Room Acoustics Qualities PDF

Room Acoustics Room Acoustics Qualities Impulse Response In a large room, one impulse sound, like that generated with a cap gun or popped balloon, may pass a listener 8,000 times per second after it is abruptly stopped. The mapping of these sound front arrivals over time is called the impulse res...

Room Acoustics Room Acoustics Qualities Impulse Response In a large room, one impulse sound, like that generated with a cap gun or popped balloon, may pass a listener 8,000 times per second after it is abruptly stopped. The mapping of these sound front arrivals over time is called the impulse response, and the impulse response can be thought of as the acoustical fingerprint of a room. It represents what is heard from a single musical note or a single speech syllable (each of which arrives in a burst, like an impulse) and can show arrival time, loudness, reverberance, frequency content, directionality of sound reflections, and acoustic defects. Room Acoustics Qualities Impulse Response We can derive the reverberation time of a room by fitting a smooth line to the decay rate of its impulse. And we can spot a pronounced echo by identifying a strong, late reflection that, on the graph, towers vertically over its adjacent neighbours on the time axis. But even experienced acousticians have difficulty evaluating the subtleties of a room’s acoustic character with only the impulse response to examine. Comparisons between two impulse responses — whether generated in a physical model, in a software model, or in a constructed room — can illuminate the location of a surface generating a troublesome reflection, or the acoustical impact of proposed changes in the architecture of the room, such as the addition of a balcony or the opening of a door linked to a reverberation chamber. Room Acoustics Qualities Impulse Response Because speech and music are both marked by sound spurts separated by short periods of quiet, many of the requirements for good speech intelligibility match requirements for good music listening. Still, listening for music requires a different character of room, a different impulse response. Room Acoustics Qualities Impulse Response Room Acoustics Qualities Impulse Response Room Acoustics Qualities Reverberance The Fogg Art Museum opened on Harvard University’s campus in 1895 with a lecture hall that was functionally unusable. The room, a hard‐surfaced affair, semicircular in both plan and section, caused speech to remain audible for more than five seconds. A syllable spoken would linger in the room to muddy the next 15 syllables in the sentence! Seven hundred miles to the west and a decade earlier, Wallace Clement Sabine’s mother had taken a dominant role in her children’s education, and she enrolled Wallace in college at a young age. After graduating from Ohio State University at the age of 18, Wallace started graduate school at Harvard. Mother Sabine left her less‐ambitious husband and moved to Boston with Wallace (and Wallace’s sister, who was at M.I.T.). Upon completion of his graduate studies in physics, Sabine was offered a faculty position at Harvard. That’s when university president Charles William Eliot solicited help righting the Fogg Museum lecture room’s acoustics from 27‐year‐old Sabine, who was, at the time, researching electricity. The president asked the physics professor to bring the lecture hall in line with the beloved Sanders Theatre, also on Harvard’s campus. Room Acoustics Qualities Reverberance Sabine spent the next two years taking acoustical measurements at the Fogg Museum, as well as in other buildings on campus. While many of his contemporaries were searching for methods to render sound visible so that it could be studied, Sabine preferred to listen to sound. He used a stopwatch to time the audible sound decay, and his judgment to determine when the persistent sound level had dropped below his ability to hear it. Sabine and his students worked between the hours of midnight and 4:00 a.m. to minimize noise they might encounter from other students in the daytime. They carried three‐inch cushions from Sanders Theatre (the room with the exemplary acoustics) across campus to the Fogg, and hung them on the wall to gauge their effect on the sound’s decay, only to return the cushions to Sanders before classes started the next day. In 1897, when the university president asked Sabine to complete his study, Sabine pleaded for more time to collect data, but, tired of waiting, President Elliot insisted that Sabine fix the theater based on data already collected. Now denied permission to continue taking measurements, Sabine was forced to examine his numbers, and he experienced a breakthrough. He discovered the mathematical relationship between the size of a room, its surface materiality, and the reverberance in the room. Room Acoustics Qualities Reverberance As sound ricochets inside an enclosed space, and reflections beget reflections‐of‐reflections, sound seems to linger. The persistence of sound in a room after the sound source is suddenly stopped is called reverberance. In architectural acoustics, more reverberance is neither universally desired nor universally avoided. Each use for a space has an appropriate level of reverberance, a target to be aimed for and to achieve. Generally, in unamplified spaces, the desired reverberance is a function of the balance of speech‐to‐music planned for the room, with speech requiring less reverberance to maintain intelligibility, and music requiring more reverberance to maintain a quality called “fullness”. Room Acoustics Qualities Reverberance Of course, Wallace Clement Sabine didn’t discover reverberance, but he did give the world a window into how sound decays. He completed the first measurements of the absorption coefficients of materials, and he formulated the relationship linking a room’s geometric volume and boundary surface materiality with its reverberance. He found that the rate of sound decay in a room was the same whether he excited the room with one organ pipe, two organ pipes, or four organ pipes. In equal time intervals, sound energy decays by the same fraction of its initial value, and the loss of sound energy is always a constant percentage of the total amount of energy. Room Acoustics Qualities Reverberance The proposed formula, the “Sabine formula” for calculating reverberation time, is written as Sabine determined that the qualitative impression of reverberance could be expressed as the quantitative value of reverberation time, the number of seconds required for sound in a space to decay (once it is abruptly stopped) by some fixed decibel value. In this case, 60 decibels was chosen as the reference decay. Room Acoustics Qualities Reverberance RT is sometimes instead written as T60 or RT60. Nominally, a small office may have a reverberation time of 0.25 seconds, which means that a sound inside the room, when cut off suddenly, decays 60 decibels in a quarter‐ second. For reference: a small office may measure 0.25 seconds, a classroom may measure 0.50 seconds, a theatre may measure 1.0 second, a concert hall 2.0 seconds, At first approximation, the sound is weakened from impacts with the room boundary; therefore, a larger space, has a longer mean free path between surface impacts, and correspondingly slower decays and longer reverberation times. Room Acoustics Qualities Reverberance Spaces with fewer surfaces to absorb sound, and spaces finished with more reflective surfaces that absorb less sound, also have longer reverberation times. The reverberation time metric, one of the most important acoustic factors in performance spaces, is also the most widely applicable room acoustics measurement in the greatest number of room types. Banquet halls, classrooms, offices, and almost all types of rooms where listening is important have their own window on the reverberance continuum. Too much reverberance, and notes or syllables smear together; too little reverberance, and musical loudness or fullness might suffer. Further, because the Sabine formula is easy to calculate and doesn’t require measurement in an extant room, it can be estimated during the design phase, when adjustments to the architecture are easiest to make and have the greatest impact on the acoustics. Generally, it is best to target a reverberation time on the high side of the acceptable range for music rooms, and on the low side of the acceptable range for speech rooms. With our understanding of reverberance comes the capacity to specify and achieve an appropriate reverberation time during building design and also to adjust the room’s reverberation time from one performance type to the next. Retractable velour banners and curtains may deploy for an amplified performance, and then retract for an unamplified performance later in the evening, or panels with reflective surfaces may slide in front of room surfaces with absorptive “fuzz”. In some cases, doors, apertures, or ceiling panels open a space to additional room volume when the performance piece calls for more reverberance. Room Acoustics Qualities Reverberance Through most of the 40,000‐year evolution of music, the room didn’t react to the reverberance requirements of the music, but rather the music was composed to respond to the space in which it would be performed. As historically important and widely applicable as Sabine’s formula is, it has shortcomings. The math involved uses principles of statistical acoustics to output an average rate of sound decay because it would be prohibitively difficult to trace out every ray from every source‐receiver. Still, arriving at a statistical average fails to account for the importance of geometry, especially in non‐rectangular rooms with unusual shapes. The formula assumes a diffuse sound field: the same sound energy everywhere in the room. This might not be the condition in a convoluted room, or in a concert hall with an absorbent plane of audience on one surface, and highly sound‐ reflective boundaries everywhere else. (Concert halls typically see 50% to 90% of their total room absorption in the audience plane.) The behavior of low‐frequency sound, especially in small spaces, can be hard to predict statistically. While sound in the 63‐Hz octave band is important to listening quality, reliably accurate absorption coefficient values for that octave band are difficult to measure in many laboratories, so data in that band is too often omitted. Finally, very absorbent rooms are not as diffuse, so Sabine’s formula is most accurate when Room Acoustics Qualities Reverberance Room Acoustics Qualities Reverberance Room Acoustics Qualities Reverberance Room Acoustics Qualities Reverberance This optimal reverberation time monograph is for preliminary design purposes only. After room is designed and materials chosen, detailed octave‐band‐resolution reverberation time calculations should be conducted. The optimal reverberation times given here are targets for mid‐ frequency (average of the 500‐Hz and 1,000‐Hz octave band values) measured in the unoccupied condition. No single reverberation time is perfect for all uses of a room, so variations up to 10% from targets are common. For desired warmth in unamplified music listening, low‐frequency reverberation times should increase to something on the order of 20% longer than mid‐frequency values. To avoid undesirable “boomy‐ness” in spaces for speech or amplified music listening, low‐frequency reverberation times should nearly equal those at mid‐frequency. Upholstered seats count as “absorptive material” in this calculation. For shown graph, it is assumed that the room aspect ratio is 2H long by 1.5H wide by H high and that absorbing material measures a 0.75 absorption coefficient. The non‐absorbing surfaces in the room are Room Acoustics Qualities Clarity The acoustical quality of “clarity” is reverberance’s opposite, the differentiation of each syllable and musical note. Clarity and reverberance are highly (inversely) correlated, so rooms with high reverberation times suffer from a loss of clarity, and rooms with a low reverberation time enjoy a richness of clarity. Yet a measure of each is desired. Rather than a singular focus on the rate of sound decay, achieving clarity also demands maximizing both the direct sound and the very early sound reflections that arrive just after the direct sound. Room Acoustics Qualities Clarity The human brain combines the arriving direct sound with early‐ arriving sound reflections, increasing the distinctness of each note and allowing each syllable of speech to stand apart from those before and after it. The integration of, nominally, the first 50 milliseconds of reflections (speech) and 80 milliseconds of reflections (music) into a single fused louder image is labelled as the “Haas effect,”. The phenomenon is also called the “precedence effect.” Haas found that the auditory system uses the direct sound to locate the source, but it is the early reflections that promote clarity. Room Acoustics Qualities Clarity More recent research suggests that the 50‐millisecond and 80‐ millisecond cut-off point values (speech and music respectively) between the zone of early reflections and the zone of reverberant energy (and echo) may represent too narrow time-window. Our brains likely fuse reflections arriving up to 200 milliseconds after the direct sound, with the cut-off time a function of (a) the balance of speech to music, (b) the type of music, and even (c) the shape of the room. Whether the threshold is 50 milliseconds, 80 milliseconds, or 200 milliseconds, the time window threshold of early reflections is of course not measured after the sound is made, but rather after the direct sound arrives at the listener location. Room Acoustics Qualities Clarity This understanding has a profound effect on the shaping of rooms. To enhance clarity (and loudness), maximize the direct sound by limiting the distance between the source and receiver. Provide good sightlines to the musician or lecturer. Because human eyes and ears are on the same horizontal plane, clear sightlines to the stage typically afford the listener unblocked access to direct sound, so raked seating planes promote acoustical clarity. Maximizing early sound reflections further promotes clarity as it mitigates unwanted echo, which comes from strong sound reflections that arrive too late to support clarity, and are too loud to make up the reverberant decay. Room Acoustics Qualities Clarity The positions and angles of walls and ceiling segments should be shaped to encourage strong first‐order reflections (those that arrive after a single bounce off a sound‐reflective surface). To promote clarity and mitigate echoes, sound‐absorbing materials (as much as needed to achieve optimal reverberation times) should generally be placed at the far end of the room, distant from the source. The clarity index C80(3) measures the total sound energy arriving before an 80‐millisecond threshold, compared to the total sound energy arriving after that threshold, averaged for three mid‐frequency octave bands. We don’t include low frequencies when measuring clarity because the human auditory system performs poorly at differentiating temporal effects in bass tones, 250 Hz and below. The higher the clarity index, the clearer the sound and the better the speech intelligibility. Room Acoustics Qualities Clarity Room Acoustics Qualities Clarity Room Acoustics Qualities Clarity Room Acoustics Qualities Clarity Room Acoustics Qualities Clarity Room Acoustics Qualities Variable Acoustics For unamplified music performances, such as symphonies, audiences prefer rooms with the advantages of both reverberance (the persistence of a sound after it stops) and clarity (each note decays rapidly enough so that the next can be heard sharply), yet the two are opposing qualities. Typically they are inversely related so that more reverberance begets less clarity.  Room Acoustics Qualities Variable Acoustics The coupled‐volume concert hall with its signature impulse response, the double‐sloped decay, tries to resolve this conflict. This venue typology attempts to reconcile the competing qualities of reverberance and clarity by wrapping a normative concert hall with a coupled volume, then controlling the sonic transparency between the two rooms with doors. Musicians play on stage, and most of the sound energy is delivered to the audience in the usual way—but some of the sound energy slips past the ajar doors into the coupled volume, where it bounces between The audience hears the sound that never left the main part of the surface. concert hall, and later, the sound that leaked into the coupled volume and leaked back into the main part of the concert hall. Room Acoustics Qualities Variable Acoustics Room Acoustics Qualities Variable Acoustics Room Acoustics Qualities Variable Acoustics If the coupled volume is more reverberant than the main part of the concert hall, the late‐arriving energy that leaks back into the audience will be louder than that which never left the main part of the hall. The impulse response of a coupled‐volume concert hall can appear double‐sloped so that each note decays rapidly at first, then more slowly as the sound in the coupled volume re-enters the main part of the hall. Because of that rapid early decay, each note is expected to die quickly enough to allow the next note to be heard with a measure of clarity; and because of the slow late decay, each note is expected to linger in the room long enough to be heard with a measure of reverberance. Room Acoustics Qualities Variable Acoustics Room Acoustics Qualities Variable Acoustics As a result, the coupled‐volume having a double‐sloped decay, would lead to simultaneous reverberance with clarity. In practice, the system proves to be highly sensitive and fickle. There are dozens of coupled‐volume concert halls, but musicians, music critics, and audiences identify only a few with audible double‐sloped decays. First, for the coupled volume’s sound energy to return to the main part of the concert hall with more sound energy than that which remains in the main part of the concert hall, the coupled volume must be much more reverberant than the main hall, perhaps measuring ten times the RT! This means that the coupled volume must be large, minimal in its surface area (relative to its volume), and finished with very low‐absorbing materials. Of those that are built, halls with large concrete coupled volumes, in shapes that minimize the coupled volume’s surface area, fare best. Room Acoustics Qualities Variable Acoustics Second, the doors that separate (and link) the coupled volume and the main room must provide only a small gap for sound to leak through. If the doors are fully closed, the room behaves as a standard concert hall, one without a coupled volume at all. This may be appropriate for some musical pieces that would not benefit from a double‐sloped decay. If the doors are fully opened, the room behaves like a single larger concert hall equal in volume to the two rooms added together. This may be appropriate for other pieces that require more reverberance than clarity. The aperture size to produce a double‐sloped decay is thus somewhere between fully closed and fully opened, and it is surprisingly close to the fully closed position. Typically this means openings on the order of only 1% of the total surface area of the room. When the doors are opened to 3%, the double slope may evaporate into an impulse response that approaches the doors‐fully‐opened condition.  Room Acoustics Qualities Variable Acoustics Adapted from M. Ermann, “Coupled Volumes: Secondary Room Reverberance and the Double‐Sloped Decay of Concert Halls,” Room Acoustics Qualities Variable Acoustics Third, the background noise in coupled‐volume concert halls must be very low. Of course, limiting the background noise is an important part of any space for unamplified music listening, but it takes on added importance in coupled‐volume concert halls because if the noise floor is too high, the entire double‐sloped effect is lost beneath the noise level from a nearby road or mechanical equipment or adjacent lobby. The potential to reconcile the competing qualities of reverberance and clarity, and doing so through spatial and geometrical manipulation, remains interesting, as many questions are still unsolved, like whether the audiences prefer the double slope to a traditional single‐sloped Sabine decay. The coupled‐volume approach is one in a collection of strategies that uses a dynamic architecture to vary a room’s acoustic quality. Variable acoustics might provide a means of adapting a space to a specific musical piece, or it might be used to simulate the sound absorptance of an audience during rehearsal when no audience is present, or it might allow architects to tune a room after it is built. Room Acoustics Qualities Variable Acoustics Retractable sound‐absorbing banners or curtains deploy to reduce the RT, or retract to increase it. Alternately, sound‐reflecting panels slide away to reveal a sound‐absorbing or sound‐diffusing panel behind them, and slide back when reflections are preferred instead. Other schemes feature rotating triangular wedges with one side sound reflective, one side sound absorptive, and a third sound diffusive. In each case, the room’s operator decides which type of surfaces should be used. Room Acoustics Qualities Reverberation Time Calculation Checklist 1. Recognize what the sound recognize. If one surface covers another, or almost covers another, you need only account for the one “visible” surface. In the cafeteria example, the seated students were accounted for in stead of (rather than in addition to) the 3,000‐square‐foot area of wood floor underneath them. 2. Approximate when appropriate. Because the cafeteria is not a space for unamplified music listening, precision at early stages of design may be unnecessary. Exit signs, light fixtures, door handles, or other surfaces smaller than a door can typically be omitted in the calculation. Room Acoustics Qualities Reverberation Time Calculation Checklist 3. Substitute one material for another when required. Manufacturers make sound absorption data available for their products, but in early stages of design, when specific manufacturers have not yet been selected, data for some materials may not be readily available. Even later in design, a material, or an unusual application of a material, may be absent data. In these cases, substitute a material of similar mass, surface texture, and mounting. (What is the weight per square foot? Is there an air space behind, or is it flush‐mounted?). 4. Average the absorption data of two materials if you are uncertain which to use as a substitution. Room Acoustics Qualities Reverberation Time Calculation Checklist 5. Be accurate when calculating the room’s volume. While a 10% underestimation of the absorption coefficient of the 5,000 square feet of glass in the cafeteria results in no meaningful change in the calculated reverberation time, a 10% underestimation of the room’s volume drops the calculated reverberation time from 3.1 seconds to 2.8 seconds. 6. Consider the edges of seating blocks when calculating the area of audience surface. If a block of seated people is exposed to an aisle, include an extra three‐foot strip of audience, the length of the aisle, when estimating the audience’s surface area. This correction accounts for the audience edge portion, visible in elevation. Room Acoustics Qualities Room Shaping for Speech and Music Room Acoustics Qualities Room Shaping for Speech and Music Room Acoustics Qualities Room Shaping for Speech and Music Room Acoustics Qualities Room Shaping for Speech and Music Room Acoustics Qualities Room Shaping for Speech and Music  Room Acoustics Qualities Room Shaping for Speech and Music In speech, early‐arriving reflections assist with loudness and clarity, so a room geometry that features surfaces angled to relay incident sound back to the audience improves intelligibility. We angle surface reflections to privilege seats farther from the source, on the assumption that those seats need the most assistance. Late‐arriving reflections echo, so the room geometry must also minimize the likelihood of strong reflections that have traveled too far. If the reverberation time target dictates it, sound‐absorbing surfaces will cover some portion of the room. But which surfaces? Those that (even with shaping) still produce an echo—like the back wall and the upper‐rear portions of the side wall—are the obvious candidates for providing the “fuzz” necessary to bring the reverberation time in line. In this way, those fuzzed surfaces can both reduce reverberance and reduce the likelihood of echo. This often translates to a room that is reflective on approximately three‐quarters of the wall and ceiling surfaces, and absorbent on the remaining one‐quarter (the rear‐top portion).  Room Acoustics Qualities Room Shaping for Speech and Music Rooms for unamplified music typically thirst for longer reverberance, limiting the need for added absorption besides that provided by the audience. These rooms may or may not be shaped to direct first‐order reflections to the audience. If they are shaped, the beneficial early sound reflections may come at the cost of the late reverberance because sound energy directed back down at the absorbent audience is sound energy no longer available to ricochet around the sound‐ reflective portions of the room and provide needed sustain. A study of 17 British concert halls found mid‐frequency EDT/ RT ratios to range between.79 and 1.26. (EDT measures the first 10 decibels of decay and extrapolates out to 60 decibels, and is considered a better indicator of running reverberance.) The most diffuse rooms were characterized by similar EDT and RT values (ratios approaching one) and the most shaped rooms, directing early reflections toward the audience, measured at lesser EDT values than RT values (ratios less than one). Therefore, in shaped rooms for music, aim for a higher reverberation time, in the recognition that the shaped room form will act as a tax on the reverberance estimated by the Sabine formula. Because that equation assumes a diffuse sound field and doesn’t account for a specially shaped geometry, achieving an acceptable running reverberance requires the designer to target a reverberation time a bit higher than would otherwise be recommended.