SDA2B SPL Measurements

jcandy

Overview: In this thread I will briefly describe some frequency response measurements of a Polk SDA2B. The speakers sat unused for about 10 years with blown tweeters. Now they have been cleaned up and are fully restored. Measurements will be given for both the older SL2000T and the newer RDO-194 tweeters, as well as with and without grills. Measurements are generally taken from a SINGLE SPEAKER off axis at about 15 degrees to the left of the right speaker (an approximate listening position).

SDA effect: There was some question about what to do about the dimensional mid-woofer, and it was decided to simply "do nothing". The alternative, namely shorting the pin and spade on the interconnect, converts the dimensional driver into a second (but somewhat different in character) mid-woofer, which is very different than what's typically happening with normal stereo listening. Below about 50Hz the stereo and dimensional drivers act in unison (although the DCR of the SDA inductor reduces the output of the dimensional driver somewhat). This is the same for the tests and for normal listening. However, in the tests the dimensional driver simply rolls off above 50 Hz. So, what is missing from these measurements is the small R-L signal that would be coming from the dimensional driver in the 50Hz-2kHz range.

First, some photos:

F1nut

I use my ears, they never let me down.

jcandy

Splicing procedure: First, I wanted to get an overall picture of the SPL of the SDA2B with grill off, RDO-194 tweeter. The microphone was positioned at tweeter level, 17 degrees off axis, about 1.25m from the speaker, and a far-field measurement was taken. Then, near-field measurements of both mid-woofers and well as the passive radiator were taken, summed, then multiplied by the numerically-calculated baffle diffraction curve to convert them to far-field form. Then the near and far-field results were spliced at 350Hz (I think) to give the final SPL trace. The weak link the is diffraction calculation, but I think the results given are not too bad.

Discussion of results: The results show a huge peak-dip-peak in the crossover region at 2kHz. This, combined with some bass peaking at 60Hz is going to give the speaker a "characteristic" sound :rolleyes:. With regard to the 60Hz peak, I am a little unsure of the accuracy because of how the SDA effect was ignored. But the default assumption is probably that there is real bass peaking there, and this will get worse if the air-core SDA inductor is replaced by one with lower DCR.

jcandy

Reversing the polarity: First, a sanity test to see that the tweeter is connected with the correct polarity. This test -- the reverse-null test -- is supposed to show a large dip at the crossover frequency when the tweeter polarity is reversed (taken 180 degrees out of phase). We perform the test with the RDO-194 tweeter and the grill off.

Results: The red curve shows the in-phase result, and the black curve the out-of-phase result. Although the red curve is certainly the correct orientation, the difference is alot less than one normally sees in a good design. For example, compare this in-phase connection with this this out-of-phase connection.

jcandy

New versus old:In this plot we compare the RDO-194 (blue) against the SL2000T (orange) with the speaker grill on. This was done by keeping the microphone in the same place, with all the amplifier and gain settings fixed. Obviously, we expect the curves to be identical below the crossover frequency.

Results: Honestly, there is not a huge difference In my mind, the sound of the speaker ought to be dominated by the uneven SPL in the crossover region. Other than that, the SL2000T is 0-2dB brighter in the 500Hz to the 12kHz region. The differences beyond 10kHz are likely inaudible to most.

jcandy

Grill diffraction: the speaker grill has the potential to cause all sorts of problems if the speaker has a nice, flat response without a grill. On the other hand, some manufacturers (Paradigm) integrate the driver face-plates with the grill contruction so that the response is smoother with the grills on.

Results: We show the results for both the RDO-194 (blue with grill, red without) and the SL2000T (orange with grill, purple without). The main effect is to produce a small dip in the response at 4.5kHz. The effect is the same for both tweeters because its geometrical, not electrical. Interestingly, the difference with and without the grill is comparable to with and without the RDO-194 tweeter. The addition of the grill attenuates the high end about as much as switching from the SL2000T to the RDO-194.

inspiredsports

Help me out here; I thought these were dead on 09/05/2010 when you posted . . .

http://www.polkaudio.com/forums/showthread.php?t=106357

Now you have both versions of tweeters, and have recapped and burned in to the point you can present technical measurements ???

heiney9

So the tests posted in this professional review of the 1C a fairly close cousin to your 2B's weren't enough? They don't seem to have the same findings as you. In fact the findings are pretty far off when you read the "Lab Results" compared to your "discussion". I'd say your method is suspect.

http://www.polksda.com/sda1creview.shtml

I'll trust my ears and my gear.

But knock yourself out.

Joe08867

I thought the tweeters you had were all bad also? Your posts are looking more and more suspect.

Get the story straight already!!

heiney9

You say there is not a huge difference in the two tweeters? You should try listening to them. There is a HUGE difference.

Have you tried to listen to the speakers for an extended time period to see if what you hear actually sounds like waht you measured or are you one of those who plots and reads graphs to decide how something sounds?

If you are the above, that's fine..............it takes all kinds, but many here prefer to listen over reading and plotting graphs.

You should read my signature, if you haven't already. I am a FIRM believer in that statement!

H9

Conradicles

Nice speakers...good luck with the sale.

Face

jcandy wrote: »

New versus old:In this plot we compare the RDO-194 (blue) against the SL2000T (orange) with the speaker grill on. This was done by keeping the microphone in the same place, with all the amplifier and gain settings fixed. Obviously, we expect the curves to be identical below the crossover frequency.

Results: Honestly, there is not a huge difference In my mind, the sound of the speaker ought to be dominated by the uneven SPL in the crossover region. Other than that, the SL2000T is 0-2dB brighter in the 500Hz to the 12kHz region. The differences beyond 10kHz are likely inaudible to most.

You're only telling us part of the story by only measuring SPL, how about distortion? Besides, 1db in either direction is audible, hence where the unit came from.

inspiredsports

Conradicles wrote: »

Nice speakers...good luck with the sale.

But won't he have to come up with a cure to magically flatten out those SPL graph lines to get maximum $$ ?

jcandy

Joe08867 wrote: »

I thought the tweeters you had were all bad also? Your posts are looking more and more suspect.

Get the story straight already!!

I have 4 sets of tweeters, 2 bad. Original (1990), first replacement (1997), second replacement (1999) and Tuesday. The second replacement set, although they were ordered in 1999, just turned up on Tuesday.

headrott

jcandy wrote: »

The second replacement set, although they were ordered in 1999, just turned up on Tuesday.

That's a hell of a long shipping time isn't it?

Greg

Pauly

Conradicles wrote: »

Nice speakers...good luck with the sale.

Chuckle Chuckle

Pauly

inspiredsports

jcandy wrote: »

I have 4 sets of tweeters, 2 bad. Original (1990), first replacement (1997), second replacement (1999) and Tuesday. The second replacement set, although they were ordered in 1999, just turned up on Tuesday.

Hmmm, 4 sets.

I popped one SL2000 in 1988 (with the same NAD equipment your uncle had) when I was young and dumb.

4 sets . . . somethings very, very wrong here.

jcandy

Face wrote: »

You're only telling us part of the story by only measuring SPL, how about distortion? Besides, 1db in either direction is audible, hence where the unit came from.

Yes, its true that SPL curves are only part of the story. I could do a distortion sweep if there is interest. Polar plots are important too, but both polar and distortion plots are tougher to interpret.

heiney9

I'd be curious to know how much break in time the RD0194-1 tweeters have on them before taking measurements. You should measure again after about 100-150 hours since that's when they really start to come into their own.

H9

inspiredsports

jcandy wrote: »

Yes, its true that SPL curves are only part of the story. I could do a distortion sweep if there is interest. Polar plots are important too, but both polar and distortion plots are tougher to interpret.

Back to a point made thousands of times here.

It's binary.

They sound good or they don't.

jcandy

inspiredsports wrote: »

Hmmm, 4 sets.

I popped one SL2000 in 1988 (with the same NAD equipment your uncle had) when I was young and dumb.

4 sets . . . somethings very, very wrong here.

Two sets were "popped" over a decade. Much of the use was my aunt listening to AM talk radio, if you can believe that. My uncle doesn't really like music. It stranger even that you think.

sda2mike

this geek's favorite recording is probably some esoteric test cd.:rolleyes:
most enjoyed while contemplating graphs, charts and curves of several speaker design he'll never listen to..sell those 2b's to a good home...soon!

michaeljhsda2

The OP's drivel sounds similar to that of candylicker35 or whatever his screenname was. Coincidentally or not he has the word candy in his screen name.

Joe08867

michaeljhsda2 wrote: »

The OP's drivel sounds similar to that of candylicker35 or whatever his screenname was. Coincidentally or not he has the word candy in his screen name.

It's funny you mention that. I was looking up some of Candliquor's posts to see if they were similar yesterday.

The only difference is this Candy has graphs to show his work.

Both seem a bit looney tunes to me.:rolleyes::p

F1nut

jcandy wrote: »

The addition of the grill attenuates the high end about as much as switching from the SL2000T to the RDO-194.

That alone proves that whatever you're doing, you're doing wrong.

Off to the list.

jcandy

heiney9 wrote: »

I'd be curious to know how much break in time the RD0194-1 tweeters have on them before taking measurements. You should measure again after about 100-150 hours since that's when they really start to come into their own.

Tweeters do not normally change significantly with break-in. Driver break-in is normally suggested with woofers and mids (when there is a surround and a spider), and this is usually leads to a drop in fs. If you drive a woofer hard then measure the T/S parameters, fs will drop, but if you let the driver "cool off", fs will return largely to its previous equilibrium value. The notion that 150 hours is required for a tweeter to somehow "sound right" is nonsensical.

jcandy

F1nut wrote: »

That alone proves that whatever you're doing, you're doing wrong.

Off to the list.

Grill diffraction is well-known and uncontroversial:

http://www.rjbaudio.com/Audiofiles/grill%20frame%20diffraction.html

heiney9

jcandy wrote: »

Tweeters do not normally change significantly with break-in. Driver break-in is normally suggested with woofers and mids (when there is a surround and a spider), and this is usually leads to a drop in fs. If you drive a woofer hard then measure the T/S parameters, fs will drop, but if you let the driver "cool off", fs will return largely to its previous equilibrium value. The notion that 150 hours is required for a tweeter to somehow "sound right" is nonsensical.

Sorry but I strongly disagree, all components change as they are "run in". Sorry if your plots, graphs and microphones aren't able to show that. Again, that's why I use my ears and brain to interpret.

Your complete faith in measurements is nonsensical. Measurements, even if done correctly, (which I suspect your's are flawed) don't begin to tell the whole story.

Again, that's why I trust my ears.

H9

jcandy

sda2mike wrote: »

this geek's favorite recording is probably some esoteric test cd.:rolleyes:
most enjoyed while contemplating graphs, charts and curves of several speaker design he'll never listen to..sell those 2b's to a good home...soon!

Geek? Have we arrived at the name-calling and ad hominem stage already? The problem is that, as nice as they are, the SDA2Bs are not some kind of magical entity, nor are they "spectacular" by any stretch of the imagination. The coloration was obvious after a few minutes of listening, and is now corroborated by objective measurement. They are impressive speakers, great for their era, but with an obvious and non-negligible problem with the mid-tweeter crossover. Compare the simple SDA crossover to this, for example:

http://www.zaphaudio.com/ZD5.html

jcandy

Joe08867 wrote: »

Both seem a bit looney tunes to me.:rolleyes::p

You guys seem awfully touchy. Its almost like a religious cult here.

heiney9

That's cool that you don;t care for them. Time to sell them and move on. Or are you going to spend your time on CP trying to convince all of us that actually like the SDA's that they are not all that great and we should buy something else?

Seriously, you've made your opinion known, it's time to move on otherwise your other motives will become quite obvious and the troll calling some are referring to will be quite obvious. Or you are trying to get 100 posts so you can list these for sale.

Agree to disagree and move along.

H9