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Loudspeaker Impedance?

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Anonymous
June 13, 2005 7:44:54 PM

Archived from groups: rec.audio.tech (More info?)

I have a question regarding loudspeaker impedance. It is normally
rated as 8 Ohms. What does this mean? Is this the impedance magnitude
at some frequency?

I know that a dynamic speaker consists of a coil in a permanent
magnetic field. The current through the coil produces a magnetic field
that interacts with the permanent field, producing a force. The force
acts against the air resistance of the speaker cone. So, we at least
have the inductance of the coil, a back emf, stray interwinding
capacitance, coil ohmic loss and an acoustic radiation resistance.

Any experts on this?

-JJ

More about : loudspeaker impedance

Anonymous
June 13, 2005 10:34:36 PM

Archived from groups: rec.audio.tech (More info?)

Arny,

Yes, it is a classic mass-spring-damper mechanical system.

So, what does a manufacturer mean by "8 Ohm speaker"? Is this intended
to mean that that the impedance magnitude is about 8 ohms at 1 kHz?
There must be some intent.

-JJ

Steve Urbach wrote:
> On 13 Jun 2005 15:44:54 -0700, wizard12342002@yahoo.com wrote:
>
> >I have a question regarding loudspeaker impedance. It is normally
> >rated as 8 Ohms. What does this mean? Is this the impedance magnitude
> >at some frequency?
> >
> >I know that a dynamic speaker consists of a coil in a permanent
> >magnetic field. The current through the coil produces a magnetic field
> >that interacts with the permanent field, producing a force. The force
> >acts against the air resistance of the speaker cone. So, we at least
> >have the inductance of the coil, a back emf, stray interwinding
> >capacitance, coil ohmic loss and an acoustic radiation resistance.
> >
> >Any experts on this?
> >
> >-JJ
> Speaker impendence varies with frequency. My Altec 605A speakers
> (nominal 16 ohms)have a peak (64 ohms) impendence at Mechanical (cone)
> resonance frequency in FREE AIR. When installed in a Bass reflex
> Ported enclosure designed for this speaker, the impendence curve goes
> from the Mountain peak at 25Hz to a 2 hump camel shape with the dip
> at 25Hz. (I plotted this by hand years ago on 2 different system. Very
> tedious then)
>
> , _
> , | \ MKA: Steve Urbach
> , | )erek No JUNK in my email please
> , ____|_/ragonsclaw dragonsclawJUNK@JUNKmindspring.com
> , / / / Running United Devices "Cure For Cancer" Project 24/7 Have you helped? http://www.grid.org
Anonymous
June 13, 2005 11:07:05 PM

Archived from groups: rec.audio.tech (More info?)

wizard12342002@yahoo.com wrote:
> I have a question regarding loudspeaker impedance. It is normally
> rated as 8 Ohms. What does this mean? Is this the impedance
> magnitude at some frequency?

It depends. If the speaker is rated according to one of several
generally similar standards, such as IEC 60268-5 or national
derivatives thereof, it has a very specific meaning, to wit:

"The rated inmpedance specified by the manufacturer nor-
mally represents the lwest value of the modulus of the
impedance in that part of the rated part of the frequency
range, where the maximum power is to be expected, and is
normally not more 20% higher than the lowest value of the
modulus of the impedance at any frequency within the rated
frequency range"
IEC 268-5, 7.2 Rated Impedance

The problem being is that not a lot of manufacturers pay any
attention to standards like this. So, unfortunately, the
answer to your first question is, "it depends."

> I know that a dynamic speaker consists of a coil in a permanent
> magnetic field. The current through the coil produces a magnetic
> field that interacts with the permanent field, producing a force.
> The force acts against the air resistance of the speaker cone.
> So, we at least have the inductance of the coil,

Which is not a major contribution to the impedance until high
frequencies.

>a back emf

Forget back EMF. While it is, in and of itself, part of the
mechanism involved, a simple adherence to "back EMF" will
lead you quite astray.

> stray interwinding capacitance,

Which has almost NO contrinuting effect whatsoever.

> coil ohmic loss

One of the most important components.

> and an acoustic radiation resistance.

Which for vast majority of direct-radiator loudspeakers is
utterly insignificant.

> Any experts on this?

Well yeah.

Rather than getting yourself all wound up in a knot worrying
over a lot of the insignificant pieces and misleading stuff
like you mentioned above, start with a 30,000 foot and it
will all make better sense.

Consider a loudspeaker to be, really, two transformers hooked
in series. The first is an electrical-to-mechanical transformer.
Just like a transformer, it converts from one level (in electrical
terms) to another (but in mechanical terms). And, JUST LIKE A
TRANSFORMER, the impedance you see on one side is reflected on
the other, simply scaled by the transformer. Most importantly,
the MECHANICAL impedance on the secondary is manifested as
an ELECTRICAL impedance on the primary. And, to descend one
more level closer ot the ground, the mechanical side has a
network of mechanical components such as mass (the cone or
the mechnical inertance of the port), stiffness (the driver
suspension and the cabinet stiffness) and friction (suspension
losses, absorbtion and so on).

As mentioned the electrical-to-mechanical transformer works
just as nicely as a mechanical-to-electrical transformer, and
the combination effect of all these masses, stiffness and
frictional losses that live on the secondary side of the
transformer in the mechanical domain directly manifest them-
selves on the primary side as electrical capacitances, induc-
tances and resistances.

Well, what, you might ask, is the "transformer?" Very simple:
it's the electrical motor, the voice coil and magnet. And the
"turns ratio" is simply a quantity called the "Bl product,"
the product of the field strength and the length of wire
immersed in that field.

In the simplest case of a driver, where you have a moving mass,
a sitffness and a firctional loss, this mqnfiests itself in
the electrical domain as a simple parallel resonant circuit,
where the mass reflects itself as a capacitance, the sitffness
as an inductance, and the friction as a resistance.

And, indeed, you will find that that the electrical impedance
of such a driver (ignoring, for the purposes of simplicity,
nonlinearities) IS IDENTICAL TO A PARALLEL COMBINATION OF A
CAPACITOR, AN INDUCTOR AND A RESISTOR, all in series with
another resistor, the DC electrical resistance of the voice
coil.

And by identical, I mean just exactly that: an amplifier sees
no difference, either under steady state or transient conditions,
between the circuit I just described, and a real driver.

The effective component values for a typical mid-sized woofer
might be on the order of a dozen millihenries, 400-500 microfarads,
and a dozen or three ohms (and the 6-8 ohms for the voice coil
DC resistance), and are MUCH larger than the effects you mention
(voice coilm inductance, which is maybe 1 millihenry or so,
stray capacitance, on the order of a few nanofarads, and so on).

The result is an impedance which at VERY low frequencies is
dominated by the simple DC resistance of the voice coil, but
so you go higher in frequency and approach the mechanical
resonance, rises and is inductive until you reach resonance,
where the impedance it at a maximum and resistive, and then
the impedance falls with increasing frequency and is capacitive
until it starts to approach the DC resistance of the voice coil
resistance, and only then does the raw voice coil electrical
inductance start to make any difference, and the impedance
starts to rise again.

Through all this, the impedance, though rising and falling with
frequency, is still predominantly resistance, as indicated by
the fact that the impedance phase angle rarely if ever approaches
or exceeds +- 45 degrees (45 degrees indicates an equal amount of
resistance and reactance, more than 45 degreesm and the reactive
part dominates, and the impedance can NEVER exceed 90 degrees
(or, if there is ANY loss in the system, even get to 90 degress).

Now, you wonder where that second transformer is: Well it's the
mechical-to-acoustical transformer. That transformer physically
is the emissive area of the cone, and it's coupling ratio is,
generally very small in direct radiator loudspeakers, which is
why such speaker aren't very efficient. As a result, the acoustical
impedance (radiation impedance) is poorly coupled to the mechanical
domain and has but a verty small effect on the mechanical impedance,
and thus has a very small effect on the electrical impedance.
Measure the impedance of a loudspeaker in air and in a vacuum and
(irgnoring the case of a bass reflex, for example), you'll find
verry little difference in the impedance (reflex speaker can
be considered to use the mechanicla properties of the box
volume and port to opperate: we're instead referring to the
radiation impedance having a small effect).

Now, through all of this, it's important to note that in the
vast majority of electrodynamic speaker, one element really
predominates, and that is the voice coil DC resistance. It's
where most of the power is dissipated (and gets appropriately
hot).

Now, notice through all that explanation, I never ONCE used
the term "bacl-EMF." This is because it's unnecessary and
misleading. People look at back-EMF as if it is the be-all
and end all and wind up making all sorts of erroneous and
misleading conclusions. Yes, it's the physical mechanism
at work in the electrical-to-mechanical transformer, but it's
no more mysterious, magic or powerful and, in fact, works
EXACTLY the same way as the magnetic interaction in a real
transformer: they both involve the relative motions of con-
ductors, currents and magnetic field. Nothing more, nothing
less.

Instead, if you're looking at the loudspeaker from its
electrcial terminals, simnply look at it as one big electrical
circuit, consisting of capacitors, inductors, and resistors,
because that's EXACTLY what the amplifier sees. And, indeed,
if you're working in the electrical domain, you can look at
the electrical portions of the circuit, the DC resistance, the
inductance, even the crossover compoenents, as mechanical
elements of stiffness, mass and friction., and similarily
in the acoustical domain.
Related resources
Anonymous
June 14, 2005 12:19:33 AM

Archived from groups: rec.audio.tech (More info?)

wizard12342002@yahoo.com wrote:
> I have a question regarding loudspeaker impedance. It is
normally
> rated as 8 Ohms. What does this mean?

It means that the speaker's impedance is rated at 8 ohms,
and often little else.

> Is this the impedance magnitude at some frequency?

Maybe, maybe not. There are such things as speakers rated at
8 ohms whose impedance is not 8 ohms at any frequency. There
are other speakers rated at 8 ohms whose impedance is 8 ohms
at from two to 6 frequencies.

> I know that a dynamic speaker consists of a coil in a
permanent
> magnetic field. The current through the coil produces a
magnetic
> field that interacts with the permanent field, producing a
force.

Yup, the core of a speaker is an electromagnet.

> The force acts against the air resistance of the speaker
cone.

The force also acts against the mass of the speaker cone and
the springiness of the cone's suspension.

> So, we at least have the inductance of the coil, a back
emf, stray
> interwinding capacitance, coil ohmic loss and an acoustic
radiation
> resistance.

Plus the other stuff I mentioned.
Anonymous
June 14, 2005 4:46:21 AM

Archived from groups: rec.audio.tech (More info?)

On 13 Jun 2005 15:44:54 -0700, wizard12342002@yahoo.com wrote:

>I have a question regarding loudspeaker impedance. It is normally
>rated as 8 Ohms. What does this mean? Is this the impedance magnitude
>at some frequency?
>
>I know that a dynamic speaker consists of a coil in a permanent
>magnetic field. The current through the coil produces a magnetic field
>that interacts with the permanent field, producing a force. The force
>acts against the air resistance of the speaker cone. So, we at least
>have the inductance of the coil, a back emf, stray interwinding
>capacitance, coil ohmic loss and an acoustic radiation resistance.
>
>Any experts on this?
>
>-JJ
Speaker impendence varies with frequency. My Altec 605A speakers
(nominal 16 ohms)have a peak (64 ohms) impendence at Mechanical (cone)
resonance frequency in FREE AIR. When installed in a Bass reflex
Ported enclosure designed for this speaker, the impendence curve goes
from the Mountain peak at 25Hz to a 2 hump camel shape with the dip
at 25Hz. (I plotted this by hand years ago on 2 different system. Very
tedious then)

, _
, | \ MKA: Steve Urbach
, | )erek No JUNK in my email please
, ____|_/ragonsclaw dragonsclawJUNK@JUNKmindspring.com
, / / / Running United Devices "Cure For Cancer" Project 24/7 Have you helped? http://www.grid.org
June 14, 2005 6:28:13 AM

Archived from groups: rec.audio.tech (More info?)

On 13 Jun 2005 18:34:36 -0700, wizard12342002@yahoo.com wrote:

>Arny,
>
>Yes, it is a classic mass-spring-damper mechanical system.
>
>So, what does a manufacturer mean by "8 Ohm speaker"? Is this intended
>to mean that that the impedance magnitude is about 8 ohms at 1 kHz?
>There must be some intent.

It means that's about the minimum impedance, within the audio band.
Anonymous
June 14, 2005 2:15:35 PM

Archived from groups: rec.audio.tech (More info?)

wizard12342002@yahoo.com writes:

> I have a question regarding loudspeaker impedance. It is normally
> rated as 8 Ohms. What does this mean? Is this the impedance magnitude
> at some frequency?

There is a convention to the use of the term "nominal impedance",
specifically covering the range in spectrum where majority of the
musical spectral power occurs. Typically speaker impedance
is 8 ohms (most home speakers) or 4 ohms (most car systems and
some home speakers).

You must keep in mind that 'nominal impedacnce' is not defined in IEC,
and not recommended to be used. That is why the IEC concept of 'rated
value' is so useful. There is a very detailed definition and
explanation of this term in IEC60268-2. The IEC standard (IEC60268-3)
allows any "increase" above the rated value, but limits the
"decrease". The standard does not allow the impedance to fall below
the 80 % of the nominal value at any frequency, including DC.

> I know that a dynamic speaker consists of a coil in a permanent
> magnetic field. The current through the coil produces a magnetic field
> that interacts with the permanent field, producing a force. The force
> acts against the air resistance of the speaker cone. So, we at least
> have the inductance of the coil, a back emf, stray interwinding
> capacitance, coil ohmic loss and an acoustic radiation resistance.
> Any experts on this?

More details:
http://www.epanorama.net/documents/audio/speaker_impeda...

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/
Anonymous
June 14, 2005 2:38:06 PM

Archived from groups: rec.audio.tech (More info?)

Arny Krueger wrote:

> Maybe, maybe not. There are such things as speakers rated at
> 8 ohms whose impedance is not 8 ohms at any frequency.

You mean a 16 ohm speaker? Care to elaborate?

--
Eiron.
Anonymous
June 14, 2005 7:21:46 PM

Archived from groups: rec.audio.tech (More info?)

Arny Krueger wrote:
> wizard12342002@yahoo.com wrote:
> > I have a question regarding loudspeaker impedance. It is
> normally
> > rated as 8 Ohms. What does this mean?
>
> It means that the speaker's impedance is rated at 8 ohms,
> and often little else.
>
> > Is this the impedance magnitude at some frequency?
>
> Maybe, maybe not. There are such things as speakers rated at
> 8 ohms whose impedance is not 8 ohms at any frequency.

Could you enumerate examples of such?

> are other speakers rated at 8 ohms whose impedance is 8 ohms
> at from two to 6 frequencies.

and not 1 or 7? :-)

> > The force acts against the air resistance of the speaker
> cone.
>
> The force also acts against the mass of the speaker cone and
> the springiness of the cone's suspension.

If we're going to go to this level of detail, why not be a bit
more complete?

In the simple example of a single driver, that force, below
resonance, acts primarily against the mechanical stiffness
of the driver's suspension, a reactance. Above resonance, it
acts primarily against the mass of the speaker cone, again
a reactiance. At resonance, it acts primarily against the
frictional losses of the driver suspension.

For speakers in enclosures, you can then add the enclosure stiff-
ness, the port mass, leakage losses and the like, but one hopes
the idea gets across.

And across the entire range, a very SMALL part of this force acts
against the driver's radiation impedance, which consists of both
a resistive and a reactive part.

> > So, we at least have the inductance of the coil, a back
> emf, stray
> > interwinding capacitance, coil ohmic loss and an acoustic
> radiation
> > resistance.
>
> Plus the other stuff I mentioned.

Which, as I posted elsewhere, FAR exceed these minor effects.
June 15, 2005 4:17:05 AM

Archived from groups: rec.audio.tech (More info?)

On 14 Jun 2005 10:15:35 +0300, Tomi Holger Engdahl
<then@solarflare.cs.hut.fi> wrote:

>wizard12342002@yahoo.com writes:
>
>> I have a question regarding loudspeaker impedance. It is normally
>> rated as 8 Ohms. What does this mean? Is this the impedance magnitude
>> at some frequency?
>
>There is a convention to the use of the term "nominal impedance",
>specifically covering the range in spectrum where majority of the
>musical spectral power occurs. Typically speaker impedance
>is 8 ohms (most home speakers) or 4 ohms (most car systems and
>some home speakers).

Most "high end" home speakers are 4, though.
Anonymous
June 15, 2005 11:58:01 AM

Archived from groups: rec.audio.tech (More info?)

Eiron wrote:
> Arny Krueger wrote:
>
>> Maybe, maybe not. There are such things as speakers rated
at
>> 8 ohms whose impedance is not 8 ohms at any frequency.
>
> You mean a 16 ohm speaker?

No, I mean an 8 ohm speaker whose impedance never actually
goes below 8.1 ohms.
!