# Can you measure Impedance with an Ohm Meter..

Discussion in 'Electronic Basics' started by Denny B, Jul 17, 2003.

1. ### Denny BGuest

Is it possible to measure Impedance with an ohm meter.
For example can you measure the Impedance of a 4 ohm
car speaker with an ohm meter?

When I tell people they cannot measure Impedance with an
ohm meter, you need an impedance meter and the impedance
must be measured with the speaker in operation, with a signal generator
supplying a fixed frequency, this always leads to disagreements
with the person who demands an ohm meter to measure what he calls
Impedance, when I tell him an ohm meter can only measure the resistance
of the coil there is always disagreement.

I tell them Impedance is AC resistance and what they measure
with an ohm meter is DC resistance. Disagreement here again.

Will somebody take me up on this and clarify the situation.

Denny B

2. ### \(one of the\) Chris EvansGuest

Denny, you are perfectly correct on all points, impedance is an AC
phenomenon and can only be measured using AC test signals. A component of
the resulting value will be dependant upon the DC resistance of the coil,
since AC signals are affected by DC resistance, but there will be a
component of the result that shows only with AC stimulation and this
component will have different values at different frequencies. A well
designed speaker in a well designed enclosure will exhibit a more or less
constant impedance across the intended range of working frequencies, but
this value will not be the same as the DC resistance of the coil. There are
a couple of ways that you may try to justify this (apart from the simple
"try it and see").

1) The speaker coil has resistance and inductance and capacitance. A DC
measurement will sense the resistance but not the inductance and capacitance
of the coil, however an AC measurement will sense all.

This is a gross oversimplification but illustrates the point that there
*must* be more than resistance involved.

2) Imagine applying DC only to the speaker. The speaker cone will deflect
by some amount and then remain stationary and silent. There will be a
standing voltage and current which will allow the calculation of a
resistance. We are supplying electrical energy to the speaker which is all
turning into heat in the coil (there is nowhere else for the energy to go)
and this is a resistance phenomenon. Now imagine applying a pure AC signal
to the speaker (no DC). The speaker cone will vibrate and is therefore
radiating sound energy continuously. As in the previous case we are
supplying electrical energy to the speaker, and as in the previous case some
of that energy is heating the resistive component of the coil in the same
way as before, but the speaker is now also radiating energy in the form of
sound and this energy must be coming from the electrical stimulation we are
supplying.

In the first case the electrical energy is dissipating in only one way, and
we can express that as a resistance only. In the second case the electrical
energy must be dissipating in the resistance in exactly the same way as
before, but we are now also dissipating energy from the AC source in the
form of sound and this energy must be coming from the electrical input. The
additional energy demand to suppply the sound "appears" to the electrical
input as an impedance (more correctly a dynamic impedance, since it is
necessarily a phenomenon which only shows when the cone is moving) which is
in addition to the resistance that is "seen" in the DC case. The impedance
of a speaker will always be greater than the resistance, to account for the
additional energy that must be supplied to make the sound.

The above simplifies the physics, since there are other effects: the coil
and wiring will hace some capacitance but this is very small; some energy
will also be lost to the physical movement of the cone (frictional lossess
and the energy needed to flex the cone suspension which will appers as heat
in the cone and cone suspension); inductive coupling between the coil and
surrounding metalwork (including the magnets) will induce currents which
will dissipate as heat in the metals. There are probably more I have not
your thought of. Well designed speakers in well designed enclosures will
minimise the other effects, and in fact an ideal speaker would have dynamic
impedance and no resistance (since resistance represents wasted energy and
thus inefficiency).

Chris

3. ### RatchGuest

You are correct. Impedance is the vector like sum of resistance and
reactance. Reactance is 90° out of phase with respect to resistance. A
battery driven ohmmeter can only measure DC resistance. It could be said
that it measures the impedance at 0 HZ where the reactance is zero.
Impedance is not AC resistance by itself. AC resistance occurs at higher
frequencies when the charge flows more near the surface of the conductor and
less at the center. This reduces the cross section of the conductor and
thereby increases the resistance. The resistance and reactance both reduce
current, but in a different way. Resistance is caused when the charge
carriers collide with the material lattice causing it to vibrate faster.
This causes a loss of drift velocity and kinetic energy for the charge
carriers, and a increase in kinetic energy for the material lattice. The
overall effect is to increase the temperature of the material and decrease
the current. Reactance is caused by a back voltage occuring, which
decreases the current and stores energy in either a electromagnetic or
electrostatic field. The energy is taken and returned with no loss during
parts of the AC cycle. If the coil is attached to a speaker, the impedance
will change because energy is being used to produce sound. This is called
acoustical coupling. Ratch

4. ### Dan FraserGuest

No you cannot measure impedance with an ohm meter but in the case of
speakers, you can guess reasonably well.

8 Ohm speakers often have a DC resistance between 4 and 6 ohms.

4 ohm speakers have DCR between 2 and 3 ohms

and so on.

The speaker impedance will always be higher than the DCR but not a lot
higher.

Therefore I can make a good guess about the impedance of a speaker by
measuring the DC Resistance [DCR] with an ohm meter.
--
Dan Fraser

From Costa Mesa in sunny California
949-631-7535 Cell 714-420-7535

Check out my electronic schematics site at:
http://www.schematicsforfree.com
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