You are correct.

With speakers, resistance varies with frequency, so the rated impedance is
usually referred to as "nominal" impedance, or functional average of the
load on the amp.

DC resistance is as you said, merely the resistance of the voice coil and
usually lower than the nominal impedance. It is safer to design low
impedance systems around the DC resistance as then you have some fudge room
on the load the amp sees and the actual impedance will be higher than the DC
resistance.

-- FLINT

"Denny B" wrote in message
...
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.

Thanks in advance
Denny B

On Wed, 16 Jul 2003 22:57:36 -0600, "Denny B"
wrote:

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.

Thanks in advance
Denny B


Hello,
That's easy; you're right and they're wrong. Although impedance can be
calculated by using an AC voltmeter to measure the signal voltage and
an AC ammeter to measure the current, then dividing the voltage by the
current.

""Denny B" wrote in message


Is it possible to measure Impedance with an ohm meter.

Only at DC.

For example can you measure the Impedance of a 4 ohm
car speaker with an ohm meter?

Ironically, most speakers have a nominal impedance that is about the same or
a tad more than their DC resistance. A 4 ohm car speaker will have a DC
resistance that is close enough to 4 ohms to convince most people.

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.

You're right. They are wrong, at least in theory.

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

You're right. They are wrong, at least in theory.

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

The ironic part is that the DC resistance of most speakers is close to the
nominal impedance.

htmlinput type crash/html
begin On Wed, 16 Jul 2003 22:57:36 -0600, Denny B f wrote:
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?
no. An ohmmeter uses a dc test signal.

Robert Rowton wrote in message . ..
On Thu, 17 Jul 2003 12:29:05 +0000 (UTC),
(Richard D Pierce) wrote:

In article ,
Robert Rowton wrote:
Actually, now that I think of it (Doh!), impedance can be calculated
with just an AC voltmeter and a series resistor.

GO to google.com, search the rec.audio.tech archives for the
word "impedometer."

Interesting reading, thank you.

Strictly speaking, you can measure the impedance MAGNITUDE only
with a generator, a series resistor and an AC voltmeter.

True, determinations based upon indirect measurement, and their
resulting inferences, are necessarily incomplete.

No, that's not what I meant and, in the context I was writing, your
statement is not correct.

When you use the impedometer method to measure impedance, you can
get a DIRECT measurement of the modulus of impedance or impedance
magnitude to any desired accuracy your time and equipment allow you
to. With care and the right equipment, 3% accuracy is trivially
easy, 1% is routine.

Now, what I meant by my comment was that without slightly more effort,
you cannot measure, for example, the difference between the real and
reactive (imaginary) part of the impedance or, equivalently, the phase
angle of the impedance. Add a scope or a phasemeter, and you now can.

But having ONLY the impedance magnitude in hand, you can derive the
impedance phase. Since it can be shown that the the effective transfer
function of the impedance magnitude is a minimum phase function, you
can then say that the impedance phase is a unique and completely
derivable function of the impedance magnitude.

There is no inference to this and the dervied phase function is NOT
going to be any less complete than the original measured impedance
magnitude data.

Going to an even more fundamental level, there is nothing inferential
or incomplete about the methodologies used to measure impedance
magnitude. It's not clear, then, what you are trying to say in the
phrase, "determinations based upon indirect measurement, and their
resulting inferences, are necessarily incomplete" has nay relevance
in the context of measuring the impedance magnitude function described.

"Denny B" wrote in message ...
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?

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

A fair number of people have responded in ways that provide
incomplete answers, so lety's see if we can do a somewhat more
complete survey of the situation.

In many cases, with sufficient added infromation, one can often
reasonably INFER the impedance from a measurement of the DC
resistance of a driver or speaker, with as I say, a number of
caveats. More on this later.

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

In this statement, all by itself, you are completely correct. In
the context of many loudspeakers and drivers, your are still correct,
but there is a common (but NOT INTRISIC) relation between the CD
resistance as measured with an ohmmeter and the rated or effective
impedance of a loudspeaker or driver. And this is where your friends
do not understand the situation.

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,

And in this statement you are not correct, but possibly because your
understanding is incomplete. But trhe fact that you got this wrong
doesn't make your friend right. He's just more wrong than you are.

When one sees a spec about the "nominal" impedance of a speaker,
the convention (and this convention is codified in one or more
national or international standards, i.e., IEC 268/60268-5), it
means a specific procedure has been applied to derive that and, in
the vast majority of cases, it is not done with a "fixed frequency"
but rather by examining the impedance vs frequency over the operating
bandwidth of the speaker. One procedure, for example, is to take the
the first impedance minimum above the impedance peaks in the bass as
the "nominal" impedance and use that. This minimum will clearly occur
at different frequencies for different speakers and requires a broadband
measurement to find that 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.

In this he is just plain wrong and you are correct. Tell him I said so
and if still wants to argue the point, argue it with me.

All that being said, it is USUALLY the case that the measured
DC resistance is about 80% of the nominal rated impedance, but
this, again as I said, is NOT an intrinsic relation. There are
a number of factors where this could be completely wrong and
your friend would find his position leading to completely
bogus results. Here are a couple of examples:

1. A number of speaker systems are capacitively coupled or
partially so as a matter of their design. It's a valid
design technique for achieving a particular system aligment.
Consider how your friend's position falls flat on its face
when he measures the DC resistance of a fully capacitively
coupled speaker: he'll measure, once the ohmeter settles
down, essentially INFINITE resistance. Now, what does your
friend do with THAT piece of data?

Consider also the case of a parallel RC aligment circuit
in series with the speaker: the DC resistance he measures
will be that of the woofer and the resistor in parallel.
Say that resistor is, oh 8 ohms and he measures 14 ohms
with his handy-dandy little ohmeter, and his estimate for
the nominal impedance is off by a factor of two, because the
impedance above the corner of this filter and at the mid-band
minimum is 7.4 ohms. Again, his happy little theory is blown
clear out of the water.

2. Consider a system made with a single 8 ohm woofer and two
8 ohm midranges in parallel. The measured DC resistance is
7.1 ohms, yet the impedance minimum above bass and thus the
rated system impedance is on the order of 3.4 ohms. Once again,
your friend's position is burnt toast.

3. Have your friend measure a Quad ESL speaker's DC resistance.
It's VERY low. Yet the rated impedance is 8-16 ohms. An ohmeter
measures the series resistance of the input transformer. You
can now spread marmalade on your friend's theory.

4. Have your friend measure the DC resistance of any piezo electric
tweeter around. It's essentially infinite.

All of the above examples are hardly theoretical or esoteric examples:
they all represent real measurements done of real speakers and systems
that ordinary people can and do buy.

If it is your friends contention that one definitively measures
the impedance of a speaker using an ohmmeter, he is, to put it simply,
just plain wrong.

"Jerry G." wrote in message ...
An ohm meter can only measure DC resistance.

Impedance is a complex measurement, because this is concerning a
reactive device, that will change its effective resistance to frequency.
Depending on the design of the device under test, its impedance may be
linear or non-linear with variance of frequency. The device may also be
a combination of resistive and reactive as a load. This gets in to a
complex situation where the actual impedance may differ from the DC
resistance. The effective resistance then has to be determined by the
amount of total current it pulls at a particular voltage and frequency.
At different frequencies, this will also vary because of the reactive
characteristics that it also has.

Wow, this is got to be one of the best examples of a collection
of basically the right words and phrases assembled in such a randomly
disconnected and unorganized fashion as to ensure it has no relevant
meaning whatsoever as I have ever witnessed in these here parts! :-(

Yes, impedance is "complex" but the term is used to relate to the
fact that our model of impedance requires the use of complex
values and operations to describe it the way we do.

And by "complex" we do NOT mean "complicated" (though, it seems,
it's complicated to this poster), we mean the use of complex
variable of the form:

a + bi

where a and b represent values and i represent the square root of -1.
a + bi is referred to as a complex number, were a represents the
"real" part and bi represents the "imaginary" part ("imaginary"
does not mean it is the figment of someone's imagination, but
rather it is a holdover from long ago when it was thought that
sqrt(-1) did not represent any real quantity). In the case of
describing and modeling impedance methematically, the complex
impedance is made of two parts, the resistive part, here represented
by real part "a" and the reactive component, represented by the
imaginary part "bi".

Read up on reactive load, and Volt Amp theory.

What is "volt amp theory?"

And, yes, reading up is sound advice, Mr. Greenburg, you might
consider giving it a shot sometime yourself.

The math can get very complex!

The math can be simple or complicated. Both cases use complex math.
Understand the difference?

"Denny B" wrote in message
...
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.

Sounds like you have it pretty clear to me. But why would you want to
measure the impedence - just to verify if likely a 4 or 8 ohm speaker ?


geoff

"Denny B" wrote in message
...
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.

Sounds like you have it pretty clear to me. But why would you want to
measure the impedence - just to verify if likely a 4 or 8 ohm speaker ? That
you can estimate very well with an ohmeter. The 8 will be around 6, and
the 4 will be around 3 .


geoff