Forums

Speaker resistance and sound energy

Started by M. Hamed March 24, 2013
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?
On Sun, 24 Mar 2013 10:47:49 -0700 (PDT), "M. Hamed"
<mhdpublic@gmail.com> wrote:

>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.
--- No, it's their nominal _impedance_. ---
>But shouldn't the conversion to sound be accounted for somehow since it's a sink on >energy delivered?
--- The conversion of the power delivered to the voice coil by the AC source and the acoustic power delivered to ambient air by the loudspeaker cone is accounted for by the cone's being physically attached to the voice coil and the magnetic field being generated around the voice coil by the AC source which interacts with the permanent magnetic field generated by the field magnet, causing the cone to move back and forth and either compress or rarefact the ambient air. ---
>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?
--- It comes from the AC source pushing current through the voice coil, which generates the magnetic field which interacts with the field magnet's fixed field. -- JF
On Sun, 24 Mar 2013 10:47:49 -0700 (PDT), "M. Hamed"
<mhdpublic@gmail.com> wrote:

> I always see speakers modeled 8 Ohms. And this is probably their DC resistance value.
No.
On Sun, 24 Mar 2013 10:47:49 -0700 (PDT), "M. Hamed" <mhdpublic@gmail.com>
wrote:

>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?
DC resistance is usually a healthy fraction of the rated AC impedance, like 3/4 maybe. Which indicates that speakers are pretty inefficient. Of course, impedance is usually a wild function of frequency, not a steady 8 ohms. And if there's a crossover, there will be series capacitors that will look open to an ohmmeter, and inductors too. Sound is produced by the voice coil moving in the magnetic field, with a component of the motion being mechanically lossy because of the sound produced. It's like a motor driving a load, where the mechanical load reflects back as electrical resistance. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Sun, 24 Mar 2013 14:19:23 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 24 Mar 2013 10:47:49 -0700 (PDT), "M. Hamed" <mhdpublic@gmail.com> >wrote: > >>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? > >DC resistance is usually a healthy fraction of the rated AC impedance, like 3/4 >maybe. Which indicates that speakers are pretty inefficient. > >Of course, impedance is usually a wild function of frequency, not a steady 8 >ohms. And if there's a crossover, there will be series capacitors that will look >open to an ohmmeter, and inductors too. > >Sound is produced by the voice coil moving in the magnetic field, with a >component of the motion being mechanically lossy because of the sound produced. >It's like a motor driving a load, where the mechanical load reflects back as >electrical resistance.
--- No, it's like a voltage source driving a motor which is driving a load where the mechanical load reflects back as electrical resistance. -- JF
"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
On Sun, 24 Mar 2013 10:47:49 -0700 (PDT), "M. Hamed"
<mhdpublic@gmail.com> wrote:

>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?
Speakers are rated in impedance a combination of resistance and inductive reactance. And a real speaker would have different impedances at different frequencies, air density, cabinet design, cone compliance, magnet design, etc.. I think I remember reading somewhere that speakers are rated in impedance at 1,000 cycles - but doubtless that wouldn't hold true for a tweeter... would it?
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 :-)
On Thu, 28 Mar 2013 01:00:36 -0700 (PDT), "M. Hamed" <mhdpublic@gmail.com>
wrote:

>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 :-)
You were only about 20% off. In audio, that's high precision. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On 3/28/2013 10:42 AM, John Larkin wrote:
> On Thu, 28 Mar 2013 01:00:36 -0700 (PDT), "M. Hamed" <mhdpublic@gmail.com> > wrote: > >> 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 :-) > > You were only about 20% off. In audio, that's high precision. > >
You trying to start an audiophoolery fight? Mikek :-)