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Old 22nd December 2007, 08:23 PM   #1
marec is offline marec  United Kingdom
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Default Driver break-in

I've never been a voodoo believer since the excesses of the 70's. However, recently I decided to build MJK's latest open baffle project and ordered a pair of 108EZs. One of these turned out to be faulty and had to be returned. I thought I might as well "break in" the driver I had, and fed it a signal of ~ 20Hz at 2.8 volts (thanks Scottmoose...). After a couple of hours, the driver started bottoming. I reduced the signal level. After another hour, the driver was bottoming again. When the replacement driver arrives, I'll measure it's Fo before and after break in. At the moment, it appears that something is definately happening, but whether the driver changes will be apparent throughout the audible range, I don't know. I wish I'd bothered listening before and after, but i always think that comparisons separated by minutes, never mind hours, can be deceptive... However, with a "before" and "after" driver, there's opportunity for all sorts of entertainment!
Wonder if the same thing will happen with the 15 alphas that have been lying around for months?
Rgds
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Old 22nd December 2007, 10:10 PM   #2
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Probably.

Break in isn't a myth, or not entirely anyway. It can be over-rated, and applied to ludicrous things (wire anyone?). In the case of FR drivers, it certainly exists. You've got a driver with very stiff suspension (relative to, say, your average mid-bass), a light cone & powerful motor, fresh out of the box. It will take a while for that suspension to loosen up at least, and the fibres used in paper cones also mature over time. How significant it is depends on the driver, and also on the manner of loading. Got it in a horn, where excursion is tightly controlled, and break-in takes longer than if in a cabinet where the suspension gets more of a work-out. Etc.
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Old 7th January 2008, 11:47 AM   #3
marec is offline marec  United Kingdom
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Thanks for that...
I tried with a 15a.
Fs (Fd, F0?), out of the box =46.5 Hz.
I fed it 8 volts at 20Hz for 48 hrs. Result, Fs = 45Hz.
I seem to remember a post recommending break in at Xmax. How do I do this? Do I just drive until the voice coil hits the stops and then back off a bit?
I'm still waiting for my second 108ez to arrive so that I can measure & (and listen!) before & after....

Incidentally, I've got some bamboo flooring strips that I was going to create the MJK OB with. However, the stuff rings like a bell. I was thinking of damping this with a layer of MDF attached with 'No more Nails' or something similar. Is there any experience of using this material i can tap into please?

IIRC Planet 10 has built enclosures from bamboo. Any comments?
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Old 7th January 2008, 01:20 PM   #4
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Ah. The old resonance issue.

Something that people often forget is that it's often not how severely a material resonances so much as where it resonantes. If a material's (primary) resonant frequency occurs at a point where a box / whatever is working hardest & putting out a lot of energy, you're in trouble. If it occurs elsewhere, where comparatively little energy, which might excite it, is produced, then it's basically not much of an issue. A light, stiff material also releases energy quickly rather than storing it & slowly letting it go.

Go easy on the 108s. Just play some rock music or similar through them at moderate volumes for a few days to loosen things up.
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Old 7th January 2008, 02:50 PM   #5
marec is offline marec  United Kingdom
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Re-reading my post and thinking about my casual use of 'ringing like a bell' set me thinking.
If it is bell like, does this imply high Q, low frequency spread? If so. notch filter?
I got out a couple of the strips. It's not yet a board! Holding the strips between two fingers on the edges and rapping with a knuckle ( Oh, this is soooo scientific...), reveals a musical instrument. There is the loud noise of the initial impact, which isn't an identifiable note (at least, not on my piano). I imagine, then, that this is a result of many frequencies. It is of relatively short duration, but longer than the same impact on an MDF strip.
There is also a much longer duration, lower frequency and purer note which isn't generated when the strip is suspended at it's end. Several strips tested have different short duration notes, but similar long duration notes. I guess this is related to the length of the strip. Strong harmonics are generated when supporting the strips at various positions on it's edges.
The amplitude of these concerns me. What, I wonder will happen when I glue them together as a board? Should I perhaps only glue them at an end? Or both ends, suppressing the observed reonance?
Isn't building even the simplest of loudspeakers fun when you are so ignorant????
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Old 7th January 2008, 03:17 PM   #6
marec is offline marec  United Kingdom
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Please note that I've answered one of my own stupid questions. Securing the strips at one end does remove the musical thrum, but replaces it with a most unmusical rattle...
I can hear the "Of Courses" from here....
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Old 7th January 2008, 03:29 PM   #7
marec is offline marec  United Kingdom
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How sad is someone who posts to their own thread when no-one else does?????????
Scottmoose: Thanks, If low storage = low mass and if, in the case of an open baffle, preventing the backwave from reaching the front wave is a small issue (given the distance from a reflecting boundary...), should the baffle material for an OB be as light as possible commensurate with supporting the drive units? One could effectively decouple the drive units from the baffle, reducing the direct transmission of vibration from the chassis.
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Old 7th January 2008, 05:54 PM   #8
Kensai is offline Kensai  United States
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I've used cheap 12"x18" bamboo cutting boards for OBs at my desk with great success. I've been looking at the bamboo flooring as a method to make larger baffles from locally available materials (think I could get 2 pretty darned large ones from a single $70 carton of the thick, solid bamboo stuff, available in natural or caramelized). I'm of the opinion that if I can't get it done in bamboo, somehow, I'm not likely to build it (and that applies to anything). The resonance of the material should change significantly as you glue pieces together, from the increased width and the mechanical interfaces between the pieces. Good luck and let us know how it turns out.

Kensai
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Old 7th January 2008, 06:46 PM   #9
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Well now, an interesting question re the lightness etc. This might interest you: http://myweb.tiscali.co.uk/nuukspot/...speakers2.html
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Old 10th January 2008, 10:34 AM   #10
marec is offline marec  United Kingdom
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Well,an interesting few days....
Having poured 1/2glass of perfectly good Crianza into my keyboard, the laptop started typing even more rubbish than usual.I've never had a laptop apart before, so started with every visible screw on the base.
This revealed all sorts of interesting bits,but after several hours I seemed to be no closer to the keyboard! Fortunately a kindly soul borrowed a kitchen knife from our drawer, flicked off a bit of plastic trim from the top and voila, keyboard falls out! A good wash and blow dry and (nearly) as good as new. I wonder where those four screws on my desk came from?

Anyway, the replacement 108 turned up, so I stuck them in a couple of small OB's.

OMG.... What have I got here? I really can't tell much difference between the one that's had the abuse and the brand spanker. In fact I can't get past the banshee like noises they make to even make a comparison. 10 second headache!

Out for lunch I think and see what happens after a couple of hours!

Lowthers were never like this.....
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