THE ART AND SCIENCE OF CONCERT HALL ACOUSTICS

 

V.       RESULTS

Many results can be extracted from all the data collected.  Here are results for six specific architectural features are presented and discussed.  Other results are reported elsewhere (SSESCO web page at  http://www.ssesco.com/~cna/Web/Acoustics/Web-Report/Project-Overview/Acoustics-Report-Overview.htm ).

V.1.           Test #1:  Proscenia

 

            The first item examined in this study was the accuracy of a modeling program in anticipating the answer of a simple problem with a known acoustical result.  I chose to model a simple, rectangular room with a stage area that is part of the same room.  Following this model, I tested two more versions of the same room, the only difference in each being a proscenium added between the stage and audience area.  A proscenium is a wall separating these two spaces with an opening for sound and visual images to come through.  Behind the proscenium walls is the “backstage” of a theater.  One of these modified tests had a square proscenium opening; the other had an arch.

            It is known that by taking an orchestra out of the room in which the audience is, the sound is not as good.  It is also known that if a musician plays straight into a wall, the listener on the other side of that wall will not hear the musician nearly as well as if the sound traveled directly.  Modeling tests using the ODEON program confirmed these anticipated responses, with some notable elaborations.

            In fact, proscenia should be avoided at all costs for symphonic productions.  An orchestra is usually too large to fit in the proscenium opening, and therefore, parts of the orchestra are playing from behind the proscenium walls.  In both models, the proscenium created a “shadow” where direct sound from some parts of an orchestra would not be audible (fig. 6).  The portions of the audience in this shadow would be relying on second and third reflections to hear those instrumental parts, and thus, the orchestra would not achieve the desired features of balance, blend and synchronization.  Music that is not supposed to be smooth would sound smooth, and the performance would lose its edge.  It is the difference between a definition of about 10% early sound and 70% early sound, though the seats could be within 2 feet of each other.  Proscenia should be reserved generally for operatic performances, where all the performers can fit within the opening. 

Fig. 6.  Simple hall with arched proscenium and source behind the proscenia wall.  Demonstrates the “shadow” where the audience would not hear direct sound.

 

The negative effect of a proscenium can be amplified by repeated beams whose goal is to provide visual continuity along with supporting the building.  This can be seen in the model of the Carmel-Sunset Performing Arts Center.  Each separate beam has a shadow behind it where the sound definition is poor, even if the source is within the opening of the proscenium.  This effect also leads to a splitting of the lateral reflections – people sitting two seats away from each other may get entirely different perceptions of envelopment (fig. 7).  This explains why this hall is undergoing significant renovations to improve the acoustics of the hall.

Fig. 7.  Carmel-Sunset Performing Arts Center with arched proscenium and repeated proscenia-like beams.  Demonstrates the splitting of the lateral reflections.

 

V.2.           TEST #2: DECOUPLED, SEMI-DECOUPLED, AND COUPLED CHAMBERS

            My next experiment was to model the new Gallagher-Bluedorn Performing Arts Center at the University of Northern Iowa.  The goal of this hall was to be fully adjustable to all types of music, thus, to be able vary the slope of the Decay Curve was quite important.  One way the acoustician, David Kahn, proposed to do this was by the use of a separate reverberation chamber.  The goal is that when the chamber is open, sound will enter it, bounce around, and seep back into the hall as part of the Reverberation Tail (fig. 8). The opening to this chamber could be fully open (coupled), partially open (semi-decoupled) or closed (decoupled) (Kahn 14). 

Fig. 8.  Gallagher-Bluedorn Performing Arts Center from side view.  Illustrates supposed action of coupled reverberation chamber.  Arrows represent movement of sound.

 

A more open a reverberation chamber can return the sound more immediately to the main hall, while a slightly open chamber will hold the sound for a longer time (Shulman 332).  However, I was curious as to whether this really makes a big difference in the sound.  Fig. 9 pictures the decay curves for the first .5 sec in the hall as in Fig. 8, with the opening half closed, and with the opening sealed.  Note the steep initial slope which indicates that the early sound is strong. 

Fig. 9.  Three graphs illustrating the early sound of the same hall with the reverberation chamber coupled, semi-decoupled and decoupled.

 

The early sound is virtually identical, though slightly longer at the lower and middle frequencies in the semi-decoupled hall.  It is the reverberation time, however, that really notes the difference in the halls.  In general, the trend does not work as Shulman suggested it should.  Rather, in the decoupled hall, the RT’s are at sufficient levels, but are much drier (i.e. shorter) than the RT’s in the semi-decoupled hall, which is still drier than the coupled hall.  The difference can be by as much as .8 sec – the difference between Classical clarity and Romantic reverberation.  It appears that the general theory of a coupled reverberation chamber works, but that it is necessary to have a wide enough opening to get some sound in before it can bounce around.  The reverberation chamber has thus demonstrated itself to be a useful feature to make multi-purpose halls more flexible to the genre of music a group is performing.

V.3.           TEST #3: CARPETED VERSUS CONCRETE AISLES   

            The hall I examined next was the Ted Mann Hall at the University of Minnesota in Minneapolis.  Also designed by David Kahn, the hall was very similar to that of Gallagher-Bluedorn.  However, upon speaking with some of the staff at the hall, I learned of a well-founded complaint that could be examined.  The floors of the entire hall are made of concrete, upon most of which sound absorbing chairs are mounted.  As a person walks around in the hall, however, an annoying “clack-clack” carries to all corners of the auditorium.  The aisles are the only exposed area of the floor; could they be covered in carpet without an adverse effect?  Carpet and concrete are opposites in the materials used for concert halls – and carpet is considered a sound killing material that should be avoided at all costs. 

            Simulating the comparison in the Ted Mann hall demonstrated that the differences were minimal.  Apart from normal variations, fig. 10 represents the floor grid receivers with matching values for EDT. 

Fig. 10.  Two grid responses demonstrating matching values in the EDT at the floor receivers.  The top grid represents a hall with concrete aisles, the bottom with carpet.

 

 

Fig. 11 demonstrates tiny differences in the values for definition.

Fig. 11.  Two grid responses demonstrating slight variations in the definition at the floor receivers.  The top grid represents a hall with concrete aisles, the bottom with carpet.

 

            This test seemed to prove that the substance the aisles were made of was inconsequential, however, I was not convinced.  This law of floor coverings has been so ingrained into the study of acoustics that how could such a simple test disprove it? 

I ran a test on a basic box hall with specifications as uncomplicated as possible.  Once again, the only difference between the two halls was the composition of the aisle material.  Once again, the difference between the two halls was barely noticeable.  Ray tracing provided one possible explanation for the lack of a difference in the results.  Very few of the reflections even hit the floor, and most of those reflect off of surfaces where the audience is seated (fig. 121).

Fig. 121.  Ray tracing of the basic hall used to test the impact of aisle material on hall acoustics.  Highlighted (red circle) are the only two reflections that even come close to hitting the aisles.

 

Still reluctant to accept this data, I referred to the Sabine equation for Reverberation Time; it is used to make rough estimates in mostly bare halls for the reverberation time.  It is defined as:

                        RT = 0.16 V/A  sec,

            When A is defined as:

                        A = ST a T + SR a R + SN a N + air absorption   m2  

ST is the acoustical surface area of the audience and orchestra, SR is the acoustical surface area of other miscellaneous reflective surfaces, and SN is the acoustical surface area of other miscellaneous absorptive surfaces.  a is the sound absorption coefficient for that surface – all values of a fall between 0 and 1 and represent how much sound energy that material will absorb at a given frequency (Beranek 436-7).  Because the halls are identical for all values except the aisles’ absorption coefficient, all of these values can be considered constant “X” with the exception of SAislesaAisles .  Therefore, the function can be simplified to:

                        RT = _    0.16 V      _

                                 SAislesaAisles + X  

           

            The SAisles can be estimated to be approximately 1%-2% of the total acoustical surface area.  If aAisles of carpet is completely absorptive and thus has an absorption coefficient of 1.00 (it actually falls closer to 0.80); and if the concrete is completely reflective and thus has an absorption coefficient of 0.0 (more like 0.02); than it is possible to realize how the simulated results were reached.  Aisles are a feature that at the full range of their influence will only change an answer 1%-2%.

            In conclusion, it is possible to say that this simulation has dispelled the myth of avoiding carpet.  Prudent use of carpet is acceptable for small areas in order to enhance the character of the hall, though large-scale use is not recommended.                    

V.4.           TEST #4: EFFECT OF DIFFERENT MATERIALS ON INSTRUMENT SOUND

            The next feature examined with the simulation software was the effect of materials known to absorb certain frequencies on the sound of certain instruments. This was in response to some interesting materials that have been used in various halls that should otherwise sound good, and don’t.  Such a study was a good test of the auralization technology.

            Three new materials were created for the simulation – each one absorbed a certain segment of the sound spectrum (low, medium or high frequencies).  Three identical simple box rooms were used, with all but the audience floor and orchestra covered in the material being tested.  As anticipated, the frequencies being targeted for absorption with each material were significantly less powerful, with lower EDT’s, RT’s and SPL’s.  The Clarity and Definition were remarkably higher for the frequencies being absorbed, as in Figure 12. 

 

 

 

 

 

 

 

Fig. 122.  Definition in hall for frequency absorption at the middle frequencies (500Hz, 1000Hz, 2000Hz).  Demonstrates how the frequencies being absorbed effect values of the hall.

 

            When auralized, these three materials had a marked effect on the sound of a violin.  For example, when the middle frequencies were absorbed out by the room, the listener did not receive them, and as a result, the sound was very unpleasant to the ear.  Without the bulk of the notes, the sound was tinny and discordant.  When the low frequencies were absorbed, the music lost its feeling of being grounded.  The sound was unpleasantly shrill.  When the high frequencies were eliminated, the sound mellowed, and was generally more pleasing than before.  The violin sounded closer to a viola or a cello.  The use of specially absorbent materials may be useful to balance a room with the reputation for over brilliance.  

V.5.           TEST #5: ROOM HEIGHT

            Most of the rooms tested had approximately equal height, and all of them were fairly tall (greater than 10m).  Not all halls are shaped in such a way, however.  Therefore, I decided a test on the effect hall height has on sound was in order.

            I created two rooms of equal volume by manipulating the boundaries of the simple test box as used in Test #4.  One hall was 13m tall, while the other was 7m tall.  I equalized the area that the audience would sit in, in order that the size of that large an area of absorption would not be a factor in the results.  All other factors in the tests were the same.

            The most profound effect room height had was on the lateral reflections.  Keeping the sound centered around the audience, in order for the music to sound as if it is coming from all directions, means there has to be boundaries.  A shorter room with the same volume spreads the sound out more, and does not effectively produce lateral reflections.  See figure 13.

   

 

Fig. 13.  The effect room height has upon lateral reflections.  Note the lack of reflections received the rear wall in short hall.

 

            This test seemingly demonstrates that it is beneficial to have an auditorium with good height.  However, this brings back the concept that the smaller the hall, the better the sound.  A short hall may be better, if the volume does not have to be equal.

V.6.           TEST #6: BUBBLE HALL

            Based on my research and the direction I believed acoustics was taking, I attempted to anticipate an ideal shape of a concert hall that has not been used yet.  For example, hall builders learned that fan shaped hall made for poor acoustics.  They moved on to shoebox hall shapes and are now in a phase of reverse fan shaped halls (fig. 14)

Fig. 14.  The trend towards rounder rooms, as in the reverse fan shaped hall popular in recent years.

 

            Additionally, the shape of shells has recently reflected a short fat cone with the orchestra at the source side.  Using these trends, I anticipated that halls are evolving towards a more spherical shape.  Therefore, I designed a “Bubble hall”(fig. 15) that would reflect the next step in the evolution.

Fig. 15.  Bubble hall: the next generation of concert auditoria?

 

            This design, however logical, is not a good design for a concert hall.  The lateral reflections created are amazingly good, however, the hall has the problem of being very echoey.  Imagine yourself in the dome of a large cathedral – that is an experience similar to what Bubble hall would create.

            In an attempt to correct the deficiencies with Bubble hall, I designed six other versions of it, including a hall with an elliptically shaped plan view.  Several of these modifications improved Bubble hall, but none to the extent necessary to match the great shoe box designs.  It is possible that with Boston Symphony Hall, Concertgebouw, and Grosser Musikvereinssaal that hall designers have found the best possible combinations of acoustical features.