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Old 15th February 2009, 02:05 AM   #11
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Pipoes,

Almost all decent tweeters use an underhung motor design such that the BL versus coil displacement is perfectly constant as long as you don't exceed Xmax ((Top plate thickness-voice coil winding height)/2). Any tweeter over $10 OEM cost has at least a 0.5mm Xmax. Note that this is 2.5 times the 0.2mm Xmax that you brought up earlier.

Like Ron mentioned, one of the keys to getting a direct radiating tweeter to play loud is the crossover frequency. Drop the crossover by one octave and the driver displacement needs to increase by a factor of four to keep SPL constant. Conversely, raise the crossover frequency a little bit and the maximum SPL goes up a lot. There is no point in increasing the crossover slope beyond 2nd order from an excursion standpoint. A 2nd order HP filter will limit the driver to a constant excursion below the crossover frequency.

The "1kW" impulses that Dynaudio and Morel have used for years to brag about their power handling capabilities are completely bogus. There are a number of reasons for this.

1) Look closely at the chart recorder output on the Dynaudio impulse measurement. They usually have a small note indicating that a series capacitor was used. They fail to mention this. Since an impulse is comprised of the entire frequency spectrum, this capacitor reduces the current going to the tweeter quite a bit.

2) The 1kW power is calculated from the peak voltage at the output of the amplifier driving this system. It is not the actual power delivered to the driver from the voltage across it AND the current going through it.

3) Any system with a high frequency limit (the inductance of the tweeter in this example) will round off the peak of the impulse. This reduces the amount of current from the impulse that actually flows in the circuit.

Somewhere, I have photos of a real 1kW impulse applied to a Dynaudio tweeter. It only lasted a portion of the impulse.
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Old 15th February 2009, 03:40 AM   #12
gedlee is offline gedlee  United States
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Matt

In theory a tweeter is just a small woofer and can be modeled as such. I do this all the time. What you find in practice is that a tweeter is thermally limited not excusrion limited. Its easy to see why this is.

If we look at the audio bandwidth it goes from lets say 100 Hz - 10 kHz (lower of course, but the argument still works). Its usual to consider 1 kHz as the mid point of the audio spectral bandwidth (which is a good rule of thumb from a pitch or percetual point of view). If we have a flat spectrum (which again isn't true of music, but it will make my point) then we need to consider that power (PSD, Power Spectral Density, is in volts^2 / Hz) is based on linear frequency NOT log frequency as our perception is. Now in linear frequency, for a flat spectrum from 100 Hz - 10 kHz. There is ten times as much power above 1 kHz (10,000 - 1,000 Hz = 9000 Hz) as there is below 1 kHz (1000 Hz - 100 Hz = 900 Hz). Thats a big difference, especially when you consider that the tweeter is usually a lot smaller (voice coil, etc.) than the speaker that operates below 1 kHz. This causes the tweeeter to virtually always be limited by its ability to disipate the heat from the input voltage. To the extent that it can't do this, it thermally modulates the sound, which I contend is a major factor.

In a test that I did using two two ways, one with a 1" dome tweeter and one with a 1" aperature compression driver, above 1 kHz, the dome tweeter had about 10 times as much thermal compression as the compression driver producing the same room SPL. Thats going to be a major audible difference.
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Old 15th February 2009, 07:54 AM   #13
Pan is offline Pan  Sweden
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Quote:
Originally posted by Jack Hidley
Pipoes,

Almost all decent tweeters use an underhung motor design such that the BL versus coil displacement is perfectly constant as long as you don't exceed Xmax ((Top plate thickness-voice coil winding height)/2). Any tweeter over $10 OEM cost has at least a 0.5mm Xmax. Note that this is 2.5 times the 0.2mm Xmax that you brought up earlier.


Hi!

Scan Speak 9800 = +/- 0.1mm.
Scan Speak ringradiator +/- 0.2mm
Scan Speak 9700/9900 +/- 0.4mm

Seas DXT overhung
Seas 27TDFC +/- 0.25mm underhung
Accuton C23 overhung

Quote:
Like Ron mentioned, one of the keys to getting a direct radiating tweeter to play loud is the crossover frequency.
Absolutely!

Quote:
There is no point in increasing the crossover slope beyond 2nd order from an excursion standpoint. A 2nd order HP filter will limit the driver to a constant excursion below the crossover frequency.
Yes but that means that everything below Fs will cause serious cone travel and IMD. By crossing with a higher order slope you will significantly reduce movement.

Quote:
The "1kW" impulses that Dynaudio and Morel have used for years to brag about their power handling capabilities are completely bogus. There are a number of reasons for this.
Bursts, not impulses.

Quote:
1) Look closely at the chart recorder output on the Dynaudio impulse measurement. They usually have a small note indicating that a series capacitor was used. They fail to mention this.
I don't remember if they mentioned it or not, it can't be both though. :-) Looking at the curves of the burst tests hould be enough to indicate a HP filter and they also state that power handling depends on crossoverpoint.

[quote]
Since an impulse is comprised of the entire frequency spectrum, this capacitor reduces the current going to the tweeter quite a bit.[/uote]

No impulses used, I believe they use/d 10ms sine bursts.

Quote:
2) The 1kW power is calculated from the peak voltage at the output of the amplifier driving this system. It is not the actual power delivered to the driver from the voltage across it AND the current going through it.
Most of their tweeters is 8 ohm nominal which means slightly lower impedance in the middle of the tweeeter range.. which means moe power in this range than 1kW if the reference power is calculated from 2.83V into 8ohm.

The graphs are taken with a mic into a writer so the readings are based on SPL not on calculated power. So unlesss they deliberately manipulated these graphs I think they can be viewed as correct.


Quote:
Somewhere, I have photos of a real 1kW impulse applied to a Dynaudio tweeter. It only lasted a portion of the impulse.
Any more info on that set up?

An impulse have low frequency content which a sine burst do not have. Likely what happened at that test was that the excursion was more than spec and the driver suspension was ripped?


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Old 15th February 2009, 08:14 AM   #14
Pan is offline Pan  Sweden
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Quote:
Originally posted by gedlee
What you find in practice is that a tweeter is thermally limited not excusrion limited.


Agree!

Quote:
If we have a flat spectrum (which again isn't true of music, but it will make my point) then we need to consider that power (PSD, Power Spectral Density, is in volts^2 / Hz) is based on linear frequency NOT log frequency as our perception is. Now in linear frequency, for a flat spectrum from 100 Hz - 10 kHz. There is ten times as much power above 1 kHz (10,000 - 1,000 Hz = 9000 Hz) as there is below 1 kHz (1000 Hz - 100 Hz = 900 Hz). Thats a big difference, especially when you consider that the tweeter is usually a lot smaller (voice coil, etc.) than the speaker that operates below 1 kHz. This causes the tweeeter to virtually always be limited by its ability to disipate the heat from the input voltage. To the extent that it can't do this, it thermally modulates the sound, which I contend is a major factor.

But still we must consider spectra in music and typical listening habits. Most instruments roll off above 1kHz so the actual power of harmonics wil be small going to the tweeter. We have the occassional transient but as we have seen that can be handled well, it's the long term power handling that is poor but I for one don't have music that has constant high levels (+100dB) in the higher registers.

An SPL of 90dB is LOUD at in the tweeter range and that equals 1W into a typical tweeter and there will be no thermal compression to speak of at such level with a well designed dome tweeter.


Quote:
In a test that I did using two two ways, one with a 1" dome tweeter and one with a 1" aperature compression driver, above 1 kHz, the dome tweeter had about 10 times as much thermal compression as the compression driver producing the same room SPL. Thats going to be a major audible difference.
There's a big difference between different tweeters. A soft dome without ferrofluid and a small magnet assembly will be much worse than a metal dome with ferro and a big magnet system (= large thermal mass).


/Peter
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Old 15th February 2009, 08:25 AM   #15
Pan is offline Pan  Sweden
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Here's a spectral analyzis of a grand piano. A piece played by both hands so we see a good represantation of a wide bandwith sound from the instrument. The sample is a 10s clip from a recording I did.

What can be seen is that the max level in the harmonic range (from 2-3k and up) is aprox- 20dB below the fundamentals.

This means that if we play back the piano at a realistic level of about 110dB we will have only 90dB from the tweeter and that is not constant. We will feed less than 1W into a typical tweeter and there is no thermal compression going on. Perhaps the mean power is something like 0.1-0.5W.

Of course we can come up with situations that is worse but for typical program material thermal compression from well designed dome tweeters is not an issue.


/Peter
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Old 15th February 2009, 05:21 PM   #16
gedlee is offline gedlee  United States
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Your example proves nothing and I have data, as I said, to the contrary. A dome tweeter does compress quite seriuosly on typical program material.
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Old 15th February 2009, 06:41 PM   #17
Pan is offline Pan  Sweden
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Clearly you are mistaken about the example above which was meant to clearly show that for many types of program material thermal compression is a non issue.

That you have found a dome tweeter that performs bad does not proove anything.


/Peter
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Old 15th February 2009, 06:46 PM   #18
pjpoes is offline pjpoes  United States
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Thanks for the info guys, this has better answered my original question.

Thank's Dr. Geddes, the first part of your answer at least indicated that the models I ran were fine.
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Old 16th February 2009, 01:35 AM   #19
Ron E is offline Ron E  United States
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Quote:
Originally posted by Pan
Of course we can come up with situations that is worse but for typical program material thermal compression from well designed dome tweeters is not an issue.
Here is some acoustic music that has an amazing amount of high frequency energy. Solo Harpsichord. On the log plot there is a bump between 80 and 600 and then it is essentially constant out to the antialiasing filter...
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Old 16th February 2009, 02:32 AM   #20
gedlee is offline gedlee  United States
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All of the energy to the right of 1 kHz goes to the tweeter, the rest to the woofer. Looks pretty one sided to me.
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