1. Basic audio amplifier (voltage source) capable of providing a low distortion,
0-5V signal between 10-1000Hz.
2. Audio frequency wave generator (sinusiodal signal source).
If TTL output equipped, use to trigger frequency
3. Frequency counter capable of 0.1Hz resolution and adjustable
4. Digital AC/DC multimeter capable 1mV and 0.01 Ohm resolution
(voltage drop and resistance of source resistor, respectively)
5. Analog or Digital AC meter capable of 0.1V resolution (amplifier
6. A non-inductive, 5-10W, 8 Ohm resistor (source resistor).
7. Optional: phase meter (network analysis) or oscilloscope
The TS parameters are derived directly from the impedance modulus of the driver
operating within the bandwidth that captures the response at the
free-air resonance, Fs. The 4-wire method makes this task
straightforward requiring only five measurements to be taken. We have created an Excel
4-wire constant voltage TS
that sequences the tasks and performs the calculations required
to derive the driver Quality factors Qes, Qms and Qts. A
schematic of the setup shown below. Note that the
schematic shown is from a
functioning SPICE model of a loudspeaker in series with a source
resistor driven by a sinusoidal AC voltage source with a maximum
output of 3Vrms. Drive signal voltages with magnitudes
between 0.5-1.0V are reasonable for woofers. The analysis,
to be valid, assumes the magnetic force acting on the coil is
uniform, regardless of coil position.
1. Measure DC resistance of source resistor,
Rs, and driver
voice coil, Re. Input both values in spreadsheet.
2. Place source resistor in loop, apply amplifier voltage
and set voltage level, Vs (for example 0.5 Vrms). Input
value in spreadsheet.
3. Adjust the sine wave generator to a
frequency near the specification value of Fs. Adjust until the
voltage drop measured
across the source resistor is a minima, Vm.
Exact value of Fs occurs at Vm. Input measured Vm and Fs values
4. To establish the values of Fh and Fl,
adjust the sine wave generator to a frequency below Fs that
exhibits a source resistor voltage
drop equivalent to the spreadsheet parameter, Vr. Input the value in spreadsheet as Fl
(= frequency low). Repeat the procedure for
the frequency above Fs that exhibits a source
resistor voltage drop equivalent to the spreadsheet parameter,
Vr. Input the value in
the spreadsheet as Fh (= frequency high).
5. Once Fh and Fl meet check requirements, the
values of Qms, Qes and Qts are known at the drive signal