I wouldn't count on that measurement being meaningful without having detailed information on the technical characteristics of the meter, which chances are will not be available. One important parameter being how brief a transient it is capable of capturing. Others would include its frequency bandwidth, and some indication of how it handles non-sinusoidal waveforms (e.g., true rms, or peak converted to sinusoidal rms equivalent, etc.).
That said, the power capability you need will vary dramatically as a function of the dynamic range of the music you are listening to (i.e., the DIFFERENCE in volume between the loudest notes and the softest notes). What I would suggest is that you repeat the measurements with material having the widest dynamic range that you would normally listen to. Many classical symphonic recordings, for example, have vastly wider dynamic range than most rock recordings, and therefore require vastly more power during brief peaks, for the same average volume.
Regards,
-- Al |
As a followup to my previous post, see Figure 1 here for an example of a speaker having small phase angles (i.e., having an impedance that is pretty much resistive) across most of the spectrum. Note that the phase angle never goes below approximately -12 degrees, and only goes above approximately +20 degrees in the extreme upper treble, reaching a maximum of around +38 degrees at 20 kHz. Yet its sensitivity is only moderately high, at 91 db/1W/1m. So since little of the inefficiency occurring in the conversion of electrical power to acoustical power can be attributed to inductance or capacitance, it follows that the great majority of the lost power is being dissipated in the speaker as heat. Regards, -- Al |
By definition a resistor consumes the entire load placed across it. Yes. And it converts it into heat. Speakers don't so they aren't resistive loads...not anywhere near. Speakers do consume most of the power that goes into them, they just don't convert most of that power into sound. The power that is not converted into sound is mostly dissipated as heat. The faulty logic of your statement is addressed further in the final paragraph of this post. First, though, see the plot I previously linked to, of the impedance phase angle vs. frequency for a particular speaker. See any plot of impedance phase angle vs. frequency for any other speaker. While most speakers will not have phase angles that are as close to being resistive as the one I linked to, as I previously stated, "it is rare for a speaker to have phase angles that exceed or even approach 45 degrees across broad parts of the spectrum (although that can occur across narrow ranges of frequencies). Meaning that their impedance is mostly resistive." You have HUGE winds of wire in a magnet and you think that that is mostly resistive when it is driven? In the example I linked to, the inductance of the tweeter voice coil is undoubtedly the reason for the rise in impedance phase angle in the upper treble region. As frequency decreases, a given amount of inductance becomes progressively less significant. The crossover network further complicates matters. The bottom line, for a given speaker, is the measured impedance phase angle. How can you claim that the impedance is essentially inductive or capacitive, when the measurements indicate otherwise? How can an almost pure resistive load be 5%, or even 10% efficient? THAT does not make sense. By definition a resistor consumes the entire load placed across it. Speakers don't so they aren't resistive loads...not anywhere near. Where did the other 90% to 95% go? I suspect that you'll agree with me that the impedance of an incandescent light bulb is essentially resistive, at least at 60 Hz and other low frequencies. And I suspect that you'll agree that the great majority of the power supplied to it is NOT converted into light, and its efficiency is therefore very low (roughly 10% or so per this Wikipedia writeup). Where do you think the rest of the power supplied to it goes? Ever touched a 100W light bulb that has been on for a few minutes or more? Regards, -- Al |
Since the majority of the load is mostly reactive below 200 Hz this is a terrible "resistive" load per your conclusion. It is NOT a resistive filament. No where near. It does produce a very reactive signature to the majority of the signal power being applied. Rower, the point of contention was whether the 90 or 95% inefficiency of a speaker is mostly the result of power dissipated in the speaker as heat, or is mostly the result of reactive impedance characteristics. In the severe example you provided, the impedance phase angle only exceeds 45 degrees by a very small amount in a very narrow range of frequencies, between approximately 60 and 70 Hz. Throughout most of the spectrum, including the bass region, its phase angle is much less than 45 degrees. While that kind of impedance characteristic could perhaps be legitimately referred to as "very reactive" in a relative sense, relative to many other speakers, it is not by any means "mostly reactive below 200 Hz." An impedance is not "mostly reactive" unless the phase angle exceeds 45 degrees. And there is no way that the depicted impedance phase angle characteristic, or just about any other reasonably conceivable impedance phase angle characteristic, would account for the major portion of a 90 to 95% inefficiency. Yes, I certainly agree that the speaker will be a difficult load for many amplifiers to drive, with good sonic results. But that wasn't the issue. Regards, -- Al |
09-28-12: Koestner
Would you guys think I am off by a factor of two, or possibly much larger? I have no idea. IMO, though, Ron (Rrog) correctly stated the bottom line: "The best way to find out if an amplifier is powerful enough is to listen." Some additional points: I found measurements of your amp here. They indicate that it can provide 76 watts into 4 ohms at 1% distortion. However, it is indicated that while the amp operates Class A up to the clipping point into an 8 ohm load, with a 4 ohm load it transitions to Class AB at some unspecified level that apparently is significantly below the clipping point. Conceivably that could have some effect on sound quality at power levels you would be using. Another way to look at it: Let's call it a 60W amplifier into 4 ohms. Assuming that the 92 db/2W/1m/4 ohm numbers for the speakers are accurate, it can be calculated that at listening distances of say 10 to 12 feet, 60W will result in a sound pressure level of approximately 100 db, neglecting room effects. Provided that the sound quality of the amplifier is still holding up at that level, 100 db will certainly be loud enough for most listeners with most recordings. It will also certainly not be loud enough for some listeners with some recordings, particularly (as I mentioned earlier) recordings having very wide dynamic range. For instance, I have many classical recordings on labels such as Telarc, Sheffield, Reference Recordings, etc. that at my listening position reach peaks that I've measured at around 105 db, although the average level during those recordings is perhaps in the low 70's. Keep in mind that a 30 db difference between peak volume and average volume means that 1000 times as much power is required for those peaks, compared to the average level of the recording. Hope that helps. Regards, -- Al |
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 |
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 |
10-01-12: Rower30
The dynamic range reference is from the AVERAGE SPL, not the minimum. So if your avevrage SPL is 85dB, your peak dynamic range will be 115dB, not a value most systems can manage. I had indicated that the average SPL of the recordings I referred to were "perhaps in the low 70's." I doubt that anyone would want to play them at an average level of 85 db, with peaks of 115-120 db. That is simply too loud. As a point of reference, as noted in this thread 8 hours is the limit of permissible continuous exposure to 85 db, beyond which hearing damage can be expected to occur. Some of that precious little energy is wasted as heat as you say, but the majority is imaginary in vector. What do you base that on? That would say that a speaker whose impedance has small phase angles across the audible frequency range would approach 100% efficiency. My understanding is that most speakers do not have as much as 10% efficiency. You cited a figure of 5%. And it is rare for a speaker to have phase angles that exceed or even approach 45 degrees across broad parts of the spectrum (although that can occur across narrow ranges of frequencies). Meaning that their impedance is mostly resistive. That can be seen in the measurements that are provided by John Atkinson in Stereophile's speaker reviews. As an additional point of reference, note in this Wikipedia writeup that the acoustic power radiated by a jackhammer is all of about 1 watt! If the 50 or 100 electrical watts or thereabouts that may be sent into a speaker at times were reduced to that kind of acoustic power level (or less) primarily as a result of non-resistive impedance characteristics (as opposed to conversion to heat), it would say that the impedance would have to be almost entirely either inductive or capacitive (i.e., having phase angles approaching +90 or -90 degrees) across nearly the entire frequency range. Which, frankly, is nonsensical, as well as being completely inconsistent with JA's measurements. Kijanki, LOL :-) Regards, -- Al |
