I believe this is true even if you don't think you can hear that high; I believe that human auditory processing uses higher frequencies even if it seems that we can't hear them. There is published research that shows we are affected by inaudible high frequencies.
After resetting my supertweeter level from +10dB to +15dB as some measurements suggested I should do, I had a glorious night of listening to pop and rock recordings such as Grouse "We Want to be Loved" and Klaatu "Hope" and Genesis "Ripples" (I never really dug that one before but pulled it up by accident) and "Entangled". The clarity, high definition, impactful bass, rhythm, intertransient silence, palpability, liquidity, and continuousness were incredible. Background words were clear as day, bells and other metal objects sounded real and not harsh or overwhelming.
Just as a friend of mine has said, good highs will do more good for the bass than anything. Also, extended extreme highs generally do not cause "harsh" sound, they actually generally facilitate "smoother" less fatiguing sound. It had previously been similarly exciting when I raised the tweeter level from +2dB to +10dB a couple of days ago, I noticed similar effects, but less clearly.
Was it too much, tooooo tooooo much perhaps? Maybe, I'm not sure yet. But I think that if I have not found exactly the correct settings yet, I have found a mark I want to have on the dial, to bring back whenever I want the "twice as real" sensations.
*I once worked for an audio storeowner who often described audio systems...either as criticism or compliment...as "larger than life and twice as real." Whether this was praise of an awesome system or condemnation of an editorializing system depended on the context. In this case, I like my "double reality" so far, but this could change...I've had lots of experiences getting fatigued by high frequency sounds in the past, it's one reason I prefer vintage Acoustat speakers to more modern and obtainable Martin Logan. I have often found Martin Logan speakers to be fatiguing, never so with Acoustats, though the Acoustats conversely seem to lack the nth degree of transparency, which is why I have gotten interested in experimenting with supertweeters in the past two years.
Before getting too concerned about the high supertweeter output level, note that even with this level I was rarely getting one LED on the crossover to light up, and I believe my Elac supertweeters are capable of handling hundreds of watts. Also, to help keep noise down, I have either a 6dB or 10db attenuator on the supertweeter amplifier (an Acurus A250). Can't remember which I'm using right now, but I think it's still the 6dB attenuator. The midrange amplifier level is +3.4, and I think the midrange amplifier has more gain, though the midrange speakers are far less efficient. The Elac tweeters can be crossed over as low as 8Khz, but I'm crossing them at 14.8Khz (plus built-in crossovers). The tweeters are rated to 35Khz and have essentially flat response from 8-35Khz. I think everyone needs good omni ribbon tweeters, and I'm keeping mine, thank you very much, one of my most amazing Audiogon finds. Unfortunately, these days it seems most audiophile are too busy counting their tourmaline cable elevators to bother with supertweeters.
Right now my sound measuring devices are not agreeing on whether this level is correct. My iPhone RTA application is insisting that I've lost my mind, showing 18Khz now about 15dB too high. On the other hand, I have some real but long out-of-calibration Class A sound measuring devices, and they seem to be suggesting if anything not going far enough.
It was using my cracked-glass Bruel & Kjaer 2203 with a 4165 microphone (20Khz free field response) that gave me the inspiration. It seemed to suggest that above their last high frequency resonance around 13.7khz, the Acoustats fade away far too quickly. I tried crossing over the supertweeters at various points, but have now ended up crossing over at 14.8Khz as it seems to prevent the supertweets from contributing significantly to the that last Acoustat resonant peak. With 14.8Khz, there is no serious dip as I roll the oscillator from 13 to 16Khz, yet generally both speakers do not play with the same frequency tone, it's either one or the other, which is very desireable. (However, these tests were done with the +10dB level and now have to be repeated for the +15dB level.)
But whatever level I tried, it didn't seem like I was getting enough output at 16Khz, 20Khz, 22Khz, whatever frequency you care to measure. In that band, where I can still get flat measurements, it seemed like the +10dB setting was still underpowering the supertweets about 6dB below the highs in the 10-13Khz range coming from the Acoustats. Usually when confronted with this sort of data, I seek to split the difference and try something like +12.5dB. But this time, I decided to go all the way that I can go easily, by setting the DCX output level boost to +15 dB, the maximum it allows. That seems to work very well, see subjective comments above, though by measurements with the 2203/4165 combo, I could still add another couple dB to the top end (not so easy to do without changing some equipment) to make it better.
I also have a General Radio meter, one of their last and best ones, and it shows a similar pattern. Surprisingly, the 1" GR electret microphone seems to have even better "high frequency response" than the Bruel & Kjaer, and seems to show I now have flat response to 24Khz. The GR microphone is a random incidence type, not really supposed to be used for measuring speakers (or anything on-axis) but rather backround noise. However, this actually means that, on-axis, it has boosted high frequency response. Used as a random incidence microphone, the GR would only be good to about 16Khz. But it's not clear what the readings actually mean since I am not using this instrument "as directed".
So the instruments don't agree, or I'm not sure exactly what they are saying, but my ears say "don't forget how to do this, it's magic."
It has been incredibly hard to use my Bruel and Kjaer. Possibly my unit itself is flaky (I have two other units that are even worse) and also it turns out to be somewhat difficult to measure super high frequencies in the far field because of beaming, reflections, and interference effects. The meter is constantly moving over a 10dB or greater range as you move the meter slightly or change the frequencies slightly. So it's very hard to pin down my observations to concrete numbers, but as I am scanning the oscillator up and down I can get an idea of where the relative peaks and nulls are.
But that meter does seem to tell me that the acoustats roll off quickly above 14Khz, and I think they shouldn't do that. Possibly they need to have their internal coupling capacitors replaced.
I believe my hearing still goes to 16Khz, but possibly only at quite high levels, I was having trouble hearing above 14.5k from the listening position. But I could clearly tell when the supertweeter was extending the response; with the supertweeter switched on it didn't sound like the response was dropping dead above 14khz, but extends up higher, exactly where is not clear since my hearing is rolled off. Right next to the supertweeter, if i move my head I can hear sonic interference effects up to 16Khz or so.
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