What is this room gain stuff anyways?

[GordonNHT]

I've read that room gain affects low frequencies. So what frequency is that? I'm sure it depends on the room size. I've also read that high frequencies will be attenuated due to absorption in the room. I should be able to measure these things. So I setup a small 2 way speaker on one side of the room and measured the frequency response (FFT 500ms window, lots of smoothing) at 1', 2', 4', 8' and 16'. My room is approx 17' x 24' with 9' ceiling and neither live or dead sounding (ok maybe slightly live). Now according to theory, in an anechoic environment, the response should drop 6db for every doubling of distance (I am assuming a point source). Any deviations will be due to the room. See attached PDF.
So what does it mean? It looks like above 10Khz or so, it really does drop at 6db per doubling. Everywhere else it drops by less than 6db. Thus I conclude that generally speaking, room gain can affect all frequencies up to 10Khz (in my room) also high frequency absorption is really just another way of saying reduction of room gain at high frequencies. Does this make sense or is there some other explanation?

[auplater]

my understanding of room gain is it's nothing more than a geometric artifact, i.e., loss of "free space" radiation as frequency decreases , with secondary adjustments for non-linear effects, including non-linear microphone response with intensity, room boundary losses (non rigid surface, etc.), deviation from being a perfect radiator, that sort of thing.

So the measurements you made may be compromised by said secondary non-linearities.

John L.

[SY]

I assume that, as a check, you used a shorter gating and got a quasianechoic set of curves which show the normal 1/r2 drop in intensity?

Unlike you, I'm not a pro-feshunul speaker guy. But your results aren't hugely surprising. Using a light analogy, the radiant intensity falls off as 1/r2 as long as you're in free space. That's true for sound, too, because it's only a function of conservative forces.

Now, we put a whole bunch of reflectors up and things don't look so simple. With the gating you're using, which was intended to completely mix direct and reflected sound, as you move away from the speaker, the direct sound decreases but you're also moving closer to a reflective boundary. So the drop off won't be as fast until reaching a frequency where the reflections are significantly attenuated (like 10k).

As a side note, line sources are touted as having a 1/r characteristic, which is probably true in free space. It would be interesting to measure the actual in-room characteristic compared with a point source to see what the advantage really is.

[GordonNHT]

Aha, finally an excuse to build a line source. I've never experimented with them before. I wonder how a dipole line source will differ from a monopole line. I'll let you know.

[Drew Eckhardt]

Originally posted by GordonNHT
I've read that room gain affects low frequencies. So what frequency is that?

The fundamental resonance, which is c / 2 / longest room dimension.

I'm sure it depends on the room size. I've also read that high frequencies will be attenuated due to absorption in the room.

The reverberant field strength is a function of total radiated power (which can be derived from on-axis SPL and speaker directivity), room size, and how absorptive the room is.

Drivers become increasingly directive as the wavelengths approach the driver dimensions. 10KHz wavelengths are about 1.25" long which is getting real close to your 1" dome tweeters' size. Given flat-on-axis response this means that the power response is dropping.

This isn't room gain.

[planet10]

Most of your room gain is going to be <100 Hz... you have no data that low.

dave

[auplater]

Originally posted by GordonNHT
Aha, finally an excuse to build a line source. I've never experimented with them before. I wonder how a dipole line source will differ from a monopole line. I'll let you know.

One of the significant reasons I built mine was their ability to disapear in the room and present an extremely realistic soundfield compared to conventional speakers... the image floats in the room and is very stable as you move around.

BG-75 / ATC planar dipole construction

John L.

[SY]

Up until a couple months ago, I was using line source dipoles. A very attractive sound. At the moment, I'm using something closer to a point source (modified NHT 3.3) and the sound is very different. I'm not sure which I prefer; they each seem to do different things well. The lne source dipoles gave a huge soundstage, very open and expansive. The point sources seem to be more revealing of miking details and give great image precision. I wish I could have both...

I'm starting on another pair of dipoles using the Super Zero "woofers," 24 to a side.

[auplater]

Originally posted by SY
Up until a couple months ago, I was using line source dipoles. A very attractive sound. At the moment, I'm using something closer to a point source (modified NHT 3.3) and the sound is very different. I'm not sure which I prefer; they each seem to do different things well. The lne source dipoles gave a huge soundstage, very open and expansive. The point sources seem to be more revealing of miking details and give great image precision. I wish I could have both...

I'm starting on another pair of dipoles using the Super Zero "woofers," 24 to a side.

Curious you found the point source yielding greater image precision... I've found just the reverse.... not only is the image stable w/ the dipoles, but the location of the individual instruments is very precise and inner details (especially in well recorded orchestral works) are revealed to an extent I've not experienced with conventional boxes. All of this is, of course, moot with lousy source material (although some mp3's actually sound pretty good as long as the music isn't too complex and the bit rate is 128K or greater).

John L.

[myhrrhleine]

At very low frequencies the sound can be the same wavelength as the wall spacing.
The soundwave bounces back and forth in phase, adding together and causing a 6dB rise in SPL at that frequency due to the in-phase addition