Re: [Phys-l] stereo speakers

["John Clement" <clement@hal-pc.org>]

Speaker impedance is fairly complex.  It rises and falls from values close
to the rated value to higher values.  In addition it is not pure Ohmic as
has already been mentioned.  Part of the curve is due to the resonance of
the woofer and box, while other parts are due to the necessity of using
crossovers to split the midrange and highs to squawkers and tweeters.  Some
designs have actually split the spectrum into smaller slices each handled by
a dedicated speaker.

There have been some exotic designs which have only a single speaker.  The
older model Ohm waveguide speaker is such a beast and can reproduce square
waves, sort of.  In either case some manufacturers publish their impedance
curves.  The impedance is higher generally outside of the bass range because
the high frequency drivers are often more efficient than the woofer.

Actually the impedance of the amplifier is designed to be much lower than
the speakers.  This helps damp random speaker vibrations and the ratio of
the speaker/amplifier impedance is called the damping factor.  Traditionally
it was designed to be greater than 10, which was more difficult to achieve
with output transformers.  They used to quote the damping factor along with
other specs, but I think with transistor amps. That has gone away.

Of course for average students this may be way too much detail, but a few
advanced students may get interested in it.  Oh, and an 8 ohm speaker may
actually go as low as 6 ohms, while being around 8 over a large portion of
the bass range.  You are almost totally dependent on what the manufacturer
quotes.

John M. Clement
Houston, TX


> I was discussing Ohm's law today, and a few students were asking about
> stereo speakers and their resistance. Textbooks do few problems involving
> speakers. From my understanding, a typical speaker is 8 ohms. This will
> get a certain amount of current and voltage, providing a power (P = IV) to
> the speakers. If one replaces the speaker with one that has a lower
> resistance, does this mean that the current (and power) will increase for
> a given voltage? Or is it that for a given power (volume), the current
> will increase while the voltage will decrease (since R is now less)? And
> the implication is that when the volume is turned up (P increases), an
> amplifier must have a better power rating to supply the sound if the
> speaker resistance is lower.