I'm done!
As of 3pm on last Monday, I am finished with my senior capstone. It was a long two terms, but I'm highly satisfied with the results. Almost done with Augustana; just a few more weeks of class and I'm out.
For many years the concept of acoustics, the study of sound and its behavior in physical environments, has been considered a “black art” - a science filled with as much guessing and mystery as actual computations (Long xxv). Fortunately, advancements in technology and acoustical research over the past few years have opened many possibilities to improve our sounds and music. In the realm of architectural acoustics, concert halls and performance spaces are being designed to create the ultimate experience for audience and performer alike. However, many halls were built prior to this scientific revolution, making it expensive and complicated to renovate the space for better acoustical properties. Centennial Hall at Augustana College is one such hall. Centennial Hall has several architectural flaws that diminish the positive acoustical properties of the space, but measures can be implemented to correct these problems. In this paper I will identify the acoustical properties critical to a concert venue, discuss how Centennial Hall corresponds with these properties, identify other acoustical problems of the hall, and make recommendations for ways to improve the acoustical conditions in Centennial Hall.
In order to analyze the acoustics of Centennial Hall, we must begin by defining some of the important elements that constitute the acoustics of a concert hall. First, there is reflection and diffusion of sound waves, which determines the reverberation of the room. Next, sound absorption defines how the sound will behave when it encounters a surface such as a wall or ceiling. Also, the general shape and size of the performance space will factor into the distribution of sound to the hall.
Sound travels away from its source as a wave, usually omni-directional as in strings or woodwind instruments, or projected by means of a bell as in brass instruments or the human voice. One way to visualize sound waves is rays traveling from the sound source and interacting with surfaces they hit (Brooks 49). Reflection occurs when the sound encounters a surface such as a wall or ceiling and is redirected. Flat walls reflect sound like a mirror. Concave surfaces reflect in a way that refocuses the sound at a different location. Convex and rough surfaces scatter the sound all about the room (Rossing, Moore, and Wheeler 526).
The human ear senses not only the direct path of sound from the source, but also reflections that may have bounced off of multiple surfaces before reaching the listener. A large time difference between the direct sound and its reflection causes an echo effect in which the listener perceives the reflections as separate events (Brooks 19-21). In a good concert hall, reflections reach the listener before the previous ones decay which is perceived as one continuous sound; an effect called reverberation. The time it takes a sound to decay to silence after the source ceases is the reverberation time (Rossing, Moore, and Wheeler 529-531). Several factors such as hall size, shape, and construction can affect the amount of reverberation in a room. A room is considered “live” when the reverberation time is longer; around 2 seconds is ideal for concert halls. Too much reverberation can make music sound muddled while too little reverberation can make music sound dry and without warmth (Brooks 21).
Another behavior of sound waves is diffusion. This phenomenon occurs when certain shaped surfaces reflect the sound waves all about the room. Effective diffusion will eliminate echoes and uniformly distribute sound throughout the hall referred to as “envelopment.” Envelopment is “the impression that the sound is arriving from all directions and surrounding the listener.” (Long 681). Many surfaces can act as diffusers, such as ceiling coffers, pillars, statues, and geometric shapes placed throughout the hall that are designed to diffuse sound. However, modern architecture has drifted away from these ornamentations, eliminating critical diffusive properties of concert halls (Brooks 129).
Sound absorption is another key acoustical property in concert hall design. Only a certain amount of a sound wave’s energy reflects off a surface. Some of that energy is “absorbed within the material” of that surface. Varying materials have different absorption coefficients, the quantified amount of sound lost to the material (Long 249-250). Non-porous materials such as glass, wood, and painted concrete have small absorption coefficients, meaning they reflect back much of the sound. Porous materials such as cloth and carpeting have high absorptions and reflect back little of the sound (Rossing, Moore, and Wheeler 531). Even the air that the sound travels through will absorb some of the sound energy. This can be varied based on relative temperature and humidity (Brooks 19). Control of these environmental conditions requires heating, ventilation, and air conditioning systems which create other barriers in the quest to achieve good acoustics.
One more property of acoustics necessary for this analysis is the size and shape of the concert hall. The four most common shapes of concert venues are: rectangular or “shoebox;” diamond, in which the side walls expand out in the middle; fan, in which the side walls expand out to the rear of the room; and horseshoe, in which the side walls curve into the rear wall. Shoebox halls are considered to be the best sounding halls because sound reflections create an ideal musical envelopment for the audience, although other shapes have also proven successful (Long 658). Hall size is important in determining reverberation and loudness. The volume per seat of the hall generally decreases as the number of audience seats increases. The average number of seats in halls with good acoustics is around 1750 to 2200 and 250 to 275 cubic feet per seat. New designs for concert halls are increasing the seating capacity because more seats mean more revenue to pay for the hall’s costs (Long 656-658). The larger size of these halls may make their acoustics less controllable or predictable.
Now that some basic knowledge of architectural acoustics and parameters of an ideal concert venue are established, I will explain how Centennial Hall exhibits these properties and compares to the desired model. Centennial Hall is fan shaped. The width on stage is 75 feet and the side walls extend out to a width of 114 feet at the back wall (“Augustana” 3). Because of the expanding distance of the lateral walls, sound reflections will favor the sides and rear of the auditorium. The center seats receive fewer reflections from the walls and must rely on those from the ceiling (Beranek 28). With an average height of 23.5 feet in the front and middle of the audience, the highest point in the hall, audience members in the center section will receive sound reflections late (“Augustana” 6). When a performer on stage plays a note, the time difference between the direct sound and the first sound reflection will be so great that the listener in the center section may hear an echo instead of reverberation. This can be solved by lowering the ceiling in that section or by installing overhead sound reflectors called “clouds” (Brooks 81). With a 1600 person seating capacity and about 200 cubic feet per seat, an increase in volume would help regulate loudness and improve reverberation time. This can be accomplished by raising the ceiling at the back of the auditorium where there is unused catwalk space. Nothing can be done concerning the general shape of the room unless the hall is demolished and rebuilt, in which case a shoebox shape would be preferable. Therefore the acoustics must be adjusted by other means.
The current diffusive properties of Centennial Hall are minimal. Along the first 25 feet of lateral walls there are floor to ceiling pegboard prisms. These diffusers also completely line the rear wall. With the minimal degree that these devices protrude into the listening space, they have little effect on the direction of sound reflections. The ceiling of the hall also offers no diffusion. Placing diffusive objects along the walls and suspended from the ceiling would alleviate this situation. Some such objects would include: protruding geometric shapes like boxes and pyramids, convex surfaces, and coffered wells that resemble empty picture frames (Long 726). With more diffusion, the sound filling the audience space will distribute more evenly and listeners will experience more envelopment from the music.
The surface material of these pegboard diffusers produces several negative acoustical properties. The holes in the pegboard are large enough that a significant amount of bass frequencies are attenuated behind the paneling. Also, the hard, wooden surface of the board reflects much of the higher frequency sounds of an ensemble, creating a harsh and over-brilliant sound (Rossing, Moore, and Wheeler 741). With these two effects occurring, audiences perceive music to be without “warmth” or “tonal color” (Long 654). Adding materials such as carpet and drapery absorbs high frequency sounds and reduces acoustical glare. An adjustable drapery system that covers the rear wall would accomplish this and also provide a means of altering reverberation time for different musical works and ensembles (Rossing, Moore, and Wheeler 540).
A common complaint among those who perform in Centennial is that they cannot hear one another on stage. Because the stage is so wide, diffusion is critical for performers to hear other members of the ensemble. However, the only significant diffusive objects around the stage area are the pipes of the organ. Not even the ceiling above the stage will offer useful sound reflections to the ensemble members because it is too high. Performers in Centennial compensate by ignoring what they hear and relying on the conductor for issues of balance and blend. Ideally, a permanent band shell would reflect sound back at the ensemble so they could make musical adjustments based on listening. The shell would also project a larger amount of music toward the audience; bass sounds in particular (Long 720-721). Small, moveable shells are currently used to adjust the ensemble sound, but their size and limited number cannot encompass a full orchestra or band effectively. The best solution is to construct a permanent band shell that covers the whole stage and reflects the ensemble’s sound back onto itself.
The main purpose for constructing Centennial Hall in 1958 was for performances by the Handel Oratorio Society. This accounts for the dimensions of the stage floor and risers. The risers allow performers in the back of the ensemble to have sight lines with the conductor and to prevent any direct sound from being shielded by performers in front of them. A recommended 4 to 5 foot riser depth allows for adequate room for a seated instrumentalist, their instrument, and a music stand. The riser depth of Centennial Hall is only 3 feet (“Augustana” 3), which is enough room for vocalists, but not enough for instrumentalists (Long 664). Besides diminished mobility and comfort, some of the performer’s music is absorbed into the backs of the performers in front of them. The currently used remedy for this situation is temporary wooden step fillers that provide 1 to 2 feet more room. However, because the step fillers must rest on the following riser step, that next riser will allow less room for performers, thus rendering it ineffective. Also, these fillers are rather unsightly to the audience. An appropriate solution would entail permanently extending the risers another foot. However, this limits the amount of stage space available and risks covering the piano lift. Reducing the number of risers from 6 to 5 would compensate for this adjustment. With these changes, ensembles would be more comfortable when performing and more sound would be projected into the audience.
Experts in the field of architectural acoustics stress that the reduction of ambient and mechanical noise should be the primary concern for redesigning concert halls. Noise is extraneous, undesired sounds that interfere with an audience’s ability to listen to the music (Rossing, Moore, and Wheeler 703). The human hearing system is incredible in that it subconsciously uses selective hearing to filter out background noise from that in which we want to listen. The whirr of a ventilation unit, the hum of overhead lights, the clanking of hot water pipes are all noises that rarely register in our hearing because they are filtered out as unimportant, but even the slightest noises can negatively limit the amount of music heard. A musical composition with sections as quiet as a whisper can be as climactic as any grand fortissimo. If there is a slight amount of noise in the hall during the performance, the performers must play louder in order to compensate being masked. The presence of background noise narrows the useable dynamic range of the performers.
In 1979, acoustical engineer Lawrence Kirkegaard came to Centennial Hall for a basic analysis of the hall. The most significant flaw that he found was that noise levels produced by machinery were alarmingly high. Measurements on stage and in the audience indicated that noise levels exceed what is acceptable for proper listening of concerts. It was primarily suspected that the excess noise was coming from ductwork in the air conditioning system (Kirkegaard 1-2). Air flowing through the metal ducts causes vibrations that rattle the ductwork, the catwalk, and roof. Air flow in the system also creates turbulence that masks quiet sounds. Special lining can be placed inside the duct that reduces vibrations and turbulent noise (Long 481). Another noise source found by Mr. Kirkegaard was the exposed mechanical room underneath the stage (Kirkegaard 2). Operating machines pass noise into the audience through a grille in front of the stage. Relocating the mechanical room to somewhere not connected with the hall would be ideal in eliminating vibrations. Transplanting that large amount of machines would be difficult, so isolating the machines through acoustical shielding and vibration reducing materials would be beneficial (Long 451-453).
With the advent of new understandings of acoustics, we are more adept to ascertain why a concert hall sounds the way it does. When audiences and performers become frustrated by acoustical flaws, corrective measures can be implemented after analysis of the hall’s architectural properties. Based on the current acoustical situations of Centennial Hall, my above recommendations would create a better environment for enjoyable music performances.
Sources
“Augustana Auditorium.” Blueprint. Childs & Smith Architects and Engineers. 18 Dec. 1957.
Beranek, Leo. Concert Halls and Opera Houses: Music, Acoustics, and Architecture. 2nd ed. New York: Springer-Verlag, 2004.
Brooks, Christopher C. Architectural Acoustics. Jefferson: McFarland & Company, 2003.
Kirkegaard, R. Lawrence. “Re: Centennial Auditorium, Augustana College.” Letter to Glen Brolander. 3 April 1980.
Long, Marshall. Architectural Acoustics. Elsevier, 2006.
“Centennial Hall.” Chart. Prestley and Prestley. Ca 1981
Rossing, Thomas D. Moore, F. Richard. Wheeler, Paul A. The Science of Sound. 3rd ed. San Francisco: Addison Wesley, 2002.
This is why we can't have nice things!
For many years the concept of acoustics, the study of sound and its behavior in physical environments, has been considered a “black art” - a science filled with as much guessing and mystery as actual computations (Long xxv). Fortunately, advancements in technology and acoustical research over the past few years have opened many possibilities to improve our sounds and music. In the realm of architectural acoustics, concert halls and performance spaces are being designed to create the ultimate experience for audience and performer alike. However, many halls were built prior to this scientific revolution, making it expensive and complicated to renovate the space for better acoustical properties. Centennial Hall at Augustana College is one such hall. Centennial Hall has several architectural flaws that diminish the positive acoustical properties of the space, but measures can be implemented to correct these problems. In this paper I will identify the acoustical properties critical to a concert venue, discuss how Centennial Hall corresponds with these properties, identify other acoustical problems of the hall, and make recommendations for ways to improve the acoustical conditions in Centennial Hall.
In order to analyze the acoustics of Centennial Hall, we must begin by defining some of the important elements that constitute the acoustics of a concert hall. First, there is reflection and diffusion of sound waves, which determines the reverberation of the room. Next, sound absorption defines how the sound will behave when it encounters a surface such as a wall or ceiling. Also, the general shape and size of the performance space will factor into the distribution of sound to the hall.
Sound travels away from its source as a wave, usually omni-directional as in strings or woodwind instruments, or projected by means of a bell as in brass instruments or the human voice. One way to visualize sound waves is rays traveling from the sound source and interacting with surfaces they hit (Brooks 49). Reflection occurs when the sound encounters a surface such as a wall or ceiling and is redirected. Flat walls reflect sound like a mirror. Concave surfaces reflect in a way that refocuses the sound at a different location. Convex and rough surfaces scatter the sound all about the room (Rossing, Moore, and Wheeler 526).
The human ear senses not only the direct path of sound from the source, but also reflections that may have bounced off of multiple surfaces before reaching the listener. A large time difference between the direct sound and its reflection causes an echo effect in which the listener perceives the reflections as separate events (Brooks 19-21). In a good concert hall, reflections reach the listener before the previous ones decay which is perceived as one continuous sound; an effect called reverberation. The time it takes a sound to decay to silence after the source ceases is the reverberation time (Rossing, Moore, and Wheeler 529-531). Several factors such as hall size, shape, and construction can affect the amount of reverberation in a room. A room is considered “live” when the reverberation time is longer; around 2 seconds is ideal for concert halls. Too much reverberation can make music sound muddled while too little reverberation can make music sound dry and without warmth (Brooks 21).
Another behavior of sound waves is diffusion. This phenomenon occurs when certain shaped surfaces reflect the sound waves all about the room. Effective diffusion will eliminate echoes and uniformly distribute sound throughout the hall referred to as “envelopment.” Envelopment is “the impression that the sound is arriving from all directions and surrounding the listener.” (Long 681). Many surfaces can act as diffusers, such as ceiling coffers, pillars, statues, and geometric shapes placed throughout the hall that are designed to diffuse sound. However, modern architecture has drifted away from these ornamentations, eliminating critical diffusive properties of concert halls (Brooks 129).
Sound absorption is another key acoustical property in concert hall design. Only a certain amount of a sound wave’s energy reflects off a surface. Some of that energy is “absorbed within the material” of that surface. Varying materials have different absorption coefficients, the quantified amount of sound lost to the material (Long 249-250). Non-porous materials such as glass, wood, and painted concrete have small absorption coefficients, meaning they reflect back much of the sound. Porous materials such as cloth and carpeting have high absorptions and reflect back little of the sound (Rossing, Moore, and Wheeler 531). Even the air that the sound travels through will absorb some of the sound energy. This can be varied based on relative temperature and humidity (Brooks 19). Control of these environmental conditions requires heating, ventilation, and air conditioning systems which create other barriers in the quest to achieve good acoustics.
One more property of acoustics necessary for this analysis is the size and shape of the concert hall. The four most common shapes of concert venues are: rectangular or “shoebox;” diamond, in which the side walls expand out in the middle; fan, in which the side walls expand out to the rear of the room; and horseshoe, in which the side walls curve into the rear wall. Shoebox halls are considered to be the best sounding halls because sound reflections create an ideal musical envelopment for the audience, although other shapes have also proven successful (Long 658). Hall size is important in determining reverberation and loudness. The volume per seat of the hall generally decreases as the number of audience seats increases. The average number of seats in halls with good acoustics is around 1750 to 2200 and 250 to 275 cubic feet per seat. New designs for concert halls are increasing the seating capacity because more seats mean more revenue to pay for the hall’s costs (Long 656-658). The larger size of these halls may make their acoustics less controllable or predictable.
Now that some basic knowledge of architectural acoustics and parameters of an ideal concert venue are established, I will explain how Centennial Hall exhibits these properties and compares to the desired model. Centennial Hall is fan shaped. The width on stage is 75 feet and the side walls extend out to a width of 114 feet at the back wall (“Augustana” 3). Because of the expanding distance of the lateral walls, sound reflections will favor the sides and rear of the auditorium. The center seats receive fewer reflections from the walls and must rely on those from the ceiling (Beranek 28). With an average height of 23.5 feet in the front and middle of the audience, the highest point in the hall, audience members in the center section will receive sound reflections late (“Augustana” 6). When a performer on stage plays a note, the time difference between the direct sound and the first sound reflection will be so great that the listener in the center section may hear an echo instead of reverberation. This can be solved by lowering the ceiling in that section or by installing overhead sound reflectors called “clouds” (Brooks 81). With a 1600 person seating capacity and about 200 cubic feet per seat, an increase in volume would help regulate loudness and improve reverberation time. This can be accomplished by raising the ceiling at the back of the auditorium where there is unused catwalk space. Nothing can be done concerning the general shape of the room unless the hall is demolished and rebuilt, in which case a shoebox shape would be preferable. Therefore the acoustics must be adjusted by other means.
The current diffusive properties of Centennial Hall are minimal. Along the first 25 feet of lateral walls there are floor to ceiling pegboard prisms. These diffusers also completely line the rear wall. With the minimal degree that these devices protrude into the listening space, they have little effect on the direction of sound reflections. The ceiling of the hall also offers no diffusion. Placing diffusive objects along the walls and suspended from the ceiling would alleviate this situation. Some such objects would include: protruding geometric shapes like boxes and pyramids, convex surfaces, and coffered wells that resemble empty picture frames (Long 726). With more diffusion, the sound filling the audience space will distribute more evenly and listeners will experience more envelopment from the music.
The surface material of these pegboard diffusers produces several negative acoustical properties. The holes in the pegboard are large enough that a significant amount of bass frequencies are attenuated behind the paneling. Also, the hard, wooden surface of the board reflects much of the higher frequency sounds of an ensemble, creating a harsh and over-brilliant sound (Rossing, Moore, and Wheeler 741). With these two effects occurring, audiences perceive music to be without “warmth” or “tonal color” (Long 654). Adding materials such as carpet and drapery absorbs high frequency sounds and reduces acoustical glare. An adjustable drapery system that covers the rear wall would accomplish this and also provide a means of altering reverberation time for different musical works and ensembles (Rossing, Moore, and Wheeler 540).
A common complaint among those who perform in Centennial is that they cannot hear one another on stage. Because the stage is so wide, diffusion is critical for performers to hear other members of the ensemble. However, the only significant diffusive objects around the stage area are the pipes of the organ. Not even the ceiling above the stage will offer useful sound reflections to the ensemble members because it is too high. Performers in Centennial compensate by ignoring what they hear and relying on the conductor for issues of balance and blend. Ideally, a permanent band shell would reflect sound back at the ensemble so they could make musical adjustments based on listening. The shell would also project a larger amount of music toward the audience; bass sounds in particular (Long 720-721). Small, moveable shells are currently used to adjust the ensemble sound, but their size and limited number cannot encompass a full orchestra or band effectively. The best solution is to construct a permanent band shell that covers the whole stage and reflects the ensemble’s sound back onto itself.
The main purpose for constructing Centennial Hall in 1958 was for performances by the Handel Oratorio Society. This accounts for the dimensions of the stage floor and risers. The risers allow performers in the back of the ensemble to have sight lines with the conductor and to prevent any direct sound from being shielded by performers in front of them. A recommended 4 to 5 foot riser depth allows for adequate room for a seated instrumentalist, their instrument, and a music stand. The riser depth of Centennial Hall is only 3 feet (“Augustana” 3), which is enough room for vocalists, but not enough for instrumentalists (Long 664). Besides diminished mobility and comfort, some of the performer’s music is absorbed into the backs of the performers in front of them. The currently used remedy for this situation is temporary wooden step fillers that provide 1 to 2 feet more room. However, because the step fillers must rest on the following riser step, that next riser will allow less room for performers, thus rendering it ineffective. Also, these fillers are rather unsightly to the audience. An appropriate solution would entail permanently extending the risers another foot. However, this limits the amount of stage space available and risks covering the piano lift. Reducing the number of risers from 6 to 5 would compensate for this adjustment. With these changes, ensembles would be more comfortable when performing and more sound would be projected into the audience.
Experts in the field of architectural acoustics stress that the reduction of ambient and mechanical noise should be the primary concern for redesigning concert halls. Noise is extraneous, undesired sounds that interfere with an audience’s ability to listen to the music (Rossing, Moore, and Wheeler 703). The human hearing system is incredible in that it subconsciously uses selective hearing to filter out background noise from that in which we want to listen. The whirr of a ventilation unit, the hum of overhead lights, the clanking of hot water pipes are all noises that rarely register in our hearing because they are filtered out as unimportant, but even the slightest noises can negatively limit the amount of music heard. A musical composition with sections as quiet as a whisper can be as climactic as any grand fortissimo. If there is a slight amount of noise in the hall during the performance, the performers must play louder in order to compensate being masked. The presence of background noise narrows the useable dynamic range of the performers.
In 1979, acoustical engineer Lawrence Kirkegaard came to Centennial Hall for a basic analysis of the hall. The most significant flaw that he found was that noise levels produced by machinery were alarmingly high. Measurements on stage and in the audience indicated that noise levels exceed what is acceptable for proper listening of concerts. It was primarily suspected that the excess noise was coming from ductwork in the air conditioning system (Kirkegaard 1-2). Air flowing through the metal ducts causes vibrations that rattle the ductwork, the catwalk, and roof. Air flow in the system also creates turbulence that masks quiet sounds. Special lining can be placed inside the duct that reduces vibrations and turbulent noise (Long 481). Another noise source found by Mr. Kirkegaard was the exposed mechanical room underneath the stage (Kirkegaard 2). Operating machines pass noise into the audience through a grille in front of the stage. Relocating the mechanical room to somewhere not connected with the hall would be ideal in eliminating vibrations. Transplanting that large amount of machines would be difficult, so isolating the machines through acoustical shielding and vibration reducing materials would be beneficial (Long 451-453).
With the advent of new understandings of acoustics, we are more adept to ascertain why a concert hall sounds the way it does. When audiences and performers become frustrated by acoustical flaws, corrective measures can be implemented after analysis of the hall’s architectural properties. Based on the current acoustical situations of Centennial Hall, my above recommendations would create a better environment for enjoyable music performances.
Sources
“Augustana Auditorium.” Blueprint. Childs & Smith Architects and Engineers. 18 Dec. 1957.
Beranek, Leo. Concert Halls and Opera Houses: Music, Acoustics, and Architecture. 2nd ed. New York: Springer-Verlag, 2004.
Brooks, Christopher C. Architectural Acoustics. Jefferson: McFarland & Company, 2003.
Kirkegaard, R. Lawrence. “Re: Centennial Auditorium, Augustana College.” Letter to Glen Brolander. 3 April 1980.
Long, Marshall. Architectural Acoustics. Elsevier, 2006.
“Centennial Hall.” Chart. Prestley and Prestley. Ca 1981
Rossing, Thomas D. Moore, F. Richard. Wheeler, Paul A. The Science of Sound. 3rd ed. San Francisco: Addison Wesley, 2002.
This is why we can't have nice things!