Speaker impedance and sensitivity

Yes this is a repeat of an earlier thread.

So have a precised repeat of my earler response:

Low speaker sensitivity = more amplifier watts.

Low speaker impedance = more amplifier guts.

@voodoochillin - Yes, I think you are thinking about this correctly. For instance, the Harbeth P3ESR has a sensitivity of 83 db. The math shows you would need 250 watts at a listening position of 10 feet to produce 100db of sound with that speaker. Most reasonable people don't listen at those levels for very long. Personally, I wouldn't go crazy with the watts. Instead, look for amps that have robust power supplies and output stages that produce high current. I owned Harbeths for awhile and they loved a high current 50 watt Class A amp I had at the time. 

I asked about this when my 15ohm Falcons needed less power than my 6ohm Harbeths with both having the same efficiency rating. BTW, I was using tube amps!

Ralph Karsten ( ATMA-SPHERE fame) gave a full explanation!

atmasphere  

These days sensitivity (2.83V/1 meter) is how speakers are measured. There was a time when efficiency (1 Watt/1 meter) was the spec instead- back when tubes were King.

2.83 Volts into 8 Ohms is 1 Watt. That is why the seemingly arbitrary number.

When you cut the impedance in half to 4 Ohms, 2.83 Volts is now 2 Watts (which is a 3dB increase). So the impedance makes a difference! You have one speaker that is 6 Ohms and the other is 15. If the sensitivity is the same, for each doubling of impedance the efficiency is increased. So by my calculations the 15 Ohm speakers should sound about 4.5dB louder because they are 4.5dB more efficient.

When you are talking tube amps, because they can’t double power as impedance is halved (like many solid state amps do) the efficiency spec is more important. That means you have to derive it from the sensitivity spec and the impedance of the speaker. How I arrived at my numbers above is 6 is halfway between 8 and 4 Ohms so that’s a matter of 1.5 dB instead of 3 dB (which it would be if the 6 Ohm speaker were 4 instead).

The difference between 8 and 16 Ohms is its doubled, meaning another 3dB difference. I added the 1.5 to 3 dB to get my answer. Its not quite right since the speaker is really 15 Ohms, but its close enough for government work.

Sensitivity is not about power, it’s about output assuming a perfect amplifier, and is measured at 2.83 Volts, which could be 1 watt or 8 depending on your speaker.  It is often confused with efficiency, which IS about power and is measured at 1 Watt output.  Assuming the amp is good and speaker moderate the two are reasonably well interchangeable.

Impedance curves are about current vs. frequency. Where the curves are worst is where the amp has to work the hardest, and if the amp wilts under pressure you’ll hear reduced output at some frequencies, usually the bass.

Speaker makers feel no particular pressure to make their products easy to drive, with a few exceptions like Fritz, which is famously easy to drive with any amplifier.

British monitors are designed for low-level nearfield listening and have an altered bass response. They do not play loud and if fed lots of power quickly generate thermal compression. They are a relic of audios past and are not that useful today. Though Im sure someone will say these wee little things are the bee's knees so YMMV.

Another consideration not always followed from manufactures is

phase angle relative to impedance and its effects, interesting to say the least

within the impedance, sensitivity, and amplifier selection topic.

Hi---I had the same question a couple of weeks ago. Here's the link to my post with some excellent explanations. I hope this helps.

https://forum.audiogon.com/discussions/sensitivity-question-2