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Re: Tom Danley at AES Chicago

Posted by tomservo (M) on February 23, 2007 at 06:49:51

In Reply to: Re: Tom Danley at AES Chicago posted by Paul Eizik on February 22, 2007 at 18:41:44:

Hi Paul,

Paul, you hit the nail on the head.
When a direct radiator is below the knee in its radiation resistance curve, it feels a changing radiation resistance vs frequency.
In order to make “flat” response in that range, one must roll off the radiator velocity to compensate.
The filter that does this is internal to the driver, consisting of the VC’s Rdc and the driver’s moving mass which is converted by the motor to a capacitance.
Some stuff on line.

Fig#3
http://www.akabak.de/Texte/aes102.pdf

http://www.silcom.com/~aludwig/Sysdes/Thiele_equiv_circuit.gif

When you have a minimum phase problem, you can fix it with a complementary minimum phase fix, this is when EQ works perfectly.
Here, one is fixing the effect of a frequency dependent resistance which does not have the same reactance as a filter with the same slope. Compensating that with an RC filter, does fix the amplitude response BUT leaves the phase of the RC filter which is not canceled out by anything.
At the meeting, I had a TEF measurement of a small woofer, showing the acoustic phase mid band is around –90 degrees over a wide range.

Now, while this phase shift prevents the driver from replicating a complex input waveshape (like a square wave, impulse or even music for example), some direct radiators operate PAST the knee in the curve. RCA fan sent me a curve once of a 8 inch full range driver that made a smooth transition thru that range and above, could reproduce a square wave over some range.
Now, this is a rare exception by a very skillful design and component properties.

The problem is that when drivers operate in breakup, there behavior becomes very frequency dependant so this is normally avoided.
On the other hand, the absence of perturbation at crossover and some range of proper operation is likely what makes single drivers so attractive for the home.

Keep in mind that some very popular measurement systems make the assumption that the speaker IS minimum phase and in which case, the phase response can be found by the amplitude response. On the other hand, the Time Delay Spectrometry process measures both amplitude and phase separately and so it shows what the driver actually does.
When one has things like a changing radiation resistance that does not have the normal phase change associated with it, that this is not seen properly unless actual acoustic phase is measured.
Lastly, unlike electronics speakers spread signals out in time significantly, keep in mind that it is nothing like universally agreed that speakers shouldn’t do this or that a speaker even should be able to preserve the waveshape of the input signal.

Best,

Tom Danley

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