[MBK]

Hi everyone,


Lately I've found myself reading more and more acoustic threads, as well as working my way through the excellent Master Handbook of Acoustics. In an effort to clarify my understanding, and that of other forum members like me who are starting to get a handle on this stuff, I'd like to summarize what I understand so far. Please correct me, add your own comments, etc.


Considering acoustics in the construction of theaters has a few primary goals:


1. Prevent sound transmission into and out of the theater


2. Smooth out frequency response to minimize room imparted distortion



To prevent sound transmission into and out of the theater the following methods are often used:


1. Room within a room or staggered stud construction. RSIC clips are often used on the ceilings and walls here to provide a second layer of decoupling.


2. Increasing the mass of the walls and ceiling by using double layered drywall. Increasing the mass of the floor by using acoustimat.


3. Green Glue in between two layers (sandwiched in the drywall). This can also be used for a double layer subfloor


4. Exterior grade doors with weather stripping


5. Minimizing exits into and out of the room by building soffits and running wire "within" the room rather than punching holes in the wall.


6. Separating the theater HVAC system from the "house" HVAC system where possible.


To smooth out frequency response within the room must deal with both high frequency and low frequency noise.


A theater should be neither too "live" or too "dead". Testing the RT60 of a room will help determine the level of "liveness" (NOTE: What is a recommended RT60? I've heard around 1 second, but I could be making that up)


High Frequency smoothing:


1. Make the front wall a "dead" wall by covering it in linacoustic or similar material. This is generally done behind an acoustically transparent screen wall. Often times this dead wall treatment is brought out a few feet into the room to help prevent first order reflections.


2. Run linacoustic or similar material along the sidewalls to ear height.


Low Frequency smoothing:


1. Bass traps in corners or along the walls (screen wall included). These traps will help prevent excitation of low frequency room modes that can destroy bass response at specific frequencies. For these treatments to be effective, a significant portion of surface space must often be covered. Bass traps can be bought commercially, or built using absorptive materials such as compressed fiberglass.


2. If possible, build the room so that the ratio of height to width to length minimizes mode formation. There are a number of online calculators to help with this.


3. Equalization (e.g. Beringer Feedback Destroyer).


Hopefully people can add to this, and correct me where I'm wrong!

 

[bpape]

Not bad. Just a couple things.


When you put bass traps in corners, if they are not tuned to JUST absorb bass, they will absorb everything. All frequencies are efficiently absorbed in corners so beware if you don't want/need broadband absorbtion. Properly constructed bass traps can be very efficient if properly placed within a room. You don't necessarily have to eat up a ton of wall/floor space. It really depends on how bad the issues are and how deep they are. Generally, the deeper the issue, the bigger the trap one way or another.


When you build the room, you consider the ratio of dimensions, yes. However, you must also consider where you will be sitting in relation to the room, in relation to the screen, and in relation to the speakers. Getting them all correct at the same time is a lot more effort than just worrying about axial modes.


The front wall should be dead but not just at high frequencies. You would like to extend it down into the upper midrange if possible. The point of doing the front wall is to absorb first reflections that are close enough in time to cause smearing. Reflection points on the ceiling and wall are also treated if possible. Some just do the side wall and back wall points and don't do the ceiling.


Running Linacoustic along the wall to ear height is one approach. This provides distribution of absorbtion throughout the space which is good. There are other methodologies that can work just as well. This one is very easy to implement without a lot of math. It does, however, take up a great deal of the surface area of a room leaving little room for spot treatments if needed without overdeadening certain frequencies.


When dealing with RT60, the goal is to get the room's reverb times down to whatever level you choose EVENLY across the entire spectrum. There is a lot of information to suggest that the target time is based on a combination of what the room is used for AND it's volume.


If you have something you CAN'T deal with or don't choose to due to space or asthetic limitations a parametric equalizer can be of great value. Just make sure to get one where you have FULL control over all of the bands. Some parametrics still restrict the range you can adjust as a center point so there is little overlap. The BFD, the Symmetrix, etc. allow fully overlapping bands to provide exceptional control. I personally run a Symmetrix just on my sub with all 5 bands centered between 23Hz and 55Hz.

 

[MBK]

When deadening the front wall to prevent first reflections, what are some common techniques?


For high frequencies we have linacoustic. For mid to low range we would be talking about compressed fiberglass generally, correct? The fiberglass traps should ideally cover the wall and be installed over the linacoustic?


Perhaps you can elaborate a bit on seating position. I know that you would not want your seats to be in either peaks or nulls, but am a bit unsure of how to calculate this.


Also, I know that columns can be used to both interfere with first reflection points (by placing the screen-facing side of the column just beyond the point?), and can also be used to store bass traps.


Would anyone care to contribute as to methods for using columns for acoustic tuning? How about stages and risers?

 

[bpape]

To get into the middle frequencies, you generally either need more thickness or space it out from the wall a bit. You will be getting some broadband absorbtion already from the drywall, people, furniture, etc. How much more or less you need is determined by your room's volume, what the room is to be used for (music, HT, both), and the absorbtive value of all of the rooms surfaces and contents.


Contrary to where a lot of people put their seats, you don't want seats exactly in the center of either dimension. This puts your ears in a null of the primary axial mode and a peak for all even harmonics. Also, sitting right against a side wall is not desirable. When sitting against a boundary, you are getting reinforcement of a lot of frequencies that you don't necessarily want. In addition, you are putting yourself in a position for early reflections to become a bigger issue. With absorbtion up to ear level, this is not as much of an issue in the highs. Sitting against the rear wall not only provides unwanted reinforcement, but also makes it a terrible place for getting a sense of the surround field. If you try to keep your seats at say x/7's of the room's dimensions, you are pusing the peaks and nulls you are sitting in up above the Schroeder frequency of the room where modes act a bit differently and are much closer together. Balancing proper viewing distance, screen size, speaker placement, and seating placement at the same time is the trick. If you have to compromise, to me it's easier to sit in a peak because that can be tamed with trapping or EQ. Sitting in a null is much harder to deal with.


Columns are there to break up large flat surfaces facing each other as well as to break up sound running parallel to their faces. They are also used as ideal places for housing acoustic treatments so they are not visibile and to mount speakers and lighting without putting a hole in the 'envelope' of the main room. This helps to minimize sound leakage into and out of the room.


Stages are put in for a couple of reasons:

1. It identifies a clear boundary for children.

2. It can provide decoupling from the main floor if the sub is placed on the stage.


In any case, care must be taken to assure that the enclosure is dead so it does not act like a drum and cause response issues.

 

[MBK]

If I understand correctly, the Schroeder frequency is the boundary between reverberant room behavior and discrete room modes. We would want to sit in a location where the peaks and nulls are above this frequency because then the peaks and nulls we are exposed to are more the characteristics of reverb (something the human ear can deal with fairly well) and relatively uninfluenced by low frequency room modes which push or suck frequency in a very audible way.


How is the x/7ths position arrived at? I know that the center of a room is poor because it would put you at a null of the first mode (1/4 wave length)

 

[Savant]

MBK,
Quote:

Originally posted by MBK 3. Green Glue in between two layers (sandwiched in the drywall). This can also be used for a double layer subfloor

There are other materials you can use besides Green Glue to accomplish this.
Quote:

Originally posted by MBK A theater should be neither too "live" or too "dead". Testing the RT60 of a room will help determine the level of "liveness" (NOTE: What is a recommended RT60? I've heard around 1 second, but I could be making that up)

A 1.0 second RT, assuming you mean midband, is quite high for a Home Theater. And I would say that RT should only be used as a guideline. Indeed, many would argue about whether it even exists in small rooms since "reverberant" implies that reflections have built up to where any single reflection is insignificant relative to the combined effect of the many hundreds of reflections from various surfaces. It is much more important to address early reflections in small rooms — which is very much in line with the other things about which you are writing — than it is to get overly concerned with RT. Good control of early reflections will inherently reduce RT. But reduction of RT does not always yield good control of early reflections. Take an example from the "large room" world. Many houses of worship are constructed and finished with acoustic tile ceiling, carpeted floor, padded pews or seats and hard walls. The RT in these rooms is usually well within the "ideal" range. But myriad reflection problems such as flutter echoes will remain.


Scaling this down, so to speak, to a small room like a Home Theater, you can put a ton of absorption on the walls and ceiling, but if it's not properly selected and placed, you could still have early reflection problems. The RT will hardly matter at that point.
Quote:

Originally posted by MBK 2. If possible, build the room so that the ratio of height to width to length minimizes mode formation. There are a number of online calculators to help with this.

Technically, you are not minimizing mode formation. You are decreasing modal spacing and increasing modal density. In truth, proper selection of room dimensions will provide more modes. But the spacing will be tight so there are minimal gaps between modes. This provides a smoother low frequency response relative to a room where the modes are less dense and more widely spaced in frequency. Poor room dimensions will yield more pronounced low frequency problems that are more difficult to control. Good room dimensions will provide a smoother response and any pronounced problems should be relatively easier to control.
Quote:

Originally posted by MBK 3. Equalization (e.g. Beringer Feedback Destroyer).

I haven't subscribed here for too long. But I generally view this approach to be a last resort. A properly designed room should not require any sort of equalization.


As for seating position, I appreciate bpape's x/7th approach. However, I would also point out that the better the low frequency control is in the room, the less seating position will ultimately matter. Since the goal is consistent sound in the room, where you sit should only be decided relative to the sound field and the screen. IMHO.


Finally, use of the Schroeder frequency should be undertaken with great care. When Schroeder established this frequency, he was writing specifically about concert halls and other large venues. This frequency often does not "translate" well to small room design. I find the Davis frequency to a better approximation. Which is simply the highest 6th axial mode in the room, or:


3*c/Lmin


where c is the speed of sound in air and Lmin is the smallest room dimension. You can read up on this in Sound System Design by Don and Carolyn Davis. Overall, whether using Schroeder or Davis, please keep in mind that the frequency calculated is only a guideline.

 

[MBK]

Thanks Jeff,


I did see that I miswrote in my first post. Of course modes cannot be eliminated, you can only build to minimize problematic modes.


Could you please elaborate on your statement about selection and placing of absorption? Obviously we are looking for broadband absorption to eliminate early reflections across many frequencies. Placement could follow the mirror trick for all seating positions. Do you have recommendations of placement or material?

 

[Savant]

MBK,
Quote:

Originally posted by MBK Do you have recommendations of placement or material?

For placement, the mirror trick technique is generally what I had in mind. RPG's Room Optimizer program also shows graphically where different panels should be placed in the room. I don't always agree 100% with the type of treatment it tells you to place, but the geometry is clearly provided. Of course, you can also figure placement out by trial and error, but that can get pretty time-consuming.


As for types of materials, I think it's pretty obvious what I would recommend. Isn't it? (I will refraim from going into any detail since that would show my obvious bias!!! )

 

[bpape]

Good stuff Jeff. I'll have to check out Davis.


As for the x/7ths...


When you sit at x/7ths of a rooms dimensions, you are only in a peak at the 7th harmonic of the fundamental room mode for that dimension and not directly in any single null. Generally, unless you have a HUGE room, by the time you get to the 7th harmonic, you are up around 150Hz or so. Up there, traps to address those kinds of issues are much more managable in size.


There is nothing magical about 7, it just is enough to push the issues higher in the spectrum - similar I suspect to why Davis (if I understand correctly) recommends the 6th harmonic of the smallest dimension (usually height.) If you think about a normal 8' ceiling, 3/7ths is at about 42" which is pretty close to normal ear height.


7 is also an easy number to work with as it will put you relatively close to the center of the room without incurring a lot of problems. In a lot of rooms, just putting 2 chairs arm to arm with the arms in the center of the room will put you far enough off to avoid issues while remaining close for viewing and listening purposes.

 

[MBK]

Very cool stuff! Thanks for the responses so far. I'm hoping that this thread can serve as a starting point for people trying to wrap their head around basic acoustics for home theater. Hopefully people will keep it going.

 

[Rutgar]

Quote:

Originally posted by Savant MBK, Finally, use of the Schroeder frequency should be undertaken with great care. When Schroeder established this frequency, he was writing specifically about concert halls and other large venues. This frequency often does not "translate" well to small room design. I find the Davis frequency to a better approximation. Which is simply the highest 6th axial mode in the room, or: 3*c/Lmin where c is the speed of sound in air and Lmin is the smallest room dimension. You can read up on this in Sound System Design by Don and Carolyn Davis. Overall, whether using Schroeder or Davis, please keep in mind that the frequency calculated is only a guideline.

Are you sure this is correct? Using that formula, I calculate that the "Davis" frequency for my planned room would be 212 Hz. Isn't that kind of high for modal cutoff?

 

[bpape]

I was kind of wondering the same thing. If I use the shortest dimension (the height) and take the 6th axial mode or 8', that puts the cutoff at around 423Hz!

 

[Rutgar]

DOH! I used the width demension of my room, which is 16 feet, to get 212 Hz. Using the height (10 feet), that number is 340 Hz. Which is even higher. I don't know, but it's still my understanding that you only really need to be concerned with the modes below 200 Hz.

 

[Terry Montlick]

Quote:

Originally posted by Savant A properly designed room should not require any sort of equalization.

You don't know how much I'd love this to be true, Jeff!


We both see a lot of home theater rooms. In your experience, how flat have you gotten subwoofer frequencies with acoustical treatment alone?


Regards,

Terry

 

[BasementBob]

Terry:

Quote:

savant wrote: A properly designed room

I'd imagine it depends on one's definition of a properly designed room.

Start with a 6m tall space, dedicate the top 2m of interior space to acoustical treatment, one dimension 70' long with splayed walls, ...


A better question is does a properly treated room require equalization?

 

[Savant]

Wow, you guys sure do have lots of question!


bpape: I do understand that underlying concept behind the x/7th thing. And I think it's a good rule of thumb! Also, you may want to hold out on Davis for a few months (unless it's at your library). The 3rd edition is coming out soon. The one and only Gene Petronis is assisting the Davises with this newest edition. It should be stellar.


Rutgar: The Schroeder frequency and the Davis frequency both predict pretty high cutoff frequencies. I don't believe either one can finitely predict where a room is no longer modal. My own personal opinion on the matter is Davis ±1 octave. It depends on so many things that, at best, an equation like on of these can only get you in the ballpark. Two virtually identical rooms could have completely different behavior depending on construction, treatment, etc. There definitely needs to be more research on the matter. I believe some rooms with 8' ceilings could exhibit modal characteristics as high as 423 Hz given the right conditions. I also believe some rooms will stop behaving modally after the 2nd or 3rd axial mode. Bottom line: There's a huge, not well-defined gray area with this.


Terry: In the some cases, I've gotten ±3 dB down to about 25 Hz. I will admit that that is rare and takes some pretty substantial treatments in good sized rooms. But it is not uncommon to get the same sort of result down to 40-50 Hz. And, IMO, the phase (time domain) issues introduced with using an EQ are too great to consider EQ anything but a last resort. Now, perhaps some of the newer technologies will convince me otherwise. I know one or two of them reportedly restrict their correction to minimum-phase problems. This would be a good thing. Hopefully, these sorts of units have some horsepower to them. Unfortunatley, one of the systems currently being marketed is an active EQ system that's nothing more than a single-band parametric and it retails for $1,500.00 or so. Call me crazy, but I would rather spend $1,500 on something that addresses more than one frequency! And, as is often the case with EQ, it's still really only relevant to one seating position. Having said that, there's an article in the Sept. issue of Home Theater by Chris Lewis I have yet to read through. Supposedly, he reviews some of the newer systems that are getting closer to working around the hang-ups I (and others) have. Time will tell. But I'm not convinced...yet...

 

[Terry Montlick]

Quote:

Originally posted by Savant And, IMO, the phase (time domain) issues introduced with using an EQ are too great to consider EQ anything but a last resort. Now, perhaps some of the newer technologies will convince me otherwise. I know one or two of them reportedly restrict their correction to minimum-phase problems. This would be a good thing. Hopefully, these sorts of units have some horsepower to them.

Actually, all simple parametric equalizer filters are minimum phase. Being just stable second-order IIR sections, they couldn't introduce delay if they tried. It's only when you get into "fancy" FIR digital filters that you add delay, and therefore can be non-minimum phase.


That's the cool thing about equalizing individual low frequency room modes. Both equalizer and room mode are minimum phase, so there is matching behavior in both time and frequency domains! You fix one domain and the other domain gets fixed automatically.


I'm oversimplifying just a bit, because I don't see multiple room modes as completely minimum phase. I think of them as creating a "near minimum phase" system, because there will be a distance and a corresponding slight time delay between the pressure maxima of two different modes (such as axial length and width). This creates phase interaction between two such adjacent modes, which is visible as an excessive dip which wouldn't be present for a pure one-dimensional minimum phase system.


The minimum phase equalizer doesn't have this problem, and that's a good thing! If it did, it would necessarily try to boost those nasty dips while attenuating the peaks.


Of course, you are still stuck with the problem of equalizing for a single listening position. Very low frequency room modes provide some wiggle room.


Regards,

Terry

 

[Savant]

Terry,


Excellent explanation of the minimum phase stuff!
Quote:

Originally posted by Terry Montlick Of course, you are still stuck with the problem of equalizing for a single listening position. Very low frequency room modes provide some wiggle room. [/b]

Agreed. And that's really the crux of it. As I said, I am looking forward to some alone time with Chris Lewis' article. And I'm at AES right now, so perhaps I'll steal some time to go listen to some demos...if any of the manufacturers are brave enough to put one on!!!

 

[bpape]

I'd be interested in your experiences Jeff. You say that SOME rooms could stop being modal after 2-3x axial modes where others migth be as much as +1 octave from Davis (let's see - 423 for 8' + 1 octave = 800+ Hz! - thats a lot of wiggle room!). I'm not disputing it, just wondering under what conditions those extremes might occur.


As for parametrics, I purchased a Symmetrix single channel 5 band with totally overlapping bands. I use all 5 bands between 16Hz and 55Hz just on my sub. One of the best investments I ever made. The room was not bad before but when I added this beast sub, I could get way below the lowest axial mode of the room (23Hz) at serious levels. The parametric helped tame the beast without castrating it.

 

[Savant]

bpape,


That's the problem: There are so many variables to consider, it is often very difficult to (a) define where the modal cutoff is in the room and (b) verify it.


Very small rooms, like vocal booths in studios, could well be modal to a very high frequency. Luckily, those sorts of higher modes don't necessarily have to be treated with the huge devices sometimes required in larger rooms.


Very large rooms could have nice modal density, but exhibit modal behavior to a relatively higher frequency due to the inconsistencies in large format sound systems in the bottom two octaves.


But I'm just speculating, mostly. I never rule anything out. I usually look at a modal response at least up to the range of the Davis frequency. Most of the time, the room is ot exhibiting modal behavior, even at the Davis frequency; modal behavior seems to taper off around an octave lower.


But again, it would be irresponsible to rule anything out until you have all the facts. And that usually takes measurements to verify.


Does this help?