Are room modes really bothersome with high spl movie sound tracks?

What is "base"? Do you mean "bass"?

I hear that a lot as well, but you may also want to look into controlling the reflections in the mid and high frequencies. By just doing that, it won't sound as "loud" but will actually measure the same. It will be a cleaner sound-As long as you arn't producing a distorted sound in the first place.


dbl

The way I understand it is you want to treat room modes not to get "more" bass but to get cleaner, more even bass (and actually technically "less" bass). You mentioned long decay times which is tied to the room modes - the frequency being produced is propagated by the room creating a standing wave that does not dissipate when the subwoofer stops producing the sound - giving you a long decay time, a louder note than is intended, and boomy/ringing bass = bad. The notes is louder at that frequency than it should be since the wave is bouncing off the walls in phase causing the wave to get reinforced (louder). You can't think of room modes as a cell phone that gets drowned out when the music is playing loudly enough, it's more like an amplifier of certain notes so no matter how loud you've got it turned up those notes will be louder than they're supposed to be (and will require EQ to treat). Actually in my experience room modes are more troublesome at loud playback than at modest levels. I've got a room mode right around 40hz which sounds great when listening to music at lower levels but when turned up it's like all you can hear is that 40hz tone overpowering everything else in the track

I find tubby boomy bass more bothersome as level increases.

You're observant, but you're missing the important clues.

Good bass = equal bass at all frequencies, from all seats. This never occurs in untreated, concrete-walled listening rooms.

Clues
- concrete room has lots of bass
- 12" sub barely OK in wooden house, but loud in concrete
- 75W sub flexes plate glass

When you excite a resonant system, such as the air in your concrete room, the energy tends to pool where it dissipates least: standing waves. These start at the longest 1/2 wavelength that fits in your room, and continue up in frequency. However, it's only in the low bass that we see individual modes, modes that are difficult to suppress.

When energy hits a transition, it has three choices, reflection, absorption and transmission. To have a resonance, you must have a reflection.
- concrete reflects sound well
- wooden construction allows transmission
- flexing of plate glass is an example of absorption (conversion to heat).

I can tell from your post that you think your concrete listening room has really good bass. I can tell from your tone that you're unlikely to be open to honest opinions about your concrete room. I'm also not able to listen for myself.

What I can tell you is that when I measured my bass frequency response, it was unusually flat above 10Hz. It's taken me years to understand why. My room is:
- matchstick construction
- only one strong reflector (a fireplace)
- which is opposite staggered walls (a big doorway)
- lots of plate glass (3-frame sliding glass door)
- paneling was installed over furring strips
- corner cupboards in 2 corners, bookcase in the others
- carpetted and padded hardwood floors

Note that I've got 2 diaphragmatic absorbers (paneling, plate glass), corner treatments that disrupt bass reflection, plus the positive effect of carpet/pad, curtains, cloth furntiture, etc. in a non-reflective room.

Have fun,
Frank

Room modes are mostly important as a bass phenomenon. They are related to the wavelength of the sound, the dimensions of the room, and the position of the speaker/listener. Modes cause peaks and nulls in three dimensions. That means that the loudness of certain frequencies are positionally dependent. Thus, some seats may be good, but others will have either uncontrolled peaks at that particular frequency or be radically lower in volume at that frequency in another listening positions. This phenomenon is highly undesirable in a theater with multiple seats.


The design of a recording studio is irrelevant to the design of a theater.


On average the bass should be the same, it's just more evenly distributed in space.

Yet, it isn't evenly distributed across all listening positions. If you were to measure the bass response across multiple frequencies in multiple locations in the room you would see how crappy the bass distribution is and how far it is from the soundtrack's intent.


The decay characteristics of a room have little to do with 'modes' or standing waves but more to do with how much of the energy content of the sound field is absorbed by the room. It is the rooms ability to 'dampen' the sound. This is important at all frequencies but is most important above mid-bass, and especially important in the frequencies important for dialog (ie 1-3K)


Fixing the room makes it better, not necessarily louder.

Are you sure your number are right?
In imperial measurements we have 6'L x 6.5'W x 9.5'H?

That's a closet.

I know guys with subs that big.

A couple of comments:

An enclosed space will have modes when energized. A room will have modes dictated by its dimensions, the lowest frequency will be determined by the longest wavelength supported by the boundary. A concrete room will contain the energy for a longer period of time than a room whose shell can flex.

A room will have modes, this is unavoidable, however, you may be able to reduce their destructive influences in many ways. You can control their frequency extension and distribution via dimensions. You can avoid placing yourself and loudspeakers in areas that will exasperate problematic modes. You can introduce shell construction systems that will dampen low frequency energy, along with shell resonances. You can introduce interior acoustic treatments that are designed to absorb low frequencies. Once you've done all that you can passively, calibrate the system at your standard listening level.

What is that? are you sure about its absorption coefficient?

If you are trying to absorb sound energy using a "resistive" material like foam or fiberglass, it will need to be a 1/4 wavelength thickness of the lowest frequency targeted. I'm not sure of your dimensions or calculations, but with your 9' 5" ceiling height, you'll have a fundamental mode at about 59 Hz. A 1/4 wavelength at this frequency would mean material about 4' 8" thick. Any fabric covering will also determine upper frequency absorption coefficients.

I would suggest using acoustic panels that incorporate diaphragmatic absorption along with fiberglass as a thin, broadband treatment.

Frankly, I can't image building a room that small. The ceiling height is ridiculously low, the room is barely wide enough to fit furniture, the length is too short. The overall feeling of the room will be claustrophobic, and anyone with any amount of claustrophobia wouldn't tolerate the room for the length of a movie.
More ironic is the use of IB subs, which will could require an enclosure as large as the primary room.

Brain fart on that one... It's 6' 3" I was thinking about this earlier and the room will have less area than two 4' x 8' sheets of plywood laid down side by side with a ceiling 2" over my head - sounds like a fish house to me It's your room though

75 inches is 6'3". That is a very low ceiling. You want to put the seating on a platform? Even person of below average height would bump his head.

79" is 6'7" wide. The love seat in my family room is 5'6" wide. That leaves a bit over 6" on each side, which is far to narrow to get around.

You plan on using a projector? How big is your screen?

I thought my 14x19x7 room was small but damn!!!

One of the major components of musical preference is a sense of space, defined in the literature as Apparent Source Width. The greater the ASW, the more we humans enjoy the sound. This requires space, distance from listener to walls/floor/ceiling, distance from speakers to walls, and uniform, broadband wall reflections, not resonant ones at single/narrow frequency ranges.

400 cu ft is a closet. Nothing will change that. If that's all the space you have, use it, but I think it would make a nice little workshop.

Have fun,
Frank

Parametric eq is used in exactly that way. Here's the downside... Modes create not only peaks but also nulls within the room. These can be just as deep as the spike is tall.

Imagine that two of your seats happen to (or need to be) positioned such that one is in a 59hz peak of 10db and the other is in a 59hz null of -10db. What are you going to do with your eq now?

This thread is novel and a bit nutty! It reminds me of those video game places where kids sit inside a race car like thing with speakers that are almost like headphones. You have a space that is more like a van, or a really big set of headphones. You have an interesting task. You're also creating a big challenge.

If I were to create a tiny room like that, I'd probably put in a plasma, build in the speakers, make the whole thing like one big bass trap so it's a room within a room and cover all surfaces with absorption.

Firstly, about bass traps. You can hear modes at any level, it doesn't matter what level or if it's movies, rock music, "audiophile" music, whatever. When you hear the kind of bass you can only get in a well trapped room, then the modes will annoy you. It's a bit like when you're in a building with a central HVAC system. When it goes off you suddenly notice how loud it was, because suddenly it's quiet. Right now you are used to hearing modes ringing, but after you treat them you won't want to go back. You've probably had a similar experience elsewhere in audio.

I find rock music is enhanced with bass traps as much as anything else. Kick drums are tight and punchy. LFE has more control. Albums that never quite got there with the bass are a delight to listen to now. The same is true with movies. Everything improves. In fact, at higher levels the ears become more sensitive to bass and any problems are further magnified.

Bass traps can help reduce nulls, so in that sense they don't necessarily give you less bass, but overall you are likely to find that you sense a net decrease in bass. The reason is that in the time domain, the modes decay at a faster rate. Before trapping, they ring out in time and this creates the impression of more bass and more boom. So with traps you will actually sense less bass and I've found I'm running the subs harder. It's a bit like being able to turn your system up more and enjoy it at a louder level because it's more clean after say getting more headroom, more efficient drivers, etc. The overall effect is I now get bass that is more dynamic, more visceral and with more impact. I run them harder to get there, but because I've cleaned up the room artifacts it works far far better.

You can do a sim for modes in REW and it's quite a good tool. I did a quick sim and the bass looks ok, but the low midrange has some dips.

If you have a concrete room, you have real problems. However, every room pretty much needs trapping anyway, it's just in a concrete room you need it more. Once it's done, however, you may find you keep a bit more bottom end gain. One local retailer here has a concrete room with 4 large bass traps. The result was good, and their 10" 4th order bandpass (no more output than a sealed box) hit with a big stick in a big room. Far more impact than a 10" sub in a normal room.