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 online ready 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 distorted. Prepared the link as shown in the figure below, turn on the generator and test them out on your speakers.



fr measure
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 suspension speakers). Make a note to the point the generator frequency: that is the resonance frequency fr of your speaker.


Qt measure
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 other. 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 Qt. To enter you must type the decimal point (not a comma).

Va (mVolt)
Rc (Ohm)
Vfr (mVolt)
Re (Ohm)
Vf1f2 (Volt)

fr (Hz)

f1 (Hz)
f2 (Hz)



At this point we will have the two fr and Qt data necessary to apply the method of installation speakers.
Good job!


loudspeakers installation


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