Just like the title says - drivers with wide impedance curves. It might seem like an odd question, but I've noticed some drivers, tweeters especially, have very narrow impedance peaks while others have very wide ones. Some are so wide they resemble an equilateral triangle. I've noticed it's usually the more expensive tweeters that have the wider peaks.
What is it about them physically that widens their impedance peak? What effect does this have on their sound (if any)?
I've noticed in passive crossovers the impedance peak is avoided or notched out. How would you notch such a wide peak? If you didn't and instead used an asymmetrical crossover (2nd order on woofer, 3rd tweeter), assuming that the impedance rise had the right slope and also over the right octave, could the 3rd order on the tweeter become a second order and integrate well with the woofer? (also assuming the tweeter frequency response was flat to impedance peak)
What is it about them physically that widens their impedance peak? What effect does this have on their sound (if any)?
I've noticed in passive crossovers the impedance peak is avoided or notched out. How would you notch such a wide peak? If you didn't and instead used an asymmetrical crossover (2nd order on woofer, 3rd tweeter), assuming that the impedance rise had the right slope and also over the right octave, could the 3rd order on the tweeter become a second order and integrate well with the woofer? (also assuming the tweeter frequency response was flat to impedance peak)
High Qms = high and narrow impedance peak. Ferrofluid reduces Qms.
The peak can be removed by adding a RCL network (resistor, capacitor and inductor in series) in parallel with the driver.
The peak can be removed by adding a RCL network (resistor, capacitor and inductor in series) in parallel with the driver.
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Ah, so a stiffer suspension is what shortens and widens it?
Edit: didn't see the ferrofluid addition. Loaded, left, came back, replied.
Edit: didn't see the ferrofluid addition. Loaded, left, came back, replied.
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Specifically I'm working with an ATC tweeter right now (one of two they make). It's their s-spec tweeter, the better one. It has an extremely strong magnet, no ferrofluid, and one of the widest and tallest impedance peaks I've seen. 25 ohms at resonance (~950hz) and triangle shape to 8 ohms at 2000hz and 400hz. I didn't measure it all the way out, but at 4500hz it's down to 5ish ohms (6 ohm tweeter). Very wide
No, just thinking.
Say a tweeter with a peak impedance of 25 ohms at 1000hz, is 12 ohms at 1500 and 600hz, 9 ohms at 400hz and 2500hz, and falls to its nominal impedance of 6 ohms at 200hz and 4000hz.
In a tweeter, the moving mass and suspension compliance determine its resonant frequency.
The larger the mass, the lower the frequency.
The higher the compliance, the lower the frequency.
Decreasing mass or compliance increases it.
But you can't increase either by too much, else no movement.
At resonance, the mass works with the suspension. The mass moves out and is is caught by the elastic suspension, which stretches, stops, then contracts, pulling the mass inwards (where it is caught again by the suspension and the cycle repeats). But not indefinitely. There are losses, and once these add up to the initial input, motion ceases.
Close to resonance the mass still works well with the suspension.
To have the mass move:
Above resonance - the suspension needs to be stretched faster and helped to contract
Below resonance - the suspension's contraction needs to be opposed in both directions.
Both require more energy than at resonance, where the force from the suspension does not need to be opposed.
Above resonance the coil pushes the mass through the counter force of the suspension.
Once at full displacement, the suspension is helped by the coil to more quickly launch the mass back inwards. The cycle continues
Below resonance the coil is always trying to return the mass to center.
The coil does all the work of moving the mass outward, plus resisting the suspension.
The suspension does all of the work moving inward. But does too much, and the coil counters the extra.
The further above or below resonance, the harder the coil has to work to counter. The closer to resonance the frequency, the easier the coil's job.
At resonance, impedance is highest
If a tweeter is 6 ohms, but 24 ohms at resonance, that means 1/4 the power is required at the resonant frequency for the same output (compared to frequencies significantly above or below it).
At resonance the tweeter is moving with less current required because the work is being done mechanically - electrically this has to appear as lower impedance.
As compliance increases, the impedance peak rises.
The easier the coil can move, the further it will want to move at resonance, the less power will be required to have it continue moving. With lower compliance (stiffer) suspensions, the resonant frequency is mechanically coupled to the adjacent frequencies
I think the maximum height and the height compared to compliance (all else being equal) depends on BI - bad magnetic coupling wouldn't allow for the highest peaks.
From this, I think the ATC tweeter I have has a high moving mass, with a low compliance suspension, and an extremely strong magnet.
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Not that it's important for a tweeter, but I'm thinking the damping factor would go down by as much as you'd expect it to go up with an actual 24 ohm speaker
If you understand the physics, above resonance the suspension and damping become less and less significant and the force generated in the motor (coil and magnet) accelerates the mass. This is referred to as the mass controlled region. As the frequency drops below resonance both mass and damping become less important and the motor force is balanced by the suspension compliance. Hence, the compliance controlled region. At resonance mass, compliance and damping are all equally important.
First sentence of the second paragraph I said it wrong - instead of coil returning to center, it's suspension, with the coil working against it to keep the frequency lower than Fs.. the next two sentences have it said the right way though.
I didn't quite understand before the post, I was thinking "out loud".
It's good to have terms now to describe each mode of operation. Thank you
I didn't quite understand before the post, I was thinking "out loud".
It's good to have terms now to describe each mode of operation. Thank you