Room Acoustics Qualities PDF

Room Acoustics Room Acoustics Qualities Loudness  Concertgoers listening to music unamplified are justifiably greedy: They demand access to their share of the sound energy in the room, and the same symphony, playing the same piece, will vary in sound level, depending on the a...

Room Acoustics Room Acoustics Qualities Loudness  Concertgoers listening to music unamplified are justifiably greedy: They demand access to their share of the sound energy in the room, and the same symphony, playing the same piece, will vary in sound level, depending on the auditorium. Room Acoustics Qualities Loudness  While some symphony halls, especially those under 1,000 seats, may have too much loudness, in most cases (and in almost all larger halls) we work to increase the acoustical quality of loudness in the room because there is often not enough sound energy per seat.  Halls enjoying more loudness have: - less sound absorption (especially from the audience plane), - more early sound reflections (especially those arriving laterally), and - a shorter distances between stage and seat. Room Acoustics Qualities Loudness The sound pressure level from reflected energy can be estimated by the formula: Therefore, sound pressure level at a point in the room rises with the sound power of the orchestra, the sound reflectiveness of the room surfaces, the length of the reverberation time, and the proximity of the orchestra. Typically, the overriding factor is A, the total acoustic absorption in the space. Room Acoustics Qualities Loudness The acoustic quality “loudness” is measured with the metric sound strength (G), which is the sound energy measured at a seat, relative to the sound energy from the same source at ten meters in a free field. Suppose a dodecahedral loudspeaker produces a sound level of 70 decibels at a ten‐meter radius in a free field environment. That same loudspeaker, with the same calibration, is brought into a hall and set up on stage, where it produces a sound level of 74 decibels at a seat ten meters away. We then say that the hall has a G of +4 decibels (74 minus 70). Room Acoustics Qualities Loudness  Sound strength is almost entirely a function of the room constant, or total absorption in the room measured in sabins.  Preferred values of Gmid range from +4 decibels to +7.5 decibels, with the most‐admired concert halls measuring a median value of +6, and the least‐admired concert halls measuring a median value of +3.  People are rather sensitive to small changes in loudness: Subjective psychoacoustic studies suggest a just‐noticeable‐ difference human response threshold for G of about a quarter‐decibel to a half‐decibel. Room Acoustics Qualities Loudness To minimize room absorption: A) use massive building materials with low sound absorption coefficients, B) minimize the area of sound‐absorbing surfaces such as curtains, and C) minimize the total area of surfaces that exposed to the sound, for a given volume of room. Fact: the audience plane provides between 50% and 90% of the total sound absorption in a concert hall. Design: 1. to promote loudness for the audience, you need to reduce the absorption of the audience itself. 2. due to the thickness of their upholstery, some audience seats may absorb much more sound than other audience seats, so chair selection is important for loudness. Room Acoustics Qualities Loudness The absorption by the audience? The absorption by the audience is a function of the area of the audience plane, rather than the number of seats, so a denser, more compact audience with smaller mean distances between seats translates to less absorption for a given room occupancy. Also, likely a shorter distance from the source. Room Acoustics Qualities Loudness The absorption by the audience? Examples: Many of the successful older halls, built in times of smaller people and lesser comfort expectations, benefit from a compact audience area. Further, for a given number of seats, a configuration with fewer, and larger, audience blocks absorbs less than one with more, and smaller, audience blocks. This is because the edges of the audience block?! How? As the sides of the chairs are exposed to an aisle, themselves can be seen by the sound as a strip of absorbing surface Room Acoustics Qualities Loudness  The total effective absorbing area of an audience with minimal number of blocks approaches 1.1 times the total audience area as measured in plan. Example: In that case, 1,000 square feet of audience seating should be calculated using 1,100 square feet of audience seating to account for the exposed aisle sides.  Conversely, if many aisles separate many audience blocks, the effective absorption approaches 1.4 times the audience plan area, and for the same example of 1,000 square feet of audience seating, we’d use 1,400 square feet when making reverberation time predictions. Room Acoustics Qualities Loudness Geometry of the volume: With a steeply raked audience plane, and due to the geometry of the spreading direct sound, more available sound will be absorbed (better approximating a plane perpendicular to the path of the traveling sound). With a flatter audience plane, more of the direct sound passes over and can reflect off surfaces.  Room Acoustics Qualities Loudness Geometry of the volume: Room geometries that enhance loudness: 1. minimize the distance between source and receiver, 2. minimize the total area of room surfaces, and 3. maximize early arriving direct sound. Room Acoustics Qualities Loudness Sound strength values drop by as much as six decibels from the front to the rear of concert halls.  To avoid that, (at least partially mitigate the geometry of the common rooms), balconies bring the audience closer to the sound source, (similar in concept to denser seating arrangements). Values of G drop under deep balconies,  so balconies should remain shallow with small overhangs relative to their height over the audience below them. Reducing the seat count for the room—making a room for fewer people—increases sound strength because it diminishes both the audience absorption and the mean distance to a seat. Room Acoustics Qualities Loudness Over‐stage canopies can provide the early first‐order sound reflections known to increase sound strength, Narrow rectangular room can offer a lateral‐arriving sound reflections. Thus, shoebox‐shaped concert halls have, on average, higher sound strength levels. Room Acoustics Qualities Loudness shoebox‐shaped concert halls Room Acoustics Qualities Loudness  At receiver positions close to the source, the direct sound dominates;  At remote positions the reflected sound dominates.  The distance from the source at which the direct sound energy level matches the reflected sound energy level is known as the “reverberation radius.”  In a typical concert hall, this will be on the order of 15 Room Acoustics Qualities Loudness  At receiver positions close to the source, the direct sound dominates;  At remote positions the reflected sound dominates.  The distance from the source at which the direct sound energy level matches the reflected sound energy level is known as the “reverberation radius.”  In a typical concert hall, this will be on the order of 15  Audience provides the primary source of sound For preliminary design purposes, Room Acoustics Qualities absorption in performance a concert hall with 1,800 seats spaces. So as audience size Loudness increases sound strength and a 2.4-second unoccupied reverberation time requires a decreases. room volume of approximately 815,000 cubic feet. It can also expect an audience area of just under 15,000 square feet and a Gmid of just less than 5dB. Rooms for chamber music are: - smaller (less than 700 seats), - louder (G values of 9.0 to 13.0 decibels), and less reverberant (unoccupied RT values of 1.9 to 2.3 seconds). Opera halls are: - quieter (Gmid values of -1.0 to 2.0 decibels) and - less reverberant (unoccupied RT values of 1.5 to 1.9 seconds). Because the audience-size and performer-space requirements often dictate room area, the ceiling height variable controls  Audience provides the primary source of sound absorption in performance spaces. For preliminary design purposes, a concert hall with Room Acoustics Qualities So as audience size increases sound 1,800 seats and a 2.4-second unoccupied strength decreases. Loudness reverberation time requires a room volume of approximately 815,000 cubic feet. It can also expect an audience area of just under 15,000 square feet and a Gmid of just less than 5dB. Because the audience-size and performer- space requirements often dictate room area, the ceiling height variable controls room volume in concert hall.  Audience provides the primary source of sound For preliminary design purposes, Room Acoustics Qualities absorption in performance a concert hall with 1,800 seats spaces. So as audience size Loudness increases sound strength and a 2.4-second unoccupied reverberation time requires a decreases. room volume of approximately 815,000 cubic feet. It can also expect an audience area of just under 15,000 square feet and a Gmid of just less than 5dB. Rooms for chamber music are: - smaller (less than 700 seats), - louder (G values of 9.0 to 13.0 decibels), and less reverberant (unoccupied RT values of 1.9 to 2.3 seconds). Opera halls are: - quieter (Gmid values of -1.0 to 2.0 decibels) and - less reverberant (unoccupied RT values of 1.5 to 1.9 seconds). Because the audience-size and performer-space requirements often dictate room area, the ceiling height variable controls Room Acoustics Qualities Balconies Room Acoustics Qualities Balconies  Balconies relocate seats that would otherwise be at the rear of the room to a position closer to the source.  When protruding from the rear wall, they break up a surface that might otherwise produce an echo.  Side balconies redirect sound that might otherwise have moved to the top of the room, back down to the audience instead, where it heightens loudness. Room Acoustics Qualities Balconies Deep balconies, however, do more harm to the room’s acoustics than good.  They choke off the seats underneath them visually and aurally, restricting sightlines to the ceiling and creating an “acoustical shadow” beneath the overhang.  Room as a whole loses reverberance because sound that passes underneath the deep overhanging balcony fails to get back out with enough energy to contribute to the reverberant tail of the decay. In this way, the under‐ balcony volume’s absorption profile approaches that of an open window. Room Acoustics Qualities Balconies Room Acoustics Qualities Balconies Room Acoustics Qualities Balconies Room Acoustics Qualities Balconies Room Acoustics Qualities Sightlines Room Acoustics Qualities Sightlines Room Acoustics Qualities Warmth Listeners to unamplified music prefer robust low‐frequency content, a quality termed acoustical warmth. Many wall and ceiling assemblies bend as panel absorbers, and attenuate more in the bass tones than at the speech frequencies, so warmth is primarily achieved through careful material selection. Rooms without sufficient low‐frequency reverberance and low‐ frequency loudness are thus said to lack warmth; Rooms with excessive low‐frequency energy are said to be acoustically “dark.” Room Acoustics Qualities Warmth Room Acoustics Qualities Warmth Gypsum board: Because of its thickness, mass, and mounting, a gypsum board assembly absorbs sound in the 125‐Hz octave band at a rate about five times that of a masonry or concrete assembly. That’s because the low‐frequency sound sees the gypsum segments spanning between joists and studs as panel absorbers transferring acoustical energy into mechanical bending. This is particularly acute in the case of single‐layer lightweight gypsum board, which has an absorption coefficient of 0.29 at 125 Hz. So, to achieve warmth in a room, design brick, stone, or concrete surfaces, or surfaces with thick plaster over another material (rather than over a lath and airspace, which would render the plaster a panel absorber like the gypsum board). Room Acoustics Qualities Warmth Gypsum board:  The undesirable low‐frequency absorption associated with wood spanning battens, over an airspace, has since been discovered.  Where wood is still preferred in concert halls, it should be adhered to stiff massive materials, provided that air pockets are minimized behind the panelling, and the adhesive is sufficiently stiff so that the panel and substrate are seen by the sound as a single element. Room Acoustics Qualities Warmth  Low frequencies have long wavelengths, and long wavelengths don’t reflect off small surfaces, so using large surfaces is also part of a strategy to promote warmth.  Where smaller surfaces of similar angles to adjacent surfaces are present, long wavelengths may see the segmented planes as a single curved surface. Room Acoustics Qualities Warmth We gauge warmth with the bass index, comprising the sound strength at 125 Hz (G125) in decibels, minus the sound strength average for the middle frequencies of 500 Hz and 1,000 Hz (Gmid). Rooms with more low‐frequency loudness, relative to their middle‐ frequency loudness, will have higher bass indices. The highest‐regarded concert halls have bass indices between -2.0 decibels and +0.5 decibels. The human perception just‐noticeable difference likely lies between 1.0 and 2.0 decibels. Room Acoustics Qualities Warmth The kind of low‐frequency boost that is desired in rooms for unamplified music is unwelcome in rooms with loudspeakers. Electronic amplification suffers excessive boomy‐ness in the presence of the low‐frequency support required for unamplified music. When more low‐frequency energy is warranted in an amplified room, it can be added digitally. If a venue will be sometimes amplified and sometimes not amplified, consider low‐frequency absorbers that can retract and deploy (for instance, heavyweight, sufficiently furled, mechanized velour banners with airspaces between the banners and walls). Room Acoustics Qualities Concert Hall Types Room Acoustics Qualities Concert Hall Types Room Acoustics Qualities Concert Hall Types Room Acoustics Qualities Concert Hall Types BQI (Binaural Quality Index) Room Acoustics Qualities Concert Hall Types Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression  It is our bodies bilateral symmetry that privileges sound arriving from the sides of our heads, where our ears are directed.  Lateral reflections from the side walls trigger a binaural response, a sense that sound is coming from all directions and that we are immersed in the sound.  This sense of immersion in music is called spatial impression. Environments that lack spatial impression sound as if the listener is outside the room, hearing the music through a small open window. Room Acoustics Qualities Spatial Impression Most important acoustical characteristics of good rooms for listening: - Spatial impression, - Reverberance, - Loudness, - Warmth. While it is technically possible to have too much sound arriving from the side, by far the more common problem is insufficient lateral‐ arriving sound. The best‐reviewed concert halls in the world have meaningfully more sound arriving from the side, so designers work to achieve ever‐increased lateral sound reflections. (Rooms with more loudness are also judged to enjoy more spatial Room Acoustics Qualities Spatial Impression  Geometry is paramount to generating lateral sound reflections, so music rooms should position sound‐reflecting surfaces near, and to the side of, audience seats.  Narrow rectangular halls measuring on the order of 75 feet wide are best‐suited to deliver side sound.  Spaces with large rear balconies that render the back wall absorptive by the audience generate environments where the reverberant sound seems to come from the front of the room; these rooms are penalized in their reputation because of the directionality of their reverberation. Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression Halls with side balconies also promote lateral sound because of the face of the balcony (which can be angled to direct reflections to the audience) and because of the underside of the balcony protrusion. Sound that would otherwise reflect off the wall toward the ceiling will double‐bounce off the wall and the balcony underside, only to return to the audience from the direction of the listeners’ ears. Room Acoustics Qualities Spatial Impression Rear balconies with particularly deep overhangs have the opposite effect, starving the acoustical shadow underneath the balcony from sound reflections arriving from the upper portion of the walls. Room Acoustics Qualities Spatial Impression The overall form of the room also contributes to the portion of sound arriving from the sides. Tall rooms allow double‐bounces off the ceiling and side wall, while short rooms bring ceiling sound to the audience without the benefit of a side‐wall reflection. Room Acoustics Qualities Spatial Impression lateral fraction (LF) method is used to measure the spatial impression. It uses a special bidirectional figure‐eight microphone that measures sound from two opposite directions. The figure‐eight microphone is oriented to receive sound from the sides, and is paired with a (normative) omnidirectional microphone that measures total sound arriving from all directions. Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression Binaural quality index (BQI) is also used to measure of spatial impression. It also ranges from zero to one, and it also increases with increasing spatial impression. The BQI uses a model head with anatomically correct ears, and tiny microphones embedded into those ears. Left and right channels are measured or recorded, then post‐ processed to tease out how closely the sound fields at the two ears correlate with one another. BQI averages the 500‐Hz, 1,000‐Hz, and 2,000‐Hz octave bands, and is limited to sounds arriving within 80 milliseconds of the direct sound. Room Acoustics Qualities Spatial Impression The early lateral fraction measured in rooms generally ranges from 0.05 to 0.50, meaning between 5% and 50% of the sound arrives from the side. Average values were found to be 0.18, and target LF values range from a minimum of 0.10 to a maximum of 0.35. Generally, higher values are better, but in rare cases one can have too high a value and sound sources may be difficult to localize. Room Acoustics Qualities Spatial Impression The most‐admired concert halls measure BQI values of 0.65, while the least‐admired halls average 0.45. The just‐noticeable difference measured in subjects judging BQI was found to be 0.065. It should be noted that one would expect both the lateral fraction and the BQI to increase near the side walls of an auditorium, where more of the sound approaches from the sides, and the sound field differs more at each ear. Yet, these are not considered the best seats acoustically. Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression Recent research has showed that there are two different phenomena associated with the binaural umbrella of spatial impression; these are: Early lateral reflections arriving within the 80‐millisecond threshold after the direct sound arrives contribute to a quality known as apparent source width (ASW); late lateral reflections arriving after the 80‐millisecond threshold contribute to a quality known as listener envelopment (LEV). Before 1960 it was believed that the late sound was most important. Between the 1960s and late 1980s, it was believed that the early sound was paramount. Now, both early and late sound are important, but differ in their effects. Room Acoustics Qualities Spatial Impression A broad apparent source width (the early side‐arriving sound, also called auditory spaciousness or source broadening) gives listeners the sense that they and the orchestra occupy the same space, and that the orchestra is playing together as an ensemble. It is measured by the lateral fraction taken over the first 80 milliseconds, or by the BQI (which always measures the first 80 milliseconds). Room Acoustics Qualities Spatial Impression The later sound represented by listener envelopment is heavily influenced by the late lateral loudness (GLL), although over time the direction of successive wave fronts becomes ever more omnidirectional. Because late sound level is heavily dependent on total room absorption, and total room absorption for concert halls is heavily dependent on audience area, music rooms with small audience areas enjoy high levels of listener envelopment. Measures of the lateral fraction after 80 milliseconds are used to quantify listener envelopment. Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Spatial Impression Room Acoustics Qualities Intimacy Big rooms generally sound big, and small rooms generally sound small. Surely some of that distinction comes about in the respective reverberance levels inherent to rooms of different geometric volumes. But it is also believed that the early‐arriving sound contributes to a sense of acoustic intimacy. By bringing earlier early reflections, designers can make a big room sound like a smaller, more intimate one. Intimacy is measured by the initial time delay gap (ITDG), the length of time in milliseconds between the arrival of the direct sound and the arrival of the first sound reflection. Shorter ITDG durations are associated with more intimate rooms. To provide smaller ITDG values, position sound‐reflecting surfaces in close proximity to listeners so that the reflected sound might arrive earlier. ITDG values in large concert halls range from about 20 milliseconds (meaning that the first reflected sound arrives 20 milliseconds after the direct sound) to about 60 milliseconds. Room Acoustics Qualities Intimacy Small rooms are intimate by their very nature, so ITDG values are much lower in chamber music halls, which range from 8‐millisecond ITDGs to 27‐millisecond ITDGs. Fan‐shaped rooms generally have higher initial time delay gap values, and therefore are believed to sound less intimate than rectangular rooms of a similar size. Because their side‐wall geometry fails to direct first‐order sound reflections back to the middle of the hall. Room Acoustics Qualities Diffusion