Acoustics (and practical) questions for an auditorium

[seeker_ktf]

I know this is OT for "studio acoustics" but I was prompted to post here so it's OK.

[Wes Lachot]

Unfortunately, the carpet will only absorb high frequencies, which will tend to make low and mid frequency problems more apparent. But it sounds like that decision has already been made.

I was hoping to be able to pass two washer backed bolts through the 703 and then cover the front face with muslin, wrapped around to the back and glued on. Then, use some as yet undetermined method to attach them to the wall.

Muslin is not fire-proof, and will not pass code for a public building. You must use a real acoustic fabric with an A fire rating.

The simplest way to attach fabric-covered panels is with impailing clips. They can be mounted to the wall with molly bolts, and they have little sharp knife-like projections that stab into the fiberglass and hold it in place.

How important is bass trapping for a room this size?

Less important than for a small room, but you still want to balance the absorption across the frequency range. That's why the carpet has me worried.

How well with the 703 stay up without a frame?

If you're not sure about these sorts of structural issues, it may be time to hire a professional.

Is 703 good enough? Ethan seems to favor 705FRK for it's accelerated LF absorption.

For installation in public spaces, 705 is usually the answer due to it's rigidity.

Good luck.

--Wes

[Ethan Winer]

Seeks,

Besides Wes' great advice, I assume you got my email reply yesterday?

--Ethan

[Rod Gervais]

Seeker,

I could not agree with Wes enough.....

Determine what it is that you wish to do - and then contact an engineer to assess what is feasible.

It's not possible for any of us "sitting here" to know what will or will not work from that perspective.

One other comment - it may be possible to use the fabric you wish if it is treated to meet the rating requirements in your area.

Before installing any wall coverings - you should check with your local fire marshall to verify what you are doing and any documentation he may require for his final acceptance.

Happy Hunting

Rod

[jazzman_in_pa]

Seeker, it's terrific that you've offered to help your church solve its acoustic problems.

But large-space acoustics is a complicated affair, where paying for experience is a most wise decision. If you don't already have accurate readouts of the RT60 in that room at various frequencies and various locations, put a temporary halt to your plans and call in a professional. A good one is dirt cheap considering what you get: years of experience of what does and doesn't work. You instincts appear good, but an experienced professional can save you a lot of money by prescribing just the right amount of treatment in just the right locations for just the right frequencies at just the right balance. How did you arrive at the figure of 2,560 square feet of 2" 703? How many square feet of carpeting will be installed? Do you have a cumulative spectral decay plot of the room that you can post here?

If you don't know an acoustician you can trust, consider contacting Joe de Buglio. He's in Toronto, but church acoustics is what he's been doing for - ahem - God knows how long, and he's done them all across North America.

http://www.jdbsound.com/index.html

He has plenty of success, and has worked with church volunteers to help the church save money.

Lee

PS: I'm not related to him and don't get any kickbacks.

[seeker_ktf]

Guys, thanks for the responses.

I got pretty much what I expected, and I've printed the replies out just to prove the point. The biggest problem in this whole situation is the design. Basically, nobody seems to think things like this are important until they realize that it's a mess, and of course, by then it's too late. It's obvious they never did any planning towards this situation.

I only got involved because I've done a lot of recording there, and they released an album last year that I mixed. Even still, I'm just a hobbyist. As I sit here tonight writing this, it seems rather obvious that if I'm the best they could come up with for advice they're scraping pretty low. Sometime tomorrow afternoon I'll be letting them know what the status is. It'll be a disappointment for sure, but hey, reality is like that sometime. There are some good acoustic engineers in Houston. They might just pull a rabbit out of the hat after all.

Thanks for the heads up on the fire code too. I hadn't thought of that at all. I'm very glad I asked a few questions. I would have hated to have the fire marshal red tag their building based on advice I gave them.

It's funny, but now that I've read my original post and your replies, I see the horrible pattern I never thought I'd fall into. I generally hang out on a different recording site, but I recon it’s the same here. It's always a case of being in way over your head. Questions like, "I got a gig to record a 30 piece orchestra and we're starting in two days. What kind of microphones should I use?" or "I need to find a way to make my computer plays notes on my keyboard for a show I'm doing this weekend. Someone told me that I should use MIDI. Will MIDI do this and where can I get one?" Thanks for being gentle. ;)

[Wes Lachot]

Seeker,

I think maybe you're being a little hard on yourself. You did the church a huge favor by coming on here and asking some very intelligent questions. Maybe the answers aren't gonna be what they wanted to hear, but you did your job well.

Churches can be real funny about acoustics. There always seems to be a lot of ambivalence about doing any sort of acoustical treatment, even when the situation is so poor that speech is essentially unintelligable. Good luck.

--Wes

[Rod Gervais]

Seeker,

You have done well by asking the right questions.

The church should be happy to find the "truth" - now they can make decisions on how they wish to proceed without throwing money away.

A fool believes he knows the answers - and falls into a quagmire of trouble that eventually destroys what he tried to accomplish.

An intelligent man asks questions and learns.

You are an intelligent man. The church is blessed to have you helping with this.

:p:

Happy Hunting

Rod

[PRR]

In general, churches (and schools) must go through at least 3 phases before they come to grips with their acoustic and PA situation (if they ever do). Your church is probably in phase 1: asking one of the congregation (or friend) to "do something" without qualifications or budget. Next phase is to hire an "expert" who spends a lot of money on the wrong things. Few churches get to phase 3. One I know at phase 2.9 is over 200 years old.

So do what you can to mitigate gross flaws, but don't knock yourself out. Work on education and awareness. Accept that they have to stumble a few times before they get it right.

And to be blunt: that is another reason to Get Out and Hire A Pro. When your no-buck efforts don't produce miracle results (or worse, get red-tagged by the fire or building authotity), you don't want to hear the complaints no matter how gently they may be offered. (Much less any legal liability if someone gets hurt.)

So for general understanding rather than Advice:

> 80 feet wide

A bit wide for good acoustics.

Pick a point on stage and a point in the audience. Measure the direct path from one to the other, then find the length of the bounce-path from a side wall, and get the difference. If it is more than 35-40 feet, the wall-bounce hurts more than it helps.

Brooks (below) has a theory that old big-rooms worked because they could not span much more than 40 feet across. Modern truss designs allow much greater spans, but also much longer bounce-paths.

Oh wait. A rough triangulation of a 100'x80' room gives a worst case near-wall bounce path difference of only 28 feet, which is not bad. The far-wall path is longer, as high as 88 feet for an unlikely situation. And at that extreme, the near-wall image is much stronger than the far-wall image.

Also, you are never far from a ceiling-bounce, a VERY good thing. (Though it may not help folks in the sides of the room because of the angled ceiling.)

Not too bad. Anyway, you are stuck with the width.

> the back wall (drywall) is curved on a circular arc, with the circle's center being exactly center of the front wall. If you stand at the place where the preacher would be, you can hear your whispers reflected back to you quite nicely.

That's awful dumb. However it really only affects the center-stage. If you mark a 3-foot radius around the Focal point and avoid putting sources or listeners there, it is not a great problem. It would be worse if the focal point was IN the room rather than on the edge of it. (I have that room.)

Anything you can do to significantly break-up the arc will help. But for a tall single-curve arc, you do NOT have to cover the whole wall (and probably should not). The prime area is face-high, level with the preacher's mouth down to the body of a cello or guitar amp. Reflections high on the arc wall go up to the ceiling and are broken up. Reflections low on the wall smack the back row of the audience and are absorbed. Put a helper center-stage and hold a mirror flat to the arc-wall. If helper can see himself, cover that area. You will find that a mirror high or low on the wall will not reflect the helper to himself. (Unless the arc is also slanted, in which case a ladder and mirror is the easiest way to find the critical stripe.)

If the last 20 feet of floor is clear, the people 20+ feet from the back wall will hear a long-delay reflection, but it will not be strong because they are far outside the focus. So you really only need to deal with a 4 to 6 foot high stripe across the back arc.

At mouth level, 1 foot square carpet tiles on 50% of the area will significantly cut the echo. But be aware of fire codes, because carpet on walls can be very flammable and is in the panic-zone for people exiting. Fire-rated fabric covered fiberglas is probably a better idea. Flat panels NOT focused on the center-point will also break up the reflection: perhaps paintings or plaques mounted with a 2x4 behind one side. Statuary is great but semi-naked cherubs may not suit your style; anyway you need a herd of cherubs for a wall that wide.

Another worship-aware acoustician is Christopher N. Brooks http://www.orpheus-acoustics.com/ He has a "...dream of acoustical excellence...". His book may be very useful for educating your organization. His personal faith is Jewish but he seems to respect and understand all styles of worship. He is also very much into fixing the problem instead of just throwing money. For a modest fee, he will come give you a lecture/concert about acoustics. (He's in Pennsylvania so travel expense may make it impractical until your group decides to get serious.)

Basic questions:

What is the reverberation time? Clap your hands loud and count one-potato, two-potato until the sound dies. Note if the treble or bass lingers more or less than the rest. (*)

How loud are the blowers? Brooks thinks (and I tend to agree) that in most modern rooms, blower noise is THE worst acoustic problem, at least for unamplified or lightly amplified sound.

How is the intelligibility, without amplification, over the whole audience area? (You may have to test with empty seats: reverb will be less of a problem when the house is full.) Could you give a Chemistry lecture and understand 85% of the tricky words?

What do people complain about? (They may not give the right reason for dissatisfaction: listen and interpret.)

(*) A rough guess for a room like that gives midrange RT60 = 3 seconds empty, 2 seconds with 500 people. If the walls and ceiling are all gypsum board, you may have bass suck-out: 125Hz RT60=1.5sec when midrange RT60 is 2sec. Empty, the carpet will dominate and suck-out the highs with little or no effect on midrange or bass. But slight variations in material or mounting can cause large changes in RT60 and the plot of reverb against frequency: I can fudge it to say bass=3, 500Hz=4.3, treble=2, which is a pretty nasty curve. Not bass-ripe, and apparently dead in the treble while in fact your vowels are drowning the room and your consonants.

Midrange RT60=2sec is about right for a trained speaker saying things the audience already knows (which is in fact what most worship services do). In large rooms, RT60=1.6sec would be better if you have to understand every word. Anywhere from 1.5 to 2.5 may be acceptable for mixed sermons and semi-acoustic music. Directional loudspeakers can throw sound through much more reverberant spaces, but sound system design is much simpler if the room is pretty-good without PA.

The room average height is 24 feet and IF you only play to a full-house, I know just from the height that the room is "good". A bit low for orchestra, a bit high for speech, about right for mixed use. But without bodies it could be very different.

[Kemble]

Hey Seeker-
From one volunteer to another: I am praying for you! Good luck with this endeavor. Our gymnatorium is well treated, but only to a point.

Here's some REAL advice on this matter (I think this will be more important at times!): remember why you are doing this (assuming it is to serve God by serving your church), and remember WHO you are serving. I must say that is has been difficult to constantly hit brick walls and have to deal with people who just don't get it- and just keep on smiling and say "Thank you Lord for giving me an opportunity to practice patience! :D "

My $.02.

Jeff Z
(currently putting band-aids on Church on the Rise in Westlake, OH)