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?
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?
room gain
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.
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.
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.
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 said:
The fundamental resonance, which is c / 2 / longest room dimension.
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.
GordonNHT said:
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.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=95603
John L.
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.
I'm starting on another pair of dipoles using the Super Zero "woofers," 24 to a side.
SY said:
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.
dipoles
No doubt ... they need space to realize their benefits. Unless you can place them out into the room, the rear wave image enhancement seems to collapse, and interfere rather than augment the soundfield. Most naive listeners who hear mine state they're the first speakers that sound as good from behind as in front.
As psuedo linesources, they also fill the entire house with convincing sound, much different from conventional boxes.
24 NHT zero's?? That's the Scan Speak 18W/8545, right? What are you going to use for the mids-treble range? I've got 2 more of the ATC 25" Neoplanars on the way to play with, since they worked so well in my mains. I may replace the cones in my old Infinities, or maybe use them for HT surrounds...
John L.
SY said:
No doubt ... they need space to realize their benefits. Unless you can place them out into the room, the rear wave image enhancement seems to collapse, and interfere rather than augment the soundfield. Most naive listeners who hear mine state they're the first speakers that sound as good from behind as in front.
As psuedo linesources, they also fill the entire house with convincing sound, much different from conventional boxes.
24 NHT zero's?? That's the Scan Speak 18W/8545, right? What are you going to use for the mids-treble range? I've got 2 more of the ATC 25" Neoplanars on the way to play with, since they worked so well in my mains. I may replace the cones in my old Infinities, or maybe use them for HT surrounds...
John L.
GordonNHT said:
http://www.trueaudio.com/st_spcs1.htm
http://www.stereophile.com/reference/39/index2.html
best,
graaf
Thanks for the links. I guess it is generally agreed that so called "room gain" is due to boudary effects at low frequencies in our small spaces. But my measurements show that reflections in mid and high frequencies also cause in increase in measured SPL. Why is this not room gain? I suppose it's matter of who invented and defined the term.
Hi,
I'm not too sure, but I don't think boundary reflection and the humps and bumps these cause is anything to do with room gain.
Imagine a sealed box speaker in a room that is the same volume as the box.
The pressure changes inside the box will be the same as the pressure changes outside the box, provided the room and box are BOTH sealed to the same standard.
Now decrease the sealed box to 10% of the room volume.
The room pressure changes will be approximately 10% of the internal pressure changes inside the speaker box.
There comes a point in this box to room volume ratio where the gain of room pressure is no more than would be felt if the room were not sealed. I suspect that box to room volume or possibly cone area & Xmax (Vd) to room volume and room sealing effectiveness are all tied into this.
Reflections and their inherent subtractive/additive interference effects will add on to any room gain effect.
But, I could be (very) wrong.
I'm not too sure, but I don't think boundary reflection and the humps and bumps these cause is anything to do with room gain.
Imagine a sealed box speaker in a room that is the same volume as the box.
The pressure changes inside the box will be the same as the pressure changes outside the box, provided the room and box are BOTH sealed to the same standard.
Now decrease the sealed box to 10% of the room volume.
The room pressure changes will be approximately 10% of the internal pressure changes inside the speaker box.
There comes a point in this box to room volume ratio where the gain of room pressure is no more than would be felt if the room were not sealed. I suspect that box to room volume or possibly cone area & Xmax (Vd) to room volume and room sealing effectiveness are all tied into this.
Reflections and their inherent subtractive/additive interference effects will add on to any room gain effect.
But, I could be (very) wrong.
I built a monopole line array and performed the same test as before measuring frequency response at 1', 2', 4', 8' and 16'. The results are a big mess. Comb filtering along with unavoidable reflections off the floor and ceiling yeilds ugly curves. More smoothing. I have never designed a line array before but I would say it is a bit more difficult than a small 2-way. In any case this does show that the SPL does fall off much less than a point source. In fact at 2700hz the level changes by only about 3 db from 1ft to 16ft.
Well, that's quite cool- I casually mention that a particular measurement would be interesting, then you actually go do it! For your next task, please have your payroll department direct deposit your paychecks into my account.
Seriously, thanks for that- this is another thing that "everyone knows" but it's rare to see real data.
Seriously, thanks for that- this is another thing that "everyone knows" but it's rare to see real data.
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