How to Measure Speaker Impedance in an Audio System with a Multimeter

Hi!

How do I measure the impedance of a speaker in an audio system?


Thanks!

disconnect the speaker, and put an ohmmeter across the terminals. an 8 ohm speaker will read about 6 ohms or so.

Or if you have a LCR meter, you can measure Z directly... Or measure R, C and L separately and calculate the value of Z. But in a speaker L and C are almost negligible and you can measure only R and be fairly close to the actual Z.

Depends on why you want to know:

If it's just to verify if it's an 4Ω, 8Ω or 16Ω speaker:

* http://www.youtube.com/watch?v=cjis8GOuZl8

If you're designing crossovers or some other esoteric activity, then:

* http://www.youtube.com/watch?v=Sf0rx_-MipM
* http://www.youtube.com/watch?v=WS4wJmwf2gY

Hi there, Mr Asoke Kumar Mazumdar, Asst.Professor in Electronics Engg. (according to your page on this site)

It seems strange that an Asst.Professor in Electronics Engg. needs to ask such an incredibly simple question,

Please pardon me if I am misunderstanding you, but is it possible that you are boasting about a qualification that you do not actually possess ?

your observation is well taken about this guy, i don't usually look at their profiles if the poster will ask simple questions assuming they are a hobbyist.

but let's give the guy more room to explain.

Hi Asoke


Impedance changes with frequency (Quite important this There is no fixed impedance for a range of frequencies across the audio spectrum) normally these are quoted as true impedance and complex impedance mathematically defined as

Z = (R^2 =X^2) ^1/2
Or R = Z (Cos pheta)

For the most purposes you can use the following practical method

To measure speaker impedance you need the following
A variable frequency generator btw 10 hz and 40 kHz You also need either a oscilloscope or a test meter to measure the voltage manually

You want a 10 0hm resistor rated at 1% tolerance with a power rating of ½ watt or greater You also want a 200 ohm to 1000 ohm resistor 1% tolerance ¼ watt

The first step that you have to achieve is calibrating the DVM to read any ten units on the DVM that is 10% of the scale

Connect the signal generator to the 200R to 1000Ohm resistor on the hot side so that it appears you have a resistor in series with the Signal generator

Next connect the 10 ohm 2watt 1% resistor btw the signal ground side of the generator and the 200R resistor leading the signal out line Adjust the signal generator to give you an output on the meter of 1 volt After calibrating the meter with this value , Your meter on the ac volts range will reflect the impedance in ohms so for example 1 volts will be 10 ohms for example 0.8 volts will be 8 ohms, 0.4 volts will be 4 ohms

See Diagram A of the uploaded PDF file for better explanation



Now connect the speaker you are testing in place of the 10 ohm 2 watt 1% resistor obviously keeping the series resistor or R1 in place and removing the 10 ohm resistor

See Diagram B on the uploaded PDF file to explain this

Next plot an impedance curve on some graph paper recording each value that is the output on the ac volt meter for each frequency step of 10hz or 100 hz starting at 10 hz and slowly increasing the signal generators frequency to 25 khz , 30 khz in 10 or 100 hertz steps making sure you record each voltage out put on your graph plotted to frequency or simply use a spread sheet Excel

Use semi log paper to achieve this and take enough readings to produce a smooth curve

At one point you will have the resonant frequency This will look like a very steep curve Your average impedance will be the sum of all recorded ac volt values divided by the number of readings

Further information can be obtained from these books

Designing and building your own Speaker system
David B Weems ISBN 0-8306-3374-X

Enhanced Sound project
Richard Kaufman ISBN 0-8306-9017-4

The Loud Speaker Cookbook
Vance Dickenson ISBN 1-882580-10-9

This actually quite interesting because you will find nearly all manufacturers

PS all of them , always quote an impedance of a loudspeaker but never do you see them quote impedance at what frequency You will find the figures quoted especially on imports

Gone are the days of warfdale and decent speakers (Good old fashioned well made speakers costing lots of money ) These are the days when we also used to re cone because it was worth it

We only ever see trash (You have to laugh at some of this or cry I’m not sure ) but you will the figures quoted are in actual fact an absolute pack of fibbers or exaggerations designed to sell a speaker not advertise its true specs

Interesting isn’t it

Just shows you how bad our engineering standards are , not good at all really because everyone who wants to learn only ever hears the above No one ever really show you how or more importantly why

But if you want to know more go and buy these books The Vance dickenson book always, always works for loudspeaker cabinet design

Ive used this many a time and produced some of the best sounding boxes which have one many a competition especially in cars even with cheap speakers would you believe You can make any speaker sound very good if you do your maths correctly and dont just assume

Its not bad standards, but simply a lack of standards (or its enforcement) when it comes to loudspeakers. One reason could be that people who buy loudspeakers usually couldn't be bothered less about T/S parameters...for them it should just sound good (and by good it means lots of bass and/or loudness). There is no authority that polices manufacturers of speaker units, so basically its marketing hype and looks that actually gets the sales figures.

Its not as if looks don't matter...but open up any hi-end box and in all probability you will find a couple of cheap capacitors (and coils, if is one of those better ones) to implement the crossover.

That said, for most audiophiles it is also a matter of prestige/brand/ego in that cheap (i.e., less expensive) automatically means bad and that only adds to the "mystique" and confusion surrounding this whole speaker business. Only when we build our own speakers and go through the process of designing a box methodically as against hammering together a box and sticking a driver into it, do we realise how much BS is sold in the name of "hi-fi".

Coming to impedance of a speaker - there can be two things here: One is the impedance of the individual drivers and the other is the overall impedance of the whole speaker box (i.e., considering the drivers and crossover). For one the overall impedance is not of much significance apart from indicating what the amplifier's drive capabilities should be (for example is the overall impedance is quoted at 4 ohms, the amplifier should be able to drive any load impedance of 4 ohms or less).

Individual driver's impedance curve is a more meaningful property and this is used to determine that driver's T/S parameters which in turn are used to design a suitable box for it. There cannot be any single impedance for the driver (or loudspeaker) since it is frequency dependent and therefore most manufacturers only quote the impedance based on the value measured at about 500-600 Hz where the impedance is at a minimum and does not change much for frequencies an octave either ways. This is usually about the DC resistance plus 20%...there is a fair bit of "fuzziness" in this because an 8 ohms driver may have a DCR anywhere between 5-7 ohms.

Frank ,, its not possible to know 100% electronics even if you are Master or done Phd .. so its allowed to ask anything whatever comes in to your mind no matter its a simple question or hard one .. i think you got my point ,,thanks

hmmm... but I agree with Frank, an Asst Professor in Electronics Engineering should know this. Unless we have misinterpreted his question: could it be that he is really asking: "How is the impedance of a speaker typically measured in the industry, i.e. is there a standard technique?"

That is something that might not be known by an Asst Professor. I mean, is there a standard frequency that a speaker's impedance is spec'ed at? Perhaps his confusion arises from the _lack_ of a standard method.

A loudspeaker is a complex load - both individual drivers and their combinations along with crossovers present a frequency dependent load to the amplifier. And therefore there is no one impedance that can be called correct. So the usual norm is to simply state the impedance as being 15-20% greater than the nominal DC resistance of the voice coil. This is convenient, since the DC resistance is the smallest value that the coil can have and since the loudspeaker is operated with AC signals, its impedance will always be greater than the DCR due to the inductive reactance of the coil. The frequency at which the minimal impedance (DCR + lowest inductance) occurs lies mid-way of the relatively flat region of the impedance curve - this is the region where the impedance changes very gradually from being capacitive to becoming inductive - the region starts about an octave away from the resonance frequency.

For the most part, the frequency at which the speaker's nominal impedance is quoted is not important...its only purpose is to estimate if the amplifier being used with it will be able to drive it without overheating or overloading or clipping.

See this: http://en.wikipedia.org/wiki/Electrical_characteristics_of_dynamic_loudspeakers

However, it is possible to have a loudspeaker that has a fairly constant impedance across the frequency band. It requires use of impedance correction networks for each driver in the loudspeaker box. This network compensates the impedance variation of the driver beyond the resonance frequency so that now the driver presents a fairly resistive load to the crossover. This makes the crossover design somewhat simpler and predictable but then the impedance correction network itself more than nullifies any simplicity of the crossover.

Yes Dharmik, I agree, that is why phrased my reply in a very gentle manner, asking for clarification.

With a little google, I have discovered that such a person with the stated qualifications does actually exist, and that his specialisations do not include audio electronics.

I offer my apologies to Asoke for any feeling of insult I may have caused.

So Asoke, the simplest answer is this: The vast majority of loudspeakers are either 4, 8, 16 or 32 Ohms. As mentioned elsewhere on this page, the speaker's impedance consists of it's resistance and its inductive reactance.

However, as the inductance is usually small in relation to the resistance, to determine the impedance for most practical purposes, just check the DC resistance, and take the impedance as being whichever number, 4 or 8 or 16 or 32, is nearest to the measured value.

You can estimate the impedance somewhat like below:

4 ohms nominal = DCR 2.7 ohms - 3.9 ohms
6 ohms nominal = DCR 4.0 ohms - 4.7 ohms
8 ohms nominal = DCR 4.8 ohms - 6.0 ohms

However there is a fair bit of overlap between the "ranges" and therefore what is 4 ohms according to one manufacturer might be 6 ohms according to another. It is best to be conservative and assume the lower impedance in the interest of amplifier safety.

I am a isbn barcode designer and sometimes i need to measure the impedance of a speaker in an audio system.I'd like to share my tips with you.

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.
Step 1

Connect the red probe to the positive input and the black cable to the negative input on your multimeter. Change the units to ohms.
Step 2

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.
Step 3

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.
Tips

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.

Warnings

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.