Most interesting piezo test

[gdougherty]

it's ~20db down from signal level.

If you can't hear it forget about it. Down 20dB is insignificant.

My worry is that I have heard things in the past. My most recent install build with melded arrays got damping material behind the cone and waterproof treatment. Tweeters in that have been beautiful and problem free. The mobile rig with 4 OT12's occasionally gives me problems and it's on clean piano patches in a certain note range or vocals (especially female) where the problem comes through. What I hear sounds like a higher frequency distortion and despite it being down so far, that's eerily similar to what I'm seeing. I've been relying on my eyes and the RTA to pick up repeatable spikes that shouldn't be there more than my ears so I'm unsure as to whether I'm imagining I can hear it but I do know it's there and shouldn't be.
The real issue right now is that I either need to salvage some of these and bring them up to spec, or buy more because I've come up about 10 short of the 112 I need. If I can spend a bit of TLC and improve the overall sound anyway, it seems worth the test.

[gdougherty]

So, tested and found an intact horn I'd set aside with pretty major upper harmonics in the 5-6K range.
Removed the element and applied a coating of rubber cement to dry around the edge of the horn where the element mates to it.
Tested again, still had spikes.
Pulled it, applied more rubber cement to ensure the seal was good and crammed some polyfill behind the tweeter cone using a long wooden skewer to push it in. Polyfill is moderately packed in so it's tight but not bursting out at the edges.
Tested again, no harmonic resonances, clean signal from 2.5-16K.

Because these are for outdoor use I think I'll also apply some of the Helmsman clear waterproofing I used on my install build. No reason to ruin tweeters in a rainstorm. I'll see what I can do about getting video up of the process sometime.
http://picasaweb.google.com/georgedman has a pic in the audio folder of where the polyfill goes in the piezo.

[LelandCrooks]

So, tested and found an intact horn I'd set aside with pretty major upper harmonics in the 5-6K range.
Removed the element and applied a coating of rubber cement to dry around the edge of the horn where the element mates to it.
Tested again, still had spikes.
Pulled it, applied more rubber cement to ensure the seal was good and crammed some polyfill behind the tweeter cone using a long wooden skewer to push it in. Polyfill is moderately packed in so it's tight but not bursting out at the edges.
Tested again, no harmonic resonances, clean signal from 2.5-16K.

Because these are for outdoor use I think I'll also apply some of the Helmsman clear waterproofing I used on my install build. No reason to ruin tweeters in a rainstorm. I'll see what I can do about getting video up of the process sometime.
http://picasaweb.google.com/georgedman has a pic in the audio folder of where the polyfill goes in the piezo.

Dude, I thought I was anal just testing them.

More power to ya!

[gdougherty]

Dude, I thought I was anal just testing them.

More power to ya!

Yeah, but you have a good stockpile and people paying for the testing (unless you're building for you). I'm stuck with the ~130 I have on hand or spending more time and money getting others in. Plus, if it improves these, there's the possibility that it'll improve the non-problematic ones as well in otherwise typically unnoticed ways.

[LelandCrooks]

Yeah, but you have a good stockpile and people paying for the testing (unless you're building for you). I'm stuck with the ~130 I have on hand or spending more time and money getting others in. Plus, if it improves these, there's the possibility that it'll improve the non-problematic ones as well in otherwise typically unnoticed ways.

If I came off as harsh I didn't mean to. I love tweaking stuff. I've seen these mods before on piezos. There's a webpage somewhere devoted to it. You're correct, about the time involved, nobody's going to pay for that kind of time intensive labor.

[gdougherty]

Yeah, but you have a good stockpile and people paying for the testing (unless you're building for you). I'm stuck with the ~130 I have on hand or spending more time and money getting others in. Plus, if it improves these, there's the possibility that it'll improve the non-problematic ones as well in otherwise typically unnoticed ways.

If I came off as harsh I didn't mean to. I love tweaking stuff. I've seen these mods before on piezos. There's a webpage somewhere devoted to it. You're correct, about the time involved, nobody's going to pay for that kind of time intensive labor.

No, not harsh at all. Just noting that economics are driving my anal retentiveness in this case. If I'd ordered 160 or more and still had a bunch of good ones to pick from I might not bother.

[Rick Lee]

So, tested and found an intact horn I'd set aside with pretty major upper harmonics in the 5-6K range.
Removed the element and applied a coating of rubber cement to dry around the edge of the horn where the element mates to it.
Tested again, still had spikes.
Pulled it, applied more rubber cement to ensure the seal was good and crammed some polyfill behind the tweeter cone using a long wooden skewer to push it in. Polyfill is moderately packed in so it's tight but not bursting out at the edges.
Tested again, no harmonic resonances, clean signal from 2.5-16K.

Would you recommend this procedure for all of the piezos? I don't have software for testing and I'm planning on building 4 DR250s or 280 in the near future.

[gdougherty]

So, tested and found an intact horn I'd set aside with pretty major upper harmonics in the 5-6K range.
Removed the element and applied a coating of rubber cement to dry around the edge of the horn where the element mates to it.
Tested again, still had spikes.
Pulled it, applied more rubber cement to ensure the seal was good and crammed some polyfill behind the tweeter cone using a long wooden skewer to push it in. Polyfill is moderately packed in so it's tight but not bursting out at the edges.
Tested again, no harmonic resonances, clean signal from 2.5-16K.

Would you recommend this procedure for all of the piezos? I don't have software for testing and I'm planning on building 4 DR250s or 280 in the near future.

It's a fair amount of work and in these few circumstances it certainly made an improvement on the RTA plot. As Bill noted, the harmonics are further down. In some cases the rubber cement sealing seemed to be enough and in others it seemed like the polyfill helped. I'd found the polyfill idea on a piezo tweaker's posting and they claimed it made a substantial difference. The idea is similar to other things I've seen, most notably the B&W Nautilus tweeter implementation where the tweeter is in it's own teardrop shaped housing with damping material packed in behind the tweeter.

Given the variations in the piezo tweeters, I can't definitively say that I've heard any major difference in sound. It makes a visible difference on the RTA in some cases and so I've chosen to go ahead and put the effort in. I'm also putting a coat of waterproofing on the tweeters and the 2512's though you wouldn't need to worry about that in a DR like you would an Omni speaker variant. OTOH, I've never claimed to be a golden eared listener, and I'm unfortunately not one of those sound techs that can hear feedback or ringing and tell you what frequency to cut. I hear and know well enough to put together a good mix but beyond that I haven't developed my ear.

[mloretitsch]

I just tested 48 in a row for upcoming ot12 builds. With great interest I have followed this thread

I use a two step process...I play a music track with piano and vocals first and listen for any harshness or oddities, then i use TrueRTA to sweep them quickly with a condensor mic.

The ones that sound really good to me have the mentioned smooth curve and response to 2khz. The harsh ones show a heavy spike at 3-4k and another at 7-9k and little response below 3khz. I would say only about 5 of the tested units have the nice smooth extended response however.

I will have to try the polyfill trick for fun on my discarded ones and see how it goes. I allowed most of the ones to stay in the batch only tossing the ones with really steep spiked and one that had almost no output over 7khz.

Much like any other driver, there are some fairly wide manufacturing tolerances.
-Matt

[LelandCrooks]

The harsh ones show a heavy spike at 3-4k
-Matt

That will blare at you with pink noise in comparison to the right ones. Find one that's good and use it for a reference. Real quick once you get used to the sound you're looking for.