Thanks very much for the Pi 7 plans. I came across (and purchased) a a set of drivers from a late 60s Curtis Mathis stereo console unit. The woofers are a stamped basket 12" with paper cone and cloth surround, no markings other than "29B49-1" which I presume is the model number. The tweeters are a 2x6 metal horn tweeter, no markings other than "V/60". The crossover consists only of a single capacitor (4.7 uf 25VDC), no coil or anything else. The drivers are in pristine condition and I would really like to incorporate them into a Pi 6 (Theater) design. Can you offer any suggestions on determining the specs on these speakers ? Also- please email the plans for the Pi 6.

Thanks !

-Vello

You'll need a signal generator and meter or scope. Put a test resistor in series, something between 10 and 1000 ohms.

Find Re. It is the DC resistance of the voice coil.
Find Fs. It is the frequency where impedance is highest.

To find mechanical and electrical Q values, the following formulas are used:

Qms = Fs * (Zmax/Re)0.5 / (Fh - Fl)
Qes = Qms / (Zmax/(Re - 1))
Qts = Qms * Qes / (Qms + Qes)

where

Fs is the resonant frequency of the speaker in free air (Hz)
Zmax is the impedance of the speaker at resonance in free air (ohms)
Re is the DC resistance of the voice coil (ohms)
Fh is the frequency above Fs where speaker impedance is (Zmax*Re)0.5
Fl is the frequency below Fs where speaker impedance is (Zmax*Re)0.5

Note: Fl and Fh can also be found at the points where voltage across the test resistor is equal to Vq in the follwing formula:

Vq = (Vmax*Vmin)0.5

where

Vmin is the voltage across the resistor at the speaker's resonant frequency
Vmax is the voltage across the resistor at a frequency far from resonance

To find Vas using the sealed box method, the following formula is used:

Vas = Vb((Fb / Fs)2 - 1)

where

Vb is volume of the sealed cabinet (ft3, m3 or liters)
Fb is the resonant frequency of the speaker in the box (Hz)
Fs is the free-air resonance of the speaker (Hz)