in a scientific manner, which is what we have all come to admire in you.

xschop wrote: »

Hey @VSAT88 Passing on MegaDETH and Black Sabbath, but going to listen to some smooth jazz on my new CRS+ this weekend thanks to @dkfreebird.

It was great meeting you Rob. I can’t wait to
see what you do with them. When I’m done with the bedroom remodel I will get the plates installed.