When does a loudspeaker get too big for a room?

[Cosmik]

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.

Finer details aside, what this tells us, though, is something remarkable. It says that bass reflex is not a slam dunk boost or extension of the bass because it (presumably?) doesn't give the same pressurization gain in real listening rooms that a sealed speaker does.

It also tells us that a small sealed speaker that, on paper, may look bass-light because of its -3dB frequency response figure will, in the appropriate room (maybe with some gentle EQ), give beautiful bass all the way down to 20Hz or whatever (maybe not so remarkable, because headphones do this, too...). People obsessed with 18" subwoofers may really be wasting their time unless their rooms are huge.

It also occurs to me that the best sealed rooms ever encountered are probably anechoic chambers. Any implications for discrepancies between speaker specifications versus the reality in real listening rooms?

 

[Don Hills]

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. ...
...
As it says here:

That musicanddesign quote sums it up well. The problem is that the speaker's "free air" contribution and the "room gain" contribution aren't complementary. They don't balance each other out for any sane combination of speaker displacement capability and room size.
For example, I have a design with a 12 inch woofer with a 12 mm xmax. It's capable of 112 dB SPL at frequencies above the LF cutoff.
The "pressurisation" SPL of that speaker at xmax in an 8 x 10 x 13 foot room is about 102 dB. It'll be lower for larger and/or leakier rooms. For grins, I worked out the size of room required for the "pressurisation" SPL to equal the "free air" SPL. A bit over 8 x 6 x 6, or the size of a large closet...

 

[Don Hills]

...
It also occurs to me that the best sealed rooms ever encountered are probably anechoic chambers. Any implications for discrepancies between speaker specifications versus the reality in real listening rooms?

Anechoic chambers become progressively less "anechoic" as the frequency drops. You need a huge room with serious depth of damping wedges. I suspect Mr Toole can speak from practical experience on this point.

 

[Don Hills]

It's only too big when your ole lady tells you it is, ...

Many men wish they had that problem.

 

[Don Hills]

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

How long is your piece of string? (*)
You've had some good advice from others. The size of speaker you can squeeze in depends a lot on the speaker design and the geometry this dictates - how far from the back wall and how far from you they should be. Ask people for their opinions and recommendations. If you find the bass response too boomy / heavy in your particular room, DSP should be effective at taming it.

(*) Calibrated Acoustic String

 

[Blumlein 88]

 

[RayDunzl]

I can see the point, but what's the point?

I wonder if there are any videos of someone clamping their head in a paint shaker?

Wow, upon listening again, I think the bass on that shaker is as good as in that truck.

 

[RayDunzl]

When does a loudspeaker get too big for a room?

Is this a trick question?

It either is, or it isn't.

 

[stunta]

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)

This makes sense. Does the "convergence point" also explain why some monitors are called "near-field"?

Is this a trick question?

Is this a trick question? I don't think I am technical enough to be asking trick questions in this forum.

 

[Vintage57]

Times 2.

 

[Blumlein 88]

This makes sense. Does the "convergence point" also explain why some monitors are called "near-field"?

We know the answer is of course 23.

23 mm is headphones.
23 inches is near field monitor
23 meters would be a really far field monitor like at a rock concert.
23 km has something to do with Ray's shout-o-meter index. ??

Oh, and don't confuse the convergence point with the singularity.

 

[Sal1950]

Times 2.

PF DSOTM approved I sure.

 

[Bjorn]

It's not the size of the speaker that really matters but it's the directivity and low frequency capability. Obviously there's some relation between size and low freq. output, but not always. My midbass horn is 122 cm in width , but have no real deep bass and is easier to integrate in a small room than any commercial speaker due to its directivity which is constant and with a high Q.

 

[Frank Dernie]

Based on the noise and vibration research I have done I have always assumed that the extent to which a speaker excites the various room modes must be dependant on the position of the speaker in the room, in the same way the timbre of a string instrument, ie the magnitude of the combinations of fundamental and harmonics, is influenced on where on the string you pluck/bow.
In my room conventional speakers carefully placed to minimise the excitation of the fundamental room modes are still more peaky in the bass than the Tune Audio Anima horns placed in the corners of the room to give a partial(? faux) extension to the bass horn. the horns don't have as much extension though, with 30 Hz being 7dB down on 40 Hz, whereas the conventional speakers still have some output at 20Hz.
Both are pretty big though.

 

[Bjorn]

Based on the noise and vibration research I have done I have always assumed that the extent to which a speaker excites the various room modes must be dependant on the position of the speaker in the room, in the same way the timbre of a string instrument, ie the magnitude of the combinations of fundamental and harmonics, is influenced on where on the string you pluck/bow.
In my room conventional speakers carefully placed to minimise the excitation of the fundamental room modes are still more peaky in the bass than the Tune Audio Anima horns placed in the corners of the room to give a partial(? faux) extension to the bass horn. the horns don't have as much extension though, with 30 Hz being 7dB down on 40 Hz, whereas the conventional speakers still have some output at 20Hz.
Both are pretty big though.

Placement of listening position matters much more in regards to "moving out" of room modes than the placement of speakers.

A speaker that goes lower in frequency will trigger more room modes. That's part of the reason why smaller speakers or dipoles, which don't excite the lowest fundamental modes, are easier in regards to bass issues. Basically something that can be achieved with any speaker using a high pass filter.

 

[svart-hvitt]

Just curious: How far away from the speakers and how near do people sit?

 

[DonH56]

Modes are a function of room dimensions, but equally important (or vexing) are reflections and SBIR (Speaker Boundary Interference Response), and SBIR is dependent upon speaker placement as well as listening position. Even room modes can be changed with speaker (typically subwoofer) placement.

One thing I often mention is that there is always a null in the center of the room; best to place the main listening position (MLP) out of the center at maybe the 1/3 or 2/3 spot.

 

[Bjorn]

One thing I often mention is that there is always a null in the center of the room; best to place the main listening position (MLP) out of the center at maybe the 1/3 or 2/3 spot.

Not always. Depends on the structure of the walls and layout of room too. I'm actually sitting close to the center in my home theater and it's the place that yields the flattest bass response.

 

[DWPress]

My room is 16x20' with 11' ceilings and my home-built speaker cabinets are nearly 6 cubic feet. This is my main studio work space, no room for them on the floor so for the past 26 years they have been suspended from the ceiling with eye bolts and 5/8" chain and are 3.5' from the rear wall and 1.5' from sidewalls. There's just no other place for these beasts, I'm stuck with what I have and make the best of it. Five years ago I decided to go with a fully active XO and did this with a MiniDSP 2x8. The ability to manipulate frequency range, phase, time and EQ for each individual driver in a 3 way system in real time is really astonishing. Since I can't move the speakers, I manipulate the enclosures and occasionally swap drivers. The pic below is an early incarnation that merely shows the space these things are in. The current incarnation in progress has different 6" mids in separate sealed enclosures attached/hung on the inside edges of the existing boxes with an open baffle area for testing some ribbon tweeters vrs different 7x10" horn tweets (with correctly oriented Tractrix as opposed to the pic below) that are in the current boxes. Yes, they are ugly. Doesn't really matter what they look like, it's in my studio! With those two extra channels on my 2x8 I also control a Dayton 15" sub and a small 10" sub in home-built enclosures. The big sub is off most of the time (those are also 15" drivers in the main cabinets) but now that it's summer and 3 windows are open most all the time it is beneficial to have it on at lower listening levels because of the loss of "cabin pressure" in the room.

MiniDSP products such as this require being more proactive and making many multiple measurements (in multiple places) with calibrated mics and programs like REW but I think they also offer other simpler room correction options and there are plenty of other vendors out there to suit your particular needs. You just need to figure out what your needs are!

To tie this all together the OP asked about big speakers, active systems and later, the effects of room volume/pressure on low frequencies. Just my 3 cents....

 

[DWPress]

A later incarnation: