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Relate sensitivity/impedance to speaker efficiency

Can you help me relate speaker sensitivity and impedance to how efficient speakers are relative to one another?

What I mean is, given 2 speakers with the same or similar sensitivity (say 89 or 90), if one has a nominal impedance of 4 and another has a nominal impedance of 6, would the higher impedance speaker be easier to drive? Would the higher impedance speaker perhaps offer more flexibility in amplification (perhaps allowing the use of tubes?

What matters more for ease of amplification - a speaker with higher sensitivity or a speaker with a higher nominal impedance? (i.e. given similar nominal impedance, going from a speaker with a sensitivity of 87/88 to one with a sensitivity of 90/91; or given a similar sensitivity, going from a speaker with a nominal impedance of 4 to one with a nominal impedance of 6 or 8?)

I realize the answer to these questions is probably more complex, but are there some general rules to use as guidelines before actually trying the speakers out?
nnck

Showing 4 responses by atmasphere

atmasphere's avatar
atmasphere
11,633 posts
 
There are a couple of other things to consider, in addition to the points that Al makes. some are here:

http://www.atma-sphere.com/papers/paradigm_paper2.html

As I understand it though, both speakers are friendly to tubes. However impedance plays a couple of other roles. Output transformers in tube amps tend to be more efficient when driving higher impedances. Certainly speaker cables become less critical as the impedance of the speaker is increased. The result of this is that (all other things being equal) as you raise the speaker impedance it will appear to become smoother and more detailed, regardless of the amplifier used.

While 4 db may not seem to be much, that is more than doubling the amplifier power to make the same sound pressure. That can be a significant cost! So it should not be ignored.
atmasphere's avatar
atmasphere
11,633 posts
 
Nnck, First lets look at the two options as if they are both on the Power Paradigm (efficiency, 1 watt 1 meter).

Under this model the Acoustic Zen is 90/91db. The other speaker is 84/85db if it is also 4 ohms. If you want a tube amp the latter is going to be hard to live with unless you are near-field.

Now lets look at it as if both are under the Voltage Paradigm, (sensitivity, 2.83V 1 meter). Since the Zen is 6-8 ohms, and if the other speaker is 4, in this particular case the difference in sensitivity is about 3-4 db, which might make things easier to live with. But you should note that the reason the sensitivity spec exists is to accommodate amplifiers that can double power as the speaker impedance is cut if half. Tubes won't do that! Tube power is also expensive and while 3 db does not seem like a lot, it would be a mistake to think that you won't hear it.
atmasphere's avatar
atmasphere
11,633 posts
 
Hi Al, I was involved in a rather ambitious speaker cable project about 25 years ago, wherein we used a time delay reflectometer to analyze a variety of cable geometries. What we found is that characteristic impedance does indeed play a role (not nearly so important as it does at RF frequencies though) in the performance of the cable.

If the load is highly reactive, then the characteristic impedance can be important if the amplifier is otherwise unstable with the back EMF. Just a side note- obviously I am way OT here.

Nnck, my apologies- you were correct and it was me that was getting confused with all the conjecture. But as it has turned out, the less efficient speaker is on the Voltage Paradigm and the more efficient one is on the Power Paradigm, so now we find that it is true that one is about 4 db more efficient than the other, despite the sensitivity of them being almost the same.
atmasphere's avatar
atmasphere
11,633 posts
 
Agreed!
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