FFT spectra are usually crude. Even many apparent continuous displayed spectra are often based on small sample of some kind.
To really get down to the bare metal, you need to turn the oscillator knob by hand and take readings. Unfortunately, this is far easier said than done. At any given precise frequency, the level may fluctuate over a 20dB range just by moving a half meter or so. So which is the "correct" reading? (Here I can see, a warble tone generator would be very very nice...)
Anyway, I got the B&K Oscillator (cheap ugly but working) and Genrad 1933 meter (cost about as much as a car when new around 1973). I've done all my previous sweeping and tuning on the right channel. Now it's time to verify that I haven't messed things up too badly in the left.
Right now I'm playing 14Hz at the minimum output level of the B&K at gain 95 on the Tact.
14 Hz 80dB (effortless. speaker hardly moving)
16 Hz 85dB I can hear it
15 Hz 84dB barely audible
17 Hz 86dB
19-20 Hz 89dB some rattling, chuffing, or something
18 Hz 87dB
21 Hz 94dB vague noise, too loud
22 Hz 93dB too loud
22-23 91
25 89
30 85 sounds plenty loud
45 90 local peak
71 85 local peak
58 80 min, but similar all around here
78 60dB (ok, i tuned this by had to minimize the meter)
89 91
Saturday, January 22, 2011
Friday, January 21, 2011
More Crossover Tests and Thoughts
Thursday night I tested my new high supertweeter output level using a General Radio 1933 Sound Analyzer, and it suggests I am doing OK.
The Genrad has built-in switchable octave band filters. I played pink noise and tried the different bands, pointing the random incidence microphone toward the ceiling mostly. The highest band, 16Khz, was pretty much in line with the other bands. If I turn off the supertweeter, the 16Khz band goes way down.
In the first test i did, the 16Khz band was 2dB higher than the 8kHz band. I could easily make it level with the 8kHz band by turning down the supertweeter level a few dB. But then in a later measurement, with the microphone pointed slightly differently, the 16kHz band was 1dB lower than the 8Khz band. The highest bands were the 32Hz and 64Hz bands, they were at least 5dB higher than the others. (Those bands are still in need of room correction with my Tact preamp, but having them boosted slightly makes for better sound anyway.)
More listening to pop recordings such as "Bass Erotica" suggests the new crossover alignment is really on track if not perfect yet. As currently configured, the system is more fun to listen to than it has ever been. It's good all around (if still a bit lumpy in the bass) but the rhythm, pace, and intertransient silence have never been equaled in any of my previous systems.
I particularly think the idea of spacing the low frequency crossover around the 90-100Hz vertical room resonance is fabulous. You are going to have that resonance anyway, so you might as well take advantage of it to take some load off your speakers. Separating the low pass and high pass crossover points reduces the effect of interference that can occur when both speakers are playing simultaneously.
During Crossover Redesign Days 2-3 I spent much time sweeping the low end with my B&K oscillator, and polished the crossover points slightly, moving the subwoofer lowpass from 72 to 84 (to reduce a depression in that range) and moving the panel highpass from 100Hz to 104Hz (because there was still a peak at 100, but when I raised highpass to 107Hz I start getting a depression).
I might try fiddling with this some more...I could possibly eliminate any peaking around 90Hz entirely, but since I am getting to the point where depressions open up if I lower the lowpass or raise the highpass, it would seem I may have already gotten about as much benefit from this trick as I can. And it does nothing for the main room resonance at 45Hz, that will have to be fixed by the Room Correction System.
I've tried different crossover points for the supertweeter also, currently 14.8khz seems to work best (at least when I tested it at the old +10db level).
One concern has also come up in my mind. It seems pretty clear the Acoustats are rolling off pretty steeply above 14kHz. I had thought they were supposed to have high frequency response to 18 or 22kHz. Perhaps mine need some capacitor replacement.
The Genrad has built-in switchable octave band filters. I played pink noise and tried the different bands, pointing the random incidence microphone toward the ceiling mostly. The highest band, 16Khz, was pretty much in line with the other bands. If I turn off the supertweeter, the 16Khz band goes way down.
In the first test i did, the 16Khz band was 2dB higher than the 8kHz band. I could easily make it level with the 8kHz band by turning down the supertweeter level a few dB. But then in a later measurement, with the microphone pointed slightly differently, the 16kHz band was 1dB lower than the 8Khz band. The highest bands were the 32Hz and 64Hz bands, they were at least 5dB higher than the others. (Those bands are still in need of room correction with my Tact preamp, but having them boosted slightly makes for better sound anyway.)
More listening to pop recordings such as "Bass Erotica" suggests the new crossover alignment is really on track if not perfect yet. As currently configured, the system is more fun to listen to than it has ever been. It's good all around (if still a bit lumpy in the bass) but the rhythm, pace, and intertransient silence have never been equaled in any of my previous systems.
I particularly think the idea of spacing the low frequency crossover around the 90-100Hz vertical room resonance is fabulous. You are going to have that resonance anyway, so you might as well take advantage of it to take some load off your speakers. Separating the low pass and high pass crossover points reduces the effect of interference that can occur when both speakers are playing simultaneously.
During Crossover Redesign Days 2-3 I spent much time sweeping the low end with my B&K oscillator, and polished the crossover points slightly, moving the subwoofer lowpass from 72 to 84 (to reduce a depression in that range) and moving the panel highpass from 100Hz to 104Hz (because there was still a peak at 100, but when I raised highpass to 107Hz I start getting a depression).
I might try fiddling with this some more...I could possibly eliminate any peaking around 90Hz entirely, but since I am getting to the point where depressions open up if I lower the lowpass or raise the highpass, it would seem I may have already gotten about as much benefit from this trick as I can. And it does nothing for the main room resonance at 45Hz, that will have to be fixed by the Room Correction System.
I've tried different crossover points for the supertweeter also, currently 14.8khz seems to work best (at least when I tested it at the old +10db level).
One concern has also come up in my mind. It seems pretty clear the Acoustats are rolling off pretty steeply above 14kHz. I had thought they were supposed to have high frequency response to 18 or 22kHz. Perhaps mine need some capacitor replacement.
Thursday, January 20, 2011
Twice as real*...setting the supertweeter level
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.
Wednesday, January 19, 2011
Another Audio Blogger
Somewhere, probably scanning DIYAudio I came across DanTheMan's Blog.
Like me, Dan likes to investigate loudspeakers.
Here's a link to hislist of interesting audio sites
Like me, Dan likes to investigate loudspeakers.
Here's a link to hislist of interesting audio sites
Crossover Redesign, Day One
The 3 day MLK weekend featured cold rainy weather, so I had the perfect alibi to spend 2 days measuring, re-thinking, and re-designing the crossover of my main Living Room System.* This effort is still continuing (now in something like Day Three or more as I am writing this) and I think it has already been extremely beneficial to the sound and will soon make possible a Wonderful New System once I rerun Room Correction.
(*Nominally I tend to use Linkwitz-Riley crossovers whenever possible; they have many advantages starting from the fact that the drivers are always in phase with each other. Currently I use the LR48 slope built into my Behringer DCX 2496 crossover; in addition to being a Linkwitz Riley crossover it is very steep and the drivers are kept in the same polarity. But quite often I adjust crossovers to perform additional frequency countouring by using different crossover frequencies for high and lowpass, etc. This is needed to correct speaker and/or room response, and that is the subject of this post.)
The state of things in late 2010 had clearly gone downhill when I made the measurements (see back a few posts) that showed startling lack of bass response below 100Hz. What happened? I had not only previously measured this as actually boosted below 100Hz but was using a Tact RCS (Room Correction System) 2.0 system to realize my "room curve" which is essentially flat with bass boost below 30Hz. What happened was that over the course of the year, small changes tended to roll back the bass levels. Why were these small changes being made? Because despite reasonable measurements and RCS, the bass response had time smearing properties because of the previous way I had "designed" the crossover between subwoofer(s) and electrostatic panels.
That was waaay back in 2008, I had played pure bass tones from my Kurzweil synthesizer. One of the concerns that I had then was lack of bass in the lowest regions to be reproduced by the panels, in the 100-200Hz range. I used Ivie IE30, Behringer DEQ 2496, and other devices to assess the frequency response and possible ways of setting the crossover. But that was so long ago, I can't really remember much about it except that it proved to me it was OK to use a single subwoofer crossed as high as 86 Hz.
Then later, when I bought the second subwoofer in January 2010, I decided to use higher frequency crossovers to avoid damaging the panels like I had in March 2009 (Acoustat panels cannot arc but you can burn up the transformers with enough sustained high volume levels). To allow me to play as loud as possible with no worries, I pushed the crossover frequency all the way up to 121 Hz. (Previously, I had experimented with crossovers as low as 40Hz.)
By this time, I was already using Tact RCS 2.0 room correction, but I felt I could get the best results out of correction by providing plenty of "material" (sound output) for the RCS algorithms to work with. I would make up for the lack of uncorrected response in the region 100-200Hz by raising the subwoofer lowpass all the way to about 165Hz. So the uncorrected response was now rather "thick" in the bass, but I had no fear about this believing that RCS would fix it up.
Well, RCS did seem to fix it up (though I no longer have any measurements to prove that, wasn't blogging then). But the RCS corrections were apparently very fragile. If you weren't sitting in the correct spot (which moved to mid-room) the bass could get quite boomy. So over time, I turned down the bass, leading to the current situation.
All new work is starting from NO RCS CORRECTION to make it more robust this time. RCS will be added to a good working system to make it even better, rather than being added to a system which merely has enough "material".
Pink noise measurements like the ones in the previous post strongly suggested that the high bass lowpass was not good. When the lowpass was brought down to 121 Hz used by the highpass, response got a lot better.
For several days, I reveled in the idea that maybe I should stick with idealized crossovers, just set the controls so high pass and low pass are the same. But it has passed now, because I found something even better.
Even with low and highpass set to 121 Hz, there was still considerable peaking around 100Hz as shown just below.
So I got to thinking, what if I lowered the lowpass even more. Now this is really going in the opposite direction from what I did last time, but I thought now it might be worth testing.
Sure enough, I can lower the subwoofer lowpass as low as 72Hz and still get seemingly nice response up to 121 Hz where the panels start crossing out. In fact, it is still slightly peaky if I do that, but the peak is lowered to about 80Hz and much smaller.
Uncorrected, this is far better sounding than the straight 121Hz crossover, and has much less peaking in the 100Hz region.
Now some people might complain that I am not crossing over "correctly" because high and low pass frequencies are not set the same. But the proof is in the pudding, I get far flatter response separating the low and high pass. Can I get away with this, and should I?
Yes and yes. It turns out that, very conveniently, speakers and rooms have errors mostly of the type called "minimum phase". If you add a minimum phase error to a minimum phase correction that perfectly cancels it for flat frequency response, it turns out you have also perfectly corrected the phase response as well. (Of course, nothing is ever "perfect" but it can be close.)
Now what is happening in this case? I believe the room has a strong resonant node in the 80-110Hz region caused by ceiling bounce (the ceiling ranges from 8'2" to 9'8"). This has half wave nodes in the 45Hz region, and full wave nodes in the 90Hz region.
A much smaller factor may be that the subwoofers have resonance in the 80-110Hz region. I have not tried to measure such a resonance, but I do sense that the SVS PB-13 is not as perfectly flat to 200Hz as the website suggested.
I currently think the room resonance is the larger factor, and it is very convenient that I can cancel it out by pushing down the lowpass cutoff of the subwoofer even below the highpass cutoff of my panels. I have used this trick previously in other systems, but never documented it as well as I am doing here.
Later in the day, the strange notch frequency around 4-6K began to bother me. But first I asked myself "is this real, or an artifact of the iPhone RTA measuring device." I tried running a spectrum of my bedroom system, based on Revel M20 speakers, which I had previously measured with an Ivie IE30 as being nearly perfectly flat above 1Khz.
Well, that spectrum showed exactly the same notchout at 4-6K, followed by a boost from 8K-13K, followed by a notch, followed by a boost up to 20K. In short, it looked almost exactly the same.
I then checked a spectrum I had run of a friends system (a very committed audiophile) in December. Same features in the response above 4K.
Three systems with the same response above 4K? Not likely, especially as I had previously measured one of those systems as being flat, not what was being shown. I conclude that the iPhone RTA measurement above 4K can't be trusted, or it means more-or-less flat if it shows the pattern shown above.
I think the bass response of RTA is fairly trustworthy, however, and it looked different on each of the three systems tested.
But it is good to beware, even if you like to do objective audio measurements as I do, that any particularly measurement can be, and usually is, flawed in more than one way. That's also why I have many different measurement instruments that do roughly the same thing, because they each reveal different things, and not one of them is perfect. If I had lots more money, I could afford more perfect measuring instruments, they are extremely expensive.
A real time analyzer with lots of bands is one of the most handy tools for setting up an audio system. However, it can also be misleading. It could be misleading even if it had flat response. Frequency coutours of a speaker or room are obscured by the fixed frequency bandwidths. One can get much closer to the actual sound by using a sine wave generator. Plus, with a sine wave generator, I can make spectrum level and continuity judgements by ear, and also use one of many Class A microphones I have to measure sound pressure level.
On day two, I brought out the B&K Function Generator to see what I had done and to make it even better...
(*Nominally I tend to use Linkwitz-Riley crossovers whenever possible; they have many advantages starting from the fact that the drivers are always in phase with each other. Currently I use the LR48 slope built into my Behringer DCX 2496 crossover; in addition to being a Linkwitz Riley crossover it is very steep and the drivers are kept in the same polarity. But quite often I adjust crossovers to perform additional frequency countouring by using different crossover frequencies for high and lowpass, etc. This is needed to correct speaker and/or room response, and that is the subject of this post.)
The state of things in late 2010 had clearly gone downhill when I made the measurements (see back a few posts) that showed startling lack of bass response below 100Hz. What happened? I had not only previously measured this as actually boosted below 100Hz but was using a Tact RCS (Room Correction System) 2.0 system to realize my "room curve" which is essentially flat with bass boost below 30Hz. What happened was that over the course of the year, small changes tended to roll back the bass levels. Why were these small changes being made? Because despite reasonable measurements and RCS, the bass response had time smearing properties because of the previous way I had "designed" the crossover between subwoofer(s) and electrostatic panels.
That was waaay back in 2008, I had played pure bass tones from my Kurzweil synthesizer. One of the concerns that I had then was lack of bass in the lowest regions to be reproduced by the panels, in the 100-200Hz range. I used Ivie IE30, Behringer DEQ 2496, and other devices to assess the frequency response and possible ways of setting the crossover. But that was so long ago, I can't really remember much about it except that it proved to me it was OK to use a single subwoofer crossed as high as 86 Hz.
Then later, when I bought the second subwoofer in January 2010, I decided to use higher frequency crossovers to avoid damaging the panels like I had in March 2009 (Acoustat panels cannot arc but you can burn up the transformers with enough sustained high volume levels). To allow me to play as loud as possible with no worries, I pushed the crossover frequency all the way up to 121 Hz. (Previously, I had experimented with crossovers as low as 40Hz.)
By this time, I was already using Tact RCS 2.0 room correction, but I felt I could get the best results out of correction by providing plenty of "material" (sound output) for the RCS algorithms to work with. I would make up for the lack of uncorrected response in the region 100-200Hz by raising the subwoofer lowpass all the way to about 165Hz. So the uncorrected response was now rather "thick" in the bass, but I had no fear about this believing that RCS would fix it up.
Well, RCS did seem to fix it up (though I no longer have any measurements to prove that, wasn't blogging then). But the RCS corrections were apparently very fragile. If you weren't sitting in the correct spot (which moved to mid-room) the bass could get quite boomy. So over time, I turned down the bass, leading to the current situation.
All new work is starting from NO RCS CORRECTION to make it more robust this time. RCS will be added to a good working system to make it even better, rather than being added to a system which merely has enough "material".
Pink noise measurements like the ones in the previous post strongly suggested that the high bass lowpass was not good. When the lowpass was brought down to 121 Hz used by the highpass, response got a lot better.
For several days, I reveled in the idea that maybe I should stick with idealized crossovers, just set the controls so high pass and low pass are the same. But it has passed now, because I found something even better.
Even with low and highpass set to 121 Hz, there was still considerable peaking around 100Hz as shown just below.
So I got to thinking, what if I lowered the lowpass even more. Now this is really going in the opposite direction from what I did last time, but I thought now it might be worth testing.
Sure enough, I can lower the subwoofer lowpass as low as 72Hz and still get seemingly nice response up to 121 Hz where the panels start crossing out. In fact, it is still slightly peaky if I do that, but the peak is lowered to about 80Hz and much smaller.
Uncorrected, this is far better sounding than the straight 121Hz crossover, and has much less peaking in the 100Hz region.
Now some people might complain that I am not crossing over "correctly" because high and low pass frequencies are not set the same. But the proof is in the pudding, I get far flatter response separating the low and high pass. Can I get away with this, and should I?
Yes and yes. It turns out that, very conveniently, speakers and rooms have errors mostly of the type called "minimum phase". If you add a minimum phase error to a minimum phase correction that perfectly cancels it for flat frequency response, it turns out you have also perfectly corrected the phase response as well. (Of course, nothing is ever "perfect" but it can be close.)
Now what is happening in this case? I believe the room has a strong resonant node in the 80-110Hz region caused by ceiling bounce (the ceiling ranges from 8'2" to 9'8"). This has half wave nodes in the 45Hz region, and full wave nodes in the 90Hz region.
A much smaller factor may be that the subwoofers have resonance in the 80-110Hz region. I have not tried to measure such a resonance, but I do sense that the SVS PB-13 is not as perfectly flat to 200Hz as the website suggested.
I currently think the room resonance is the larger factor, and it is very convenient that I can cancel it out by pushing down the lowpass cutoff of the subwoofer even below the highpass cutoff of my panels. I have used this trick previously in other systems, but never documented it as well as I am doing here.
Later in the day, the strange notch frequency around 4-6K began to bother me. But first I asked myself "is this real, or an artifact of the iPhone RTA measuring device." I tried running a spectrum of my bedroom system, based on Revel M20 speakers, which I had previously measured with an Ivie IE30 as being nearly perfectly flat above 1Khz.
Well, that spectrum showed exactly the same notchout at 4-6K, followed by a boost from 8K-13K, followed by a notch, followed by a boost up to 20K. In short, it looked almost exactly the same.
I then checked a spectrum I had run of a friends system (a very committed audiophile) in December. Same features in the response above 4K.
Three systems with the same response above 4K? Not likely, especially as I had previously measured one of those systems as being flat, not what was being shown. I conclude that the iPhone RTA measurement above 4K can't be trusted, or it means more-or-less flat if it shows the pattern shown above.
I think the bass response of RTA is fairly trustworthy, however, and it looked different on each of the three systems tested.
But it is good to beware, even if you like to do objective audio measurements as I do, that any particularly measurement can be, and usually is, flawed in more than one way. That's also why I have many different measurement instruments that do roughly the same thing, because they each reveal different things, and not one of them is perfect. If I had lots more money, I could afford more perfect measuring instruments, they are extremely expensive.
A real time analyzer with lots of bands is one of the most handy tools for setting up an audio system. However, it can also be misleading. It could be misleading even if it had flat response. Frequency coutours of a speaker or room are obscured by the fixed frequency bandwidths. One can get much closer to the actual sound by using a sine wave generator. Plus, with a sine wave generator, I can make spectrum level and continuity judgements by ear, and also use one of many Class A microphones I have to measure sound pressure level.
On day two, I brought out the B&K Function Generator to see what I had done and to make it even better...
Monday, January 10, 2011
Setting the bass crossover
I improved the response of the right channel from the first spectrum to the second one shown by moving the low pass crossover frequency for the subwoofer from around 160Hz down to 121Hz. The high pass crossover on the electrostatic panels remained the same 121Hz in both cases. Previously I crossed over the subwoofers higher than the panels deliberately in order to fill in what I perceived as a low point in response between 120Hz and 200Hz. In the overall scheme of things, that depression (still visible in second spectrum) doesn't seem as important as the 80-150hz peak that arises from this crude attempt to reduce it, and the bass sounds cleaner (less resonant) with the 121 Hz low pass. Been thinking about pushing crossover even lower, but I don't like to put too much of the low frequency spectrum into the panels as their low frequency transformers can melt, so 121 is about as low as I want to high pass the panels for reliability reasons.
The performance was also improved from previous spectra by increasing bass generally in both channels (through the crossover) and individually in the right channel by setting the volume control on the sub. (I can't remember if these changes got in the first spectrum above, possibly not, though it actually looks like there is some improvement there.)
Also, the supertweeter level was reduced about 5dB, that affects pretty much only the two highest frequency bars, where it is sometimes hard to see any change. The supertweeter is not intended to be very audible, instead just at the margins of audibility, and filling in the upper frequencies which we don't think we hear but might have perceptible effects anyway. Normally, the supertweeters help remove the bandwidth limited "plasticy" sound from the Acoustats; recently they may have been adding some undesireable aggressive steeliness. On other analyzers, it often appears like the supertweeter is improving the response of the highest frequency bar by making it flush rather than lower than the preceding bar, thus extending the high frequency response and making it less peaky.
As the last paragraph makes clear, though spectra look impressive and "scientific" when presented, you have to remember that each measurement is but an imperfect sample of the processes involved, and that better measurements can often put things into a rather different light. Visible and rationally analyzable spectra greatly facilitate progress in audio design, but should never be considered the end goal. The end goal is better sound, and the spectrum is only a tool that helps modify a system towards that goal.
I would classify the frequency response in the second graph as "pretty good", and much better than the average system, but still in need of work for the ultimate level of performance (what I was claiming to provide previously...now again a work in progress since my discovery of low bass in a previous post).
The suckout in the 4K-6K region is troubling, probably deserves more investigation, though most likely is a speaker limitation that can't be cured but can be corrected by the Room Correction System.
Thursday, January 6, 2011
Frequency Response Errors have accumulated
Frequency response is something I consider very important. This system is corrected to be flat with a slight bass boost below 30Hz, through the use of a Tact 2.0 RCS preamp with user room curve settings. I remember pink noise measurements in the past (taken in the same way) which were virtually flat (unfortunately, I have no permanent record of them anymore). The above curve was taken with pink noise from Stereophile Test Disc Two and the iPhone application called RTA by Studio Six Digital.
But not this time. Now I am showing 20dB of response variation from the highest highs to the lowest lows, with mostly a gradually rising response from lows to highs, but very low response in the bass below 100Hz where the subwoofers are playing.
So what happened?
So what happened?
Possibly somehow my Tact has gotten messed up. With the room correction set to bypass, I get more sub 30Hz bass than with correction turned on. Possibly the Tact lost its memory in a power surge, or possibly the changed response was a result of the accumulation of small changes.
Funny how I have made small changes over the year, now ending up with this, clearly not what was intended. I would have not expected much different from flat response. Seems like a quarterly frequency response test is in order to avoid drifting away from desired frequency response through seemingly limited corrections.
Back in mid 2010 when I was playing the Santana "Supernatural" album and concerned about way-too-much-bass, I discovered that right subwoofer level had possibly been tampered with, turned straight up, which was not what I remembered it should be. So I found a reduced level I thought was better, combined with less bass boost used in my room currection curve. I figured 3dB reduction in bass, tops, and I was simply restoring the original corrected response. It sounded much better on Supernatural without any bass cut, and I didn't notice any problem with other recordings later.
To restore frequency response, I will begin with no correction, and set driver levels and crossover points for a fairly decent overall response. Then room correction measurement and room curve adjustment will be performed.
Room correction measurement is a complicated process, and I haven't bothered since early last year.
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