impedance vs sensitivity

Sensitivity and Efficiency are two specs that try to express the same thing from different points of view. The different points of view are the Voltage Paradigm and the Power Paradigm, which are opposing concepts of how to design and test amplifiers and loudspeakers. For a complete explanation see

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

Efficiency is 1 watt measured at 1 meter. Sensitivity is 2.83 volts measured at 1 meter. If you work the math, 2.83 volts is 1 watt into 8 ohms. Into 4 ohms it is 2 watts, IOW if you have a speaker that is 92 db 1 watt/1M, it will be 95 db 2.83V/1M, if it is also a 4 ohm device.

The Voltage Paradigm, which mostly describes transistor amplifiers, has no use for the Efficiency measurement, which is a Power Paradigm specification. Amplifier power and impedance are not the same thing; if you have a low impedance speaker that is also high sensitivity, a low power tube amplifier will be able to drive it just fine as long as it is able to get a good match to the speaker.

FWIW these days there is little argument for 4 ohm speakers in high end audio. This because regardless of the amplifier technology (transistor, tube or class D), the amplifier will sound better and perform better on higher impedances. Of course, if you are unconcerned about sound quality, and more interested in sound pressure, than 4 ohm speakers will be more attractive if you also own a solid state amplifier. The other argument against 4 ohms is the speaker cable- they are extremely critical for best results on 4 ohms, while at 16 ohms they are not nearly so. Making a speaker to be higher impedance, all other things being equal, is an easy way to make the speaker appear that it is smoother with greater detail, always a desirable combination.

Yes Kijanki, in the 1950s 16 ohms was common and there were 32 ohm speakers as well. Four ohms really didn't get popular until sometime in the 1970s.

Yeah- those LUXMAN amps were pretty cool!

Blindjim, Here is how it seems to work: regardless of the amplifier, there are speakers it will work with and others that it will not. In the case of an 8 ohm speaker that has dips, the dips are of no consequence to a tube amp as long as its a crossover we're dealing with. What **is** of consequence is when the impedance maintains a sustained drop, for example when there are dual woofers in parallel (like in the B&W 802). There is no tube amp in the world that can play a B&W 802 properly (there are some that do OK) on account of that issue.

Here's why- if you use the 4 ohm tap, there will be ringing in the mids and highs because the output transformer is not loaded correctly. If you set for the 8 ohm tap, you won't be able to get enough power to the woofers- it will likely be a good 4-6 db down! The B&W was designed for an amplifier that can make constant voltage into any load (double is power when impedance is cut in half).

Conversely a transistor amp of 250 watts cannot make bass on an ESL (Sound Lab for example) because the impedance of the speaker is a lot higher in the bass than it is in the mids and highs. That transistor amp might be limited to no more than 30-60 watts into that load!

So matching is important, and yes, we do try to check and see what a potential customer is using on this account. Of course I like getting a sale, but it will do no-one any good if the amp won't work with the speaker, so I've had to steer sales away from us a number of times in the past on this account. After years of this, I realized that there was a bigger issue- that equipment matching, tubes vs transistors, objectivist vs subjectivist are all the same conversation- thus the link I dropped earlier.

Making power with tubes has always been expensive, and I have really come to value efficiency in a loudspeaker as long as it does not impair detail and bandwidth. That is a sometimes a tall order, but the speakers exist, but probably a topic for a different thread.

Blindjim, if I understand correctly, the Sonata IIIs have a substantial 4 ohm region in the bass, and 8 ohms (nominally) across the rest of the range. A tube amp is going to struggle with that 4 ohm region unless there is some provision made for that. Even though most of the range of the speaker is in the 8 ohm region, 4 ohms is what I would consider this speaker to be because you have to accommodate that in order to make it work.