Thank Dick,

Exactly what I was looking for.

George

"Richard D Pierce" wrote in message
...
In article E6Y_a.255$ae6.100@lakeread06,
George Collins wrote:
I'm looking for a source (on-line preferred) that explains how to measure
the Vas parameter using a sealed test box. Any suggestions of places to
look?

In brief, the complete method goes like this:

1. Measure voice coil resistance Re

2. Find the frequency at which the impedance is at
a maximum in the low frequency range (and verify
the impedance phase at being 0, if you can). This
is the resonant frequency Fs. Measure the impedance
at this frequency, call it ZMax.

3. Find the ratio of the DC resistance to maximum
impedance:

Ro = ZMax / Re

4. Find the two frequencies above and below Fs where
the impedance is equal to Re * sqrt(Rc), call them
Fl and Fh. As a check, confirm that:

Fs = sqrt(Fl * Fh)

to within a few percent.

5. Calculate the mechanical Q at resonance:

Qms = Fs * sqrt(Ro) / (Fh - Fl)

6. Calculate electrical Q at resonance:

Qes = Qms / (Ro - 1)

7. Calculate total Q at resonance:

Qts = Qms * Qes / (Qms + Qes)

8. Repeat the measurements above with the driver
mounted in a tightly sealed, unlined, unstuffed
test box with a volume of Vt. Note that ANY leaks,
such as porous dustcaps, secondary vents, inadequate
seals, etc., will all adversely effect the accuracy
of the measurement. Measure Fc, the resonant frequency
in the test box, the impedance ratio at resonance
Rc, the two frequencies above and below resonance
where Z = Re * sqrt(Rc), Flc and Fhc, and from that
calculate the mechanical and electrical Q in the
test chamber Qmc and Qec.

9. Calculate the equivalent volume of compliance:

a = Fc * Qec

b = Fs * Qes

Vas = Vt (a/b - 1)

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| Dick Pierce |
| Professional Audio Development |
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