absorption

[byronyasgur]

our new building is 15m by 13m and has an average ceiling height of about 3.5m - the walls and ceiling are all gypsum/plasterboard. We expect 150 people seated on simple padded chairs. I am advising that the floor should be carpeted but the pastors favor linoleum for ease of cleaning.

i am trying to figure how much absorption i need - i have calculated that the rt60 will be 1.7s at 2000hz ( with carpet floor ) - i have seen recommendations of a rt60 of 1.0s for amplified church music and speech - further calculations suggest that i need to put in a considerable amount of absorption to reduce this to 1.0s - on the other hand some people are saying that i need as little absorption as possible in church because it enhances people singing along - though i wonder do they mean more traditional church - coming from a live rock music background myself we nearly always rathered controlling the reverb from the desk - i always felt that very live rooms usually made life difficult unless you were doing something that was not that loud and more acoustic - the music in our church is similar to hillsong or delirious or matt redman which is not very acoustic or traditional

any help with figuring how much ( if any ) absorption to put in appreciated

another thing is that unexpected expenditure in several areas means that we are on an extremely low budget - in fact it's practically 0 but i'm sure i can get around them to do what needs to be done to make sure the sound is clear, and clear sound is the primary goal here

[cmchamp]

You need a balance of absorption and diffused reflection in order to keep a balance between deadness desired for your contemporary service and natural room reflections for speech.
You also need to look a the full spectrum of reflections/reverberation in the room, not just a single frequency or average.
Rooms that have an acceptable average can have an un-ruley bottom end.

[byronyasgur]

thanks - i know what you mean about looking at the full spectrum but i wanted to get a rough average for rt60 and then think about whether i need more trapping ..... but to be honest maybe that's the wrong way of doing it - i have found tons of information about solving problems but very little about preventing them at design stage - is there no way of judging what treatments are needed in a room without listening to it/testing it - ultimiately what i'm trying to do is make an estimate of how much adbsorbtion/diffusion and bass trapping i will need in the room and then add/take away/move etc while the room is in the early stages of use ( ie with congregation etc ) ... i was thinking that if i got a rough idea of what i should be aiming for as regards rt60 @2k then i could always add some bass trapps ( plus there's a pretty big bass trap planned into the stage cavity ) ... but i still need to know how much diffusion vs absorbtion i need - the other 2 things im thinking is that i might leave the audience area more live than the stage area and also that i might do polys for diffusion which i could then fill with material if i needed more absorbtion later

[Brad Weber]

Have you got a few years and thousands of dollars to invest? If you do then you can learn how to properly do what you are wanting to do as far as measurement, analysis and prediction. If not then you either need to hire someone who has that expertise and ability or maybe approach things differently.

In all seriousness, the calculations that you are likely using for RT60 incorporate several assumptions that are not entirely valid for most worship spaces and thus leads to inaccuracies in the predicted results. There are more complex approaches that can be more accurate for many spaces but that relates to a more complex analysis.

Beyond that, even if you achieved the 'ideal' RT60, and even if you did so for all frequencies, that does not by itself define a good acoustical environment. While a potentially important factor, Reverberation Time is just one factor in a room's acoustics. Don't get so focused on just one factor that you lose sight of the 'bigger picture'.

Absorption and other applied acoustical treatments can have difficulty 'making up' for the general room shape, dimensions and volume, so the time to start considering acoustics is while those aspects of the space are still being determined. The more potentially relevant factors that are defined without considering the impact on acoustics, the more difficult to try to address any related issues. And that leads into...

At least in my experience, there are typically budgetary, aesthetic and practical considerations that may also factor into any decisions regarding acoustical treatments. I find that many churches are not actually interested in achieving specific numerical goals as they are is simply getting the 'biggest bang for he buck' in terms of cost effective solutions that help improve the acoustical environment while also addressing any other concerns or limitations.

The point of all this is to consider whether maybe you need to not worry so much about achieving a specific RT60 value and focus more on what can realistically be done to most benefit the space in general.

[byronyasgur]

Brad...

thanks very much but ......

Have you got a few years and thousands of dollars to invest? .... If not then you either need to hire someone
no and no chance


In all seriousness, the calculations that you are likely using for RT60 incorporate several assumptions that are not entirely valid for most worship spaces and thus leads to inaccuracies in the predicted results.
Yes but i assume ( maybe wrongly ? ) that these inaccuracies are not incredibly huge ..... and i'm only trying to get the rt60 roughly right or as best as i can

Beyond that, even if you achieved the 'ideal' RT60 .. and even if you did so for all frequencies, that does not by itself define a good acoustical environment.
Like i said im not looking for an ideal rt60 ... I am well aware that diffusion and many other things affect the room - im not solely looking at rt60 here i was really just using that as a starter place ( probably because i figure - again maybe incorrectly - that [average]rt60 is the basis of the sound of the room - if i get the rt60 roughly right across the mid and high ranges and add to that trapping to controls the bass response effectively then along with an appropriate amount of diffusion the sound must be starting to sound reasonable at that poing)

While a potentially important factor, Reverberation Time is just one factor in a room's acoustics. Don't get so focused on just one factor that you lose sight of the 'bigger picture'.
like i said i thought it was the best place to start - your average punter will notice this above all other sound features of a room ( again maybe im wrong - i would very much like to hear your opinion and also where you think i should start instead and why )

Absorption and other applied acoustical treatments can have difficulty 'making up' for the general room shape, dimensions and volume, so the time to start considering acoustics is while those aspects of the space are still being determined. The more potentially relevant factors that are defined without considering the impact on acoustics, the more difficult to try to address any related issues. And that leads into...
yes i know that it would have been better to design the room with acoustics in mind but this room is already 90% built and i had nothing to do with it's design (plasterers are arriving in the next few days and other than that i think there's very little left to do in terms of construction that would make much difference to the sound) - the only things of relevance that i can see at this stage left to be decided upon are the flooring ( carpet vs linoleum ) and the type of chairs - other than that unfortunately everything needs to be "fixed" as best as possible with treatment - i wont go into the details of how we ended up in this situation as it's a long boring and slightly silly story ( also the pa system is coming from our old building )

At least in my experience, there are typically budgetary, aesthetic and practical considerations that may also factor into any decisions regarding acoustical treatments.
I have convinced my pastors ( admittedly too late to affect the building itself ) that we need to put in some sound treatment and they are happy to trust me with the aesthetic aspects of the sound treatment - from a practical and budgetary point of view we need to spend as little as is possible in terms of money and floor space to get the maximum "bang for buck" - ( ultimately bang for buck in this case is simply a decent balance between the sermons being intelligible, the sound from the worship team being controllable and the room being live enough for the congregation to enjoy singing in it ) - i know that's asking for the world on a string but i am simply going to do the best i can within the constraints

I find that many churches are not actually interested in achieving specific numerical goals as they are is simply getting the 'biggest bang for he buck' in terms of cost effective solutions that help improve the acoustical environment while also addressing any other concerns or limitations.
exactly, and our church is certainly no exception - do you suggest just putting a random amount of absorption and diffusion around the place and adjusting it after each service until we're happy or ???

The point of all this is to consider whether maybe you need to not worry so much about achieving a specific RT60 value and focus more on what can realistically be done to most benefit the space in general.
as i've explained im not trying to achieve a particular rt60 at all - i was just looking for a target / i would very much welcome any suggestions regarding how you would suggest benefiting the space in general like you say

thanks

[Brad Weber]

Depending on the space, the inaccuracies of the Sabine equation can be significant. However, the point I was really trying to get across is that you could have a room with an 'ideal' RT60 and yet still have many problems. A common example might be a room with lots of people in it for the volume of the space. Based on the absorption of the people and air, the traditional Sabine equation might predict a decent Reverberation Time. However, the reality could not only be a longer reverb time or a double slope reverb, but the room could also exhibit flutter echo and slap back echo from the walls and possibly some intelligibility issues from the sound system coverage hitting the reflective walls and ceiling. In such cases it might be more effective to try to address those issues and compromise on the Reverberation Time.

So rather than starting with the RT60 value, you may want to start with what you may need to do to address some of the other potential issues and then see how that turns out in terms of Reverberation Time. for example, if you have significant energy from the sound system hitting the rear wall then you might want to try to treat that heavily to prevent slap back to the stage.

Since you mentioned the walls and ceiling being gypsum board and/or plaster, one interesting tidbit is to look at the absorption of many common gypsum board constructions. You may find that between the sound that is transmitted through the wall and the diaphragmatic action of the gypsum board flexing you actually get a surprising amount of low frequency absorption. In rooms with large amounts of gypsum board I will sometimes mix the number and thickness of the gypsum board layers used in order to vary the low frequency absorption of those surfaces a bit and avoid having a huge dip in the room response at certain frequencies.

[byronyasgur]

yes i was kind of thinking that the gypsum might be acting to trap the bass and was guessing that the room might have a reasonably tame lf response - the only way i know of to test for flutter is to go around the room making noises an listening for flutter - i found one distinct one and one that i couldn't quite pinpoint - i temporarily fixed the one i found with a bit of insulation so i figure i can fix this one with a permanent piece of absorption or diffusion and i know that issues like this can be a problem but i will obviously need to fix issues like those two

i was planning on a lot of absorption on the back wall to prevent slap-back however i have the problem that the pastors are *intent* on installing ( they're already installed ) 3 large glass windows in the back wall, however the glass is not in yet and i have convinced them to mount the glass at as much of an angle as possible not sure if that will do much good but in any case i will cover as much as possible of the rest of that wall with absorption

i am undecided as to whether i go for carpet or linoleum on the floor

i am planning on putting absorbtion behind and beside the speakers on the wall to try to adresss comb filtering - other than that i don't know whether intelligibility would be improved by absorption, diffusion or no treatment

[Brad Weber]

Originally Posted by byronyasgur

i was planning on a lot of absorption on the back wall to prevent slap-back however i have the problem that the pastors are *intent* on installing ( they're already installed ) 3 large glass windows in the back wall, however the glass is not in yet and i have convinced them to mount the glass at as much of an angle as possible not sure if that will do much good but in any case i will cover as much as possible of the rest of that wall with absorption

Think about where the sound sources are in relation to the windows and where the resulting reflections will go. It's not unheard of to have people try to improve things by angling reflecting surfaces but to do so in a manner that actually makes the results worse.

Originally Posted by byronyasgur

i am planning on putting absorbtion behind and beside the speakers on the wall to try to adresss comb filtering - other than that i don't know whether intelligibility would be improved by absorption, diffusion or no treatment

Where else will the energy speakers hit surfaces and reflect to listeners to be received with some delay compared to the initial signal? Maybe the ceiling and/or side walls? Creating the optimum acoustic for a space involves looking at such reflections and addressing whether they are desired or detrimental based on the use of the space and then addressing them accordingly with diffusion and/or absorption.

[byronyasgur]

thanks brad

... and then addressing them accordingly with diffusion and/or absorption.

are there guidelines for any one given situation to help determine whether absorption or diffusion should be used (edit : i mean i know you would probably need to use a combination of both throughout the room, but how would i know which to use in a particular position for instance a particular spot on a wall or ceiling etc )

[cmchamp]

Sabine's formula "Assumes" evenly distributed absorption and what we might consider a small room.
Fitzroy and Eyring formulas are more appropriate for larger rooms with unequal absorption on surfaces.
I know I said I'd make a quick model, and it would be quick if I had time at the right computer to do it.
C.

[byronyasgur]

thanks guys but i still dont really understand why the average amount of reverb in the room is so seemingly unimportant - i know bass response, diffusion, comb filtering, flutter, slapback and everything else is also important but in my experience the average joe will only notice whether the room is "echoy" or not "echoy"

[Brad Weber]

Trying to get a Reverberation Time equation that will be valid for a wide variety of conditions has been a goal of acousticians for years, but for now there are multiple equations that are each potentially more applicable under certain conditions. For example, in a rooms with very disparate materials Millington-Sette might be more accurate while for rooms with high absorption on two opposite surfaces and lower absorption on the other surfaces, such as an office space with sheetrock walls, carpeted floor and acoustical tile, Fitzroy-Kuttruff (a modified Fitzroy equation) might be better.

Adding to this is that the method of obtaining absorption information for materials has fairly recently come under much greater scrutiny. The traditional method is that a material sample is placed in a known reverberation room and the change in reverberation time of that room along with the area of the sample allow you to calculate the associated absorption coefficients. However, when dealing with three dimensional objects such as people, seats or convoluted foam, then what is the actual exposed surface area? It has also been discovered that in some cases that edge treatments, how the samples are distributed (e.g. one large sample or several smaller pieces with space between them) and so on can affect the results. Adding to that, many real world applications do not involve a reverberant source but instead involve sources with some directionality relative to the absorption surface and that can affect the practical absorption achieved.

The point of all this is that while Reverberation Time is still an important factor, we have also become more aware of some of the limitations and deficiencies in the current methods used to predict RT60.

Added: It should also be noted that even when looking at decay times acousticians look at more than just the RT60 or the time for the signal to decay 60dB. They may look at how the sound initially decays, the overall shape of the decay, the slope of the decay, whether there are specific anomalies in the decay and so on. A room where the sound decays 50dB in one second and then the last 10dB over two seconds could sound much different than a room where the sound decays 20dB per second or where it decays slowly for two seconds and the drops even though all three situations might have the same RT60 value. And more commonly now, acousticians look at Impulse Responses that essentially show what is happening in terms of the sound arriving at a particular point.