How to Measure Speaker Impedance
By Steve McDonnell
Most speakers are either 4-ohm, 6-ohm, 8-ohm or 16-ohm speakers. Ohms are a measure of impedance, which represents the amount of power the speaker draws. Impedance is equal to voltage divided by current. This means that a 4-ohm speaker draws more current than a 6-ohm speaker; more current gives it more power and usually enables it to play loudly more easily. If you connect 4-ohm speakers to an amplifier that can only drive 6-ohm speakers, the speakers will draw too much current and the amplifier will probably overheat and shut itself off. You can measure the impedance of a speaker with a multimeter.
Connect the red probe to the positive input and the black cable to the negative input on your multimeter. Change the units to ohms.
Unplug the speaker if it has a power source. Identify the lead wires that connect to a terminal on the cone of the speaker and the wires that connect to an input jack on the speaker. Disconnect the input jack wires if possible.
Identify which side of the terminal on the speaker cone is positive and which is negative. Connect the red probe from your multimeter to the positive side and the black probe to the negative side. Read the impedance on the multimeter and round the number up. For example, if the multimeter reads 3 ohms, it's a 4-ohm speaker. If it reads 14 ohms, it's a 16-ohm speaker.
- Round the reading on your multimeter up Multimeter readings might be slightly different than a speaker's rating.The actual impedance when a speaker plays music varies significantly by the frequency of the sound. Actual impedance from an 8-ohm speaker might range from 5 ohms to 50 ohms.
- Since most speakers are 4-, 6- or 8-ohm, buying a receiver or amplifier rated for 4 ohms provides the greatest compatibility and ability to run multiple pairs of speakers at the same time.
- Look for warning stickers or labels on receivers or amplifiers that advise you not to connect a speaker with less than an 8-ohm rating. You might also have difficulty connecting more than one pair of 8-ohm speakers to these units.
Steve McDonnell's experience running businesses and launching companies complements his technical expertise in information, technology and human resources. He earned a degree in computer science from Dartmouth College, served on the WorldatWork editorial board, blogged for the Spotfire Business Intelligence blog and has published books and book chapters for International Human Resource Information Management and Westlaw.