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Old 9th January 2012, 06:23 PM   #701
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We do agree on SPICE, Wavebourn. However, careful what you say around here. '-)
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Old 9th January 2012, 06:31 PM   #702
DF96 is offline DF96  England
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Quote:
Originally Posted by Wavebourn
and did not change since then
and won't change in the future either. Built in to most Fourier analysis is the assumption that the sample presented will be/has been repeated ad infinitum. The Fourier transform of a toneburst is very different from the Fourier series of the constituent sine wave.
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Old 9th January 2012, 06:45 PM   #703
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Quote:
Originally Posted by DF96 View Post
and won't change in the future either. Built in to most Fourier analysis is the assumption that the sample presented will be/has been repeated ad infinitum. The Fourier transform of a toneburst is very different from the Fourier series of the constituent sine wave.
While it is relevant for Fourier math, it is irrelevant for the topic: it is enough that it was infinite long and stable before we started our observations/measurements. And it does not matter if it ends, or not after we measured.
I myself use a pair of Wavetek functional generators, one to VCA of another, to observe transients.
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Old 9th January 2012, 06:46 PM   #704
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Quote:
Originally Posted by john curl View Post
We do agree on SPICE, Wavebourn. However, careful what you say around here. '-)
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Old 9th January 2012, 08:55 PM   #705
tvrgeek is offline tvrgeek  United States
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Originally Posted by Pano View Post
Thanks for the details, TVR. It just sounds like you're talking about phase, but I'd like to see what you've got. 10Hz is pretty low.

If you can measure at the amp output terminals (with speaker attached) it might be easier to see the effect than with a mic. And that would allow you to test into a resistive load, as well. If you can show that this difference corresponds to some of your subjective evaluations of the amps, that's worth something.
You are missing the point. I am looking at what the amp/driver SYSTEM is doing. By changing only one component of the system, the amp, I see the system behave differently. It is not phase as it changes. It is a representation of the driver's ability to get into motion from a stop. BTW, I did these at 4KHz, not 10 Hz.
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Old 9th January 2012, 09:06 PM   #706
Pano is offline Pano  United States
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No, I didn't miss the point, don't worry. I did misunderstand about the 10 cycles, tho.

Measuring at the speaker terminals may give you a better look at what's going on, which is why I suggested it. Whatever the driver cone is doing, you should see it there electrically. I can even see cabinet resonances in my impedance sweeps. If, however, you are seeing something in the acoustic measurements that is not at the driver terminals, that would be good to know, and surprising. If the amps are doing different things, it ought to be showing up at the speaker terminals. You will see the amp/driver system that way - without the uncertainties of air and microphones.

And if you do see these differences as measured at the speaker terminals, then substitute a resistor as load and the difference go away, you've found something important. I.E., the amps measure the same on a resistive load, but not on a real speaker load.
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Old 9th January 2012, 09:12 PM   #707
tvrgeek is offline tvrgeek  United States
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Originally Posted by DF96 View Post
A 10 cycle pulse of 4kHz, starting from zero, has a lot of transient high frequency components - not just 4kHz. Reproduction will depend to some extent on the HF rolloff of the amp, so it is possible that this is some or most of what you are seeing. If this correlates with perceived audio quality on music then it could be interesting.

Also, HF rolloff may have two components: voltage gain, and output impedance. To what extent is your tweeter sensitive to driving impedance?
All very true. Starting at 0 volts AND 0 current as it is just turning on could have quite an interesting spike in current, or not. That would be very interesting to measure. I could see where the difference to supply near instant maximum current would be a reasonable cause. Thinking on the keyboard out loud, output zobel differences? Location the feedback is picked up from? I don't have a good current probe, just my buggered amp-clamp with a voltage output. Guess I should troll e-bay for a real one.

The particular tweeter is the Seas 27TBFC/G, a ferofluid filled metal dome. I would not suspect it to be terribly difficult to deal with. It's one of their better ones before moving to the Excel line where they get really serious. I have about 6 other tweeters in the ~ $100 range on the shelf. It would be good to test them.
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Old 9th January 2012, 09:14 PM   #708
tvrgeek is offline tvrgeek  United States
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Originally Posted by Pano View Post
No, I didn't miss the point, don't worry. I did misunderstand about the 10 cycles, tho.

Measuring at the speaker terminals may give you a better look at what's going on, which is why I suggested it. Whatever the driver cone is doing, you should see it there electrically. I can even see cabinet resonances in my impedance sweeps. If, however, you are seeing something in the acoustic measurements that is not at the driver terminals, that would be good to know, and surprising. If the amps are doing different things, it ought to be showing up at the speaker terminals. You will see the amp/driver system that way - without the uncertainties of air and microphones.

And if you do see these differences as measured at the speaker terminals, then substitute a resistor as load and the difference go away, you've found something important. I.E., the amps measure the same on a resistive load, but not on a real speaker load.
I agree with these comments too. Note I am comparing the amp output with the mic output. I was not looking at the difference in amp input to output. I should do this too.
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Old 9th January 2012, 09:21 PM   #709
benb is offline benb  United States
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Originally Posted by M Gregg View Post
...

Distortion....Humm an open window with a lorry driving by....we can tolerate some distraction<<<is this why after a point distortion is not a problem..Then again the ticking clock comes to mind removed by the mind after a point so you only notice when it stops... As long as the distortion is constant..with no change can the mind remove it like the clock..

Regards
M. Gregg
What you're describing is how uncorrelated the "distortion" is with the signal. Things such as harmonic distortion and baffle edge diffraction are highly correlated to the signal a hifi is trying to reproduce. That sort of thing makes it easier to tell if some "distraction" such as someone who coughs is in the room with you, or if the coughing is on the recording and being reproduced.
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Originally Posted by Pano View Post
I don't understand why it would. As far as I can tell, it does not. 10 cycles is short and hard to analyze, but I made a 4KHz burst 12 cycles long - zero to zero crossing - and it appears pure, no other tones.

What am I missing?
The sudden (instantaneous!) startup and stop of the waveform adds a "click" to the signal, and this shows up as wideband noise in the spectrum. This is why windowing was developed for practical FFT measurements.

If you ramp up the level during the first six cycles then ramp down the level for the last six cycles, it will substantially reduce the generation of signals other than your 4k burst.

This article should demonstrate. The spectrum of what you're generating is shown for the rectangular window here.
Window function - Wikipedia, the free encyclopedia
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Old 9th January 2012, 09:42 PM   #710
benb is offline benb  United States
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Originally Posted by DBMandrake View Post
Pano, with his "Sound Quality Index" ?

Toole, with his speaker ranking studies ?

Note: not a single measurement, but a single aggregate figure of merit derived from a group of measurements. That's what I'm saying is futile, because I think you simply can't boil down so many competing and largely orthogonal factors into a simple scalar figure of merit, especially if personal preference is indeed a factor.

And there's not much reason to, unless your goal is to replace controlled listening tests with measurements to see how speakers rank against each other in listener preference. A single boiled down figure of merit gives no guidance into what needs improving in each speaker, so doesn't help the designer.


I'm 100% with you there, you're arguing the same side of the argument as I did in my longer post...? I want to find those things that most matter too, however I don't see any point in a single figure of merit for ranking purposes. Keep the handful of important measurements (whatever they are) separate.
...
Someone should find out what they are and how important each one is. At first glance it doesn't seem possible, but I think with measurements AND listening tests of lots of speakers (and adequate questionnaires for the listeners, this may be the hardest part), it should be possible to find out by taking all this data and doing a regression analysis.

Formal listening tests are a lot more expensive and time-consuming than taking measurements, and if a few semi-reliable "figures of merit" can be correlated between the two, I'd think a designer could make more progress by using such "figures" calculated from repeated measurements as design changes are made, and then find out how these correlate with listening tests.

"Data mining" is a Big Thing for businesses thesedays (for things such as finding out what customers Really Want so they can sell more of it), and it looks like an interesting topic:
Data mining - Wikipedia, the free encyclopedia
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