Measure fr and Qt of a system loudspeakers
For those who do not want to make too many surveys it is important to at least
detect the resonant frequency of the speaker something for everyone.
To do this we will need a generator of sinusoidal signals that we will find
here then we will use the amplifier of our audio system (hoping to have
low output impedance ...) connecting the output of the sound card of our
pC to a line
input of our amplifier.
At this point you need a digital AC voltmeter (a common and inexpensive tester
is fine) I've also scale values in mV (millivolts) and a value sample
resistance of about 10 ohm meters with 1% tolerance and dissipated power
at least 1W.
The 10 ohm value is far from binding, the range of values that can fit is wide
(values between 4 and 20 ohms are fine) ... what is essential is the tolerance
to 1% if the ...
value of this resistance is not accurate relief that we will do will be
Prepared the link as shown in the figure below, turn on the generator and test
them out on your speakers.
Now that you have the connection choose depicted a test volume up to you (no
need to be high) and then, without further change the level, start with
frequencies from 120Hz to go down and follow the instruction given by the
voltmeter: no need to write down the values
if you do not care detect Qt, simply change the frequency until you find one
where the voltmeter will give the highest voltage indication (this with air
Make a note to the point the generator frequency: that is the resonance
frequency fr of your speaker.
After detecting the resonance frequency fr as above, we will fix varying the
volume, the voltage Va detected at the amplifier terminals to a value of 100mV
then, without touching the volume will need to measure the voltage downstream of
Rc (ie the loudspeaker terminals
) to the resonance frequency fr, we will call this value Vfr and will note.
The next step will detect the resistance in DC of the speaker that will be
equivalent to the resistance of the voice coil woofer + that of the crossover
coil in series with the woofer itself and which we will call the King. To
measure it you need to set in "Ohm" your meter and place
the ferrules on the speaker terminals (obviously disconnecting the amplifier).
From the value detected it will then be reduced by the resistance of a voltage
meter (average 0,2-0,3ohm) that simply by touching the tips you will find each
By entering these first four data in the underlying spreadsheet we will derive
the value of Vf1f2.
Taking care to keep always constant the value should be assumed to 100mV (will
be verified / corrected from time to time because it can change in function of
the linearity / impedance of your amplifier output) detect that the two
frequencies f1 and f2 for which the voltage at the terminals
speaker worth Vf1f2.
At this point you can compile the underlying spreadsheet that must be completed
by entering all the data on a light blue background to get the correct value of
To enter you must type the decimal point (not a comma).
At this point we will have the two fr and Qt data necessary to apply the
method of installation speakers.