Date: Tue Mar 10 08:45:46 2009 Back to Contents ------------------------------------------------------------------------

Author: Zani, Gerald

Subject: Re: stereo speakers

Post:

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Anthony,

Audio amplifiers and audio speakers are not a good example of Ohm's Law.

You asked:

"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?"

Yes, But...

And you asked:

"Or is it that for a given power (volume), the current
will increase while the voltage will decrease (since R is now less)? "

Yes. =20

The current will increase if you use a lower IMPEDANCE (not resistance!) =
speaker but not in a simple Ohm's Law fashion. =20
The output voltage of the source amplifier will get pulled down by the =
load.
The audio waveform will get distorted and despite drawing more power it =
will be not as loud or as pure sounding.

A speaker is not a good introductory example of Ohm's law because it is =
not a purely resistive load. The phase between the voltage and the =
current does not allow the use of the term resistance.

The output of an audio amplifier is not a good example of an ideal =
voltage source. Audio bridging is different than the more typical =
impedance matching of a power supply to a load.


In audio terms this is called bridging the source to the load. With =
audio connections one cares about bridging the source with the load in =
such a way that the source does not lose it voltage. The impedance of =
an audio source is often much lower than the impedance of the audio load =
and contrary to common thought the impedances are not matched as they =
would be with a power supply.

Some Articles:

Impedance matching with speakers =20
D. Rae Carpenter, Jr.=20
Phys. Teach. 30 516 (1992)=20

How things work=97Never throw a 2-inch speaker away: It has many uses =20
H. Richard Crane Editor=20
Phys. Teach. 30 375 (1992)


- J

-----Original Message-----
From: tap-l-owner@lists.ncsu.edu on behalf of Anthony Lapinski
Sent: Mon 3/9/2009 8:17 PM
To: tap-l@lists.ncsu.edu; phys-l@carnot.physics.buffalo.edu
Subject: [tap-l] stereo speakers
=20
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 =3D =
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.

Is this correct? Can anyone provide some "basic" physics information =
about
speakers, amplifiers, and Ohm's law?



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