Please Read and express your feelings and opinions....

Atmasphere 4-19-2018
I've been able to measure effects of fuses and power cords, which has often put me at odds with some that make apocryphal claims about them, but also those that I usually regard as 'objectivists' that won't take anything seriously without good measurements. In the latter case, they don't seem to want to cause their hand to move and make the measurements, instead proclaiming that a fuse or power cord can't make a difference. Without the measurements, their proclamations seem to me as bad (and for the same reason) as those that make the apocryphal claims.

... Which serves to illustrate, IMO, that when it comes to many of the more controversial things in audio the truth is usually a good deal more nuanced than viewpoints that tend to be expressed by those at or near both extremes of the belief spectrum, and usually lies somewhere in between.

Thanks, Ralph. Best regards,
-- Al
 

To add a data point to Steve’s (Fleschler’s) input, my 300B-based class A biased VAC Renaissance 70/70 MkIII amplifier provides a choice of 6 different feedback settings, ranging from 0 (no feedback) to 7.5 db. All of those choices represent relatively modest amounts of feedback, even for a tube amp (for example ARC amps commonly have feedback in the area of 10 to 14 db or so), but nevertheless I prefer the 0 db setting. I suspect one reason for that is that my speakers (Daedalus Ulysses) have an exceptionally flat impedance curve. Presumably that tends to lessen their sensitivity to differences in amplifier output impedance, which in turn will vary among the different feedback settings, and will be highest at the 0 db setting. I’d imagine that with a lot of other speakers one of the other feedback choices would be preferable, depending on how the impedance of the speaker varies over the frequency range, and also on how much bass damping it requires (damping factor being inversely proportional to output impedance).

Regards,
-- Al

Those who may be interested in some background underlying the exchanges above between Ralph (Atmasphere) and CJ1965 will find the following thread to be relevant, beginning with the post by Atmasphere on 4-17-2018 and continuing to the end of the thread:

https://forum.audiogon.com/discussions/a-question-on-speaker-driver-efficiency

I believe that the posts in that thread by Ralph, CJ1965, me, and others will provide readers with a good perspective on the various protagonists.

Regards,
-- Al
 

I believe that the references to force in this discussion are unnecessary and are contributing to confusion, and I believe that Atmasphere is correct. A short while ago I posted as follows in the other thread I referenced above, in relation to this matter:

Almarg 4-20-2018
Assuming that a speaker is operating in a reasonably linear manner, meaning for example that it is not being over-driven to the point that thermal compression becomes significant, it seems to me that the relation between acoustic power out and electrical power in will remain constant to a close approximation. And electrical power in will be proportional to the square of the applied voltage.

Therefore it would seem to me (and I believe also to Erik, Atmasphere, and Kijanki) that since a 50% reduction in applied voltage will result in a 75% reduction in electrical power in, which corresponds to a 6 db reduction in electrical power in, the result will be a 6 db reduction in acoustic power out.

My comment was seconded in that thread by Erik_Squires, who is particularly knowledgeable and experienced in speaker design.  An excerpt from his post:

Erik_Squires 4-20-2018
... the SPL at a reference distance, measured in dB, changes in proportion to the power OR voltage when either is expressed as dB assuming there is no compression in the driver.

Said another way, for a single driver:

Delta V dB = Delta W dB = Delta SPL dB

That's what's so cool about dBs!

Regards,
-- Al

CJ1965 4-20-2018
There’s a reason the graph shown in the web page for db conversion that Erik linked to has a value of -6db for voltage gain/loss and -3db for sound power. One look at the two formulae for sound power and electrical power will explain why if you care to devote the time to examine them closely.

I'm not sure what you are referring to.  The only graph on the page addressing db conversion that Erik linked to is this one, and it just deals with voltage, not power.

In any event, if (as it appears) you are saying that a 6 db reduction in electrical power into a speaker results in a 3 db reduction in SPL at a given distance, rather than resulting in a 6 db reduction in SPL at that distance, we'll have to agree to disagree.

Regards,
-- Al
 

Regarding the debate about the relation between SPL and speaker input power, I found the following sub-page at the site which provided the calculators that were referred to earlier:

http://www.sengpielaudio.com/calculator-efficiency.htm

Entering various parameters into either of the two calculators closest to the bottom of that page (one entitled "Sound Pressure Level and Amplifier Power" and the other in the section entitled "Electro-Acoustic Sensitivity") clearly confirms what Atmasphere, Erik_Squires and I have all said on this subject. Namely that if the input power to a speaker is changed by a given number of db, SPL at a given listening distance will change by the same number of db. (As previously stated, this of course assumes that the speaker is not being driven hard enough to cause thermal compression in the drivers to become significant).

I also found the following writeup at PSB’s site, which provides additional confirmation. About 2/3 of the way down the page a table is provided showing power vs. volume for an unnamed 87 db speaker used as an example. Note that 40 watts results in a volume of 103 db, while a 6 db reduction in that power level (to 10 watts) results in a volume that is 6 db less (97 db). While a 3 db change in power, from 1 watt to 2 watts, changes the volume by 3 db, from 87 db to 90 db. And a 20 db change in power, from 1 watt to 100 watts, changes the volume by 20 db, from 87 db to 107 db.

http://www.psbspeakers.com/articles/Guide-to-Speaker-Specifications

On another note, Erik, thanks for injecting some sorely needed humor into this thread, with the donuts post :-)

Regards,
-- Al