Speaker - frequency and impedance

[indulis]

A little off subject, but I have a question.............

Obviously a speakers impedance isn't measured at DC. At what frequeny are speakers characterized for impedance??

 

[audioguru2]

Speakers have a resonance frequency where the impedance is up to 5 times normal. The rated impedance of a speaker is at its lowest which occurs a little above its resonance frequency.

 

[indulis]

Resonance with what? What is the other element, and how is it held constant?

 

[audioguru2]

Resonance with what? What is the other element, and how is it held constant?

The cone of a speaker mechanically resonates at a low frequency. The resonant frequency of a tweeter is much higher. The resonant frequency is increased by a small size of the enclosure for the speaker, where air in the enclosure is an additional spring.
If the enclosure size is correct for the speaker, then the extremely low output impedance of an amplifier will damp the resonance.

 

[walid1]

Hi guru

you deal with us as we a big experts, u must simplify your answers i'm hardly understand u. u must take a low level persons (like me) into your account.
thank you

 

[audioguru2]

u must simplify your answers i'm hardly understand u.

Sorry, Walid. We were talking about a speaker's impedance (it is an inductor so its impedance is higher at high frequencies) and I mentioned a speaker's mechanical resonance.

Speakers have a low frequency mechanical resonance. At the resonant frequency, a speaker's cone  vibrates freely with hardly any input. Because it vibrates freely, it has a high impedance at the resonant frequency. The frequency is determined by the weight of the cone and coil and the amount of springyness in its parts that returns the cone to its resting center position. Air in an enclosure for a speaker adds to the spring, so increases the resonant frequency. The smaller the enclosure, the higher the resonant frequency.

A resonating speaker is a generator. The resonance is stopped (damped) by shorting it. The extremely low output impedance of an amplifier with low resistance in the wires "shorts" the speaker's resonance.
Try it. Hold a speaker and tap its cone. It will resonate. Then short the speaker and tap it again. It won't resonate. Connect the speaker with low resistance wires to the output of an amplifier and tap its cone. It won't resonate. ;D

 

[walid1]

this is a good and simple answer, i understand it , thank u audioguru

 

[indulis]

Be careful here……… mechanical and electrical resonance ARE NOT THE SAME THING!!! Electrical resonance is a second order phenomena (2 reactive elements). As for the dampening effect of a shorted voice coil, how about the induced field in the coil opposing the magnets flux. If you removed the magnet and shorted the coil… what do you think would happen??? NOTHING, the cone would continue to oscillate!!

 

[audioguru2]

Be careful here……… mechanical and electrical resonance ARE NOT THE SAME THING!!! Electrical resonance is a second order phenomena (2 reactive elements). As for the dampening effect of a shorted voice coil, how about the induced field in the coil opposing the magnets flux. If you removed the magnet and shorted the coil… what do you think would happen??? NOTHING, the cone would continue to oscillate!!

A speaker has such a low capacitance that its electrical resonance must be extremely high, maybe RF.
Didn't I mention that: "A resonating speaker is a generator". Short a generator and see how hard it is to move it.
Try it. Spin a DC permanent magnet motor. It spins. Then short it and try to spin it. It's stuck!

We were building speaker systems at work. One didn't work, its magnet wasn't magnetized! ;D