WARNING: This post contains generalizations! To start: 0dB 1khz tone amplified @ 25 watts = 100dB SPL -3dB 1khz tone amplified @ 25 watts = 97db SPL -6dB 1khz tone amplified @ 25 watts = 94 dB SPL To make a -3dB 1khz tone "as loud as" the 0dB tone, You'd need to double power. So: 0dB 1khz tone amplified @ 25 watts = 100dB -3dB 1khz tone amplified @ 50 watts = 100dB -6 dB 1khz tone amplified @ 100 watts = 100dB Right? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Let's say we have an amplifier as follows: 6.0vac input signal will result in 100watts per ch. @ 4ohm. Regardless of input, to make 100 watts @ 4 ohm You'll need 20vac out of the amp's voltage rails (wattage = voltage^2 / resistance). So, we know that with a 1khz 6.0 vac input signal, the voltage rails will be at full swing (no clipped wave) of 20vac. This is the limit of what this amp can do (AC wave). Now, amplifiers don't see a dB reference, they only see input voltage & amperage. Now, a -3dB tone out of the HU will pass less V & A into an amp then a 0dB tone (given same HU Vol. Pot. setting). The impedance of the input stage of an amplifier won't change. Assume that a HU will output 6.0 vac (through the patch cables) @ the amplifier's impedance, with a 1khz tone recorded @ 0dB.. playing vol. 30. Now, if all other things remain constant, but You decrease the media info by 3dB reference, then the HU will no longer be outputting 6.0vac @ vol. 30. Let's assume that with a -3dB 1khz tone @ vol. 30 from the HU means an input voltage of 5.0 vac into the amp's input impedence. The resulting SPL is 3dB down from a 6.0 vac signal. But the rule of thumb is that to gain 3dB You have to double power. So, to lose 3dB You'd have to cut power by half. Now, the rails of the amp putting out 14vac accroding to that generally accepted rule. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This is where I get truly lost: It's generally accepted that most commercial media is not recorded at 0dB reference, but actually somewhere well below that level. However, there is ALOT of information in the media for any given moment. So there are transients & events in the media that could result in a higher output signal by the HU (at vol. 30). --==BIG VOID IN KNOWLEDG BASE==-- This "stronger" signal into the amplifier will result in more voltage thrown across the rails of the amp, and therefore higher output from the amp. If You set the sensitivity of the amp with a 0dB reference (or 6.0Vac input), then You are now clipping the waveform - albeit momentarily. If you set the sensitivty of the amp with a -3dB reference (or 5.0Vac input), then the voltage of the rails is still of the AC variety... no clipping / distortion. However, Your average SPL is also 3dB down. Lets say 97dB instead of 100dB. This is where having larger amplifiers with "headroom" comes into play. You can set things up for a higher wattage to compensate for the lower reference level of the signal... resulting in the same SPL as that of a of a "less powerful" amp which is being further "stressed". God I hope that makes sense... -Nick
