Watts up with that?

The 100 db calculation was for the pair of speakers, and reflected the 3 db increase. On the other hand, I should mention that it assumed a 6 db reduction in SPL per doubling of distance, which might be a bit pessimistic (i.e., too large a number), considering the multiplicity of drivers the speakers have, that are spread out over a considerable height.

Assuming the 6 db reduction per doubling of distance is valid, though, which corresponds to 20 times the logarithm of the ratio of two distances, for the 9 foot distance you indicated the calculation works out to about 101 db, for the two speakers.

Also, if and when you perform the oscilloscope measurement, keep in mind that the word "peak" has to be applied with care. Amplifier power and voltage levels are specified on an rms (root mean square) basis, and on the assumption that the waveform is a sine wave. For a sinusoidal waveform, the number of rms watts is calculated based on a voltage equal to 0.707 times the maximum ("peak") voltage that is reached by the waveform. So the word "peak" in that context means something different than the "maximum" power level corresponding to a musical "peak," which refers to rms power and not instantaneous peak power.

In other words, what would be most meaningful is to determine the maximum voltage level that is reached under worst case listening conditions, multiply that number by 0.707, and apply the E^2/R formula to the result. Applying the E^2/R formula to the maximum voltage level that is reached would work in the direction of making the amplifier seem more underpowered than it may actually be, by a factor of about 2.

Regards,
-- Al

Thanks Al, you're really helping a lot. So is everyone else too. It's probably what I expected, I'm fine listening to jazz and vocals at normal to slightly louder volumes, but when I want to play heavy classical pieces to stir me up, I should be aware that this amp does not have large reserves. I still think the oscilloscope is cool and I may get one to play with.

Your speaker isn't a resistor. The REAL part of the complex impedance value is what is doing "work (making music). So be very careful to use the impedance as a resistive load...it is far from that. Speakers are only 5% efficient, so that means the majority of the impedance is imaginary in nature and does not do work.

A reasonable SPL is near 85 dB with 1 watt at 1 meter with a 1 KHz tone. Seems good to me. But, as you increase volume or decrease the frequency, power requirements go up dramatically. To not clip peaks on music (test tones are not dynamic) you aften time need 10 times the average power.

It is this dynamic power requirement that demands attention. When music moves from 1 watt to two watts average, for instance, you need an amp ten time bigger than the last one! A rule of thumb is every 3dB average SPL increase needs twice the power as the previous level. Most music will NEVER see a 30 dB dynamic range for this very reason. No amp can manage it. With digital you could do it, but should you?

If you listen to "normal" SPL around 83 dB and 93 dB peaks (where I listen on most music, and with typical 10dB dynamic range) with 92 dB SPL rated speakers it looks like you should have decent headroom with 30 watt continuous amps as they usually provide more than the instantaneously.

Rower, thanks for your comment, but I disagree with some of your statements:

Speakers are only 5% efficient, so that means the majority of the impedance is imaginary in nature and does not do work.

Much of the inefficiency reflects real (resistive) impedance, that consumes power but converts most of it into heat, rather than sound.

When music moves from 1 watt to two watts average, for instance, you need an amp ten time bigger than the last one! A rule of thumb is every 3dB average SPL increase needs twice the power as the previous level.

This statement is self-contradictory. An increase from 1 watt to 2 watts IS a 3db increase (as is an increase from 10 watts to 20 watts), and requires twice as much amplifier power (as the second sentence indicates), not an amp that is ten times bigger.

Most music will NEVER see a 30 dB dynamic range for this very reason. No amp can manage it.

I could show you waveform diagrams on my computer of the Sheffield Lab recording of Prokofiev's "Romeo and Juliet," which clearly depict a difference in volume between the loudest notes and the softest notes of approximately 55 db. That corresponds to a power ratio of 316,000 times. At my listening position, the softest notes are around 50 db, and the loudest are about 105 db. My 65W amp and 98 db speakers have no problem at all dealing with that. MANY other symphonic recordings in my collection EASILY exceed 30 db of dynamic range.

The 10 db typical dynamic range you refer to is probably typical of (or even greater than) the dynamic range of the majority of rock recordings that are released these days, but does not apply to a lot of other kinds of material.

Regards,
-- Al

Speakers are only 5% efficient, so that means the majority of the impedance is imaginary in nature and does not do work.

Everybody knows that speakers play on one wall better than on another. It is because 90 degree rotation eliminates imaginary (parasitic) impedance.