Subwoofer size vs. room size

[JensToft91]

Hi techtalk

I've been searching for an article or thread that explains the relastionship between subwoofer size and roomsize.
I can't seem to find anything good.

I've seen statements like: And F3 of 45 Hz i enough, provided you have enough Vd. What is enough?

Best regards,
Jens

[buggers]

What is it that you are trying to do?

[billfitzmaurice]

There is no direct relationship between room size and sub size because there's no direct relationship between F3 and maximum SPL with sub size. Where room size enters the equation is cabin gain, which averages about 8dB/octave below the frequency where the longest room dimension is 1/2 wavelength. The smaller the room the higher the F3 you may use for the same net result.

[JensToft91]

What is it that you are trying to do?

I guess I just want to know the theory. I'm thinking about building a subwoofer - but I already have a PC12-NSD placed directly behind the couch. That's not exactly a small sub.

[JensToft91]

There is no direct relationship between room size and sub size because there's no direct relationship between F3 and maximum SPL with sub size. Where room size enters the equation is cabin gain, which averages about 8dB/octave below the frequency where the longest room dimension is 1/2 wavelength. The smaller the room the higher the F3 you may use for the same net result.

So i all comes down to desired SPL - and that changes with frequency, with room dimensions?

Example: My PC12-NSD sounds smaller in my new bigger living room, than it did in my small apartment. So is that because the room dimensions is smaller, which make the cabin gain start higher in frequency? So the cabin gain is higher at 20 HZ in the small room than it is in the larger room.

So if i want 95 db at 25 HZ at my listening position, and my PC12 can do that - adding another subwoofer won't sound different? (I know it will smooth out the rest of the response, and help with localization). But two subwoofer wont "Energize" the room more than a single one, playing at the same SPL?

[billfitzmaurice]

95dB is 95dB. To 'energize' the room more means higher SPL.

[JensToft91]

95dB is 95dB. To 'energize' the room more means higher SPL.

So if I don't want it to be louder, the only reason to add more subwoofers will be to smooth the frequency response, and maybe decrease localization?

If thats true, and i think sub 30hz is loud enough, I can use a smaller subwoofer to smooth the response in the 30-80 hz region?

[dwigle]

We might be getting in the weeds. How big is your room (LxWxH)? How many openings to other rooms that can't be closed with a door or typically aren't closed?

I have a 12" RSHO with 1000 watts in an 11x14x8 small office and it will pressurize the room to the point of pain. Conversely, it's weak in my 26x14x9 theater. My theater has (2) high output 15" drivers to achieve similar results.

It sounds like a second sub may be desirable if just to decrease localization (not unimportant). I personally don't like mixing sub types (rather have 2 identical subs/enclosures), but others may opine differently.

[Fishchris]

What is "enough" for some people, might not even be close, for others. I think this is totally subjective to the individual.

[billfitzmaurice]

I can use a smaller subwoofer to smooth the response in the 30-80 hz region?

Probably not, in most cases you'd want multiple subs to have the same response and sensitivity.

[JensToft91]

This is confusing to me. If 95 db is 95db, does it matter what sub it's coming from?

[JensToft91]

We might be getting in the weeds. How big is your room (LxWxH)? How many openings to other rooms that can't be closed with a door or typically aren't closed?

I have a 12" RSHO with 1000 watts in an 11x14x8 small office and it will pressurize the room to the point of pain. Conversely, it's weak in my 26x14x9 theater. My theater has (2) high output 15" drivers to achieve similar results.

It sounds like a second sub may be desirable if just to decrease localization (not unimportant). I personally don't like mixing sub types (rather have 2 identical subs/enclosures), but others may opine differently.

The room is 123 m3 / 4661 cubic feet - so it's quite large i guess, judging on audioholics subwoofer rating.

Edit: Larger than your theater with dual 15's.
The loudest movies i watch is at about -15 db - with the sub running 5db hot.

[billfitzmaurice]

This is confusing to me. If 95 db is 95db, does it matter what sub it's coming from?.

Two subs placed less than a half wavelength apart each producing 95dB will total 101dB.

[LOUT]

Where room size enters the equation is cabin gain, which averages about 8dB/octave below the frequency where the longest room dimension is 1/2 wavelength.

Could you write a couple of examples of this with some numbers? For some reason I'm having a difficult time wrapping my head around it despite recognizing it's pretty straightforward. I'm hoping seeing a couple examples will knock the cobwebs loose or something, lol.

[billfitzmaurice]

The actual equation is 12dB per octave increase in sensitivity where the longest room dimension is 1/2 wavelength, but that assumes a very tight room. Real world results are closer to 8dB IME. https://www.youtube.com/watch?v=Zs4eeZsNEYQ

[andy19191]

So if I don't want it to be louder, the only reason to add more subwoofers will be to smooth the frequency response, and maybe decrease localization?

If thats true, and i think sub 30hz is loud enough, I can use a smaller subwoofer to smooth the response in the 30-80 hz region?

Distributed subs that smooth the response may be creating or absorbing sound depending on the frequency and location. Also at some frequencies some subs may be working strongly while others are pretty much off. This means a larger total cone area is generally needed compared to what one might expect based on experience of using just mains or a single sub. Fortunately at the lowest frequencies below the frequency of the lowest room resonance the sound pressure is pretty much rising and falling constantly throughout the room with the subs pretty much able to all work together. This prevents the cone area requirement from getting too excessive.