Speaker resistance and sound energy

[M. Hamed]

I was reading the ARRL handbook (2010) and on page 4.7 there was this sentence:

"it can be said that any device that dissipates power has a definitive value of resistance"

This made me think. I always see speakers modeled 8 Ohms. And this is probably their DC resistance value. But shouldn't the conversion to sound be accounted for somehow since it's a sink on energy delivered?

I thought maybe an inductance should be added to the model but then an inductance is lossless, so where does the sound producing energy come from?

 

[Ken]

I always see speakers modeled 8 Ohms. And this is probably their DC resistance value.

No.

 

[Phil Allison]

"M. Hamed"

I was reading the ARRL handbook (2010) and on page 4.7 there was this
sentence:

"it can be said that any device that dissipates power has a definitive
value of resistance"

This made me think. I always see speakers modeled 8 Ohms. And this is
probably their DC resistance value.

** Not exactly.

8 ohms ( or whatever) is the"nominal impedance" of the speaker - usually
measured with a 250 or 400Hz tone.

The DC resistance of the voice coil is about 80% of that number or about 6.4
ohms.

The impedance of most ( bass or full range ) speakers in the range of 250 to
400 Hz is RESISTIVE too.

So, we have two resistances that dissipate heat, one the copper wire and the
other due to losses in the moving suspension, magnet assembly and sound
radiation.

Sound radiation accounts for barely 1% of the input power, in most cases.

A good circuit model for a loudspeaker is not a simple one, it would have
many inductances, resistances and capacitances involved - plus the
resistances would each have temperature co-efficient.


.... Phil

 

[M. Hamed]

Thank you for the thorough explanation. As it often happens my false assumptions led to false conclusions.

I remember a couple years ago I created a toy speaker out of magnet wire and a strong magnet. I remember how I carefully wound the coil to measure 8 Ohms DC. How naive I was

 

[amdx]

You were only about 20% off. In audio, that's high precision.

You trying to start an audiophoolery fight?

Mikek

 

[Fred Abse]

No, it's like a voltage source driving a motor which is driving a load
where the mechanical load reflects back as electrical resistance.

I like the way they used to (maybe still?) describe horn loudspeaker
transducers as "motors".

 

[[email protected]]

I like the way they used to (maybe still?) describe horn loudspeaker
transducers as "motors".

The "motor" part of a loudspeaker is the linear motor, not the entire
assembly.

 

[Fred Abse]

^^^^^^^^^^^


The "motor" part of a loudspeaker is the linear motor, not the entire
assembly.

See above

 

[[email protected]]

See above

The cone is part of the transducer (the entire loudspeaker). It is
not part of the motor.