From: DPierce@TheWorld.com (Richard D Pierce)
Subject: Re: Cancel speaker resistance?
Sender: email@example.com (Mr Usenet Himself)
Date: Tue, 17 Sep 2002 11:13:59 GMT
Organization: Professional Audio Development
X-Newsreader: trn 4.0-test72 (19 April 1999)
In article <3D868192.F61269AB@webaccess.net>,
Chuck Simmons wrote:
>> I've studied quite a few impedance curves, and they do strongly tend
>> to follow a pattern. Peak in the LF range dip, Peak in the middle
>> range, dip, peak at the high end.
>The first two peaks are mechanical. The first peak is the first
>principle mode of the diaphragm and suspension. Very small speakers
>don't have the second peak which I suspect is due to the second
>principle mode of the diaphragm. The third appears to be electrical
>rather than mechanical because it is present in voice coils not
>attached to diaphragms. A voice coil with a stiff mass load and a spring
>suspension has only two peaks corresponding to the spring mass resonance
>and to the self resonance of the coil and stray capacitance.
You're right about the first peak at 40 Hz: it's the fundamental
cone mass/system compliance resonance. The system complisance is
the combination of the mechanical suspension for the cone and
the acoustic compliance of the enclosure.
You're wrong about the second peak, though. This is a two-way
loudspeaker system, a woofer and tweeter. THe second peak at
1800 Hz is a combination of things. First the rise above 200 Hz
is the beginnings of the lossy inductance in the voice coil.
That eventually gets shunted by the lower impedance of tweeter
voice coil. If both drivers present a resistive load above
mechanical resonance, and a constant-voltage crossover topology
were used, then the imepdance would be constant. However,
neither condition is likely to hold, especially in that it is
most likely NOT a constant-voltage crossover alignment, so you
have the second peak at 1800 Hz. The third rise in
impedance above about 6 kHz is primarily the lossy inductance of
the tweeter voice coil.
The self resonance of the coil and stray capacitance never
really happens, at least not in the audio band, and if it ever
does, it is a VERY low Q. One of the problems is that the
first-order models are insufficient to explain the gross second
order effects that drivers have. FOr example, assuming the plot
was derived from a real speaker, as opposed to a simulated load,
you'd find that the tweeter inductance rise is not the xlassic
doubling of inductive reactance with each octave, rather is
goes roughly as the square root of frequency. You will also find
a resistive component that goes as the square root of frequency.
This is due to the fact that the inductance is a complex, lossy
mechanism resulting from frequency-dependent eddy current losses
in the magnet structure: the higher the frequency, the more
significant the losses.
| Dick Pierce |
| Professional Audio Development |
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