We know that a speaker sensitivity is a measure of how much sound a speaker will give for 1 watt at 1 metre. It is interesting to note that the basic measure of sensitivity is at 1 metre and not at a typical listening distance of about 10 feet or 3½ metres.
Sound reduces over distance at the rate of 6dB with each doubling of the distance. At 2 metres distance from the loudspeaker its perceived sensitivity is reduced by 6dB. At a normal listening distance of about 10 or 11 feet from the loudspeaker its perceived sensitivity will be reduced by approximately 10dB.

Amplifier power
This is the source of much misunderstanding. Amplifier power is specified in watts, which are a measure of heating power. They have no apparent relationship to what we hear, as they are a linear measure. Loudspeakers (and our ears) perceive things in dB (decibel) steps. These are based on a logarithmic relationship.

This is the fundamental mismatch between what our ears perceive and how amplifiers are specified. The solution to the problem is to recalibrate watts into dB steps. The results are below, in a chart of watts converted to dB steps I’d copy from a hi-fi mag, for convenience i have started the chart at 50 watts.


dBW................Watts//////////////dBW................Watts

17......................50 ////////////////24....................251
18......................63 ////////////////25....................316
19......................79 ////////////////26....................400
20....................100 ////////////////27.....................500
21....................126 ////////////////28.................... 630
22....................156 ////////////////29.....................795
23....................200 ////////////////30....................1000


As we can see, as soon as we calibrate amplifier power in dB watts, we get a dramatically different view of what amplifier power really means....

First off, we can see that what looks like a large increase in amplifier power, for example from 50 watts to 100 watts, only gives an increase of 3dB.

Things get really interesting as when we get to higher powers. We start needing vast amounts of power for each dB step. For example, only 1dB (remember 1dB is the smallest change in sound pressure level that the human ear can perceive UNDER IDEAL LISTENING CONDITIONS) is the difference between 400 watts and 500 watts. If you really wanted to hear a difference above 400 watts you’d probably need to go to 800 watts (3dB) which should be audible.

We can see why amplifier manufacturers want to sweep these figures under the nearest carpet; they make most of their claims look ridiculous as they predict that most loudspeaker/amplifier combinations will have only limited dynamic range.

So the question is, what sort of dynamic range a really good hi-fi system should have?

Clint: Very informative post, thanks.

How much power one needs depends on the maximum volume desired. If for example, 100 dB is uncomfortably loud, you will probably never draw more than 50 wpc continuous. However, extra power is always welcome for transients which can draw multiples more than steady state draw. Of course, this provides a cushion against clipping. I always like to have more amplifier watts than the speaker rating.

Tanx amigo.

I concur with you. An amp ability to produce unclipped peaks of 100 - 105db is the minimum starting point for a really good hi-fi system.
Now, to put the things more pratical (real world), take your speaker sensitivity and deduct around 10db for the SPL attenuation over distance and there you go, you have arrived at the REAL in-room sensitivity of your speaker system.

Here's from ST - Electronics-related Links & Reference an interesting great on-line physics text about the decibel:

http://www.phys.unsw.edu.au/jw/dB.html

There is more to it than just a watt number. Dynamic Headroom and ability to drive 4 ohm and lower loads are important as well. The best stuff doubles power (from 8 ohm baseline rating) into 4 ohm and doubles again into 2 ohm. I don't want any amp in my system heating up my VC's.

Of course we should not use the db value as the sole criterion in judging speakers' efficiency it is only the degree of details that the sound can be reproduced.
Impedance is another important issue, kind like a compromise of the designer to allow the speaker to suck more power from the amp in order to optimise other parameters.
Generally, the lower the speaker impedance, the more power you get from the power amp. Some amps can handle a 2-ohm load, but many will overheat.

Whoops, my last reply drifted some from the OP. I also look at damping factor, slew rate, and square wave response (when possible).

Actually, I'm a big fan of mid-range amplifiers and receivers. My most powerful vintage amp is 85wpc and vintage receiver is 65wpc. They both play exceptionally well, reproduce sound that's fine to my ears, and since I don't reproduce concert-level music in my home..... they do the job.

Problem is that it is a difficult, if not impossible, thing to analyze and get any kind of scientific overview about.

The doubling of power in half the impedance doesn't go for all amps.
Think of OTL tube amps.

Some e.g. 20 W amps play louder IRL on the same speakers as some other e.g. 150W amps.

Why?

Put a 12W tube amp on horn speakers and it can play realy loud and beautiful.
Put SS on the same speakers and it sounds horrible and distorded way befor before you get to the level that the tube amp could produce on the same speaker.

And class A circuits delivers peaks more ready/transient than most other cirduits.

There is not realy any valid science and documentation in this area.
Watt and the data sheets of an amplifiers power performance is like when a car sales man uses the terms "This car drives good"; "This car drives better"; "This car drives the best". (Just a metaphor)

"dolph"

There is plenty of "valid science and documentation". Science and -*test*-('") measurements CAN BE applied to amplifiers, preamps, and sources. Whether you choose to apply the results to what you are hearing is up to you and is definitely not an absolute, as you suggest.

True, SS amps sound horrible on horns. OTOH, SS sounds great with newer drivers like domes, etc. True, tubes sound great with horns but to my ear are not as accurate with non-horn speakers. You can see the difference on square wave traces. While I think you are going in the right direction regarding amp/speaker matching, your example is a mismatch of speakers and amps.

I have plenty of high power amps, but I rarely need an output of over 2 to 5 watts. It is nice to have a lot of headroom though, and the high power amps are more desirable should I ever want to sell any of them.

Then please define everything for us, lower functioning species.


"dolph"

Power amps might be desireable for youngsters and newbies in the music loving sphere, but not to settled old, grey grumpies who listened to tens of thousands of set-up.

Headroom and power reserve is two different things by the way.

"dolph"

The above table of watts to db's are a fact not a theory.

The example you gave about OTL (Output Transformer-Less ) tube amps, is imho not a good example because even the best OTL amps will not be able to drive speakers whose impedence drop much below 4 ohm. It still works fine at, say, 3.5 ohm. But 2 ohm...out of the question.

This is simply not facts.

But leave it.

I'm not in here often enough to discuss it in details and spend time to document it.

People that don't know enough about this should make proper research anyway and then they will find out what facts are.

Have a nice weekend.

"dolph"

could you at least point us to the source of the facts?