speaker cabinet components: "parallel impedance compensation network"

[mike_mccue]

Hi,
I am trying to learn more about a parallel impedance compensation network I am reading about.

I have found lots of stuff to read about that doesn't apply to speakers and not much that can answer a basic question.

At first I thought the idea was for the parallel impedance compensation network to make sure that any wild fluctuations in the impedance of the speaker are "smoothed" over from the perspective of the amplifier when it sees the speaker as part of the whole.

But I started doing some basic math and I see that it may help to lessen the rise of impedance but will also exasperate dips.

I guess I thought the idea was to protect the amp from dips in impedance... but now it seems like it's meant to smooth resonance... I'm confused.


Here's what I was observing:


8 ohm speaker and 16 ohm resistor hooked up in parallel presents 5.33 ohms to an amp.

What happens if the speaker dips to 4 ohms?

4 ohm speaker and 16 ohm resistor hooked up in parallel presents 3.2 ohms to an amp.

What happens if the speaker rises to 12 ohms?

12 ohm speaker and 16 ohm resistor hooked up in parallel presents 6.857 ohms to an amp



I guess what I am trying to figure out is this. What is basic intention for including an impedance compensation network in a speaker design?


Thanks You.


best regards,
mike

[TransLucid]

Usually, the networks are a resistor in series with a cap, both in parallel with the speaker. They are known as Zobel networks, and are designed to flatten the impedance curve of the speaker at higher frequencies, which rises due to the inductance of the voice coil.

[mike_mccue]

Thanks, that gives me some perspective.

So, for example; as the impedance rises in the upper frequencies as shown below



The parallel network acts to reduce the effective total that the amplifier sees.


Thanks very much. It seemed like all the explanations jumped right into the technical description and assumed I knew the context.

best regards,
mike

[J M Fahey]

Short answer: the network compensates the huge resonance peak (on a guiter speaker can reach up to 45 ohms, a few even more) and the high frequency impedance rise, which can go from 16 ohms up to 24 (always referred to a nominal 8 ohm speaker).
There are no impedance "dips", the speaker simply shows the VC DC resistance at some frequency, usually between 250 and 450 Hz.
At any other frequency reactive components raise their head, making crossover design iffy.
British Hi Fi speakers traditionally compensate in complex crossovers to get "textbook perfect" response; Classic American speakers go the other way, often just gluing a couple coils and caps to a hardboard piece.
Of course, today everything is Chinese