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Old 16th February 2009, 09:43 AM   #21
Pan is offline Pan  Sweden
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These plots don't show energy but level without regard to time so the harpsichord plot only prooves that it has more HF content than grand piano, but it says nothing about a tweeters ability to handle the material at hand.

To make any judgement we need to know the absolute SPL at typical listening distance. I know that I have never put my head into an harpsichord but listen at 2-10 meter. What SPL are we talking about?

I would like to point out also that there are speakers that have other crossover points than 1kHz..

Also one needs to understand the dynamic nature of typical acoustic instruments and music.


/Peter
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Old 16th February 2009, 01:32 PM   #22
pjpoes is offline pjpoes  United States
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Peter, the plot supplied was nearly identical to yours. Your's proved that tweeters didn't take a lot of energy, why doesn't Ron's prove to the contrary? Additionally, why does time need to play a role in the term energy? It sure seems like you are trying to be argumentative for the sake of it.
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Old 16th February 2009, 06:30 PM   #23
Pan is offline Pan  Sweden
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pjpoes,

Quote:
It sure seems like you are trying to be argumentative for the sake of it.
Are you expecting me te to take time and help you understand basic physics after that?


/Peter
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Old 16th February 2009, 06:54 PM   #24
pjpoes is offline pjpoes  United States
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I'm sorry if you took offense to that. I just don't understand your argument. You told Dr. Geddes that your graph and example shows that for many types of program material thermal compression was a non-issue. Your graph doesn't represent energy expended over time, nor does Ron's, but Ron's you said didn't show it as an issue.

Obviously the conversion of electrical energy into mechanical energy happens over a period of time, and that the length of that time impacts thermal compression. What I didn't understand was why your graph showed it as a non-issue, but Ron's graph doesn't show it as an issue. Are you saying that a piano is more representative of common program material? Are these acoustic recordings even representative of normal program material? What about rock music, studio jazz recordings, movie soundtracks, etc. You make a good point though that even if reference levels indicate peaks of 105db's (as in THX movie reference levels), that this may not mean the tweeter needs to sustain this level, since the program material may not contain an equal amount of energy up that high, and most THX speakers not made by JBL have pretty high crossover points, well above 1khz. I don't know if that's true, I haven't seen a waterfall plot of a movie soundtrack before to even know how much energy exists above 1khz.

Modeling a tweeter like a tiny woofer, using measured parameters, and calculating the missing ones, I find that it's the lower treble point, around the crossover, which most drives the mechanical limits issue, and when looking at the power graphs, the thermal limits as well. With a normal tweeter, crossover points between 2khz and 3khz seem to limit maximum output to around 100-110db's, regardless of thermal issues. Efficiency plays a roll in the amount of power it takes, my example had a roughly 89db efficiency, and thus power levels to reach that higher number were in excess of 100 watts, some times many times that number if the crossover point was low enough. You simply can not run a normal dome tweeter down to 1khz at all, it reached xmax with just a handful of watts, but isn't all that common either. I didn't model one that low, but I did model it down to 1.8khz, which seems a common crossover point in "audiophile" designs, but less so in home theater designs.
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Old 16th February 2009, 09:39 PM   #25
Pan is offline Pan  Sweden
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Quote:
Originally posted by pjpoes
[B]Peter, the plot supplied was nearly identical to yours. Your's proved that tweeters didn't take a lot of energy, why doesn't Ron's prove to the contrary?
The plot themself does not proove much, you need to understand the dynamics of the instrument and the typical SPL. Just because there is much HF info like in the harpsichord plot does not mean it means "much energy" into the tweeter.


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Additionally, why does time need to play a role in the term energy?
Becasue if you have a fixed power level it means more energy the longer the duration. P=W/t.

The FFT plots does not show energy, they show relative level.

If you analyze a signal in the FFT plot it is not climbing higher and higher the longer the duration. For all we know the high frequency content could be some fast transients during a couple of seconds in the middle of the piece and the rest is ongoing low frequency stuff for minutes.

The plot shows that this and that frequency has occured during the sample at the indicated max level but not how long.. and it's the time that is interesting when feeding power into the VC (well both are interesting but I hope you get my point).

Quote:
What I didn't understand was why your graph showed it as a non-issue, but Ron's graph doesn't show it as an issue.
That's not what I said or meant. I objected against Earl's way of turning the plot into proof for making a case. He have measured a budget dome tweeter and decided that dome tweeters are inferior period!

That does not make sense IMO.

What does it matter if the harpsichord (or whatever instrument or composition) have mostly HF content if it is at a low level? One must take the dynamics of the instrument/music into consideration
and also actual SPL.

Quote:
Are you saying that a piano is more representative of common program material?
Absolutely not, but I want to show that for many situations thermal compression is not a problem since HF content in much music is low. We simply do not tolerate high levels of HF.. it is hard on the ears and music and instruments have evolved according to that.

Quote:
Are these acoustic recordings even representative of normal program material? What about rock music, studio jazz recordings, movie soundtracks, etc.
http://www.stereophile.com/reference...ot/index1.html


Of course levels and spectra differs between styles but again we normally do not appreciate high levels of HF with exceptions of clean transients. And of course large rooms needs more from the speakers and many listeners listen at levels that will give them an early onset of hearing problems.

Also many people overestimate the leves they actually listen at.

Quote:
You make a good point though that even if reference levels indicate peaks of 105db's (as in THX movie reference levels), that this may not mean the tweeter needs to sustain this level, since the program material may not contain an equal amount of energy up that high,
Exactly!

Quote:
and most THX speakers not made by JBL have pretty high crossover points, well above 1khz. I don't know if that's true, I haven't seen a waterfall plot of a movie soundtrack before to even know how much energy exists above 1khz.
Waterall graph is used to look at the decay of a system as a function of frequency. Not the same as a spectral plot or a spectrogram (which maybe was what you were thinking of?).

Quote:
With a normal tweeter, crossover points between 2khz and 3khz seem to limit maximum output to around 100-110db's, regardless of thermal issues.
I don't know your definition of normal but many tweeters are good up to 110-120dB with good performance used from 2-3k.

A 90dB sensitive speaker needs a 1000W amp to reach 120dB SPL. I seldom need more than 110dB peak though so 90dB speaker with 100W amp is good for me and a high quality dome tweeter is not a limit.


/Peter
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Old 16th February 2009, 10:04 PM   #26
pjpoes is offline pjpoes  United States
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CSD waterfalls show decay, but I was referring to the spectral waterfall plots. I forget the program commonly used over on AVS Forum for this, it's often used to show the low frequency content of movies (You know how movie guys are about bass). Anyway, it shows the change in amplitude as changes in color, like a radar graph, with time shown as x axis, and range across the y-axis. It seems like this would be a better tool to look at the high frequency content over a period of time.
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Old 16th February 2009, 10:12 PM   #27
gedlee is offline gedlee  United States
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We have been over this discussion time and time again.

My measurements used an "average" dome tweeter, not a poor one, and an "average" compression driver, not a high power one. It's ludicrous to imply that any dome can compete with a compression driver for power and thermal handling. If the dome is "good enough for you", then fine. But many people are finding that the dynamics available from a good compression driver are indeed quite audible and attractive. If you like domes, then stick with them. I've never heard one that I liked.
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Old 17th February 2009, 12:23 AM   #28
breez is offline breez  Finland
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Quote:
Originally posted by Pan
If you analyze a signal in the FFT plot it is not climbing higher and higher the longer the duration. For all we know the high frequency content could be some fast transients during a couple of seconds in the middle of the piece and the rest is ongoing low frequency stuff for minutes.

The plot shows that this and that frequency has occured during the sample at the indicated max level but not how long.. and it's the time that is interesting when feeding power into the VC (well both are interesting but I hope you get my point).
Actually the FFT plot doesn't show the peak level, but rather the average relative levels.

We can square the magnitude of the frequency response and integrate over the tweeter's bandwidth to get the total power going into it (if we set the signal's 0dB to a known voltage). And this is just average power during the whole piece (long term power handling), a high level transient could cause problems with short term power handling.
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Old 17th February 2009, 01:01 AM   #29
gedlee is offline gedlee  United States
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This is mostly correct, but just squaring the voltage assumes that you have the RMS values. The spectrum is not the RMS values. To be exact, you need to find the Power Spectral Density (PSD) over the duration of the piece and then integrate over frequency to find the average power disipated into the tweeter over this period of time. If you want to know a more instantaneous level, at a peak for example, then you would just find the PSD over that time slice and again integrate over frequency to find the power in this period.
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Old 17th February 2009, 03:15 AM   #30
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Dr. Geddes:

Is the lack of thermal compression the major reason for the sonic superiority of compression drivers over domes?

FWIW, I am using the DE250 compression driver in a Geddes 15-inch waveguide and the quality is a night and day difference from not only standard domes, but also any ribbon or quasi-ribbon (Magnepan) that I have ever heard.

Once you hear a good compression driver in a good waveguide, there is no going back to obsolete domes.
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