How did you come to this conclusion, Grim? You have several imprecise terms here, so let's work this a bit. First of all, what are you assuming is an "average sized room"? I suspect you're in Canada, and I don't know what the average sized room would be there, but here in the US, I normally think of about 20x17 feet with a 9 foot ceiling as representative of average in a single family home. (I'm assuming that no one in their right mind listens at deafening levels in an apartment setting.) Assuming the display and the main speakers are on a short wall, I would guess the primary seating position would be about four meters from the display. Maybe a little less. Assuming 4-ohm speakers of average sensitivity - let's say 86db/2.83v/meter, and assuming the large majority of the volume is generated by the main speakers, and assuming a simple fall-off of 3db per doubling of distance, that means 2 watts at 4 ohms will generate about 80db at the listening seat.
Another imprecise term is "deafening", but for action movie HT use, let's say that requires a "loud" average level and peaks 20db higher than the average. (I'm making these numbers up, obviously, but I think they're realistic.) Let's also define "loud" as an 82db average level at the listening seat, so the peaks are 102db. That is quite loud, IMO, so I'll use that as "deafening". I realize average levels that high would only occur during "action scenes", but having endured a demo of a scene from The Expendables, I think this is what some people here have in mind. So, 102db peaks at the listening seat would mean 108db peaks at one meter, so that's about six or seven power doublings to get to 108db from 86db, so 2 watts at one meter becomes something like 256 watts on peaks to hit 102db at the listening seat, assuming seven doublings.
Personally, I don't listen to movies at these levels, but this is the web site where so many people seem to think a 20x17 room might need two 15" ported subwoofers for the LFE. [Half a smiley.]
What do you think of my calculations?
Ummm, not in Canada. Is that what my VPN shows, or is it that I occasionally use British spelling? Maybe I grew up in Australia, NZ, UK, India, Malaysia, Hong Kong, Ireland, Kenya, South Africa, etc. etc.
(For future reference the size the listening space in my yurt is 23' x 14' x 9', so we're on more or less the same wavelength when using the term "average room").
Let me try to explain using reasonably efficient speakers rated at 90 dB/W/m in a room of the dimensions you provided. When driven by one watt will deliver 90 dB at three feet. If we triple the distance to 9 feet, spl at 1 watt will drop by 12 dB.
Nine feet listening distance is actually too far back for a 20' room. Why would I say this? Because you'll want to place the mains at least one foot from the front wall. Since 9 + 1 = 10, this would put you in the middle of a 20' room, which would be terrible because of modes. One third distance from the front wall (~6.7-7.0 ft would ideal, but might be a bit close for comfortable 70" big screen screen viewing. So let's say 8 feet, but we'll round up to 9 ft to keep the math simple.
To maintain 90 dBA at that distance would require 100w of power. 30W would result in an spl of about 85 dBA
from the front speakers. But since there are rear speakers too, one must consider the spl of these too.
The rear speakers in a HT system are typically placed a bit closer to the listener (it avoids nodes and notches, which could be quite significant if everything -- walls and speaker placement -- was symmetrical). So let's put the rear speakers at 3' from the seating position just to keep the math simple.
http://www.sengpielaudio.com/calculator-distance.htm
Getting 90 dBA spl at 3' would require 1 watt of power. But you likely wouldn't want the rears to be louder than the front. So let's cut the volume down by half, or to 84 dB. So the combined effect of two front drivers @85 dBA + two rear drivers at 84dBA equals an spl of 89.1 dbA... and we haven't even counted the sub woofer into the equation. Nor am I accounting for room gain, which will almost certainly occur with the speaker placement I'm using.
http://www.noisemeters.ca/apps/db-calculator.asp
Since all of the power levels I ever use are Watts RMS (everything else is misleading BS),a quality amp with 30WRMS will have the headroom to reach 60-90 watts dynamically. That's roughly the ratio between RMS and dynamic power figures in most tests.
The National Institute of Safety and Health (NIOSH) claims that sustained listening at 85 dBA will cause permanent hearing damage in 8 hrs. NIOSH also says that 100 dBA will cause permanent hearing damage in 15 minutes or less. Now dBs are log values, but it's reasonable to suggest that 89 dB over three or four hours would cause permanent hearing damage
everytime a full length movie was watched at this spl.
So there you have i!
If these calculations were done to a scientific standard, I'd be adding spl of each driver slightly differently using their trigonometric distance. I'd account for the additional spl of the sub and room gain. I'd base my calculations on 8 feet from the front drivers, which would drive spl higher. I'd also graph by frequency because, after all, one must consider Mssrs Fletcher and Munson. The results would show well on a 3D waterfall plot, had I the inclination to do so.
But I digress... the figures I've used do seem good enough to back up my earlier point, but I was off in my head by 1 dB. (I'm normally more off in my head than that, if you ask my friends!).
The trick is getting an amp that can reliably sustain 30W without clipping and with dynamic headroom. This requires a quality amp, which is why I talk quality and fall for the emptiness of a manufacturer's dynamic power rating.
I've provided the links so that you can do your own calculations. You can move the distances around -- there are undoubtedly distances you can use to minimize spl, but the ones I've used are reasonable.
So any reasonable placement of efficient speakers, when combined with a high quality amp capable of delivering and sustaining 30WRMS without clipping, will be able to deliver high sound pressure levels to an average size room.
