I have long been disturbed by an 18kHz peak that appears in my iPhone RTA measurements of the living room system:
Testing over the past two days suggests I can't fix that peak, or make other significant changes, by simple improvements to the supertweeter crossover. From long experience I highpass the supertweeters at 17330 Hz and lowpass the Acoustats at 20kHz. The result is fairly flat with a slight peak in the response at 18khz. But it seems the peak is caused by my supertweeters themselves. (Or, it could be an artifact of the iPhone RTA measurement.) It is visible in the response curve of the supertweeters by themselves, or with just the Acoustats turned off, as in this RTA snapshot:
I also have capacitors on the supertweeters that reach -3dB at 40kHz. The effect of this should be to boost the HF response gradually in their 17kHz to 40khz bandpass region. But I always seem to see the reverse in my measurements, with response rolling off either at 20kHz or slightly higher, rather than going higher and higher.
These capacitors eliminate amplifier hum and protect the supertweeters. I could possibly increase their value so that the the highpass frequency is lower, taking them more "out of the picture."
I had always figured that the relatively rising response at the supertweeter amplifier inputs up to about 40kHz would be an advantage, a +8dB rise compensating for their decreasing energy transmission from beam narrowing. But it should also be re-investigated. It may be having counterintuitive effects caused by phase changes near the acoustic crossover.
I measure the exact same peak at 18kHz, with 16kHz and 20kHz lower, with the iPhone placed as close as possible to the supertweeter.
The listening position measurements from ARTA using my calibrated microphone are much less useful, or I haven't figured out how to give them the kind of detail I see with the iPhone RTA. I've been contemplating other devices, programs, and techniques to see what's going on higher up. In my vast collection of audio junk, I have a never used Mighty Mike, based on a 1/8 panasonic capsule, which I believe does have response to 40kHz. It's complicated, and the manual and calibration file are not at hand, if they ever existed. I'd have to figure out how to wire the 1/4 phono plug, etc.
Update: Additional testing on the RTA app suggests it has about 10dB rolloff at 20kHz, but is fairly flat below that. That somewhat changes the picture. Essentially the peak at 18kHz is not a peak. It's a reflection of the rising frequency response above 16kHz. In actuality 18kHz is NOT higher than 20kHz, it may be somewhat lower. The ARTA display, which shows response rising to just above 20kHz, and then falling, may be correct, or that measurement setup may have a similar problem (response falls off beyond some frequency) but just a higher cutoff.
To test the RTA app, I measured my Revel M20's. According to published response curve, the M20 is about 3dB down above 10kHz, but then has a slight peak just before 20kHz. On a smoothed response like the 1/6 octave RTA, I would expect 20kHz to be around 1-2dB down from flat and 18kHz to be about 2-3dB down from flat. Roughly the same. Well that is not what I measure a centimeter from the tweeter
I'm showing a drop of 12dB from 18kHz to 20kHz. That suggests 10dB of error in that step. Above that, there may be some error in 18khz itself, perhaps another 3dB, then it begins to match the published response down to 4khz or so where the effect of measuring right in front of tweeter kicks in.
So there was no peak at 18kHz, but an artifact of the rising response of the tweeter above 16kHz and the falling response of the RTA app with iPhone 8 Plus microphone. This could occur in any measurement system. My tweeters may have far more extended response than I can measure now. (Dynaudio specs showed D21AF having essentially flat response to 40kHz, a rare marvel of a cloth dome tweeter. I suspected this required microphone perhaps millimeters from the driver and on center, perhaps adjusted with calipers for best response. The wavelength at 40kHz may be half the dome width, so there is strong cancellation off axis, but all the better for a peak response right at the center.)
I would have assumed the RTA app had some generic calibration for the iPhone, or maybe even each iPhone model. Perhaps not.
Most likely, the real story is that both 18kHz and 20kHz on the living room system rising above 16kHz, and they might continue rising to 40khz if measured close enough and on axis.
I continue to believe that a rise in the on-axis response of a dome tweeter is a good idea to compensate for the falling dispersion causing less reflected energy. The current level setting reflects
10 years of tuning by ear and by measurement with a goal of both making it fairly flat, somewhat tipped up, and not actually minding if the ultrasonic response as measured this way or that is above the midrange, such rises are most often associated with better sound, unlike rises in other areas, such as around 4kHz, where a dip is generally adviseable.
But it looks wise to be wary of the accuracy of the top two bands in the iPhone RTA response. I am apparently already getting what I want, a rising resonse to 20kHz and somewhere above, without any further changes.
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| Left Channel with all LR8 crossovers |
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| Right Channel with all LR8 crossovers |
Testing over the past two days suggests I can't fix that peak, or make other significant changes, by simple improvements to the supertweeter crossover. From long experience I highpass the supertweeters at 17330 Hz and lowpass the Acoustats at 20kHz. The result is fairly flat with a slight peak in the response at 18khz. But it seems the peak is caused by my supertweeters themselves. (Or, it could be an artifact of the iPhone RTA measurement.) It is visible in the response curve of the supertweeters by themselves, or with just the Acoustats turned off, as in this RTA snapshot:
When I switched to 48dB/octave crossovers on both sides two weeks ago, the same crossover frequencies still seemed to be optimum. I have now reconfirmed that with some new measurements.
The Acoustat naturally rolls off above 17kHz very steeply, possibly 48dB/octave or more. I have to lowpass the supertweeters at 17330 Hz or lower, or there is a widening depression above 17kHz.
I don't actually need to have an electronic lowpass on the Acoustats since I am relying mostly on their acoustic rolloff. But the electronic lowpass I use at 20khz seems to make very little difference in the frequency response either when the supertweeters are running or they are not. It actually seems to improve the response slightly when the supertweeters are running, curiously reducing a depression at 14kHz. This suggests to me the 20khz lowpass is improving the curve of the 17kHz rolloff so the phase change more closely approximates the acoustic crossover at LR48. When the supertweeters are running, lowpassing the Acoustats has always made the sound better, I have always believed. I figured because it reduces the high frequency reactive loading on the amplifier, but it may be because frequency response is actually improved, or interference patterns reduced.
I don't actually need to have an electronic lowpass on the Acoustats since I am relying mostly on their acoustic rolloff. But the electronic lowpass I use at 20khz seems to make very little difference in the frequency response either when the supertweeters are running or they are not. It actually seems to improve the response slightly when the supertweeters are running, curiously reducing a depression at 14kHz. This suggests to me the 20khz lowpass is improving the curve of the 17kHz rolloff so the phase change more closely approximates the acoustic crossover at LR48. When the supertweeters are running, lowpassing the Acoustats has always made the sound better, I have always believed. I figured because it reduces the high frequency reactive loading on the amplifier, but it may be because frequency response is actually improved, or interference patterns reduced.
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| Both Channels, 19kHz lowpass on Acoustats, note broad 14k dip |
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| Both Channels, 17330 Hz lowpass, 14k dip more pronounced |
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| Both Channels, Lowpass bypassed |
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| Both Channels, 20kHz lowpass, flattest of all around 14kHz |
The 18kHz peak should be investigated at the tweeters, possibly removing the rears.
I also have capacitors on the supertweeters that reach -3dB at 40kHz. The effect of this should be to boost the HF response gradually in their 17kHz to 40khz bandpass region. But I always seem to see the reverse in my measurements, with response rolling off either at 20kHz or slightly higher, rather than going higher and higher.
These capacitors eliminate amplifier hum and protect the supertweeters. I could possibly increase their value so that the the highpass frequency is lower, taking them more "out of the picture."
I had always figured that the relatively rising response at the supertweeter amplifier inputs up to about 40kHz would be an advantage, a +8dB rise compensating for their decreasing energy transmission from beam narrowing. But it should also be re-investigated. It may be having counterintuitive effects caused by phase changes near the acoustic crossover.
I measure the exact same peak at 18kHz, with 16kHz and 20kHz lower, with the iPhone placed as close as possible to the supertweeter.
The listening position measurements from ARTA using my calibrated microphone are much less useful, or I haven't figured out how to give them the kind of detail I see with the iPhone RTA. I've been contemplating other devices, programs, and techniques to see what's going on higher up. In my vast collection of audio junk, I have a never used Mighty Mike, based on a 1/8 panasonic capsule, which I believe does have response to 40kHz. It's complicated, and the manual and calibration file are not at hand, if they ever existed. I'd have to figure out how to wire the 1/4 phono plug, etc.
Update: Additional testing on the RTA app suggests it has about 10dB rolloff at 20kHz, but is fairly flat below that. That somewhat changes the picture. Essentially the peak at 18kHz is not a peak. It's a reflection of the rising frequency response above 16kHz. In actuality 18kHz is NOT higher than 20kHz, it may be somewhat lower. The ARTA display, which shows response rising to just above 20kHz, and then falling, may be correct, or that measurement setup may have a similar problem (response falls off beyond some frequency) but just a higher cutoff.
To test the RTA app, I measured my Revel M20's. According to published response curve, the M20 is about 3dB down above 10kHz, but then has a slight peak just before 20kHz. On a smoothed response like the 1/6 octave RTA, I would expect 20kHz to be around 1-2dB down from flat and 18kHz to be about 2-3dB down from flat. Roughly the same. Well that is not what I measure a centimeter from the tweeter
I'm showing a drop of 12dB from 18kHz to 20kHz. That suggests 10dB of error in that step. Above that, there may be some error in 18khz itself, perhaps another 3dB, then it begins to match the published response down to 4khz or so where the effect of measuring right in front of tweeter kicks in.
So there was no peak at 18kHz, but an artifact of the rising response of the tweeter above 16kHz and the falling response of the RTA app with iPhone 8 Plus microphone. This could occur in any measurement system. My tweeters may have far more extended response than I can measure now. (Dynaudio specs showed D21AF having essentially flat response to 40kHz, a rare marvel of a cloth dome tweeter. I suspected this required microphone perhaps millimeters from the driver and on center, perhaps adjusted with calipers for best response. The wavelength at 40kHz may be half the dome width, so there is strong cancellation off axis, but all the better for a peak response right at the center.)
I would have assumed the RTA app had some generic calibration for the iPhone, or maybe even each iPhone model. Perhaps not.
Most likely, the real story is that both 18kHz and 20kHz on the living room system rising above 16kHz, and they might continue rising to 40khz if measured close enough and on axis.
I continue to believe that a rise in the on-axis response of a dome tweeter is a good idea to compensate for the falling dispersion causing less reflected energy. The current level setting reflects
10 years of tuning by ear and by measurement with a goal of both making it fairly flat, somewhat tipped up, and not actually minding if the ultrasonic response as measured this way or that is above the midrange, such rises are most often associated with better sound, unlike rises in other areas, such as around 4kHz, where a dip is generally adviseable.
But it looks wise to be wary of the accuracy of the top two bands in the iPhone RTA response. I am apparently already getting what I want, a rising resonse to 20kHz and somewhere above, without any further changes.
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