speaker impedence

the subject you are broaching (speaker impedance) is fairly complex. any response that is shorter then approximately the length of a novel will therefore be incomplete.

the short answer is that the 8 ohm impedance is always a nominal value. what the true Z is in operation will be dependant on several varying factors.

for long winded debates, flames, and esoteric dissertations try a google groups search on the topic in rec.audio.pro

the speaker being an inductance + resistance will have a nominal R+J value which becomes a delta R+J in operation.

the series capacitor makes it a series RLC circuit, in effect a high pass filter.

Pun intended? :-)

Ben Miller

Benjamin D. Miller, PE B. MILLER ENGINEERING

The speaker impedance is much higher at low frequencies and high frequencies. It will be 8 ohms at 100Hz-3kHz or higher.

The reason why you have tweeters in speakers, is because tweeters have low impedance at higher frequencies, and better response to higher frequency singals.

See this link for an example of a speaker impedance curve:

Speakers vary all over the place. Ideally, the speaker impedance is the acoustic impedance of the voice cone as seen at the voice coil terminals. The voice coil acts like a linear motor producing back emf as the coil cuts through the magnetic field. Ideally again, the voice coil itself should have zero resistance and inductance. In practice, the coil resistance is usually a large part of the impedance seen by the amplifier. Add to this the effect of vibration modes of the voice cone at high frequencies, and complexity grows quickly.

Bill

-- Ferme le Bush

with me you never can tell. sometimes i can be a wise annulus.

Well, as Tim points out, this can be an incredibly complicated and flaming discussion.

One point though. Although the coil of a speaker would perhaps make you think it is a purely inductive component, it actually is not. Because it actually moves the PM and cone, there is 'real' work also being done, hence a 'resistive' component as well. For exactly how much, I would defer to Don Kelly who had a *long* thread on this a while back.

Bottom line is, since *some* of the energy is being transferred to a mechanical system (that may momentarily store some energy in air-pressure, cone elasticity and kinetic energy of the moving parts), the answer is *not* simple.

daestrom

daestrom, you're on point here. Don't think of the coil as an inductor, it's more a 'transformer', linking a mechanical driven device to the electronics. One can, by modifying the enclosure of the speaker (by sealing it, for example, so air cannot 'escape' from the back to the front) change the lower frequency transform function as seen by the driving point impedence at the speaker terminals. So, there is an inductive component of course, but the electrical to mechanical conversions are what makes for interesting engineering.

BTW, speakers are really lousy transducers, in terms of transforming energy from electrical to sound waves. "Lossy" doesn't come close.

As someone earlier in this thread said, if it's shorter than a book any analysis won't come close to being accurate.

'8 ohms' indeed!

---------- The electrical to mechanical conversions are straight forward (f=Bli and e=Blv work well enough). The mechanical model is not so straight forward except in theory- simply because simple models don't represent the actual behaviour and non-linearity rears its ugly head in real life. This is where the complexity exists- too many , often nonlinear, variables involved.

As to speakers being lousy transducers- right on!- but the objective of a good speaker design is independence from introduced frequency artifacts- generally at the cost of abysmal efficiency. After all , negative feedback is used with this objective in mind- better frequency response at reduced gain.

However, the modelling of electromechanical systems is fascinating. The mechanical engineers have the more difficult side of it as the electrical side can generally be considered linear with little error. Mechanically, this assumption becomes: "yeah, maybe, but..." which is a bit tougher to handle.

My opinion as an old fart and hence open to correction. --

Don Kelly @shawcross.ca remove the X to answer

The speaker is only a part of the whole audio system :

A "musical" instrument or human throat, (non -linear), produces acoustic air pressure variations, which are shaped by the environment, (recording studio or auditorium). These are partially sampled by a non-linear microphone, and converted to an electrical signal which is then amplified millions of times by an almost linear device. This electrical signal is applied to another non-linear device, (speaker), where a second acoustic air pressure variation is created. This is modified by the environment, (listening room or auditorium), and partially sampled by the most non-linear device of all, the human ear.

After all this, the nerve "signals" from the ear are interrupted by the device about which we know the least, the human brain.

Without the latter there would not be tens of "audio magazines", hundreds of audio device manufacturers, and the makers of "monster cables" and gold plated connectors would go out of business!

The speaker enclosure tries to do this. I recall talking to Paul Klipsch, the inventor of the "Klipsch horn". This was a speaker enclosure about six feet tall and four feet wide that sat in the corner of a room and used the corner walls and floor as an extension of the horn. Paul contended that there was no such thing as a "small" low frequency wave.

He's probably turning in his grave now that they are selling "book case" speakers under his name.

I've always admired the mechanical engineers who design the "moving parts" of our systems.

I've always told the engineers that worked for me that there was no such thing as an electronic failure. They're always mechanical in the final result!

Virg Wall, P.E. (EE)

Hi there Don. I checked back in, noticed the thread, and decided it might be appropriate to thank you for a few things you brought to light in our discussions. Such a pity you are a cantankerous old fart :)

northstar

PS to Daestrom if you read this - thanks to you as well.

Virg, you could have counted on me using your 'no such thing as an electrical failure' back when I earned my daily bread as an EE, but I wasn't smart enough to have thought of it that way. I'm wondering now how I would have treated that excuse when I had EEs working for me!

It's too bad those of the mechanical persuasion have to shoulder the blame, but who can deny that going up in flames is a lot more mechanical -- or chemical -- than electrical in nature.

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I learned from our discussions and you certainly learned that I was a C.O.F. :) Have fun.