The dBW Difference
When it comes to the maximum output power figure itself, most measurements in spec sheets and test reports, ours included, are given in watts, the standard electrical unit of power. Unfortunately, watts are almost useless for this purpose because they're linear while our hearing is logarithmic. For example, a receiver that delivers 15 watts at clipping is audibly more powerful (by 1.76 dB) than one delivering only 10 watts, but the 0.2-dB difference in maximum output between receivers delivering 105 and 100 watts is inaudible.
If the power figures are converted from watts to dBW, however, you can easily tell whether a difference in output at clipping is audibly significant, making meaningful power comparisons possible. Standing for "decibels referred to a 1-watt output," a dBW figure indicates how much louder a receiver can play than an amplifier with a 1-watt drive level. For example, a receiver that measures 17.25 dBW (53 watts) at its clipping level can generate sounds up to 17.25 dB louder than a 1-watt amplifier.
Any power results that differ by more than 1.5 to 2 dBW are significant, regardless of the wattage figures involved. The table on the next page gives dBW equivalents for common wattage ratings, calculated by multiplying the common logarithm of the wattage by ten: dBW = 10 ¥ log (W). In our test reports, we round every wattage figure to the nearest 0.25 dBW since smaller gradations are sonically insignificant in product comparisons.
Stating power capabilities in dBW is useful in another way: you can simply add dBW power figures to a speaker's sensitivity rating to get an approximate value for the maximum undistorted sound level that can be generated by a given receiver (or amplifier) with that speaker. With a speaker having typical sensitivity - say, 90 dB sound-pressure level (SPL) with a 1-watt input - a 17.25-dBW receiver will generate up to 107.25 dB SPL at 1 meter, which is very loud for a home theater or listening room (the SPL at the main listening positions may be somewhat lower since most people sit more than a meter away from their speakers). You can actually trade off receiver power for speaker sensitivity - as power in dBW goes down, speaker sensitivity should go up by at least an equal number of dB SPL. A very careful shopper can save money by buying a less powerful receiver and still end up with a system that can play very loud if the speaker sensitivities are high.
Here's the test procedure we follow for our receiver (and amplifier) power measurements. First, the receiver is connected to precision, heavy-duty load resistors that function like ideal speaker loads. For most of our tests the load resistance is 8 ohms, with one test (stereo output) also using 4-ohm loads. Theoretically, the output wattage at clipping should double when 4-ohm loads are used instead of 8 ohms, but usually it doesn't quite get there.
In all cases, as an input test signal we use a 1-kHz pure tone (a sine wave) that increases in amplitude from a low starting level by around 0.5 dB per second. (Dialogue normalization, which keeps one Dolby Digital source from sounding a lot louder or softer than another, is set at a standard -27 dB, as it is on nearly all movie DVDs.) To make sure the receiver will overload, we crank up the volume control quite a bit - as much as 5 or 6 dB above the reference volume setting used to make noise measurements. As the input signal rises, we watch for the point at which THD+N shoots up in a graph of output level and distortion vs. time.
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