# Use REW to calculate room gain?



## Saturn94 (Jun 8, 2013)

Is there a way to use REW to figure out how much room gain I'm getting?

I'm in the process of looking for a new sub, or a pair of subs, and understand sealed designs rely on room gain to boost the lowest frequencies.

I'm wondering if there's a way to use REW to figure out how much room gain I'm getting in my space (about 5500 ft3).

Thanks.


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## andy_c (Aug 8, 2006)

Saturn94 said:


> I'm wondering if there's a way to use REW to figure out how much room gain I'm getting in my space (about 5500 ft3).


Hi Saturn,

I found some info about estimating the room gain in a thread on another forum. It looks like what they're doing is taking a measurement of the bass at the listening position with 1 sub active, then another near-field measurement (very close to the sub itself). Then using the REW amplitude offset feature in the graphs, one plot can be shifted relative to the other so that the two line up at some frequency around 50-60 Hz or so. The near-field one often rolls off at a higher frequency than the one at the listening position if "room gain" is present.

There is definitely some debate about what's being called "room gain", as no less an authority than Geddes claims there is no such thing in ordinary rooms. Maybe what's being shown is a low-frequency room mode. I'm not sure. Regardless of what you call it, many who have measured see more bass extension at the listening position than the near-field measurement has.


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## fusseli (May 1, 2007)

Almost, with the latest beta version of REW there is a room simulator which will allow you to simulate room effects but not room gain per say. An alternative I would suggest are the free tools by Jeff Bagby, which require Excel. I have used both and they both prove to be pretty accurate compared with my in room measurements.


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## Saturn94 (Jun 8, 2013)

andy_c said:


> Hi Saturn,
> 
> I found some info about estimating the room gain in a thread on another forum. It looks like what they're doing is taking a measurement of the bass at the listening position with 1 sub active, then another near-field measurement (very close to the sub itself). Then using the REW amplitude offset feature in the graphs, one plot can be shifted relative to the other so that the two line up at some frequency around 50-60 Hz or so. The near-field one often rolls off at a higher frequency than the one at the listening position if "room gain" is present.
> 
> There is definitely some debate about what's being called "room gain", as no less an authority than Geddes claims there is no such thing in ordinary rooms. Maybe what's being shown is a low-frequency room mode. I'm not sure. Regardless of what you call it, many who have measured see more bass extension at the listening position than the near-field measurement has.


Thanks for the info. I'll check out that link. 



fusseli said:


> Yes, with the latest beta version of REW there is a room simulator which will allow you to simulate room effects. An alternative I would suggest are the free tools by Jeff Bagby, which require Excel. I have used both and they both prove to be pretty accurate compared with my in room measurements.


I have played around with that. My room is pretty irregular so my actual measurements don't match too well with the predictions. I guess the model is geared toward rectangular rooms.


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## fusseli (May 1, 2007)

It is, yes. From what I understand modeling of irregular rooms is next to impossible, and a mathematical nightmare


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## Saturn94 (Jun 8, 2013)

fusseli said:


> It is, yes. From what I understand modeling of irregular rooms is next to impossible, and a mathematical nightmare



I kind of figured that.

It was still interesting playing around with it and seeing the effect of different sub positions, seating positions, and multiple subs. In a way it encouraged me to try different sub positions.


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## fusseli (May 1, 2007)

Common sense still applies to irregular rooms. If it's big and open, room gain will be minimal. If it's a small and enclosed room, "room gain" will be greater but at the expense of worse peaks and nulls.


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## Saturn94 (Jun 8, 2013)

fusseli said:


> Common sense still applies to irregular rooms. If it's big and open, room gain will be minimal. If it's a small and enclosed room, "room gain" will be greater but at the expense of worse peaks and nulls.


That's what I understand.

My room is more big and open (living room/dining room combo, cathedral ceiling, open to upstairs landing).


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## AudiocRaver (Jun 6, 2012)

We talk about room gain but don't define it clearly. Here is how I think of it, by example (for a regular speaker):

Say a speaker's sensitivity spec is 90 dB at 1 m in a free field and the LP is 4 m away. In a free field, sound power decreases by the square of the distance, so 4x distance gives 1/16th the power or -12 dB, giving you 78 dB at the LP. If you actually measure 84 dB at the LP, you have 6 dB of room gain. I think.

At mid frequencies and higher, seems like you can measure it using distance ratios and SPLs, etc. Not sure the concept holds true in a small room at sub frequencies, with room modes and all.


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## fusseli (May 1, 2007)

AudiocRaver said:


> We talk about room gain but don't define it clearly. Here is how I think of it, by example (for a regular speaker):
> 
> Say a speaker's sensitivity spec is 90 dB at 1 m in a free field and the LP is 4 m away. In a free field, sound power decreases by the square of the distance, so 4x distance gives 1/16th the power or -12 dB, giving you 78 dB at the LP. If you actually measure 84 dB at the LP, you have 6 dB of room gain. I think.
> 
> At mid frequencies and higher, seems like you can measure it using distance ratios and SPLs, etc. Not sure the concept holds true in a small room at sub frequencies, with room modes and all.


I would agree with your definition. Room gain manifests itself by bass frequencies ringing around based upon the dimensions of the room, and for sub bass the room begins to pressurize if the room is small enough leading to good response below 30 and 20 Hz. You'd need a boatload of subwoofer power for sub 20Hz performance in an open environment, but it would also sound much better since in an open room you are hearing just what the woofers dish out and not the sloppy resonance of the room artificially boosting bass performance. Room gain does not apply to anything above a couple hundred Hz, I would imagine.

Another tip I have learned for best flat response is to use subs with a gradual roll off in small rooms where room gain is expected to fill in the low end. In other words, for the flattest FR choose sealed or small vented subs for small rooms. Leave the large vented subs with deep reaching shelf-like FR for open rooms.


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## Saturn94 (Jun 8, 2013)

Would it be reasonable to see how much rise in response I'm getting as frequency drops and use that to gauge how much "room gain" I'm getting? 

For example, if the overall sub response rises 6db from the crossover region down to 15-20hz or so, can it be said I'm getting about 6db in "room gain" at the lowest frequencies? I realize the rise may be due to factors other than strictly "room gain", but perhaps this is good enough to see how much gain I can expect from a sealed sub, or any other sub, placed in the same location?


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## Saturn94 (Jun 8, 2013)

fusseli said:


> I would agree with your definition. Room gain manifests itself by bass frequencies ringing around based upon the dimensions of the room, and for sub bass the room begins to pressurize if the room is small enough leading to good response below 30 and 20 Hz. You'd need a boatload of subwoofer power for sub 20Hz performance in an open environment, but it would also sound much better since in an open room you are hearing just what the woofers dish out and not the sloppy resonance of the room artificially boosting bass performance. Room gain does not apply to anything above a couple hundred Hz, I would imagine.
> 
> Another tip I have learned for best flat response is to use subs with a gradual roll off in small rooms where room gain is expected to fill in the low end. In other words, for the flattest FR choose sealed or small vented subs for small rooms. Leave the large vented subs with deep reaching shelf-like FR for open rooms.


I take this to mean I might be better served by a larger vented design?

Btw, my current sub is a 17x47 vented cylinder design. I've never heard how a sealed design would perform in my room, hence my hesitation to go thought route. That said, Tom V. at PSA said he thinks a pair of his XS30 sealed subs would work well in my room (I sent him a video of my room so he could see it for himself).


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## fusseli (May 1, 2007)

Multiple sealed subs is another way to go, yes. With EQ and enough power that option can equal a large ported sub. A large ported sub would be more economical in comparison.


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