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Dent in tweeter domes: Affect the sound a lot?
Old 23rd February 2012
  #1
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Dent in tweeter domes: Affect the sound a lot?

My dynaudio Bm5as tweeter domes have dents in them that cover about 1/6th the total size and only small.. but the tweeter domes arent that big to begin with..

does anybody know if that would affect the sound in a major way?
Old 23rd February 2012
  #2
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Get a small piece of blu-tack, heated up frictioning it with your hands. Gently apply to the dome and pull back.

PS: To answer your question, in theory, yes, absolutely. The domes have that design for a reason. To create a pressure zone that is equidistant to the epicenter of the dome, thus projecting in the desired direction. So if you triangulate correctly, the same signal from the L and R will be in phase at time of arrival.

In practice, unless major damage, no. If you can't hear anything, probably early reflections are taking care of masking the ever so subtle change in tone.

Dont stress too much, I'd say... I know its not very slutty of me to be saying this, but I doubt a small little dent will crash your work.
Old 23rd February 2012
  #3
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You could also use a drinking straw and carefully try to suck the dents out. Maybe use blu-tack as a seal.
Old 23rd February 2012
  #4
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i've tried everything but damn.. i take that dent out and then another one comes out of nowhere :(

i tried vacuum, blu-tack, sticky tape, straw, the sucker just wont completely go up... :(
Old 23rd February 2012
  #5
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Quote:
Originally Posted by Big_Bang View Post
PS: To answer your question, in theory, yes, absolutely. The domes have that design for a reason. To create a pressure zone that is equidistant to the epicenter of the dome, thus projecting in the desired direction.
Eh, not really. Dome drivers DO NOT pulsate as a sphere, they are pistons. There are many dome tweeters which are inverted, and they work just as well as those with the dome bulging to the outside. Thus the dome structure is just structurally advantageous as it keeps the light weight element stiff. Just like a conical bass driver is stiffer than a flat driver (for that reason there are no structurally flat bass drivers...). Or eskimo igloo is a semi-sphere, not a snow tube with flat top. Basically the radiating surface could be of any shape, and the sound pressure wave front is always identical. So, it there are dents on the dome it does not matter acoustically, but structurally there might be some breakup happening as the surface flexes, which is not a good thing, and they should be fixed somehow.
Old 23rd February 2012
  #6
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Yep.

I use a vaccum cleaner (even works for some not quite so stiff mid/woofer dustcaps).

Just had to do it the other day at my dad's house; my 1 yr old went poking all the domes/dustcaps in on the hifi there. we were not nearly as thrilled about it as he was.
Old 23rd February 2012
  #7
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Quote:
Originally Posted by Petrus View Post
Eh, not really. Dome drivers DO NOT pulsate as a sphere, they are pistons. There are many dome tweeters which are inverted, and they work just as well as those with the dome bulging to the outside. Thus the dome structure is just structurally advantageous as it keeps the light weight element stiff. Just like a conical bass driver is stiffer than a flat driver (for that reason there are no structurally flat bass drivers...). Or eskimo igloo is a semi-sphere, not a snow tube with flat top. Basically the radiating surface could be of any shape, and the sound pressure wave front is always identical. So, it there are dents on the dome it does not matter acoustically, but structurally there might be some breakup happening as the surface flexes, which is not a good thing, and they should be fixed somehow.
I see where you're coming from, but I don't entirely agree. Plus, I never mentioned the design.

The pressure gradient that projects the sound is entirely dependent on the radiating surface. I'm not saying that it should be either/or concave or convex, triangular or dodecahedron. But it should be symmetrical to radiate equidistantly, in parallel to te axis of the driver's movement. If not, the radiating axis shifts, and there is loss of transmittance and tonal representation.

But hey, all I am saying is that in pure technical terms, each monitor has to be the same, so a dent will alter the image. But at what extent? I seriously don't think a small dent will render "bad" sounding image, even in an anechoic chamber.

Furthermore, it is well known that materials pushing too hard create intermodulated distortion, that is the reason why crossovers were developed. Each speaker delivers optimal sound at given frequency range. So it is to be expected that a dent will influence negatively if the speakers are pushed hard.

So yeah, if you listen to a mastered track at full blast, I expect some sort of degradation. But at that level, to generate inter modulated distortion from materials, the audio track itself will sound crap... so we're back at the beginning!
Old 23rd February 2012
  #8
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It affects the dispersion pattern (at the high freqs only) more than anything else.

For mids and woofers it's more a cosemetic thing except maybe at the top of their output (crossover region with the tweet which compounds the issue and is the only time it's important for non-tweeters - depends on how high they're crossed over though obviously).
Old 23rd February 2012
  #9
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I would still say it does not effect radiation pattern, only causes unwanted dome breakups and flexure, manifesting as added distortion and coloration. The higher the SPL, the more. It might not be all that expensive to replace the driver if 1) you can hear it happen, or 2) you worry about it too much.

In woofers and mids dents also cause cone breakup = distortion and coloration. Nothing else, but undesirable anyway.
Old 23rd February 2012
  #10
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If that was true then what's all the bruhaha about with regards to phase plugs (vs regular dustcaps) on modern mids/woofers (the point of which is to have the top end response of the lower freq reproducer match the dispertion pattern of the tweet for better system integration and therefore less smearing in the crossover region)?

Basically the shape of the dome (or bullet or horn/waveguide etc) dictates the dispersion pattern and that is important (at least for detemining the size and shape of the sweet spot as well as avoiding lobing). This may not be that important in a nearfeild application, but definately so for mains, FOH (not that you see dome tweets here, but in theory) or hifi.

It will not affect frequency response (on direct axis) much if at all though, you're right about that.
Old 23rd February 2012
  #11
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There is a straight and exact relationship between the diameter of the radiating surface, frequency and the dispersion pattern (angle and lobing). Phase plugs and waveguides used in mid and high frequency drivers serve two purposes: to make the radiating surface virtually larger to match the radiating surface of lower driver at the crossover frequency thus keeping the radiation angle constant at crossover, and to improve the efficiency of the driver resulting in higher power at given displacement and thus less distortion. As surprising as it seems, a dome, flat driver, inverted dome and a cone working as piston all send out identical waveforms, a dent in the material does not have any effect on the dispersion pattern, only lessens the structural integrity of the driver; uncontrolled resonances = coloration and distortion.
Old 23rd February 2012
  #12
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Interesting; that wasn't my understanding at all.
Old 24th February 2012
  #13
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Quote:
Originally Posted by Petrus View Post
... a dent in the material does not have any effect on the dispersion pattern, only lessens the structural integrity of the driver; uncontrolled resonances = coloration and distortion.
I certainly get the second part, but the dispersion pattern not being affected?

Hold on there Petrus... Lets say I build two identical monitors. One I put in a typical dome, but on the other an irregular trapezoid, at a 45º angle from the driver axis.

Both will render an equal dispersion pattern ? I'm finding this very hard to grasp, but I am by no means a specialist
Old 24th February 2012
  #14
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Quote:
Originally Posted by Big_Bang View Post
I certainly get the second part, but the dispersion pattern not being affected?

Hold on there Petrus... Lets say I build two identical monitors. One I put in a typical dome, but on the other an irregular trapezoid, at a 45º angle from the driver axis.

Both will render an equal dispersion pattern ? I'm finding this very hard to grasp, but I am by no means a specialist
Yes. The dispersion pattern depends on the direction of the piston like movement of the surface alone, not the shape (assume a circular driver). If this was not the case, a dome, inverted dome and a cone would have different dispersion patterns, but the they do not. With identical radiating diameter (still talking about round drivers here) the pattern is the same for all for a given frequency. The reason the drivers are domes, "right side" up or inverted, or cones, is structural and has nothing to do with dispersion patterns. If you want to manipulate the dispersion pattern, you need to use waveguides, horns etc.
Old 24th February 2012
  #15
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Quote:
Originally Posted by Petrus View Post
Yes. The dispersion pattern depends on the direction of the piston like movement of the surface alone, not the shape (assume a circular driver). If this was not the case, a dome, inverted dome and a cone would have different dispersion patterns, but the they do not.
I was under the impression that a dome and a cone DO have different dispersion patterns though, at least in the upper registers where wavelengths are shorter.

Most speaker builders / audiophiles seem to think so too, and I know that's not conclusive so I am open to being educated here, but I just don't see how it's physically possible for the dispersion patterns to be the same. For example, wouldn't diffraction at least be at play here - there's a full 180 degrees of space for the wave to form with a dome, but much less (say 90ish) with a cone. This assumes that the point source of the wave is the centre of the cone (as is generally accepted to be the case, hence recessing tweeters for physical time alignment etc) and that the cone itself acts somewhat like a waveguide (again, only at higher freqs).

Granted, none of this helps with the dented dome affecting dispersion or not argument, but I'm willing to accept your claim on that score pending further thought/research.
Old 25th February 2012
  #16
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Dome and cone are not point sources, if we are talking about tweeters, they aim to work as a solid piston (in bass and mid cones there are ways to make the radiating surface smaller as the frequency rises = polypropylene cones, which try to keep the dispersion angle constant by absorbing the higher frequencies in the cone material). There is a simple formula tying the first order lobe angle to the radiating diameter and frequency, but I can not find it to show you right now (I have lost my trusty speaker design handbook by Martin Colloms, Amazon.com: High Performance Loudspeakers (9780471970897): Martin Colloms: Books).

Domes hold on to their piston-like behavior better than cones. All tweeters have the problem that the radiating surface gets too large for the frequency when we approach 20 kHz, causing a narrowing dispersion angle and alternating out-of-phase and in-phase side lobes. The radiating surface would have to approach (almost) zero to keep the dispersion angle constant.

Getting a bit off-topic from the dents...

Anyway, the driver surface area/frequency/dispersion angle triad is very fundamental in speaker design, extremely fundamental. Line arrays are one example, where a tall pile of drivers make the vertical dispersion angle very narrow, while retaining a normal horizontal angle. This is often beneficial, and for the same reason 2 or 3-way speakers are usually vertical, not horizontal, in all good designs.

That Collom's book makes a fascinating reading!
Old 25th February 2012
  #17
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IDK if they are silk domes or not, but I fixed one like that (silk dome) a few years back. IIRC I used a light vegatible oil and rubeed it on the tweeter with my finger...then a hair dryer while I gently massaged the dome. It eventually worked out.

The problem is the dent stretched the fabric, and soon as you pop out the dent, the dome won't hold it's shape, and creases somwhere else. Heating it mildy helped "iron" it out for me.

YOU HAVE TO BE SUPER-CAREFUL doing this though.
Old 25th February 2012
  #18
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I performed mouth to mouth resuscitation on a tweeter. I was trying to think of tools I could use to suck the pushed in tweeter out, and I just planted my mouth round it and gave it a go. I was amazed to see it worked!

Don't laugh, its true! It was a bit embarrassing to explain how I fixed it to the owner....
Old 25th February 2012
  #19
Any suggestions to pull out the dustcap on woofer? I have a couple monitors (about 8") with the woofers pushed in as well.
THX
Old 25th February 2012
  #20
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For dustcaps it depends - some are stiffer than others and most are much stiffer than tewwter domes (metal ones excepted obviously).

You can try a vacuum as I mentioned above, but if it's too stiff it won't do it. Often you have to take an exacto knife and carefully separate the dustcap from the cone (i.e like the first step in a recone job; but more carefully bc you're not tossing the cone) to get a finger/chopstick in there to push it out from the back side (or replace the dustcap entirely - there's usually supply shops in most major cities that will sell such speaker parts). I have sometimes also had success making a tiny pinprick-like hole in the dome (around mid-dent) so you can stick a slim metal hook or toothpick in there and force it back into shape.

Thanks for the patience and info Petrus!
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