When does a loudspeaker get too big for a room?

[stunta]

Obviously, if it doesn't physically fit in the room, its too big but the question here is when does it acoustically become too big? In other words, given a certain room's dimensions, is there an optimal speaker size (woofer size, driver size or whatever it is)?

I am asking this because at some point my basement room will be ready and its a 22' x 13' but the ceilings are very low (7' I think, I will measure and correct this later). I want to get the largest speakers I can to maximize dynamics so this question above popped up in my mind. I do plan to do basic room EQ.

Thanks

EDIT: I am not sure if this is the right sub-forum for this. Admins feel free to move this.

 

[RayDunzl]

Show us some examples of "big" that you might be interested in to give us a clue.

 

[stunta]

I am not sure really but I think I would prefer active DSP based speakers but not sure which specific make/models I should be looking at. How about we start with something like Revel Salon2 that people rave about on here? Its not active, but I see its quite large.

 

[RayDunzl]

I would rate it "not too big".

My room is about 18 x 14 x 9, but open on the left rear corner.

I wouldn't hesitate to put a pair in here if I wanted to.

Salon 2: 53" x 14" x 23"

Mine now: 72" x 18" x 13", and standing 51 inches from the wall behind.

---

Since you want actives, how about a pair of them, see how it goes, and add subwoofers if they aren't "big" enough.

 

[Blumlein 88]

With cone and dome box speakers that are large you may need some distance for good integration of all the drivers or so the common wisdom is. Small monitors are made for near field listening like 1-2 meters. Some others are termed mid-field monitors. My rule of thumb, totally my own, so maybe I look brilliant or maybe I am stupid, but it seems a minimum distance of twice the distance between the two furthest drivers is not a bad guide.

The Revels are about 50 inches tall, but the drivers are probably within 3.5 feet of each other. So 7 feet from them. Give yourself 3 feet to get the Salons away from the walls and 7 feet from them to you and spaced apart you begin to fill in a reasonable size room I think. So maybe 13 x 15 ft or something like that. Your room would work with the Salon 2s.

Evening out bass response can be difficult with speakers having prodigious output when the room has resonances and you are unable to make it all work physically. DSP certainly might help with that.

Panel speakers can be large and still work close to you, but may need a bit more room between themselves and the rear wall.

Now maybe some truly knowledgeable people can enlighten us.

 

[Cosmik]

Without having to indulge in measurements and EQ, etc., I think there is a fortunate match between the natural roll-off of sealed speakers (-12dB/octave) and 'room gain' (+12dB/octave) starting at some frequency derived from the dimensions of the room, whereby you can achieve a substantially flat frequency response all the way down to below audibility.

The size of the speaker is a crude proxy for the frequency at which it begins to roll off, so in the old days of sealed speakers people would have worked out that a small speaker generally sounds better than a large one in small rooms. Presumably with today's ported speakers (-24dB/octave) you are just not going to get that symbiotic relationship between room and speaker.

 

[Don Hills]

... 'room gain' (+12dB/octave) starting at some frequency derived from the dimensions of the room, ...

'Room Gain' is constant. That is, it doesn't vary with frequency. It's purely the pressure difference between the room volume with the cone "out" and the volume with the cone "in", less any leakage. The apparent "gain" is because the excursion increases as the frequency drops. The "free air" 12 dB/octave drop off of a sealed box speaker is because the excursion stops increasing.

Car audio exploits this by tweaking the driver parameters and enclosure size. Done correctly, this results in a slow rolloff LF response that is compensated by the "cabin gain".

 

[Hrodulf]

What do you mean by "room gain"? Boundary gain is a real thing, however it will change depending on surface proximity, radiation geometry and surface reflectivity (rare).

Usually bigger speakers need more distance for proper summing and back ported speakers need some distance from the wall for proper port operation. Overall both small and large speakers interact with room modes largely the same. Limiting low end response can mean less trouble dealing with room modes, but overall it's pretty much the same story.

 

[svart-hvitt]

I would try and get my head around the way Genelec are thinking, how they guide on their products’ in-room performance:

https://www.genelec.com/sites/default/files/media/Studio monitors/Catalogues/genelec_monitors_in-room_performance.pdf

You will see that the nearest possible listening area depends on for example the speakers’ summation of information from the drivers, and the further you go away from the speaker, the lower the direct sound to reverberation ratio. Coaxial source-point design is outstanding in its ability to sum driver information in the very nearest field (down to say 30 cm).

Every speaker designer should have this kind of information readily available (though very few manufacturers state what’s the speaker’s in-room performance).

 

[Cosmik]

'Room Gain' is constant. That is, it doesn't vary with frequency. It's purely the pressure difference between the room volume with the cone "out" and the volume with the cone "in", less any leakage. The apparent "gain" is because the excursion increases as the frequency drops. The "free air" 12 dB/octave drop off of a sealed box speaker is because the excursion stops increasing.

Car audio exploits this by tweaking the driver parameters and enclosure size. Done correctly, this results in a slow rolloff LF response that is compensated by the "cabin gain".

There used to be a really comprehensive article but I can't find it now. There are lots of references on various forums e.g.

...room gain... occurs when the wavelength being produced by the speaker is the same as a half-wavelength of the room, and the driving mode changes from wave mode to pressure mode. This can only occur if walls are solid enough and there are no leaks or openings. The curve will depend on how lossy the room is and the individual dimensions, but a well sealed room can yield 12dB/oct slope from the half-wavelength frequency.

 

[Don Hills]

There used to be a really comprehensive article but I can't find it now. There are lots of references on various forums e.g.

... and they're wrong.

 

[Wombat]

When the best listening position is beyond the rear wall.

 

[Cosmik]

... and they're wrong.

Damn, I was relying on that article. (It was a university's course lecture notes and went into great detail).

Re. pressurization, presumably there is pressurization and there is propagation. If I move a diaphragm at high frequencies, I won't change the pressure at the edges of the room before the diaphragm starts moving the other way i.e. I won't have achieved 'pressurization gain' (or whatever the correct term is?). But at very low frequencies I can see that I will achieve "pressurization" throughout the room.

So is it just a question of cone displacement, or is there another sliding factor combining frequency and room size? Intuitively, that seems to make sense to me. i.e. for constant cone displacement I will get a boost as frequency goes down and the above pressurization effect comes into play. If I choose my speaker's corner frequency correctly i.e. where constant displacement with frequency (= reducing SPL with lower frequency) kicks in to match where the pressurization boost kicks in, I will achieve a flat(ter) response. I don't know how well these effects match, but it wouldn't surprise me if it turned out that a theoretical + and - 12dB/octave just happen to line up...

 

[Don Hills]

...
Re. pressurization, presumably there is pressurization and there is propagation. If I move a diaphragm at high frequencies, I won't change the pressure at the edges of the room before the diaphragm starts moving the other way i.e. I won't have achieved 'pressurization gain' (or whatever the correct term is?). But at very low frequencies I can see that I will achieve "pressurization" throughout the room.

So is it just a question of cone displacement, or is there another sliding factor combining frequency and room size? Intuitively, that seems to make sense to me. i.e. for constant cone displacement I will get a boost as frequency goes down and the above pressurization effect comes into play. If I choose my speaker's corner frequency correctly i.e. where constant displacement with frequency (= reducing SPL with lower frequency) kicks in to match where the pressurization boost kicks in, I will achieve a flat(ter) response. I don't know how well these effects match, but it wouldn't surprise me if it turned out that a theoretical + and - 12dB/octave just happen to line up...

Agreed, it only really makes sense at low frequencies where the wavelength is long compared with the room dimensions. But in that region the SPL due to pressurisation is proportional to cone displacement / excursion. In "free air", in order to maintain a flat frequency response as the frequency drops, the cone excursion must quadruple for each octave lower. In a room, the apparent "+12 dB /octave room gain" is because of this excursion increase. Once the frequency drops below the LF cutoff of the speaker, the cone excursion stops increasing and the free air response drops off at 12 dB / octave (sealed speaker). The "pressurisation" SPL remains constant, assuming a sealed room. Most rooms are "ported", so the pressurisation SPL drops off as the frequency drops.
The formula for calculating pressurisation SPL is simple: SPL in dB = 197+(20*log(Speaker displacement))-(20*log(Room displacement))
See the attached small spreadsheet.
Running a few numbers shows that the pressurisation SPL becomes more significant as the room size reduces. Hence the "boomy bass with big speakers in small rooms" effect.

 

[Cosmik]

Agreed, it only really makes sense at low frequencies where the wavelength is long compared with the room dimensions.

But presumably that is not a binary on or off phenomenon that flips at some frequency, and must occur gradually as frequency decreases. So I'll wager that a constant cone displacement does not give constant 'room gain' through that transition i.e. a measurement would not show the expected -12dB/octave slope in SPL. Does the transition, in fact, happen to have a +12dB/octave slope (sealed room, etc.)? - in which case, choice of sealed speakers with the right corner frequency (a.k.a. picking the right sized sealed speaker), indeed, would correct the effect.

 

[stunta]

Agreed, it only really makes sense at low frequencies where the wavelength is long compared with the room dimensions. But in that region the SPL due to pressurisation is proportional to cone displacement / excursion. In "free air", in order to maintain a flat frequency response as the frequency drops, the cone excursion must quadruple for each octave lower. In a room, the apparent "+12 dB /octave room gain" is because of this excursion increase. Once the frequency drops below the LF cutoff of the speaker, the cone excursion stops increasing and the free air response drops off at 12 dB / octave (sealed speaker). The "pressurisation" SPL remains constant, assuming a sealed room. Most rooms are "ported", so the pressurisation SPL drops off as the frequency drops.
The formula for calculating pressurisation SPL is simple: SPL in dB = 197+(20*log(Speaker displacement))-(20*log(Room displacement))
See the attached small spreadsheet.
Running a few numbers shows that the pressurisation SPL becomes more significant as the room size reduces. Hence the "boomy bass with big speakers in small rooms" effect.

I kind of understand what you are saying, but how does this translate as an answer to the original question? More layman terms please.

 

[Cosmik]

I kind of understand what you are saying, but how does this translate as an answer to the original question? More layman terms please.

Don may disagree, but I am suggesting that there is an appropriate corner frequency for a sealed speaker in a room that will give you a flat frequency response down to very low frequencies. The smaller the room, the higher that roll off needs to start. Too low and the bass will be exaggerated, and too high you will not get deep bass.

Size is a rough proxy for where the frequency of a speaker starts to roll off at the bottom end, so the original suggestion - that there is an appropriate sized speaker for the room - is approximately correct - IMO. Sealed of course. If ported I don't think there is such a neat symbiosis between room and speaker.

As it says here:

...if the woofer system is chosen to have about the same resonance as the room, rather than see a 2nd order roll off in the response below the woofer’s resonance, in the compliance controlled region, room gain effects will augment the response and tend to keep the sound pressure constant. This effect is well known behavior for sealed box woofer systems and also accounts for exaggerated bass response if the resonance of the woofer system is significantly below the room’s fundamental resonance.

 

[DonH56]

A small box with long-excursion woofer or appropriate crossover network/padding (how many small boxes achieve lower bass) and a large tower will still experience the same room effects (though displacement does matter at some point). I tend to agree with the idea that "too large" is more dictated by crossover/driver design and layout (which determines the "convergence point" for how far from the speaker you need to be to hear a cohesive wavefront) and simple aesthetics (tiny speakers in a big room look as out of place to me as huge speakers in a tiny room) than "room gain". But I am biased as I have tended over the years to end up with fairly large speakers in moderate sized rooms. Just can't give up my big toys.

IMO - Don

 

[Sal1950]

Obviously, if it doesn't physically fit in the room but -------

It's only too big when your ole lady tells you it is, and you let her be the boss.

 

[Don Hills]

But presumably that is not a binary on or off phenomenon that flips at some frequency, and must occur gradually as frequency decreases. So I'll wager that a constant cone displacement does not give constant 'room gain' through that transition i.e. a measurement would not show the expected -12dB/octave slope in SPL. Does the transition, in fact, happen to have a +12dB/octave slope (sealed room, etc.)? - in which case, choice of sealed speakers with the right corner frequency (a.k.a. picking the right sized sealed speaker), indeed, would correct the effect.

The transition does have a slope, just not -12 dB. Drive the speaker at constant displacement. Take the SPL at a given frequency in free air and add the SPL calculated from the pressurisation formula. Plot this for each frequency and you get a curve. Starting at high frequency, the curve falls at -12 dB / octave. As it approaches the SPL level from the calculation, the curve will start to level out.

The problem is that this doesn't represent real life. Real speakers don't have a constant displacement as the frequency varies. The displacement varies with frequency to arrive at a flat ("constant") frequency response. So as the frequency decreases towards the speaker's LF -3dB cutoff, the displacement, and thus the SPL due to "pressurisation", increases. This results in a peak in the bass response curve above the cutoff. Once the displacement becomes constant below the LF cutoff, the SPL also becomes constant. For all sane combinations of woofer displacement and room volume, this is a lot lower than the SPL measured above cutoff. So to get a better match between "speaker SPL" and "room gain SPL", the speaker displacement has to start rolling off well before cutoff. Another way of saying this is that the speaker / enclosure should be tuned to a much lower Q than the usual 0.7 or so. I haven't run the numbers, but I feel something close to IB (Infinite Baffle) would be optimum. Maybe "The Cult Of The Infinitely Baffled" were right...

As I mentioned in an earlier post, the SQ (Sound Quality) side of competitive car audio take advantage of this. Many car bass drivers are designed to perform best in enclosures little larger then the driver. Done right, the resulting speaker has an early droop in frequency response above cutoff that nicely counteracts the "cabin gain" hump. Smooth, extended bass is the result. Doesn't sound so good if you put it in your lounge, though.