Axiom Home Page Forums General Discussion Technical Questions What is "to fill the room" ?

I frequently see an expression "to fill the room". As in "... you can't fill a big room with that speaker...".

I'm trying to understand what it is. To me, the size of the room per se should not matter, shouldn’t it? It's not like a bucket that needs to be filled with water. The room's wall and the reflections, I think, should play a much bigger role. Indeed, if the walls are made of an absolute sound absorber it wouldn't matter whether the room is 100 cubic feet or 10,000 cubic feet. It will be the same as outside in the open space.

When I read speaker specs the difference in the efficiencies in-room and in anechoic camera is only 3 dB. This is peanuts. So the only thing that should matter is the distance from the speaker, not the size of the room. Ok, ok, those measly 3 dB..

Gena, the meaning isn't clear, but it appears to arise out of an overestimation of amplifier power and speaker sound output needed in big rooms. Taken literally, of course there has to be some sound level in all parts of the room, so in that sense it's "filled". But the desired sound level at the listener's position is the relevant consideration and this can be at least roughly calculated. In addition, your own recent tests have put good numbers out and showed that even for your large room the actual power required at your 4 meter listening position was only a bit more than a fourth of what the inverse square law would require if only direct sound was considered.

Ok, that's what I hoped to hear. So the size of the room enters primarily through the distance from the speaker, not as a volume to be filled. In particular, the height of the ceiling should not play that big of a role. Again, within the 3dB of the 'room gain'.

Gena, volume is a factor(for the reverberent field, it makes no difference to the direct sound), but not to the extent that seems to be sometimes imagined(e.g. twice the room volume in no way requires twice the amplifier power or twice the speaker output to get the same level at a listener's position). The Linkwitz and Voelker material on room acoustics that was previously linked includes volume considerations in the analysis. Incidentally, out of curiosity, even before you reported the latest SPL numbers on your room I'd run it through the speech intelligibility calculator at the MCSquared site which has some pretty good calculators I use when I get lazy and don't want to do the math(nearly all the time). I used a 120 degree horizontal dispersion factor for the M60 and came up with about a 9' "critical distance" which I fooled around with to get some theoretical sound numbers which your measured values were in good agreement with.

The way I understand it - say, I'm reading a book using a desk lamp as a light source. Well, I will see the pages almost equally well whether I'm sitting in a small room or in a huge dark castle. The lamp is right here, it does not need to 'fill' the castle.

Again, theoretically the light intensity will be a bit higher in a small room with white ceiling and walls but, by no meas, not even by a factor 2.

regards,
Gena

That's right; in the light example the total light on the book would be just a tiny bit higher in the small white room. In a sound measurement though, the strength of the reverberent sound is equal to the strength of the direct sound at the "critical distance"(the total sound is therefore 3dB higher than either at that point)and at longer distances, as the direct sound continues to fall off at 6dB per doubling of distance while the reverberent sound is essentially constant at home distances, the reverberent sound is the principal sound factor.

I've always felt that "fill the room" was relevant primarily when making a subwoofer decision. Is this accurate?

There are obviously conflicting views on this

My take is that distance matters for the mains and surrounds, while volume matters more for subwoofers. To earlier comments, sure it's only a few dB we are talking about but a few dB can translate into twice the amplifier power or twice the number of drivers in a speaker so it does make a difference...

In my case I was running M2s with a good sub and found them to be a VERY capable music system when I was sitting relatively close (say 6-8 feet) but as I moved further away and had to turn up the volume to keep the same SPL the M2s hit their limits pretty quickly. M60s in the same scenario had no problems playing cleanly at a "loud" SPL for a listener anywhere in the (13 x 23) room.

Thanks John. That is sorta what I figured. Speakers=listening area; subwoofer=room area, including (perhaps) adjoining open spaces(?).

Room volume is not dependant on what again?
Are you mad?

Sound is about the movement of air and the molecules that make up air. Sound is very much a volume related concept just as it is distance. Energy is the key word here.

Reverberation plays a large factor as the energy from those air molecules hits and reflects off of walls. The surface material then comes into play before you can make any calculations about how effective a reverberation effect will be in any given room. The preceding assumption on reverberation effect seems to only account for bare walls. Check that assumption at the door.

As one moves further away from a sound source, the SPL decreases. Why?
Molecules lose energy often through heat. They lose energy by colliding with other molecules and transferring a partial part of that energy in the process, the remaining is lost as heat (sometimes light depending on the molecules colliding). In a larger room, the amount of energy lost (and number of molecules involved) by the time a molecule hits a reflective surface will determine the amount of energy remaining by the time that sound wave hits the ear. Larger room, more volume, greater distance to each surface, more molecules available for pushing (in numbers, not density but numbers relate to a linear distance nonetheless), more resistance to an endpoint (again in a linear concept) as well. A small driver can move x # molecules per sq area, less than a larger driver of same design. Sound is about moving air and i'm sorry, but a larger driver in a larger speaker moves more air. It can "fill" a larger room with sound much easier than a small speaker with small drivers simply because it can move more air molecules. This requires more energy to do.

Easy home experiment.
Get in the tub.
Take a spoon and a lid for a 4L ice cream pail.
Push both through the water and tell me how much water moves (wave size) in your tub and how much resistance energy you require to push each of them. Yet, notice, do waves from both of these actions still make it to the wall of the tub? Of course, but at what size? At what equivalent uses of energy?
Air in rooms is finite (when sealed). There is always resistance.
Basic physics.