Speaker Impedance

[david.rogers]

How easy or otherwise is it to measure the impedance of a 2-way speaker? Is it possible without elaborate test equipment?

[NealG]

This used to be in the FAQ under Test and Measurement, crude but it works.

'It is possible to get a close approximation of the ‘speaker impedance with just a DVM, signal generator and load resistor.

Connect the ‘speaker to the amplifier with a ‘load’ resistor in series with the positive signal wire. Choose a resistor close to the manufactures stated impedance, a 7-10 Watt wirewound of 8 Ohms for example.

Now connect your signal generator to your amplifier and using a 1KHZ tone adjust both so that you drive approx. 3V AC RMS Sine Wave into the ‘speaker. Measure across the ‘speaker terminals. If the volume is uncomfortable reduce the voltage to a point where it is acceptable.

Now take two measurements for each frequency between say 20HZ and 20KHZ. Measure firstly across the load resistor (A) then across the ‘speaker terminals (B). There is no need to take readings at 1HZ steps! Up to 1KHZ measure in say 20HZ steps and above 1KHZ in 2KHZ steps until you reach 20KHZ.

Plot a chart of the frequency’s used against measurements A and B, a spreadsheet program is useful here!

The impedance of the ‘speaker is given as:

Z = R x Vs/Vr

Where Z is the impedance result, R is the Ohm value of the load resistor, Vs is the Voltage across the load resistor (A) and Vr is the voltage measured across the ‘speaker (B)

Calculate the impedance for each measurement and plot a graph of the results Y axis for the Z result and the X axis for the measurement frequency points. Alternatively use the spreadsheet program to automatically do this for you.'

[A Stuart]

Originally Posted by NealG

Connect the ‘speaker to the amplifier with a ‘load’ resistor in series...
Z = R x Vs/Vr
Vs is the Voltage across the load resistor (A) and Vr is the voltage measured across the ‘speaker (B)
'

Pedantic mode - Presumably Vs is voltage across speaker, Vr across resistor.
High speaker impedance would take most of the volts; Low speaker impedance would allow most of the volts to develop across the resistor.

[david.rogers]

Anyone got a signal generator to lend me?

Try this http://cgi.ebay.co.uk/AUDIO-TEST-TON...QQcmdZViewItem

Will.