Audio Power and Power Amplifiers

When we look at our home theater sound systems in reverse, we first see the room, then the speaker. If we stop there, we’ve just viewed the most difficult and important portions of our systems, the ones with the biggest influence on what we end up hearing. Setting that aside for a moment, let’s look backwards at what came before the speaker. We have wires, and amplifiers. Both are responsible for getting energy to the speaker, hopefully without changing it in any way from the signal that is input to the amplifier. And, hopefully, with enough power gain to provide adequate Sound Pressure Level (SPL) for the listener’s purposes.

So, what’s adequate SPL? In the world of THX, the goal is to reproduce sound “in the way that the creators intended”. That implies a similar sounding system to that of the system the creators used, and played at a similar volume. There is something called “Reference Level”, and if you were in a commercial cinema or a dub stage (a cinema used for the final mix of a film soundtrack), you’d have a specific reference level of 85dB SPL. That’s a level that is measured with a test noise source and an SPL meter at a particular seat or seats in the theater. The test noise level is made to produce an 85dB SPL reading on the SLP meter. That level corresponds to an average level that permits peaks up to 20dB higher to pass through the system to the listener undistorted. So, if we were to desire the same level in our homes, we’d want that same 85dB SLP reference level with 20dB of peak headroom. This, in fact, is what a THX Ultra2 system is intended to do in the typical 3000 cu/ft or greater home theater.

Looking at the speaker, it must have high enough efficiency (it’s ability to convert an electrical signal to an acoustic one) to get that high SPL into the room and to the ears without requiring unreasonable power to do so. A THX Ultra2 speaker must have at least an efficiency/sensitive of 89dB/W/m, which means with 1 watt applied, a microphone 1 meter away would show 89dB SPL in an anechoic room. That’s a minimum figure, many THX Ultra2 speakers can beat that. That also means that we can calculate the power required to hit reference level at any distance from the speaker, with some allowance for reflections in a real room.

Lets look at an example. The spectacular new M&K S300 speaker has a rated sensitivity of 93dB/W/m. So if we consider some gain from wall reflections, and sat 10 feet away, a single speaker would need a 75W amplifier to produce the reference 85dB SPL with the required 20dB of peak headroom. If we sat 15′ away, that amplifier now needs to be rated at 175W to do the same thing.

From this, two questions pop up. One, how far away will we sit? And Two, how much amplifier power will we need for “reference level listening”? These are easy calculations, though, and fairly predictable. I’ll throw in a a third question: Will you listen at reference level, or will you listen significantly below or above it? Turns out, at home, most people find reference level about twice as loud as they want to listen. That doesn’t mean they won’t turn it up to show off! But it does mean that typically they’ll use about 1/10 the power they paid to have. What’s that? Is that right? Yup, if you turn your volume knob down to -10, your volume will still sound loud, and that 170 watt amp you have will be running at 17 watts on peaks, and (now, don’t be shocked…) about .2 watts average. In fact, the actual SPL in the room may be a bit higher, because in this discussion we’re talking about a single speaker, but in our home theaters we have at least 5, and they do reinforce each other to create the total SPL in the room.

So, back to our questions, how much power will we need? Will an AVR with typical power ratings of 120W per channel be enough? In most cases, the answer is yes, if we stick with speakers with adequate sensitivity, and don’t sit too far away from them. Even if you do end up somewhat short of being capable of playing at theatrical reference, you may never actually do that, so before investing in lots of amp power, you may want to try out using just your AVR and see how you use it. With most AVRs at the upper end of the product line, external power amps are easily added later.

People often challenge the power ratings on AVRs on the bases that they aren’t tested “All Channels Driven”, and if they were, the power per channel would be substantially less that otherwise stated. All true, but it’s not all that important either. Soundtracks do not drive all channels equally. Estimates from the likes of Dolby place the highest power demands on the center channel, to the tune of 60% to 70% of the total SPL in the room, with the least amount of power distributed to the surround channels. On a peak basis, it is possible for even surround channels to demand quite a bit of power, but does that happen precisely simultaneously with other channels? The likelihood is exceedingly small, statistically nearly impossible. All Channels Driven tests are valid, but overly demanding, and do not represent real usage conditions.

However, if you do really want the potential of full power to all channels simultaneously (or you have a BIG room), you’ll be getting external power amps, which actually can provide that capability, and at substantially higher power levels. I’d be happy to consult with you to help select the best amps for your situation.

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