From: DPierce@world.std.com (Richard d Pierce)
Subject: Re: How are speaker impedences specified?
Organization: The World Public Access UNIX, Brookline, MA
Date: Tue, 1 Sep 1992 13:10:05 GMT

Patrick W Lauson writes:
>John Stimson writes:
>>Shiv Naimpally writes:
>>>Impedance varies which is why you'll see impedance charts in some
>>>magazine reviews. Normally the speaker literature will say something
>>>to the effect that these speakers are nominally rated at 8 ohms (or
>>>whatever).                            ^^^^^^^^^
>>>This means that on the impedance chart, this is the minimum impedance
>>>the speakers have.

>>No.  I'm not sure how nominal impedance is specified, probably just by what
>>it seems like they could get away with calling it.
>>Vandersteens (2Ci) are nominally 7 ohm, but get down to 4.
>>My Infinitys (Ref-3, anyone want to buy a pair?) are nominally 6 ohm, and
>>I've measured them down to 4 - they possibly go even lower, since they
>>seem to present a nasty load to my amp, wher Vandersteens do not.
>>
>Most companies rate their speakers at either 4 or 8 ohms. An 8 ohm speaker
>usually reads about 6 ohms DCR, and will have a minimum impedance of
>about 6.5 ohms or so....  Its a messed up system, but not nearly as messed
>up as the way some companies do their power ratings (ahem.....ADVENT).

Okay guys, how about an expert opinion (Sure, Dick, and who do you have
in mind? ;-)

There is no hard and fast rule, no international standards for rating
impedance that are in any way universally accepted, but here's a general
methodology that mnay manufacturers seem to conform to:

The impedance of a dynamic loudspeaker varies from a minimum to a maximum
in the generally accepted audio bandwidth (20 Hz - 20 kHz). Usually, the
maximum is in the bass somewhere, corresponding to the mechanical resonance
of the bass driver, and can be anywhere from 2 to 20 times the DC resistance
of the bass driver. There may be two peaks if the speaker is a vented system
(why two? left as an excercise to the student...).

Often, there will be a local minimum in the impedance between 200 and 400 Hz,
this is the "nominal" impedance of the bass driver. The impedance will then
begin to rise, a combination of the increasing impedance due to the voice
coil inductance as well as the crossover effects. A local maximum will often
be reached right around the crossover point. Then, another local minimum will
be reached around the "nominal" impedance of the tweeter. the impedance will
then begin to rise again with increasing frequency.

The situation is more complicated with three-way system with complex
crossovers, and there may be local minima that are lower than the DC
of any of the drivers.

Now, those among the speaker manufacturers that are considered "responsible"
will pick the minimum impedance point in a frequency range that "matters" as
the "nominal" impedance. What matters is often a tough choice. For example,
there is far more energy content, spectrally, from normal music in the region
between 150 and 500 Hz than there is between 5 kHz and 15 kHz. In one sense,
as an indicator of the necessity of the amp to supply power to the speaker, it
makes sense to specify the "nominal" impedance as either the average or
minimum impedance in the region of 150 to 500 Hz. 

But some receivers are marginally stable in the region of 5 kHz to 15 kHz 
when presented with loads that are both low in average inpedance and (more 
importantly) those with impedances that change a lot with frequency (although
there is an absolute maximum rate that the impedance can change with 
frequency. The theoretical maximum is doubling or halving with frequency; 
the practical maximum, since the theoretical represents a purely reactive 
load and thus cannot produce and sound, is about half that in reality).
However, it doesn't seem that it's the speaker manufacturer's responsibility
to make up for the amplifier manufacturers design incompetence.

Thus, you will most often see the "nominal" impedance rating as that impedance
that prevails in the region of the upper bass and low midrange, around 150 to
500 Hz or so.

As an aside, the concept of "4 Ohm", "8 Ohm" and other such "standard"
impedance values is more historical artifact based on some bogus assumptions
as anything else. In fact, there was a time when speakers were color-coded
by impedance, just like resistors, and there was a standard color coding
that went with each "standard" impedance:

    Impedance   Color
       3.2      white
        4       violet
       5.6      pink
       6.5      orange
        8       red
       11       brown
       16       yellow
       23       black
       32       green
       45       blue

(from EIA STandard RS-276-A, "Acceptance Testing of Dynamic Loudspeakers,"
rev of April 1980)

There is no such thing as a "standard" 4 ohm or 8 ohm impedance.

-- 
|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
|       (508) 433-9183 (Voice and FAX)      |
Newsgroups: rec.audio