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Multi-Way Conventional loudspeakers with crossovers

SB29RDC-C004 strangeness
SB29RDC-C004 strangeness
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Old 13th October 2012, 08:20 PM   #1
tvrgeek is offline tvrgeek  United States
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Default SB29RDC-C004 strangeness

I started doing some measuring for my next build. I am going to do a budget pair on SB drivers.

I ran a quick impedance test on the tweeter. How strange. WAY off spec. So, burned it in for a day with music. Let it cool and well, the Fs did drop 40 Hz. Better, but that odd bump, and the spec is still way out of published.

A bit of fooling around, and I found the issue. It is the thin plastic back cup. If I held the driver tight against a corner, the glitch goes away and the specs drop to very close to published. Next to see how I can fix it.
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Old 13th October 2012, 09:21 PM   #2
tvrgeek is offline tvrgeek  United States
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First test, three layers of Dynamat on back of housing. Helped, but not as much as holding it. ( 630 Hz) I also noticed the flange is pretty flimsy and resonates at about the Fs. Bad news. It is worse in a baffle than it is sitting on the desk. (660 Hz) Next test, clean off the Dynamat and epoxy on a piece of MDF.

Why am I worried about problems at 600 Hz? Because I intend to put in a notch filter as I am trying a pretty low crossover.
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Old 13th October 2012, 10:37 PM   #3
Dissi is offline Dissi  Switzerland
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For comparison:

Review in HobbyHiFi 3-2010: Peak at 515 Hz, 10 ohm, symmetrical without any bump!
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Old 13th October 2012, 11:29 PM   #4
tvrgeek is offline tvrgeek  United States
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Epoxied a piece of 1/2 inch MDF to the back making the thin plastic rigid. Raised the resonance to 660. Mounted in a baffle, 676.

Now, what to think about this.
#1 the rear chamber is not well designed. If it as flexible enough to cause these differences, it would be susceptible to modulation from the woofer.

#2 To prevent #1, should be in a sealed sub-chamber.

#3 Well, I can't build a speaker with me holding the driver with pressure on it. Seems like the best result is with three layers of Dynamat. I an guessing the rear of the chamber is acting like a passive radiator, and the Dynamat changed it's mass, so it changed its tuning. What I can't explain is why so much better result with me holding it. I would have thought baffle mounting would increase the load on it and lower the resonance, but the opposite seems to be true.

#4 You need to measure the impedance in the box where you will use it. If I used the out-of-box, or worse, OEM spec, to build a notch filter, it would be way the heck off.

Dissi, Well I don't have that review. I only have one driver. Their numbers are equally far off. ( Spec being 600). As I have concluded the hump is a function of the design, so I can rule out a defect or damage. Could it be that somehow my suspension has gotten that much stiffer so it's resonance is in a range where the chamber back is revelant?

Well, I guess I can go set it up in the shop and play music for a week and see if it limbers up. I will say, I have not had a tweeter this far off before. Woofers yes, not a tweeter. As they say, stay tuned.
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Old 13th October 2012, 11:30 PM   #5
mondogenerator is offline mondogenerator  England
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youre talking about the rear and not the mounting surface right? if so maybe some 2 pack epoxy putty or P40 fiberglass would sort things out. Lead flashing?

EDIT. Your last post just beat me. So the whole rear chamber is flexing that much! Perhaps its removable, and a suitable replacement could be made?
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Old 13th October 2012, 11:52 PM   #6
tvrgeek is offline tvrgeek  United States
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I was just thinking of that. Everything is removable with a big enough hammer.

I could make one from wood, fiberglass or who knows. I notice the high end Scan Speak and the new high end Satori use odd shaped chambers. Something to sleep on.
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Old 14th October 2012, 12:14 AM   #7
5th element is offline 5th element  United Kingdom
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I am surprised that driver from a company such as SB shows something as pronounced as this. Now if I bought a pair and fs was 600Hz instead of the 500Hz spec I wouldn't lose any sleep over it, but the fact that the chamber is affecting the response in such a way is worrying. I do wonder if your driver has a defect, plastic chambers tend to be very solid.

Edit - having looked at the tweeter some more, the only thing I can think is that the chamber is slightly loose in places. Zaph's measurements have fs on 500Hz too, maybe it's a quality control or shipping issue?
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Old 14th October 2012, 12:57 AM   #8
tvrgeek is offline tvrgeek  United States
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Seems well attached. I'll know when I pop it open. It is about two years old but has been in a safe environment. I sure would hope they don't age and stiffen if not used. I'll set it up for a week long burn in tomorrow before I rip the chamber off. It is not a expensive driver, but I don't want to trash it for no reason. If it does change much, then there is further proof why you have to break in before designing the crossover.
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Old 14th October 2012, 02:47 AM   #9
mondogenerator is offline mondogenerator  England
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i would agree that break in could help. The domes im using have a quoted fs of 1.8k, but it took a good few hours for them to loosen up and smoothen. A few weeks of listening and no particular break in routine. Id still build up something, even just over the entire rear lol
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Old 14th October 2012, 11:38 AM   #10
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We will see what a week will do. I'll measure after every 24 hours.
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