Acoustics, Audio and Video Considerations

by

David L. Debertin

 

 

Some Basics of Acoustics

 

1.               Hard surfaces (wood, tile, metal), tend to reflect sound: soft surfaces (carpet, upholstery, curtains, people) tend to absorb sound.

2.               Acoustical environments consisting almost entirely of hard surfaces will be “livelier” than acoustic environments where most of the surfaces are soft. The consequences are twofold:

a.      Acoustical environments with primarily hard surfaces will tend to need less amplification for a given sound pressure level than will acoustic al environments consisting primarily of soft surfaces, assuming both spaces are of equal volume.

b.     The proportion of reflected-to-direct sound will increase with the amount of hard surfaces in the space. Depending on exactly what happens, this reflected sound can be either a friend or enemy. Simple reflections from parallel hard surfaces tend to create echoes as the listener hears the sound coming directly from the source but also coming back at the listener a fraction of a second later as it bounces off the hard surface. This reflection can be an undesired echo that tends to garble speech, or it can be desired, in that this reflection is what makes musical instruments sound reverberant. Not all echoes make sound reverberant: if each echo nearly as loud as the initial sound source, and thus is not attenuated by significantly absorbed rather than reflected, the resultant sound will be “muddy” and unclear.

c.      How long it takes sound reflected off of hard surfaces to get back to the listener depends on how far away the sound source is from the hard surface reflecting it. Sound bounces back and forth between hard surfaces like a ping-pong ball  creating multiple echoes, with each subsequent echo being lower in volume than the last until finally dissipating in times perhaps as long as several seconds. The reverberancy organ players refer to is exactly this ping ponging of reflected sound over and over off of hard surfaces. The greater the distance between the musical instrument and the hard surface, the greater length of the lag, but also the greater the attenuation of the echo with each bounce. Electronic organ manufacturers at one time used endless tape loops to keep re-recording the sound of an electronic organ over and over again at progressively attenuated levels to make an electronic organ in a small room sound as if it were in a gigantic space. Nowadays, much the same thing is done employing electronics with digital sound processing on a computer chip.

d.     Because this reverberancy frequently tends to garble speech , an acoustic that works well for a musical instrument such as an organ may not work so well if clarity in understanding speech is a primary goal. Think of re-recording speech on an endless tape loop at progressively attenuated levels. One statistic from Knudsen’s 1963 paper (Reference below) is that approximately two seconds is optimal reverberance for acoustical instruments, whereas optimal reverberance for speech is only one second.

e.      Reflections off of hard surfaces that are parallel to each other such as floors and ceilings or walls resulting in regularly spaced echoes are generally heard as bad, and to create an idealized reverberant environment for acoustical musical instruments as few of the hard surfaces as possible should be parallel to each other. Thus, a rectangular box consisting of parallel hard surface floor, ceiling and walls in general going to have awful acoustics, but especially so if the box internal volume is of only modest size and if the sound bounced off the parallel surfaces is attenuated only slightly before reaching the listeners ears. That is why a gigantic box- shaped cathedral generally has better acoustics than a small high school gymnasium.  

f.       Normally, the reflectivity of sound varies depending on how many people are in the space, since people, clothing tends to act like soft materials that absorb sound. Knudsen’s paper notes that the Mormon tabernacle, famous for its fine reverberant acoustics, sounds best when there are about 2,500 people in it. Less than that and the space has too many reflections: When 5,000 or more people are present, the space is overly dampened and non-reflective. On measuring, the empty hall has a 4 second reverberation time whereas in the hall with the desired 2,500 people the reverberance declines to about 2 seconds, the latter considered all but optimal for music. 

 

Really Bad Acoustical Environments

 

I suppose that the most “classic” example of a public space is a gym in a modest- enrollment high school. All of us have been in these kinds of spaces. These generally are moderately sized boxes, with concrete vertical walls, tile or wood floors, hard surface bleachers, and perhaps metal folding chairs on the floor when used for something other than athletic events. Architects generally know that such configurations are an acoustic nightmare and sometimes attempt to improve the situation a bit by constructing the ceiling using soft, supposedly sound-absorbing acoustical tiles. The improvements from doing this are usually modest and people normally come away from events held in school gymnasiums feeling that they have just exited from a really uncomfortable and unacceptable listening environment.

 

Really Good Acoustical Environments

 

Truly great acoustic environments are not commonly found that are absorbent enough to allow speech to be easily understood and yet are reverberant enough so that music fills the space. There are so many variables that often, architects and acoustic designers simply get lucky with a particular space.  Conversely, in other instances the most carefully laid acoustical plans come up short. However, many of the best places share some common architectural characteristics

 

1.               There are a minimum number of parallel hard surfaces. In many of the more recently designed multi-use concert halls, architects often create space in which walls appear to spring up at odd angles to or otherwise interact with each other in visually exciting but sometimes disconcerting ways. The public generally thinks that these radical designs as architect/artists plying what appears to be essentially a visual not acoustical trade, but in reality these odd shapes and angles create acoustic spaces that can greatly benefit sound for the persons attending a concert, play or even a lecture in the space. Irregular paths for sound to travel between hard surfaces means that the reflected sound bounces back to the person in the hall from irregular distances at irregular times, and this is likely perceived as a desirable reverberancy not as undesirable echoes that would probably come back to the listeners ear in a disconcerting way if the hard surfaces were all parallel to each other.

2.               They contain a mixture of hard (sound reflecting) and soft (sound absorbing) surfaces in a proper proportions. There might be a combination of concrete walls, a wooden floor with carpeted areas in some places, and tall drapes, for example, but it would be rare to be in a fine concert hall where all the surfaces are either hard and reflective or alternatively, all soft and absorbent. Getting the combination just right in terms of the mix, like making a fine stew, is often not easy, but that does not mean that those who are trying to create a better acoustical environment should simply give up and say that this an impossible goal. The ideal often becomes a compromise of sorts and tradeoffs between what works well for one use versus another, clarity of speech versus reverberancy of music, for example. Designers of multi-use concert halls face the same dilemma, and they shoot for a design that works in as many different situations and applications as possible. As a congregation, we need to remember that our acoustic space each Sunday invariably is used for both speech and music, and whatever we create or do needs to work well for both.

3.               It may seem that the Mormon Tabernacle, built in 1867 but recently extensively refurbished, violates the rule which states that ideal acoustical environments contain a combination of hard (reflective) and soft (absorbent) surfaces, since the floors are wood, seating is wood, and much of the  remaining surfaces consist of what appears to be simply hard, reflective plaster. There are several not very obvious acoustical “tricks” going on in the space, however. First, the entire ceiling is an elongated dome or ellipse, with almost none of it being parallel to the floor. The ceiling constructed of plaster is not as hard and reflective as it might first seem, as it was common in the 19th century to mix plaster with cattle or horse hair to bind the plaster together and make it more resistant to cracking. (This is analogous to mixing concrete with fiberglass strands to make concrete more resistant to cracking.) The acoustical consequence here was that the horse or cattle hair plaster absorbs more sound than an ordinary plaster wall would. Beams of reflected sound are also readily broken up by the presence of various columns, pillars and other architectural irregularities. Dome-shaped ceilings present their own set of demons, as such curved shapes can concentrate beams of high frequency sound much like a curved mirror reflects and concentrates a beam of light, creating acoustical environments in some locations within a domed hall that can be very bad. No doubt the Mormon tabernacle would have more acoustical problems had the dome not been elongated and the plaster not been partially absorbent of high frequencies.

 

Faith Church

 

Faith church is very fortunate in that the hard wood ceilings rise at essentially a 45 degree angle to the floor of the space, which automatically means that the reflected sound is not going to bounce back and forth like it would in hard surface space in which the floor and ceiling are parallel. Had the floor and ceilings been parallel, I suspect that years ago we would have tried to dampen echoes and what would have been a small gym-like acoustical space by carpeting the floors, upholstering the pews and perhaps even adding curtains on some walls to further dampen what would be heard as undesirable and overly loud high frequency sound reflections bouncing back and forth at regular intervals.

 

At the same time, although all the undampened hard surfaces still bounce the sound around with a lot of reflections, at least a lot of the reflections are irregular, and in the case of music these irregular reflections are perceived as desirable reverberancy not undesired echoes. Anything that makes surfaces irregular such as the steel columns, the unevenness of walls, paintings and even canvas banners will tend to disperse reflections while absorbing some of the highest frequencies.  But these same reflections from all the hard surfaces tend to garble speech. The windows along either side of the “tube” section of the sanctuary are highly reflective, but are so close to the congregant that the sound reflecting from the glass is not delayed for very long.

 

I was noticing that when listening to Walter Huffman talking to what was a small group of people relatively close to him. The space overall was nearly empty. Even in this close configuration speech was not as easy as it should be to understand as there was a delayed echo or series of echoes bouncing off the pews, floor and ceiling. Fortunately, the quality of speech improves as the sanctuary fills with people, and we have a very different acoustical situation on Easter morning with a sanctuary packed with people than if the space contains only a dozen people.

 

However, I continue to believe that despite the love affair some seem to have with the current lively acoustical environment consisting almost exclusively of sound bouncing back and forth from completely undampened hard surfaces, this acoustic is not nearly what it could or should be given all the different kinds of things we call upon the space to do each Sunday morning, and there must still be ways to make incremental improvements that would increase the clarity of speech while not being offensive to the musicians and others who wish the space to remain as reverberant as possible. Creating and maintaining a reverberant acoustic space for music is a noble goal, but a church is much more than an acoustic space primarily to be used for unamplified music. As will be discussed below, sounds arising from amplifiers and loudspeakers pose particular problems in overly reflective acoustical environments, because of the tendency for many public address speakers to “beam” sound along narrow rays which get narrower as the frequency of the sound is increased, and these beams turn tend to bounce back and forth, creating a very uncomfortable environment for most listeners..

 

All of this needs our careful attention, in part because any alteration of the space that affects acoustics and dampens the amount of reflected sound even a little puts some of us on the defensive. I also know that at the same time there are congregants complaining that speech is often difficult to understand. I do not hold the view that Faith church has somehow come upon a perfect acoustic and that any changes to the sanctuary will reduce that level of perfection that we have achieved almost by accident. My personal view is that while the current acoustics work well for certain kinds of music and if one is seated in certain specific “sweet spots” we are not yet there in finding the combination of hard and soft surfaces given the many things the sanctuary is asked to do acoustically even on a single Sunday, and further, the idea of maintaining exclusively hard surfaces no matter what is not working overall to our advantage in a space that must work well for both speech and music.

 

 

Audio Amplification

 

Ii you want to make me cringe, begin a discussion about the entire idea of contracting out to a professional (pro) sound contractor the problem of designing and building an amplification system for Faith church. First, recognize that

1.                         Sound contractors that do this generally are thinking about providing adequate sound volume in large to very large spaces, frequently much larger than the internal volume of Faith Church. Even if the spaces they are dealing with are not large, they are normally dealing with smaller spaces that contain carpet, upholstered chairs and the like. In absorbing sound, soft surfaces tend to eat up high frequencies and amplification power, so higher-output amplifiers and speakers are needed. Alternately, a lot of the professional audio sound business involves designing systems that work well in clubs and bars, where the sound of the music must rise over the general din going on within the bar or club. In such an instance, merely getting the sheer volume of sound at all frequencies above the din of the crowd becomes a primary goal of the design.

2.                         Designing an effective amplification system for the transepts and pews back about 8 rows would be comparatively easy. In the rural church I attend in North Dakota, the pews sit in an angled pattern around the altar, and, as a consequence there are only 8 or 9 rows. A speaker that looks like a 12-inch shiny white globe hangs from the center of the high ceiling, radiating sound in all directions (omnidirectionally) down and into the space, and reflecting sound down from the ceiling to the congregants below. All of this is driven with a simple, modestly-priced 100 watt PA amplifier from Radio Shack. The ceiling is wooden and hard surface and reflects sound toward the congregation below, pews are upholstered and the floor is a short-nap carpet. To me this is a model space for the effectiveness of simple amplified acoustics, and in particular an acoustical environment in which speech is very easy to understand, not garbled. Music sounds fine too, although the fully carpeted floor reduces the reverberancy some.  The ceiling is non-parallel and wood.   

 

3.                         What makes our situation far more difficult for amplification is the presence of the long tube-like space that extends all the way back to the front doors of the church. In most of that area, the congregant seated there will be hearing a combination of live sound, sound reflecting from hard surfaces bouncing off of various points within the sanctuary and sound coming from the amplification and speaker systems. Electricity travels at the speed of light: sound travels very slow in comparison (a comparatively snails-pace 1087 feet per second). I am not certain of the exact length of our church from the very front of the sanctuary to the last pew, but a quick calculation suggests that whatever sound is created in the front of the church arrives at the last pew perhaps 1/7th of a second later. Even what seems to be a short delay creates all sorts of interesting problems. As any choir member knows, if the choir processes from the rear of the church to the front, it will believe it is keeping up with the organ, but is actually behind the organ by about a quarter of a beat when it is singing from the rear of the church. At some point mid-way up processing through the “tube”, the choir realizes that it is dragging and has to skip part of a beat so that it is in time with the organ when it reaches the front of the church.

4.                         So, suppose that we want an amplification system that addresses the needs of the congregational members who have difficulty hearing from the back rows of pews. Since electricity driving loudspeakers at the rear of the church travels all but instantaneously, but the direct route for unamplified sound is delayed by perhaps 1/7th of  second, whatever sound is coming out of any amplified loudspeaker is going arrive at the listener’s each ahead of any part of the sound radiating unamplified directly from whoever is speaking. To the extent that the live sound is a component of what the listener hears, this creates a situation whereby speech will be garbled, unless the congregant is close enough to the amplified speaker so that the live sound is almost completely attenuated. Obviously there will still be middle rows where the live portion of the sound is a bigger component of the total but as a person sits further forward in the church the delay should be less too.

5.                          

Basics of PA design

 

My uneasiness about professional sound system contractors and what might happen if we simply turned one loose to design an amplification system for Faith church stems from the fact that most professional sound contractors deal primarily with situations where simply achieving adequate volume is a paramount concern, and few would have much experience working in smaller-scale situations in which there is a combination of blended live and amplified sound, and there are lots of reflective surfaces and at least some of the congregants may be sitting close to the amplified loudspeakers. The rural church system I am familiar with  avoids the latter problem in part by mounting the speaker high in the ceiling and not near the ears of the congregation, but since the sound travels shorter distances, the differences in the live versus amplified sound is lessened.

 

A basic premise in home audio loudspeaker design is that while low frequencies tend to be omnidirectional (the location of low frequency sounds is difficult if not impossible to pinpoint, and especially so at very low frequencies), as the frequency of the sound increases, sound tends to radiate out to the listener over a progressively narrower and narrower angle, even as the source location becomes easier and easier to identify. This so-called beaminess and localization of high frequencies has led to a many decades long struggle among loudspeaker designers to build high frequency drivers that more nearly radiate high frequencies in all directions and thus be less subject to this undesirable beaminess. A simple experiment is to listen to a loudspeaker by sitting directly in front of it, and then move off 45 degrees to the side. The best loudspeaker is likely the one whose sound quality changes the least when listening from these two locations because if this is true, then high frequency sound is more broadly dispersed.

 

Amplified musical instruments do not sound like unamplified instruments in large measure because most unamplified instruments tend to radiate high frequencies in at least several if not all directions, whereas the loudspeaker causes the amplified high frequencies to beam straight out in a single line or ray. To a degree, one can improve the apparent quality of the amplified sound by taking steps that increase the dispersion of the high frequencies, so there is less of a tendency for the sound to hit the listener’s ear directly in a painful straight-line beam.

 

PA contractors generally are interested in providing their customers with the largest volume of sound for their budget. There are several consequences of this. First, PA loudspeakers are generally built to be more efficient at converting amplifier power into sheer sound volume than are home audio speakers. Secondly, PA systems designed for larger spaces are going to still use lots of amplification wattage, so the speaker has to be durable enough to handle all the current to create ample volume without burning out or falling apart.

 

This leads to loudspeaker designs for these applications to be very different from those normally used in home audio in which high frequency drivers with adequate high frequency dispersion is very important. The common PA speaker is likely a two-way systems in which a large woofer (12 inch or even larger) are combined with an efficient horn-type tweeter. These tweeters are often designed to produce sound pressure levels of 110 db or even higher without burning out, far greater than the 85 or 90 db considered loud in a den or living room by host home audio hobbyists. A type of tweeter called a Piezo horn tweeter is commonly employed in PA systems to achieve the volume without burning out from lots of wattage. The advantage these tweeters have is that practically any amount of amplification can be used, and even if they are fried, they are inexpensive to replace. These tweeters are almost never employed in home audio systems, however, because along with the sheer volume of high frequencies comes a lot of beaminess. To the extent that the loudspeaker is located some distance away from the listener’s ears, the high frequencies will tend to be dispersed and perhaps reflected before they are heard. But to the extent that a listener is sitting relatively close to a Piezo horn, the listener will probably be cringing in real pain, and especially so if the beam of high frequency sound is bouncing back and almost unattenuated off of hard surfaces.

 

The combination of a traditional PA type Piezo horn tweeter and unrelenting hard reflecting surfaces all relatively closer to the listener’s ears will likely cause congregants to writhe in real pain and complain loudly about how uncomfortable they are even as the sound system contractor congratulates himself on how much louder the new PA system is able to play than our old system did and that no one should complain about having difficulty hearing now that the new system is in place.

 

The current PA system speakers in Faith Church dating probably from the 50s produce very little in the way of high frequencies at all. I suspect that each of these consist of nothing more than a modestly-sized full-range driver, housed in a less than adequate cabinet. The lack of any real high frequencies coming from these old full-range drivers means that at least people are not cringing in pain when sitting in the pews physically close to the speakers, but the loss of high frequencies also means that people relying primarily on the amplification from the PA System are going to complain that they cannot understand very well what is being said even though the sound seems loud enough since they are sitting right next to the speaker.

 

More Audio Thoughts and Very Tentative Recommendations

 

First, let me say that I think that it should be readily possible to design an amplification system that would both serve as a single combined system for amplifying speech as well as for amplified musical instruments. There is no need for separate systems or different amplifiers and loudspeaker systems for each. Even acoustic instruments could be miked and amplified if desired.

 

I would envision the following:

 

  1. A command center mixer/amplification console located is an inconspicuous spot near the front of the church. This center control panel and amplifier is the base point for the entire sound system irrespective of source, be that microphones, amplified musical instruments, or even other sources such as video. Wires run from this central point to speakers located at various points throughout the church Ideally it would be nice to have access to this console even during services, so adjustments easily can be made to the amount of amplification. Amplified musical instruments would simply use another input on the main amplifier. The beauty of this scheme is that the volume of sound coming, say from a miked singer, can easily be adjusted relative to the volume coming from the amplified instrument

 

  1. Wires running from the console to microphone jacks at various points around the altar area wherever we see the need for possible microphone amplification, including locations where we are not currently able to do this. Ideally we could use floor jacks here. One advantage to retaining a raised floor in te altar area is that it would be comparatively simple to run wires under the raised area.

 

  1. Study carefully what is available in latest state-of-the-art design for public address speakers, with particular concern over finding a design that works well given all the hard, reflective surfaces.

 

  1. Think about perhaps locating omnidirectional (ball-like) speakers in two or perhaps three separate points in the high ceiling of the sanctuary rather than on the side ceilings nearer to the congregants.  We would need to run wires in much higher places than we currently do

 

.

Video Considerations

(section still under construction)

 

If a person wants to create a controversy, one only needs to raise the entire subject of employing video in a church sanctuary. Some of this controversy is perhaps warranted, but some of it is a consequence of inadequate or misinformation.

 

The most general controversy surrounds the whole idea of whether a video screen has any proper place in a church or in a church sanctuary at all. Nearly every church in the ELCA at some level has been at least touching on this entire issue, and there are probably nearly as many solutions as there are churches. My only other comment is that when I arrived as a young assistant professor at UK nearly 35 years ago, professors were supplied with chalk and a blackboard and told to go teach. Some 35 years later, no UK or college classroom anywhere is complete without video projection computer, document camera which serves as a high tech overhead projector plus a host of other neat electronic devices, and no professor would survive for long in a classroom who did not take advantage of at least some of the various electronic technologies now available in the classroom. My point here is that what seemed radical at one point in time over a period of 30 years became commonly accepted.

 

In practically any other situation I encounter where someone is presenting an idea in front of a group, they are surrounded with Powerpoints, photos and a host of other images of various sorts, but most ministers still preach without a Powerpoint highlighting the key items they are trying to communicate as bullet points.

 

One could ask a question as to whether there is or should be something fundamentally different from preaching versus other situations whereby a person is communicating with a group of people, and whether whatever these differences might be would they ultimately completely rule out the possibility of using video equipment in the sanctuary.

 

One of the problem is making changes is that whatever is decided upon often ends up remaining the same way for a long period of time—witness the age of the components of our amplification system. The decisions that get made at some point in time frequently limit what can be done by others at some future point in time, others who may see things differently than we happened to think was right.

 

For those interested in my summer 2007 experiments in developing sanctuary and fellowship hall video setups in the rural church in North Dakota, you are welcome to look at photos and other materials outlining some of the features on my Web site

http://www.dldebertin.com/projector/chproj.htm

 

 

In short, we may be evaluating ideas regarding things that could be done right now or not. We may correctly conclude that we do not wish to pursue a particular idea right now for what we believe to be a very good reason. But in the process we also want to make as few decisions as possible regarding what can or cannot be readily done by others who follow at some time in the future. I am in favor of attempting to make it as easy as possible for others in the future to do things even if we might not want to pursue them right now.

 

Employing video in a church sanctuary in part is controversial because of the need to locate a large screen somewhere. A legitimate concern is that is a large screen is permanently placed near the front of a church, it will draw attention to itself and away from items such as cross or altar. In short, a white screen when not being used competes for visual attention. Ideally, a congregant should be largely unaware that a screen even exists when it is not being used. That suggests if one wants to pursue such an idea, a screen ought to be as inconspicuous as possible when it is not being used.

 

This summer has been a great eye opener for me in a lot of different ways. I am part of a church in town of Plaza, North Dakota, a town with only 169 people. My parents were members there when they lived in Plaza in the 1990s, and I continue to maintain what amounts to a summer home there. In May of 2007, I approached the newly arrived minister with the possibility of putting together a projection video system for them, in part thinking that this would be a great opportunity for the youth to get together to watch movies and for the adults to socialize, keeping in mind that the town is 42 miles away from the nearest movie theater, and winters are long and severe.

 

My idea was that we could allocate an area in the fellowship space with a permanently mounted screen that would work well. The minister said “that is fine, but I was really thinking about employing video in the sanctuary space as well and I have picked out an exact spot on the wall where I believe we could fit a screen in the sanctuary. Can you come up with a system that would potentially work in both spaces? That left me scampering for ideas on how to do this—design screens for both places that could be readily used but not draw attention to themselves when not in use. For the sanctuary I ended up with a screen constructed much like a giant banner that could be partially covered with a colored banner when not in use, complete with brass pipes at the top and bottom and a brass chain held by just a single hook in the wall. The single hook was the only structural change needed. The screen in the fellowship space uses stretched white fabric over a 1 x 3 frame, and is so inconspicuous that when the minister first walked into the area where we had installed it, he did not even realize it was there. I suspect others in the congregation haven’t even noticed it is there either. The projector and video sound system all is stored on an oak cart, which blends with the rest of wood in the sanctuary.

 

 

 

 

 

 

Reference

 

Knudsen, Vern O. “Architectural Acoustics, November 1963. Available as a class handout on architectural acoustics in a physics class at

 

http://charmian.sonoma.edu/~bryant/Fall2006/Phys300%20F06/Phys300%20info%20by%20chapter/Auditoria%20WEB%20references/Architectural%20Acoustics.htm