I have seen prior threads on this, but none recently that can answer how many watts from an amp are truly necessary.
Take an inefficient speaker, say 86 w/db. at 98 db (which will harm hearing when sustained) 16 watts would be required. Even doubling this to account for transients would be available at 32 watts. Strickly
from an engineering standpoint, are more than 40 watts really necessary? No audiophile terms like bloom, and slam needed.
Loudspeaker specs can be a little misleading. According to your logic you might need 80 watts if the loudspeaker is a 4 ohm model. But that only pertains to sound output at 1kHz. It could take even more watts to produce equivalent levels from the lower midrange down into the deep bass.

As a practical matter 50 to 80 watts should be sufficient for 80% of the loudspeakers on the market in typical rooms. It's just that even greater wattage may be required if you want to optimized that loudspeakers performance.
I agree with Onhwy61's comments. I'll add the following:

First, the 86 and 98 db numbers you cited are presumably at 1 meter. Neglecting room effects, the SPL of dynamic (box-type) speakers will fall off at about 6 db per doubling of distance, meaning a reduction of perhaps 10 db or more at typical listening distances. (The rate of falloff will be significantly less than that for planar speakers, though). Add 3 db back, however, to take into account that two speakers are being driven.

Second, keep in mind that speaker sensitivities are often specified on the basis of an input of 2.83 volts, rather than 1 watt. For an 8 ohm speaker impedance, that makes no difference, since 2.83 volts into 8 ohms corresponds to 1 watt. But 2.83 volts into 4 ohms corresponds to 2 watts, so subtract 3 db to get efficiency on a per watt basis if the speaker has a nominal impedance of 4 ohms and is spec'd on the basis of 2.83 volts.

Third, as can be seen from measurements provided in conjunction with reviews in Stereophile and elsewhere, it is not uncommon for speaker sensitivity and efficiency specs to be optimistic by 2 or 3 db or more.

Fourth, and perhaps most importantly, while the doubled power capability you referred to will provide for transient peaks on many recordings having narrow dynamic range, keep in mind that SOME recordings will have enormously greater differences in volume between the peaks of brief transients and their average volume. One common example being well recorded minimally compressed classical symphonic music. I have more than a few such recordings in my collection which when listened to at average levels of about 75 db at my listening position reach brief peaks of 100 to 105 db at my listening position. A 30 db difference (105-75) between the volume of brief peaks and average volume means that 1,000 times as much power is required for those brief peaks as for that average level.

Finally, see my post here for a description of the detailed calculations that are involved.

-- Al

P.S: I hope you won't mind if I point out that it's "strictly," not "strickly." A common mistake.
1 watt at 86 db,2 watts for 89db,4 watts for 92, 8 watts for 95, 16 for 98db, 32 watts for 101, 64 watts for 104 and so on. So yes,40 is usually sufficient but keep in mind that thx is 110db and ultra thx is 112db.

Extra wattage is always a good thing with transient peaks (symbol crashed or bass drums).
Finally, you should have a "perceived" doubling of volume for every 10db increase.
All I can say is 'quality over quantity'. In my experience most audiophiles buy a lot more power than what is really needed. Spend the money on what makes your system sound 'good' as opposed to what makes it sound 'loud'.
Almarg, would you please elaborate on why SPL does not decrease at 6 db per doubling of distance on planar speakers? I thought that figure was a constant for propagation in Earth's atmosphere rather than being related to the type of device producing the sound?
77jovian, the main reason that SPL decreases as distance increases, in a reasonable home listening environment at least, is not related to atmospheric attenuation. It is the result of the sonic energy "spreading out" over a progressively larger cross-sectional area as distance increases, resulting in a given cross-sectional area receiving a progressively smaller fraction of the total energy.

In contrast to a typical box speaker, though, where a given frequency is likely to be emitted by one or two or three relatively small drivers, a large panel speaker will tend to "beam" most frequencies, resulting in a lesser degree of "spreading" as distance increases. Or putting it another way, as the listening distance increases the listener will tend to be "on axis" with a greater amount of the panel's area. And also more equidistant to the various sections of the panel's area, resulting in less difference in the arrival times of the sound emitted by the various sections, resulting in more coherent summing of the different arrivals.

-- Al
As I read your post Tennisdoc40, I was thinking about the SPL drop-off with each doubling of distance from the plane of the speaker...
I scrolled down & saw that Almarg had already addressed this point. :-)
I'm thinking exactly along Almarg's lines.....
To make the music flow effortlessly - meaning playback of all the peaks without feeling the music is contained - you will need a lot of watts. 1000X the spec'd wattage at 8 Ohms, just like Almarg wrote.
Ratio of peak to average power required, as Al said, is very
high. Amplifier's power specification alone is only useful
for listening to continuous sine-waves. Otherwise 10W
amplifier with big headroom can sound much louder than 100W
amplifier with small headroom. For instance, in class A amps
headroom is very limited. Higher output bias current would
make amplifier very big, heavy and expensive. Class AB amps
don't have this limitation - headroom can be higher.

In addition power specification are often very vague.
Continuous power of the B&O module (200ASC) in my class D
amp is 40W. FTC Power is 55W. Rowland rates it 200W while
Bel Canto rates it 300W. What is it?
This 1000x wattage issue is misleading. The math works and so does the logic, but it's not how your ears and brain hear it. Clipping and the dynamic compression that it produces can be very hearable and annoying, or it can be hearable and somewhat pleasant sounding, or it can completely unhearable. It depends on a number of factors.

Even with the newer class D amps, high quality-high wattage amplifiers are still expensive. In allocating money in a system I'm not sure it's wise to spend based upon what happens 1% of the time. I'm repeating myself, but a well designed 50-100 watt amplifier should suffice for the vast majority of circumstances.
As for playing the numbers game, you would also have to factor in the effect of heating of the voice coil. As you increase the current flowing in the voice coil, it heats up and resistance increases so that each additional increment of power delivers less and less increase in acoustic output (this is a major source of compression). Also, mechanically, as the drivers are pushed towards the limits of their excursion, there is an increase in the mechanical resistance which further increases compression. Hence, much more power is really needed that those simple formula suggest to get to a particular SPL level.

That said, for the most part, I agree with those who believe that the recommendations on power greatly exaggerate how much is needed. Give me quality over quantity any day.
I appreciate the feedback. I have a 100 wpc ss amplifier powering speakers with 91 db sensitivity and the woofer having it's own class D amplifier. I was demoing a 150 wpc amp of the same brand and preferred the sound of the lower powered amp, even though it's listed peak amperage was lower. Growing up, we thoroughly enjoyed music powered by a Sansui receiver at 30 wpc through 16 gauge speaker wire from Radio Shack.
The music flows just fine with my 60 to 80 watts or so tube amp…150 in the mosfet sub…dynamic all day (my speakers are maybe 89 db efficient although rated as higher). A note about clean dynamic power…I have super clean mui powerful amps used for live sound work, and if you ran one into virtually ANY home stereo speaker (love to demonstrate this with Magicos or something…owned by somebody else of course) that supposedly can handle some power, put a mic on a kick drum through a clean mixer and gave it some uncompressed beans (even flat with no bass boost with the amp on maybe 20%), you'd be lucky to hear a quick "splort" before the speaker self destructed. I'm not sure what my point is…but that's reality.
^It seems as though many audiophiles find reality pointless.:-)
it always depends.

in this case it depends on how loud you need to go with various recordings and assuming speakers are capable of doing it in your room without strain or compressing.

Generalizations may apply but generalizations seldom yield exceptional performance.

The devil is always in the details.

I have a 60 w/ch integrated running lower efficiency spekaers in a large room and the results are fine up to a point but are limited compared to essentially the same but somewhat larger speakers in another large room running off similar but 500w/ch amps. There is no comparison of overall dynamics and musicality at realistic listening volumes but at typical modest volumes both do similarly fine.
Brauser - you are exactly right. The amount of power (watts) needed to give the impression of adequate sound pressure levels is less than you think. If the signal applied to your loudspeakers is not phase coherent, it will seem to lack "punch". If the listener then turns the volume up to achieve this "punch", it is only an attempt to compensate for the incoherent signal. What you end up with is a louder version of an incoherent signal. If you have a coherent signal then the "wattage" becomes more efficient. In other words if you have 2 systems, one coherent and one with phase or time domain errors, driving them both at exactly the same measured wattage, the coherent system will sound noticeably louder (at least 3+ db). Timing is everything.

Signal applied "not phase coherent"?. Do you suggest that amplifier somehow shifts phase in the audio frequency range causing reduction of perceived loudness ???
Yes it does. See the white paper on distortion.
There is a link at www.h-cat.com

The "punch" is diffused by very small changes in velocity.
This spreads the energy of a percussion event over time.

For more information...
This link should take you to the white paper on distortion and timing issues...


While I agree that all electronics causes some phase shift of the music signal, I believe that the most flagrant violator of phase distortion is the speaker. Once the listener has corrected this then I would venture to fix the phase shift from the electronics.
Believe it or not - there is far more problems in the electronics than in the transducers. (mic or speaker)

The inability to locate a sound object when played through a "system" is 90% the fault of the amplifying equipment. Localizing cues naturally embedded in the source material are skewed during the amplifying process. This makes it impossible to reproduce the venue with enough accuracy to be accepted as live.

The comments posted by our tech members make a lot of sense. Another reason to favor more rather than less power is explained in the following article:


The article speaks of an amp's "safe operating area (SOA)," which is really relevant when one considers that some speakers can present really tough loads. Or using the lingo of the article, if a speakers presents very low impedance and a highly negative phase angles, especially in the low frequency/power range spectrum, the amp could be pushed beyond its SOA, and thereby produce distortion.

Just an aside, my amp is rated at 150+ WPC, and has a very robust power supply of 1040 joules. So if the amp is asked to drive a low frequency transient, it will have the power to muscle its way through the passage ... most of the time, ... my poor ears permitting.

I am not suggesting that unless one picks up a monster Bryston or Krell amp, he's doing injustice to his rig. Instead, all I'm saying is that one should be cognizant of the electrical properties of his speakers and choose an amp that can drive the speakers at comfortable/enjoyable SPLs in the amp's SOA.