How to measure EWIS or PS speaker zone impedance

Impedance is a measure of the opposition to the flow of an alternating current in a circuit; the aggregation of its resistance, inductive and capacitive reactance. Represented by the symbol Z.

An impedance meter is an absolute necessity for all PA/EWIS system installers and maintainers. An impedance meter for ?dummies? designed and manufactured by Altronics calculates and displays speaker loads in watts.

Most impedance meters on the market display the impedance in ohms, leaving the installer to calculate the system wattage.

The Q2001 Impedance Meter measures and calculates loads for both 100V and 70V line systems. Power is supplied from a 9V battery which can be recharged via the inbuilt charging socket.

Introduction

Measuring a transformer with a regular ohm meter or multimeter is very difficult and provides inaccurate results. This is because an ohm meter or multimeter uses DC to measure the resistance. Consider a transformer, which is basically a coil of wire, the DC resistance will be fairly low. If an AC signal is passed through the winding the impedance will be much higher, unless the winding is shorted. In this case the reading will be very low.

In the case of a short circuit the inductance of the transformer tries to impede the current flow through the winding. The nature of transformers will result in the impedance being reflected into other windings on the transformer being tested. This means a shorted turn on any winding will show up on the other windings.

An impedance meter is also critical for accurately measuring low impedance loudspeaker systems, incorporating multiple speakers and crossovers, and for measuring commercial 100V line speaker systems, which may include many speakers with line transformers and several hundred meters of cable. Trying to measure this type of load is virtually impossible with a standard multimeter.

How to measure impedance

1. Make sure the amplifier has been completely disconnected from the line to be measured.
2. Connect the test leads to the Q 2001 impedance meter
3. Connect the test leads to the speaker line.
4. Once your meter is connected press the TEST button for approximately 1 second, the impedance meter will display "CHECKING LOAD" for approximately 3 seconds while it checks the speaker load. You will be able to hear a tone from the speaker load that is under test. After 3 seconds the load impedance will be displayed on the LCD along with the 100V load value. After 8 seconds the 70V load value will replace the 100V load value on the LCD.

Why measure and record speaker line impedance?

It is important to measure the impedance of a speaker line periodically on a PA or EWIS system to identify if there is sufficient capacity within the amplifier for the connected load.

When should we measure impedance?

There are three occasions where impedance should be measured and recorded;

1. Speaker zone impedance should be measured during the first yearly inspection and test of an EWIS system, and then every three years thereafter.
2. Impedance should be measured and recorded on a service report after the addition, modification or deletion of speakers on a speaker zone.
3. Impedance should be measured prior to accepting an unconditional order to modify, or add speakers to an existing EWIS or PA speaker zone. The result of the speaker zone impedance should be then considered in conjunction with the availability of spare capacity on the associated amplifier.

For more information about impedance and measuring impedance, please consult with your supervisor or an appropriately trained professional.