Ideal acoustic space

[cmchamp]

buggyboy

Then, for the sake of academics, please realize this: everything that is in the soundfield (coverage pattern) of a sound source, be it speakers or an acoustic source such as an instrument or sound, has either a reflective quality or diffusive quality. Further taking this academic study into minutia, I have spreadsheets from my mentor that he used when teaching PhD classes that take into account the room humidity, number and type of light fixtures, door handles or knobs, light switches and controls, finish materials, chairs, people based on size and type of clothing.

Unfortunately, even in imaginary buildings, the laws of physics still apply.

[Gracetech]

So what is the worship style of the imaginary church? If it is contemporary which seems to be your tone then the acoustics and i believe the room will be designed one way. If Traditional or Orchestral then the room/accoustics would be designed a completely different way.

So decide the worship style for our fictitious space.

crt

[cmchamp]

He did in his first post.
goals:
-this will be a sanctuary.
-it will hold, oh, 300 people (or use the number you like).
-it will be excellent for amplified speech, capable of very loud levels without feedback.
-it will be very good for amplified traditional music (vocals,pianos, organ, acoustic guitar, etc)
-it will be tollerably good for modern music (drums, electric guitar, etc)

[buggyboy]

Thanks cory, I was coming to post and you had it taken care of already.

I'll define it even more tightly, if that would be helpful.

in the imaginary church the preacher is the primary focus, everything else is an order of magnitude less important.

the other uses for the space include
-congregational singing,
-a song leader who is basically a soloist to get the congregational singing started on key,
-some type of accompaniment for the congregational singing (piano, perhaps guitar, perhaps organ),
-occasional vocal solos for special music,
-and rarely a vocal group.

I'm less concerned about the vocal group because I assume they can be trained to keep the lips on the grill if needed.

I'm less concerned about the song leader because I assume he'll have a strong voice and "perform" that is, project his voice.

I'm less concerned about the accompaniment because most or all of that could be either close miced or run into the pa directly.


This all started because I wanted to know the relative merits of various room shapes.

thanks

[cmchamp]

Well, when I get home today, after a Booth Brother's Concert later this evening, I'll post the range of "golden ratios" for room dimensions which reduce the effect of modes/nodes in the room.

[Gracetech]

I'm a believer that every wall should be a mix of materials no matter it's usage. The idea is to never have hard reflections. You should always be looking at the room in the ration of diffused information and absorption. The ratio between the two will define the room to the styling that is desired. About the only place i ever use hard reflections is overhead on clouds. This is done to help return the congregations voice back to themselves. It's always important to slightly angle these clouds so that when the room is empty you don't develop flutter between the floor and ceiling. Even if you have carpet on the floor you can develop flutter in the lower mid-range if the two surfaces are parallel.

For Traditional i would go with a heavier use of diffusion materials and use the absorption sparingly. It would be good to make use of the clouds as bass traps so you could tune those and really soak up any unwanted LF waves.

As far as room shape i tend to go for a more length wise rectangular shape. The room can be fanned a bit but having the length wise rectangular shape helps to control site lines which also helps to keep the speaker count down by limiting the dispersion angle.

For speakers you want to make sure that the system is fully capable of the auditory spectrum from 20Hz-20kHz. Nothing is worse than walking into a space and the PA system sounds like a PA system. I want to walk into a space and be able to close my eyes and it sound like a guitar is strumming right infront of my face not sound like it's coming through a PA. An important thing to help the sound feel connected to the source is by keeping the angle of attack low. If the speakers get hung to high their will always be that disconnect to those sitting up front because the sound is hitting them in the top of the head not coming from infront of them. Keep the angle of attach withing 40degrees to the originating source of sound. Aural positioning gets even more crazy when you get into the horizontal space. Human ears can position sources within a few degrees so if you have a stereo or better yet a LCR setup then keep the sources panned to their location on the stage. Line arrays help to balance out the front to back volumes so that you can keep the vertical attack low so i would be more prone to use them. For years i have wanted to use a matrix of speakers that lined the proceninum arch around the front of a stage to basically steer the sound and make for a better sound source possitioning system but alas no one has wanted to make the investment to make it happen.

crt

[cmchamp]

So, here are dimension ratios for favorable mode distribution in rectangular rooms.
Height | Width | Length
1.00 | 1.28 | 1.54
1.00 | 1.60 | 2.33
1.00 | 1.40 | 1.90
1.00 | 1.50 | 2.50
1.00 | 1.26 | 1.59
(from Master Handbook of Acoustics, 4th Ed. F. Alton Everest, McGraw-Hill, p.277)

Fan shaped rooms are another monster all together because of the acute angles which can cause more LF build up in those corners.

There are many construction techniques and products that can provide rigidity and diffusion. Additionally, the smaller the room, the more % of diffusion would be required for natural attenuation (absorption) of sound by air and the absorption of the carpet, seating, or people.

[Brad Weber]

Maybe it would be easier to approach this in pieces, trying to address acoustics in general much less audio as well is a lot to tackle at once.

You mentioned room shape, so let's start there. As Cory noted, there are recommended ratios for the general dimensions in order to minimize the impact of room modes. In general, rooms with dimensions that are the same (worst case), multiples of one another (next worst) or both multiples of some common smaller dimension (least worse) are best avoided. However, if you do end up with these situations then angling surfaces or adding appropriate diffusion and/or absorption can be used to help mitigate the problems.

People often seem to forget that room volume is one of the factors in reverberation time. An overly large room volume, as in the cubic feet or cubic meters of space, may mean having to add more absorption than wanted to try to get the reverb time down. A room volume that is too small may make it difficult to get the desired reverberation time with the capacity or room finishes desired for other reasons. In most cases, a longer reverb time, say a cathedral or concert hall type space, require a larger per person volume while 'deader' spaces, such as contemporary sanctuaries or lecture halls, would desire a smaller per person volume.

The shape of the room affects many factors. Acoustics is obviously one, but sightlines is usually another major consideration as are issues like exiting and the sense of community. From an acoustical perspective, round rooms are usually undesired because all reflections from the walls focus back at one point. Rectangular or 'shoebox' rooms that are much longer than they are wide are very common for music performance spaces because that shape can help provide the early lateral reflections that are desirable for music performances. However, rectangular rooms are often viewed as less desirable in terms of congregational interaction and connection to the service. Almost the inverse of that is fan shaped rooms or rectangular rooms that are much wider than they are deep, which became popular for their ability to put more people closer to the stage, but which often limit the desired acoustical effect many surfaces can have as many listeners are located a significant distance from the surfaces. Fan shaped rooms also often relate to a curved rear wall that focuses on the stage or on the listener area, which is undesirable. Square rooms can experience the room mode issues noted above where common dimensions mean stacked room modes. Other shapes such as cruciform, faceted, rectangular or square on a bias and so on tend to encounter different combinations of the same issues.

An important aspect to remember in terms of room modes and room acoustics in general is that rooms are three dimensional. Surfaces that appear parallel in plan may not be vertically parallel and approaches such as 'tipping' the top of a wall inward cannot only avoid it being parallel an opposite surface but may also aid with getting desired lateral reflections to the listeners. You also have to consider the ceiling and floor. A high, peaked ceiling may be a good way to get greater room volume, but can create its own issues in terms of the lack of reflection paths from the presentation area to the listeners and undesired paths within the congregation.

[buggyboy]

Excellent comments gentlemen, plenty of food for thought.
@ Chad…
I’ve seen clouds with overlapping angled parts…sort of ///// but at a much lower angle. This should serve to reflect audience singing, if I’m correct, slightly behind the originating row. (Sound from people 18 rows from the stage should b reflected to people 24 rows from stage). If this is a desired behavior, could such angled reflectors be built into the ceiling to begin with?

You mention hanging bass traps in the cloud…these would be tuned to the rooms problem frequencies, but the surface facing the congregation would most likely be left reflective as mentioned. What frequency range is important to reflect to assist the congregation’s singing?

If your design goal is to minimize or eliminate hard reflections…I would imagine your ideal walls would not be flat at all. Perhaps some type of textured or angled surface? I’ve seen some rooms where the side walls are made of shallow accordion folds…/\/\/\/\/\/\ …or shallow pyramids of perhaps 2’x2’ protruding from the wall 4-6”.

The wall should avoid being parallel, so they would presumably widen towards the rear of the room. I wonder at what rate they should angle?

I’m not sure what you mean by “sightlines”. Is this lines of direct sound from the main speaker?
I had read several comments all over the web that indicated a long rectangle is preferable to a wide rectangle, but they never gave a reason. Wide rectangles definitely require multiple speakers aimed at the different seating sections, which introduces more fiddling with amp levels, crossovers, etc. In long rectangles the advantage is that a flown speaker cluster is angled, throwing it’s sound over a long area front to back?

Like this… ……
Speaker....\
..........\ .......\…….
.............\ ...........\…….
...............\ ................\…….
.................\ ....................... \….
...................\ ................................\

You raise an interesting issue…How can people in the front row get good sound without pushing the main speakers so far “north” (towards the back of the stage) that you create feedback. In other words, how can you have sound getting to the front pew without getting into the mics just a short distance “north” on the edge of the stage?


“Line arrays help to balance out the front to back volumes so that you can keep the vertical attack low so I would be more prone to use them.” I didn’t quite catch what you meant there. What front to back volumes do you mean?

@ Cory…
I have the master’s handbook on order as we speak. J
Hopefully I’ll be smarter having read it.

I have heard of the woes of acute angles in my reading. Is it worth the trade off to slightly angle the side walls to prevent flutter echoes and modal ringing?

@ Brad…
I’ve wondered what the effect would be in a room of angling a wall to directly reflect all sound into a treatment. Such as a stage wall angled “north” at the top to throw band noise on stage into a heavily treated ceiling area (hidden from audience view by some artistic shield). Do different frequencies reflect in the same direction off of a plain, flat, hard wall? I thought they would, as diffusion treatments big selling point is reflecting different frequencies differently. -----[[[Brad commented on this, mentioning that “tipping” a wall like this does work]]]

Regarding room volume vs. surface area. If all else is equal, larger room volumes will have longer reverb times. This can be help or a hindrance depending on the use of the space. A 50,000 square foot tv studio would require insane amounts of treatment to try to reduce the reverb time. While a 5000 square foot concert hall would have very little reverb. Modern churches with bands and such would want a rather dead space, while older styles would be ok with more reverb. Some styles (pipe organ, massed choir?) would thrive in a really, really reverberant space. Is that about right?

“Rectangular or 'shoebox' rooms that are much longer than they are wide are very common for music performance spaces because that shape can help provide the early lateral reflections that are desirable for music performances.” ---So side wall are sometimes used to provide early reflections. The purpose is doing so would be?…increasing perceived volume without requiring extra power?

The problem with that curved rear wall in fan shaped rooms is one factor, but the other you mentioned is interesting. Since non-acoustic concerns (congregational interaction) dictated a wide room shape…this in turn dictated that most people are far away from the side walls…thus reducing the possibility of using the side walls to direct early reflections…. did this chain of events give rise to the common use of “clouds” as a way of making useful reflections?

You answered my question above about “tipping” walls. I’m interested in this. Has anyone seen examples of “tipping” wall in the vertical plane to solve flutter echoes or modal ringing? Or to enhance “good” reflections?

Regarding peaked ceilings…It would seem that a peak could be designed to send sounds from one half of the congregation to the other half. (up, over, and down). This seems like it might be useful to help people hear congregation speaking (prayer requests, announcements, etc). Any comments on that? It certainly has the liability of sending distracting noise over the same path. But another advantage would be to increase the level of “other people singing” during congregational songs.



Wow, tons of great material for me to contemplate. Thanks tons!
I’m sure I haven’t thought of all the questions that could be asked next, but I’ll be thinking about it.

[Brad Weber]

Originally Posted by buggyboy

I’ve wondered what the effect would be in a room of angling a wall to directly reflect all sound into a treatment. Such as a stage wall angled “north” at the top to throw band noise on stage into a heavily treated ceiling area (hidden from audience view by some artistic shield). Do different frequencies reflect in the same direction off of a plain, flat, hard wall? I thought they would, as diffusion treatments big selling point is reflecting different frequencies differently.

Just about all materials react to different frequencies differently. If a surface is small then low frequency energy tends to diffract around it. If it does not provide sufficient transmission loss then much of the low frequency energy might transmit through it rather than being reflected. Think of sitting in front of a speaker and holding a letter size piece of paper up in front of you, it may block some high frequencies, which thus are reflected back toward the source, but doesn't have much effect on lower frequencies. The same thing would happen with your reflector, just on a different scale.

Originally Posted by buggyboy

Regarding room volume vs. surface area. If all else is equal, larger room volumes will have longer reverb times. This can be help or a hindrance depending on the use of the space. A 50,000 square foot tv studio would require insane amounts of treatment to try to reduce the reverb time. While a 5000 square foot concert hall would have very little reverb. Modern churches with bands and such would want a rather dead space, while older styles would be ok with more reverb. Some styles (pipe organ, massed choir?) would thrive in a really, really reverberant space. Is that about right?

Sort of. The point was really that classical reverberation time is directly dependent on both the room volume and the total absorption in the space. To get the same RT with one increasing requires the other to increase. But this can get more complex as most rooms do not actually have true reverberant fields, one really wants to look at more than just the overall Reverberation Time, etc.

Originally Posted by buggyboy

“Rectangular or 'shoebox' rooms that are much longer than they are wide are very common for music performance spaces because that shape can help provide the early lateral reflections that are desirable for music performances.” ---So side wall are sometimes used to provide early reflections. The purpose is doing so would be?…increasing perceived volume without requiring extra power?

This is where you start getting into issues such as warmth, envelopment, clarity, early/late energy ratios and so on. In general, it is having a room that provides a desired response for the application, which in the case of music does include the room having some desired effects on the sound.

Another aspect of this which relates to several of the comments made is that humans are much more sensitive to horizontal aspects of sound than they are to vertical aspects, primarily due to our ears being on either side of our heads rather than on the top and bottom. So when it comes to localization, the effects of surfaces, etc., horizontal paths are generally more critical than vertical paths.

Also related, the human brain tends to be a very powerful and complex processor of aural information. We continue to discover new insights into how the human ear-brain system processes information and one aspect of this is how it processes signals with different timing, response and amplitude relationships. Signal with certain relationships may be beneficial while signals with other relationships are detrimental for different applications. The general subject is way beyond dealing with in a forum environment.

Originally Posted by buggyboy

The problem with that curved rear wall in fan shaped rooms is one factor, but the other you mentioned is interesting. Since non-acoustic concerns (congregational interaction) dictated a wide room shape…this in turn dictated that most people are far away from the side walls…thus reducing the possibility of using the side walls to direct early reflections…. did this chain of events give rise to the common use of “clouds” as a way of making useful reflections?

Acoustical 'clouds' typically have two purposes; to provide beneficial reflections for the application and to avoid detrimental reflections. But as noted above, vertical reflectors do not provide the same result as horizontal surfaces.

Originally Posted by buggyboy

Regarding peaked ceilings…It would seem that a peak could be designed to send sounds from one half of the congregation to the other half. (up, over, and down). This seems like it might be useful to help people hear congregation speaking (prayer requests, announcements, etc). Any comments on that? It certainly has the liability of sending distracting noise over the same path. But another advantage would be to increase the level of “other people singing” during congregational songs.

The problem is when you look at the length of that path. A general guideline is that if a sound arrives more than 30ms after the initial sound then it is perceived as a separate event or an echo. So say you're 40' from someone across the room and it's 20' up to the ceiling 'bounce' point for the path from them to you. That means you hear one signal direct from that person to you at around 35ms and maybe 20-25dB down in level as well as that same sound reflected from the ceiling, where it has to go 20' up, 40' across and 20' down, at about 71ms and maybe 24-30dB down in level. So the reflected path is perceived as an echo of the direct path, which is usually undesired. The 71ms value is also outside what many would consider desirable for nebenficial early reflections. The goal for congregational interaction is typically less discrete, i.e. more diffusive, paths with limited delay compared to the direct paths.