It's the sound of your tweeters destroying themselves :-(
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"Also, has the 4-5 times I've heard this crackling done permanent damage to the speakers?"
Probably not, at least not yet, as things appartently sound right at medium loud levels, but if it was me, I wouldn't go past the 11 or 12 o'clock position as you might be starting to drive your amp into "clipping" once you past that 12 o'clock position and "clipping" WILL destroy tweeters.
Now if you routinely like to "crank it up", then you might want to think about getting some speakers with not only higher sensitivity (87 db is probably a little low, you might want get speakers with 90 db sensitivity or better), but also speakers that have can handle higher power levels.
If they don't seem to sound different you've probably done no damage, but repeating that exercise will surely kill them eventually.
I've done it twice to my own pair of Arro's, fortunatly with no negative long term effects.
In one instance I was driving them too hard, in another I carelessly switched sources with the volume up(I NEVER did that again!)
Here's an interesting article that explain amp power and how it relates to speaker sensitivity.
From just reading it, and putting number in the calulator (included in article), that I'm thinking that because of the Arro's 80 watt max program rating, that you shouldn't be driving these speakers beyond 95 db SPL which is loud for pop music, but not necessarly loud for rock music, no matter what amp you're driving them with.
Btw the numbers I used in the calulator were as followed:
Measured Distance from Listener to Speaker - 2 Meters
Desired Sound Pressure at Listener - 95 db SPL
Speaker Sensitivity Rating (1w/1m) + 5db - 86 db
Desired Amplifier Headroom (3db min) - 4 db
which gave me
Amplifier POWER Needed - 80 Watts
Beyond 95 db SPL you're overdriving what the Arro's stated Max Program Rating is.
A related question: Do overmodulated upper midrange notes always suggest speaker damage? I've got two pair of speakers that I've been swapping in- and out for a few weeks now, and they both make the same rattly / overmodulated distortion when playing certain passages of piano music, especially piano music just above middle-C. I don't think it's the recordings because the sounds don't seem to track across the two sets of speakers -- but it does seem an awfully specific type of dissonance to be mechanical failure of one or more of the drivers. Shouldn't I hear the same distortion with other instruments?
For clarification....Tweeters are pretty tough, so it probably is not the music volume per se that is causing the problem. Most likely it is the TOTAL signal level, including bass which never gets to the tweeter, which is causing your power amplifier to "Clip". Cliping distortion generates a lot of high frequency signal which was never part of the original music signal. Cliping kills tweeters.
Eldartford - Can clipping occur even when the amp's rated power output exceeds the max program rating of the speaker? I thought clipping occurred when the amp "ran out of power" and sent a damaging "flat top" sine wave signal to the speakers. Seems like clipping would be more likely where the amp was "underpowered" relative to the speaker it was driving. Happy to be enlighted on this point if I'm in error.
Sandman 012 - I've used Peter Gabriel's "So" CD and its next to last track (We Do What We're Told) as a helpful diagnostic for a blown tweeter. The opening has some high frequency stuff that will cause a "buzzing" distortion from a damaged tweeter. Need to get your ear in line and close to the tweeter perhaps - but if the tweeter is bad it will be noticeable at moderate sound levels. If you have this CD, worth checking...might relieve some concerns for you if the track plays on your system without breaking up.
For those worried about distortion this is quite normal...the tweeters used in most two ways will compress at high levels. 95 db SPL at the listening position is a bit much for most consumer designs. Piano is a good way to hear it becuase the piano is a percussive instrument. Thi susally does not damage the tweeters...provided you don't over do it. Many people listen to this type distortion for years without realizing it....they just think the music is too loud for their ears and do not realize it is the speaker.
Ghosthouse...You are correct that "Clipping" refers to the "clipped off" (square) tops and bottoms of the music (or sinusoidal test signal) waveform which occurs when the waveform peak voltage exceeds what the amp can put out. However, the speaker manufacturer's recommended power amp rating has nothing to do with how much power you can apply to the speaker. The manufacturer is telling you that higher power may result in damage. It would be easy to clip a 100 watt amp driving these speakers. When you do that, an unusual proportion of that power is at high frequency and gets routed to the tweeter.
Most dome-type tweeters can take some punishment, although I have succeeded in blowing out several. The ribbon tweeters such as used by Magneplanar are more fragile, which is why you never want to use a low power amp with them.
What destroys tweeters is almost invariably amplifier clipping. The tops of the waveforms are clipped off by a lack of voltage in the power supply, and the result is a squarewave or(almost)DC arriving at the tweeter that fries it. That happens when you're trying to get more power out of the amp than it can deliver. What you've probably done is exceed the linear range of the woofers(driven the voicecoil out of the magnet structure or bottomed it out in the magnet structure). If they still sound good at lower volumes, don't sweat it! I've done the same thing to my subwoofers with some sloppily recorded source material from time to time. If you want higher SPLs: Get more efficient speakers and/or some that can take the output of your 100wpc amps.