[Elecraft] speaker impedance

[Jim Brown]

On Wed, 14 Jan 2004 20:16:47 -0600, John Cooper wrote:

>I have some 8ohm .5W computer speakers and a 8ohm .25W speaker.  
>Could these be used as is or with some modification.  Also hoping someone
>could kind of explain what imdedance is to me. I believe its the resistance of the
> speaker itself, but I could be wrong.  

Impedance is the ratio between the voltage and the current in any
device. In that respect, is "sort of" like resistance, but with a very
important difference. Impedance includes resistance, inductive
reactance, and capacitive reactance. 

If you measure the terminals of a loudspeaker with an ohm meter, you
will measure part of the resistance -- to be exact, you will measure
the DC resistance of the voice coil. But in any loudspeaker, there is
additional resistance that represents the transfer of energy from the
moving diaphragm to the air. And there is resistance from power that is
lost in the motional parts of the loudspeaker. 

So far, we've only talked about resistance. Any mechanical transducer
will also have inductive and capacitive components to their impedance.
These come from the stiffness of the diaphragm, the compression of air
in the loudspeaker enclosure, and a bunch of other physical things. The
equivalent circuit of a loudspeaker is pretty complex, and typically
has several resonances that result from both electrical and mechanical
elements. The coil has inductance, and there is capacitance between its
turns. etc. 

One common way to measure the impedance of a loudspeaker is to create a
constant current source by putting a large value resistor in series
with an audio oscillator, and use that series combination to drive the
loudspeaker. As you do that, hang an AC voltmeter across the
loudspeaker and read the voltage, and use the voltage divider equation
to determine the impedance. 

The tricky part is that the impedance of ANY loudspeaker varies a LOT
with frequency. The Standard for defining impedance is the MINIMUM
value of the impedance. So to know the impedance of a loudspeaker, you
need to sweep the oscillator and watch the meter to find the lowest
voltage. For most small cone loudspeakers, this happens somewhere
between 400 Hz and 4 kHz. If it's a 2-way loudspeaker (woofer and
tweeter), you'll probably see multiple dips, one for each driver. 

Now, to the other part of your question -- can I use the loudspeakers
that are in my junkbox.  And why do you want to use a 4 ohm loudspeaker
or an 8 ohm loudspeaker or a 16 ohm loudspeaker?  

When you are building an audio power amp (which is what drives a
loudspeaker), the power you can get depends on the voltage and current
available. You can get one watt from 1 volt times 1 amp, which would be
a 1 ohm load. You could also get one watt with 3 volts and 1/3 amp,
which would be a 9 ohm load. And you could do it with 10 volts and 0.1
amp, which is a 100 ohm load. But you have to put that power into an
impedance, and the loudspeaker designer has to go through the same
choices -- how many turns on the coil, how stiff, how big, etc. Most
power amps, including the one in the K2, are constant voltage devices,
so if you want more sound, you need to load it with a lower impedance
(to draw more current at the same constant voltage). 

The K2 is designed to maximize battery life, so you also want a very
EFFICIENT loudspeaker (lots of sound for very little power).  In
general, it is far easier to get higher efficiency out of a large
loudspeaker than a small one. So if you want to hook up an external
loudspeaker to your K2 to sit on a shelf in your shack, go to a place
like Radio Shack and buy a nice little 4 ohm loudspeaker in a box and
hook it up to your K2's loudspeaker terminals. But when you're
shopping, try several loudspeakers on one of their stereo rigs. Don't
change the volume controls between loudspeakers. The one that puts out
the most sound is the most efficient. If it isn't also expensive,
that's the one you want. 

Hope this helps.
Jim Brown K9YC
http://audiosystemsgroup.com


Jim Brown
Audio Systems Group, Inc.
Chicago
http://www.audiosystemsgroup.com