Optimizing loudness, clarity, and engagement in large and small spaces

Toronto, Canada International Symposium on Room Acoustics 2013 June 9-11 ISRA 2013 Optimizing loudness, clarity, and engagement in large and small spaces David Griesinger (dgriesinger@verizon.net) David Griesinger Acoustics 221 Mt Auburn St #504 Cambridge, MA 02138 ABSTRACT It is vital in building both large and small spaces to understand that the (unfortunately rare) designs which are successful for large spaces are not successful when scaled to smaller sizes. But a few fine recital halls, practice rooms, rehearsal spaces, and halls of all sizes exist. We can learn a lot from them, particularly in the light of recent work on the neurology of hearing. This talk will demonstrate and present some hard-earned lessons leading to solutions for common problems. Successful solutions include careful attention to shape and size, absorption in critical areas, electronic acoustics, and the Arrau diffuser. 1 INTRODUCTION People go to halls for many reasons: to see and be seen, to get away from it all, to have a chance to relax and contemplate something not closely related to their work or family, to see beautiful architecture, and perhaps lastly, to hear something beautiful. For these purposes music halls must compete with museums, theatres, cinemas, and even a quiet living room with a radio. Where does sound fit in? Currently by far the most popular sound entertainment is recorded music, often reproduced through earbuds. Excluding restaurants, the most popular going-out diversion is cinema. What are the properties of cinema sound, and what can we learn from it? Recorded music and cinema sound have evolved to match the needs of their audience, and the sound vastly different from the sound in current concert venues. The sound from the HD broadcasts of the Los Angeles Philharmonic in a modern theatre is strong, clear, and enveloping, due to just the right amount of reverberation, likely added by an electronic device the author designed. The same is true of the popular HD broadcasts of the Metropolitan Opera. The idea that a cinema or drama theatre audience wants a peaceful place to relax is proven false by the sound you hear in them. Drama theaters are acoustically dry and intimate. Cinema sound is reproduced through highly directional linear phase speakers in a hall carefully designed to minimize early reflections. Speech or dialogue is reproduced through a single loudspeaker in the centre of the screen regardless of where the actor appears on the screen. Earbuds reproduce sound without any reflections at all. Why is this sound mandatory? Because cinema directors, drama directors, and music producers know that their audience wants is engagement, through sound that focuses the mind on the drama or the musical power of the performance. Speech and music evolved to create engagement sonically. The physics of how sound triggers engagement has been covered by previous papers by the author. Reflections, particularly too many early reflections, cause this attention-grabbing power to disappear. 1

The author is a musician and frequent concert-goer. He has performed in and made commercial recordings in venues of all types. More than 25 years ago he started recording the sound he was hearing while listening to concerts, currently with probe microphones on his eardrums. He has recorded sound from seats in all the venues discussed in this paper, and has made binaural measurements in many of them. The suggestions in this paper come from the author s experience listening, and re-listening to these recordings, and trying to understand why that sound arises. He is happy to let you hear them. 2 A BRIEF SUMMARY OF SOUND PERCEPTION 2.1 Clarity The author defines sound as clear when it sounds close to a listener and has the power to grab attention. When sound is clear in this sense it is easy to parse, and because the parsing process takes little time, there is enough working memory to store the information for later retrieval. With music sonic clarity enables us to localize and follow several musical lines at the same time, and to choose to concentrate on any one of them. With speech, clarity allows us to listen to any one of several simultaneous conversations, and if our name is contained in a conversation we are not attending to, our attention instantly and involuntarily switches. This kind of clarity depends on the phase alignment of the upper harmonics of complex tones; the tones that comprise the vowels of speech and the sound of nearly all musical instruments. The phase alignment is lost when there are too many early reflections, and when the same soloist or talker is reproduced through multiple loudspeakers. So to create excellent acoustics in performance venues one must pay careful attention to the timing and total strength of reflections in the first 80 milliseconds. Some reflections in this time range are clearly beneficial, but it is all too easy to have too many. In this case the acoustician should reduce the strength of the earliest, as these are both the strongest and the most detrimental to clarity. The author has presented ways of predicting and measuring when there are too many, and when there are not [2]. Clarity as defined here is a property of harmonics mostly above 1000Hz. Reflective structures can be sized appropriately to direct troublesome high frequency reflections away from areas where they are too strong, and into areas where they are too weak. For example, coffers in a ceiling or on side walls can direct higher frequencies back to the stage where musicians will appreciate them, and away from the rear of the hall, where they are problematic. 2.2 Loudness The most important sonic property after clarity is loudness. When Boston Symphony Hall opened the critics complained about the poor acoustics, and Sabine, distraught, burned many of his papers and contemplated suicide. The hall was larger than the old Music Hall, and the sound lacked the power of the old hall. The problem was solved by enlarging the orchestra, and now, with up to 10 bass viols and seven or more French horns, the BSO is awesome. Yet small ensembles in small halls 500 seats or less can be too loud when they play with modern instruments. What to do? Classical acoustics predicts that the loudness of a sound from a given excitation is inversely proportional to the total absorption. The prediction is based on the assumption that the sound absorption by a surface is independent of where that surface is located. The assumption is 2

clearly not true for first-order reflections, as these are stronger at surfaces close to the sound source, and therefore absorb more energy than surfaces further away. But the assumption is also untrue for late reverberation, which decreases in level even in a hall with uniform absorption by the decay rate, or about 2dB for every 10 meters for a RT of 2 seconds. If a hall is too loud, put absorption near the performers. This will reduce the level and improve clarity with a minimal impact on the reverberation time. If a hall is not loud enough, increase the mean free path between absorbing surfaces as much as possible. For a large hall, this means maximizing the number of non-absorbing parallel surfaces, such that sound will reflect many times before hitting the absorptive audience. Shoebox halls, if they do not cover the side walls with people, do this well. So called surround halls like Disney Hall do this badly. Strong first order reflections are directed down into the audience, ostensibly to increase the sound strength, but exactly the opposite occurs. This maximizes the absorptive power of the audience, minimizing G and G-late. The boost in loudness assumed to come from first order reflections into the audience almost never exists. I have impulse responses taken in many halls. If I delete the strongest first order reflections and play music through them there is no difference in loudness. The loudness and the total energy is almost all in the build-up of multiple order reflections and reverberation. But eliminating the first order reflections increases clarity in the rear of a large hall as the direct sound becomes more dominant in the first 80 milliseconds. 2.3 Reverberation and Envelopment When a performer plays an instrument the direct sound from that instrument is very strong compared to whatever reflected energy may be present. The strong direct sound inhibits their ability to hear the reverberation for at least 80ms. The same thing happens for the audience. When the direct sound is strong, as it is in the front of a hall or on a recording, the onsets of notes inhibit the perception of reverberation for a similar time, such that reverberation is only audible in the gaps between notes. I studied this phenomenon many years ago, and found that the most audible segment of reverberation in recordings of orchestral music occurs 120 to 160ms after the direct sound. Bradley and Soulodre s measure for envelopment, G-late, measures the sound strength in a hall 80ms and more after the direct sound. But these observations depend on the assumption that the direct sound can be separately perceived by the brain, and this depends on clarity. When the direct sound is not separately audible the reverberation IS the direct sound, and the brain hears only one type of sound. It is reverberant, but not enveloping. It is also muddy. The instruments that created the sound are not individually localizable with eyes closed, and often impossible to separate one from another. The sound is perceived as coming from the front. This is the sound you hear in most concert halls more than half-way back on the floor. 2.4 Reverberation and Musicians A major conundrum in hall design is that the sound that is optimum for the audience is not the sound that is optimum for the musicians. When I am in the audience I want to be able to hear each instrument in an ensemble clearly, and be able to localize them with eyes closed. For me the ideal venues for such performances are the spaces for which they were written, medium sized chambers with a lot of fabric on walls, floors, and people. Modern venues which work well resemble theatres more than halls. For example Jordan Hall in Boston and Sanders Theater in Cambridge, renowned for their chamber music acoustics, are semi-circular in plan, with a single 3

large balcony and high ceilings. The high ceilings and bare walls above the balconies provide the long mean free path that maximizes late reverberation, and the close audience seating provides the clarity. But musicians want to hear enough late reverberation to be able to judge how their instrument sounds in the audience, and to do this they need late reverberation. It is difficult to play when a space is too dry, which is why they often prefer to rehearse in small highly reflective studios. This is especially true for string players and singers. Pavarotti once cancelled a performance when the reverberation processor which I had designed for his stage monitor failed. Optimizing the acoustics for both the audience and the performers means keeping the early sound strength under control while maximizing the late reverberation. For a long time I thought that the only possible solution in a small hall was to absorb enough early reflections that the great majority of the audience seats had excellent clarity, and then to use electronic enhancement to provide the late reverberation that made the sound beautiful to both audience and performers. But there has recently been an invention that provides another option the Arrau Diffuser. 2.5 The Arrau Diffuser Figure 1: The Arrau diffuser as installed in the concert Hall in St. Gallen, Switzerland, and in Liceau, near Barcelona. It consists of vertical panels approximately 1 meter square suspended in a grid above the orchestra platform. The panels increase the mean free path above the orchestra, which decreases the loudness of the early reflections, increases clarity for orchestra and audience, increases RT, and increases G. In Liceau the installation uses a double layer of panels and covers the entire ceiling of a rehearsal room. Higini Arrau presented a paper on this diffuser design at the IOA conference in Dublin, and I was so impressed that I offered to re-write his submission to Acta Acustica, which is now published [1]. The measured and subjective benefits of his design are impressive. The version installed in an orchestral rehearsal room in Liceau, near Barcelona, increased the RT of the room nearly a factor of two, while improving the clarity on stage. Orchestral conditions are said to be nearly identical to the conditions on stage in the nearby hall. The Arrau diffuser works by semi-randomly increasing the path lengths sound must travel before encountering a surface, effectively raising the mean free path. If the surface encountered after the diffuser is reflective, like a ceiling, the sound must wind its way back through the diffuser, doubling the increase in mean free path. The result is something previously thought (at least by me) to be impossible: increasing the effective volume of a space without increasing the external dimensions. 4

3 A FEW EXAMPLES 3.1 Boston Symphony Hall (BSH) BSH is renowned as a venue for symphonic music because it gets so many important features right. The side walls are sufficiently narrow and unobstructed that a lot of sound can bounce back and forth without being absorbed. The rear wall is similarly reflective, so once it is encountered the sound can bounce back the front without encountering an absorptive surface. The diffusion built into the side walls will affect frequencies above 1000Hz, but not necessarily frequencies below, once again maximizing the late reverberation strength (but not necessarily the reverberation time) at low frequencies while improving clarity. The coffered ceiling is a scattering surface for upper mid frequencies, and a retro-reflector for high frequencies. It reduces the strength of early reflections in the rear of the hall without shortening the reverberation time. It is also open for lighting and ventilation for more than 30% of the surface. Medial reflections those that come from in front of or above a listener are the most disturbing to clarity, and this ceiling mitigates them. Another big difference between BSH and lesser halls is the design of the stage house. The stage in BSH is relatively wide and shallow, with steeply slanted sides and a very high slanted ceiling. As a result all first-order reflections go directly to the hall. The stage acts as part of the hall, like the stages in the Grossemusikverreinsaal in Vienna or the Amsterdam Concertgebouw. Reverberant stage houses are uniformly bad for audience clarity. The clarity of all the instruments except those in the front rows is gone before the sound even enters the hall. I have been on the stage at Avery Fisher during a rehearsal, and found the sound loud and extremely confusing. I have performed on stage with the Boston Symphony, and the sound is entirely different. It is balanced, clear, and musical. This is just what you need to achieve clarity over a wide range of seats. If you visit BSH, be sure to get a ticket at the top price. There are an extraordinary number of great seats in this hall, and the ticket price reflects the sound quality. The extra few dollars is well worth the money. The sound is excellent nearly everywhere on the floor until you get to about row AA, where suddenly all the instruments blend together into a fuzzy ball of sound. It is loud and pretty in its way, but I find the lack of clarity disturbing. Installing some 1 absorbing panels on the side walls below the first balcony would probably extend the range of good seats to under the first balcony in the rear of the floor. Seats in the front of the first balcony are wonderful, even all the way in the rear of the hall. The first lateral reflections from the side walls are blocked by the underside of the side balconies in these seats. Clarity, loudness, and superb envelopment are the results. 3.2 A Small hall in Cambridge with a Reflective Stage 5

Figure 2: Left - A small 300 seat hall in Cambridge after the installation of 2 thick absorbing panels around the bottom of the stage. Previously the sound on stage without the panels was loud and it was difficult for players to hear each other. The sound in the hall lacked clarity in spite of a reverberation time of 1 second. After the panels were installed the sound improved dramatically. Right a drama theater used for chamber music. We had the luxury of spending a week in the hall shown in the left picture with a number of musicians, adding absorption or reflectors in different areas while recording and measuring the sound in the hall. Placing the panels as seen above was the clear winner. We also tried various types of shell around the performers, but the effect of the shell was to add strong, prompt early reflections, and the sound was markedly worse. Not shown in the picture is that the ceiling is deeply coffered, such that seats further back than the fourth row do not receive a first-order ceiling reflection. Seats in the first three rows do receive it, and the clarity is poor, in spite of the seats being closer to the performers. 3.3 A Drama Theater Used for Chamber Music The right panel of figure 1 shows a fan-shaped drama theater with a high peaked ceiling and an absorbing back wall that is also used for chamber music. There had been a few complaints about the lack of reverberation in this space. I found the reverberation time to be short, below one second, but the high peaked ceiling trapped enough sound to give a sense of space. I found the sound to be a little dry but very clear, and managed to convince the concert organizer to ignore the complaints. I greatly enjoy concerts there. The acoustics are comparable to the original halls where this chamber music was first performed. Notice that there are curtains behind the performers. They absorb the reflection from the stage wall, which is the usually the strongest and earliest of the reflections to reach a listener. It is consequently the most disturbing to clarity. Vaudeville theaters uniformly used curtains in the stage house for precisely this reason they wanted speech and music to be clear, intelligible, and engaging. It is unfortunate these stage furnishings have gone out of fashion. But musicians like more late reverberation than they get in this hall. It would not be expensive to add a bit of late reverberation with an electronic enhancement system. The sound would be more beautiful and just as clear. Our experience with these systems in opera houses and halls around the world has been very positive. But you have to be careful to enhance only the late reverberation or clarity will suffer. When we added a system to the Berlin Staatsoper it ran for three years before a professional sound engineer realized that a hall of these dimensions could not possibly sound that good, and the secret was out. The critics had said the improvements were due to Barenboim s conducting, and the system ran in every performance until the hall was closed for renovation. What the new hall will sound like is anyone s guess. Adding fabric to a stage can be a clear winner. One of the continuing problems with Carnegie Hall in New York is the lack of the large proscenium curtain which covered the upper part of the opening which was removed during renovations. This curtain absorbed the high frequency reflections from behind and above the musicians while letting the low frequencies through, giving the hall clarity and warmth. The same was done to our beloved Jordan Hall at New England Conservatory, and the change was noticed immediately by our local conductors. But Jordan, at least if musicians play in front of the proscenium, is still one of the best 1000 seat halls I know. Williams Hall at New England Conservatory holds about 300 seats, is almost cubic in shape, with a single balcony and highly reflective floor, side walls and ceiling. The reverberation time is over 1.4 seconds. However the stage is completely covered with thick 6

curtains. Almost half the sound that would otherwise cause muddiness in the hall is absorbed, and the sound in the hall is both clear and reverberant. 3.4 A New Chamber Music Hall and a New Concert hall Figure 3: Left frame: A new chamber music hall in the Gardner Museum in Boston. It seats about 300. It is a cube in shape, with the audience surrounding the musicians in two rows of chairs on the floor, and in a single row of chairs on three balconies surrounding the musicians. I leaned over the balcony rail to take this picture. Sitting back in the seat I could only see half the musicians, and the sound from them was blocked by a glass window in front of my body. With this configuration most of the audience is behind the musicians. I was a reviewer for the opening concert, which was of a piano trio. See http://classical-scene.com/2012/01/24/claremont-trio-atisgm/ Sitting in the first balcony in a perfect seat in front of the group I was almost unable to hear the cello. If I leaned over the balcony rail the sound was better, but very few other audience members chose to do this. Another reviewer in a seat somewhat behind the group heard no strings at all. Right Frame a new concert hall in Helsinki. More than 40% of the seats in this hall are behind the orchestra, and some of these are so high they are unlikely to be comfortably occupied. The sound is dry in most seats. I heard a Brahms symphony in a seat somewhat to the right of where this picture was taken. The sound was clear, moderately reverberant and somewhat gentle. It was nowhere near the Brahms I recently heard in BSH, which was powerful. My hosts said this was the best seat in the house. Who were these halls designed for? In the one on the right the design was clearly for people who want to see interesting architecture, the music be damned. The sound is reportedly balanced on the floor, but the floor holds less than half the seats, and half of them are behind the musicians, an odd place to sit if you want to hear the music in balance. The hall on the right has some decent seats in the middle of the sections in front of the orchestra, although the reverberance is low. Disney hall in Los Angeles is similar, and both halls are to some degree modelled after the Berlin Philharmonic. I heard Stravinsky s Firebird in Disney in what should have been a great seat in the centre of the first orchestra section. The music was relatively clear, but not loud. One week later I was in Berlin checking the acoustics at the Staatsoper unter den Linden. I heard Stravinsky s Rite of Spring from the centre of the first balcony, and measured the sound pressure as 6dB louder than Disney. The sound in Disney was nice. The sound in Berlin was gripping and powerful. It could have started a riot. Tourists flock to the Berlin Philharmonic and to Disney Hall to see the architecture. The good seats are all occupied by subscriptions. Who is going to fill the seats behind the orchestra in Helsinki? 7

4 CONCLUSIONS If your goal is to fill the hall with music lovers, or to make music lovers and repeat attendees from people who get the courage to buy a ticket, your goal should be to make as many seats as possible have excellent clarity, loudness, orchestral balance, and enough late reverberation to be audible. Clarity is achieved in smaller halls by limiting the strength of the earliest reflections and by bringing the audience as close as possible to the performers. Smaller halls by necessity have reflections that arrive earlier than they do in large halls, and clarity is quickly lost. If in doubt emulate a good drama theater and raise the ceiling as high as possible. Small halls up to 1000 seats - tend to be too loud and muddy, and the smaller they are, the muddier they get. To bring the audience as close as possible a semicircular format and a single balcony can be very helpful. Absorption close to the performers is your friend. Absorb as much of the sound from behind and above the performers as you can, leaving the rest of the hall reflective. If the hall is rectangular add a bit of 1 absorption to the side walls where sound reflects laterally into the rear of the audience. Done carefully the RT will not decrease very much, and clarity will be improved. Avoid reflective stages or stage shells they will make the sound louder and less clear. If it is essential to have the reverberation time of a large hall do NOT take out all the absorption or make the hall long and narrow. Turn the hall on its end bring the audience in close and make the ceiling high with surfaces that increase the mean free path. The least expensive option is electronic enhancement. Talk to me about that. If nothing else will work, try the Arrau Diffuser. Large halls can have a high G and sufficient late reverberation if there are unobstructed surfaces where sound can reflect many times before encountering absorption. If this is not possible, electronic enhancement or the Arrau Diffuser will help. The very best halls are shoeboxes because they provide late reverberation. But many shoeboxes suffer from poor clarity, especially if they have enclosed stage houses. In this case some of the muddiness can be reduced by a proscenium curtain. Loss of clarity in the rear of a large hall can be reduced by using properly sized reflectors to direct high frequencies away from the seats in the rear. For example, in Davies Hall in San Francisco the plastic panels over the orchestra direct high frequencies down into the stalls, reducing the muddiness in the dress circle and balconies. But the sound on the floor is loud and harsh. If the reflectors were smaller and directed sound to the orchestra where it would be absorbed the sound in the stalls would be better. ACKNOWLEDGMENTS We gratefully acknowledge the continual encouragement and support of Dr. Leo Beranek, and all the work he continues to do to improve music venues. REFERENCES 1 Higini Arrau-Puchades, Increasing the Acoustic Volume of Performance Spaces without Altering the Internal Dimensions Acta Acustica Volume 98, Number 2, March/April 2012, pp. 309-316(8) 2 David H. Griesinger, What is Clarity and how can it be measured presented at the joint ASA-ICA meeting in Toronto, June 2-7 2013. 8