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Room Acoustics (part 4)



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VARIABLE VOLUME EXAMPLES

Shown below are examples of auditoriums where the cubic volume can be varied to match reverberance and patterns of reflected sound energy to the intended functions. The reverberation times needed for intimate drama (< 1 s) and symphonic music (> 1 .8 s) require radical changes in volume with corresponding changes in seating capacities. In the following examples, seating capacities vary from more than 3000 at the top to less than 1000 at the bottom. Below are depicted variable volume configurations at Jesse Jones Hall, Houston, Texas (CRS Sirrine, architects and BBN, acoustical consultants), variations at Edwin Thomas Hall, University of Akron, Akron, Ohio (CRS Sirrine, architects and V.O. Knudsen, acoustical consultant). In practice, there is the almost irresistible economic temptation for owners to use the largest seating capacity for all functions.

aa-136.jpg Jones Hall; Concert hall (3100 seats); Opera house/theater (1800 seats)

aa-137.jpg Thomas Hall: Concert hall ( 3000 seats); Opera house (2300 seats); Drama theater (900 seats)

BASIC THEATER STAGES

The basic theater stages are the proscenium stage (where the performing area is largely in a coupled stage-house viewed through a “picture frame” opening), the open, or thrust, stage (where the performing area extends into the audience area), and the arena stage (where the performing area is entirely surrounded by audience). In the open and arena stages, sound-reflecting walls and ceiling (or suspended panels) are extremely important to help compensate for the directivity of high-frequency speech signals. Because the human voice is more directional at high frequencies than at low frequencies, consider ably less high-frequency sound energy (10 to 20 dB lower) is radiated behind the performer than in front of the performer. In addition, this portion of the frequency range strongly influences speech intelligibility.

aa-138.jpg Open or Thrust; Proscenium; Arena. Note: For preliminary estimates of stage areas, use an area based on 15 to 20 ft^2 per musician for stages for orchestra alone. Add 800 ft^2 for chorus.

STAGE ENCLOSURES FOR ORCHESTRA

Panels constructed of thick plywood, damped sheet metal, or heavy gypsum board can be used on stages to surround (or enclose) the sources of sound. These sound-reflecting and diffusing panels (called a stage enclosure, or orchestra shell) can help distribute balanced and blended sound uniformly throughout the listening area by connecting (or “coupling”) a portion of the stagehouse cubic volume to the volume within the main hall. Stage enclosures (< 2 percent open) increase loudness by preventing sound energy from being absorbed by scenery in the fly loft and wings. The fly loft is the volume above the stage where scenery is “flown” out of sight when not in use. Therefore, this portion of the stagehouse normally is highly sound absorptive. When not needed to support music performances, enclosures should be designed to be dismantled and stored compactly without interfering with other stage functions.

The surfaces surrounding an orchestra should also contain small-scale irregularities to blend and reflect the high-frequency sound energy from the various instruments. The enclosure can contribute to good music-listening conditions onstage (i.e., provide balance between various sections of the orchestra), where it's essential that musicians and chorus members clearly hear themselves and each other to perform as a coordinated group (called ensemble). Note that the specific arrangement of musicians depends on the number of musicians and style of music to be performed. It is prudent to de sign enclosure panels so they can be adjusted (“tuned”) while the orchestra performs in rehearsal.

aa-139.jpg Plan of Stage Enclosure

aa-140.jpg Stage-house Section: Note: The fixed height of the proscenium opening H usually varies from 20 to 35 ft (and higher for opera) The fixed width of the proscenium opening W varies from 30 to 80 ft. depending on the type of theatrical or orchestral production.

FORESTAGE CANOPIES

Sound-reflecting panels, suspended in front of the proscenium, reflect sound energy from the stage to the audience and decrease the initial-time-delay gap. These panels, called forestage canopies, extend the orchestra shell into the auditorium. This extension can enhance the direct sound needed for intimacy and can also reflect sound energy from the orchestra pit back toward the pit. The openings between the panels allow sound energy to flow into the upper volume so it can contribute to the low-frequency reverberance in the main auditorium below (needed for warmth).

aa-141-0.jpg Forestage Canopy (To extend contained shell)

aa-141-1.jpg Coupled Stagehouse (With open articulating shell to allow flow of low-frequency sound energy)

The reverberation time of stagehouses should be approximately equal to or less than that of the main auditorium, unless the stagehouse is to be used as a “coupled” reverberant chamber. To achieve a reverberant stagehouse for an articulating shell, don't specify unpainted concrete block or install sound- absorbing materials on the walls of the stagehouse, and keep the fly loft free of materials which absorb sound. The side walls of articulating shells should be 5 to 10 percent open; the overhead panels should be 15 to 50 percent open, depending on the height of the panels.

References

  • C. Jaffe, “Design Considerations for a De-mountable Concert Enclosure,” Journal of the Audio Engineering Society, April 1974.
  • T. J. Schultz, “Acoustics of the Concert Hall,” IEEE Spectrum, June 1965.

ORCHESTRA PITS

Orchestra pits should be designed so music can be blended and balanced with sound from the stage. The section below shows the elements of a traditional orchestra pit located between the proscenium stage and the audience. Guidelines which give square feet per musician can be misleading in sizing orchestra pits because risers require extra space and the elbow room a musician requires varies from instrument to instrument (e.g., musicians playing cellos or trombones require more space than those playing clarinets). Nevertheless, for preliminary planning purposes, provide more than 16 ft per musician. Also shown below are pit layouts which use movable reflectors to accommodate more than 100 musicians and less than 40 musicians.

aa-143-0.jpg Orchestra Pit Details

aa-143-1.jpg Orchestra Pit Layouts

References

  • E. L. Harkness, “Performer Tuning of Stage Acoustics,” Applied Acoustics, vol. 17, 1984, pp. 92-96.
  • R. Johnson, “Orchestra Pits,” Theatre Design and Technology, Fall 1979, pp. 8-20.

BALCONIES

Balconies can be used in large auditoriums to reduce the distance to the rear seats and to increase seating capacity (e.g., narrow halls with shallow balconies can achieve optimum intimacy). The basic elements of a balcony are shown below. To prevent echoes or long-delayed reflections off the balcony face, apply deep sound-absorbing finish, tilt or slope the surface facing the stage so sound will be reflected toward nearby audience, or use diffusing shapes, such as convex elements, to scatter sound.

aa-144-0.jpg Basic Elements

Note: For good sight lines, the highest seat in the balcony should not exceed a 26’ angle to horizontal at stage floor (e.g., drawn from curtain line or bottom of motion picture screen).

Examples of Poor Balconies

Persons seated deep under a balcony (or transept in a church) can't receive useful reflected sound from the ceiling and are shielded from the reverberant sound. The listening conditions are therefore very poor because sound will be weak and dull. The example balcony designs shown below should be avoided because they shield the audience in the rear seats from most reflected sound.

aa-144-1.jpg

BALCONY DESIGN EXAMPLES

Concert Hall and Opera House

In a concert hail, the depth D of the under-balcony should not exceed the height H of the opening. This restriction on the depth helps reverberant sound energy reach listeners seated in the rear rows. In an opera house, D should not exceed 1.5H. The balcony soffit should be sloped to reflect sound toward the heads of the listeners seated underneath and to better connect, or couple, the under-balcony volume with the volume of the main hall. When the vertical separation H between side balconies is sufficient, the balcony soffits can be de signed to provide useful lateral reflections to the audience at the center of the main level seating area.

In halls with a central sound-reinforcing system, be sure the audience in the last row has line of sight to the cluster. If this can't be achieved, use under-balcony loudspeakers with signal-delay features.

aa-145-0.jpg

Motion Picture Theater

Direct reinforced sound from loudspeakers located behind the screen al lows deeper balcony overhang. Therefore, in motion picture theaters and similar facilities, D should not exceed 3H, although 2H is still the preferred limit in theaters where other functions occur.

aa-145-1.jpg

Flying Balcony

The cantilevered balcony (or flying balcony) is open at the rear, allowing reverberant sound energy to surround the audience seated underneath. D can be longer than the conventional balcony of the same H because reverberant energy will be greater at the rear rows. An early example is the flying balcony completed in 1889 in the auditorium-theater in the Auditorium Building, Chicago. Unfortunately, the elliptical ceiling in this 4237-seat reverberant auditorium (Adler and Sullivan, architects) did not evenly distribute reflected sound energy, but focused sound into the central seating area causing hot spots and interference.

aa-146-0.jpg Flying Balcony

EVALUATION GUIDE FOR MUSIC PERFORMANCE SPACES

How To Use Evaluation Guide

The scales on the evaluation guide can be used by listeners to record their subjective impressions of spaces for music performance (e.g., concert halls, churches, recital halls). Place a checkmark in the section of the scale which best represents your individual judgment of the specific attribute or condition. The primary purpose of the evaluation guide is to encourage users to become familiar with important acoustical properties of rooms where music is per formed. The guide is not intended to be used to rank the best or worst spaces because there always will be a wide range of individual judgments, even among experienced listeners and performers. Recognize also that it's extremely difficult to separate judgment of a hail from either judgment of the quality of a particular musical performance, or from longstanding personal musical preferences.

Subjective Judgments of Music Performances

Subjective impressions can be recorded for the following conditions (see evaluation guide below):

  • Clarity (listen to beginnings of musical notes and observe degree to which individual notes are distinct or stand apart)
  • Reverberance (listen to persistence of sound at mid-frequencies)
  • Warmth (listen for strength or liveness of bass compared to mid- and treble frequencies)
  • Intimacy (listen to determine if music sounds as though played in a small room regardless of actual size)
  • Loudness (listen for direct sound and reverberant sound; evaluate during louder passages for comfort conditions and weaker passages for audibility) Diffusion (listen for envelopment of terminal sounds or feeling of immersion in sound; compare conditions with eyes open and closed)
  • Balance (listen for relative strength and quality of various sections of orchestra, and between orchestra and soloist or chorus)

Sounds which interfere with perception of music performances may also be observed. The most common are the following:

  • Background noise (sounds other than music or from audience, heard during times solo instrumentalists play faintest notes, or when hall is empty)
  • Echoes (notice direction and strength of any long-delayed, discrete sound reflections)

Use a separate evaluation sheet for each seat where performance is to be evaluated. Absence of “dead spots,” that's , locations where music is very weak, and minimum variations in listening conditions throughout space indicate good uniformity. Remember, there are no absolute or “correct” answers. Subjective impressions by individuals are the only evaluations that really matter.

Overall Impression

The box at the bottom of the guide should be used to record your overall impression of the musical performance at a given seat location. It is suggested that traditional academic ratings be used: A (for best ever, a most memorable listening experience) to F (for one of the worst, a truly bad listening experience), with C for average experience. Always keep in mind that this guide is intended to be used to develop an understanding of specific music performance conditions and , by careful observation, how they may be affected by architecture.

References

  • M. Barron, “Subjective Survey of British Concert Halls,” Proceedings Institute of Acoustics, vol. 7, February 1985, pp. 41-46.
  • L. L. Beranek, Music, Acoustics and Architecture, Wiley, New York, 1962, pp. 471- 480. (The pioneering, comprehensive rating system based on detailed study and analysis of 54 concert halls and opera houses.)
  • J. S. Bradley, “Experience with N Auditorium Acoustic Measurements,” Journal of the Acoustical Society of America, June 1983, pp. 2051-2058. (Presents data from measurements in Canadian halls where several acoustical properties have been evaluated.)
  • P. H. Heringa, “Comparison of the Quality for Music of Different Halls,” 11th International Congress on Acoustics, Paris, vol. 7, July 1983, pp. 101-104.
  • T. J. Schultz, “Concert Hall Tour of North America,” Bolt Beranek and Newman Reverberation Time Data Report, BBN Labs., Cambridge, Mass., 1980.

--------------------------

EVALUATION GUIDE

(Place mark on section of scale which best represents your impression of listening condition. Use separate sheet for each seat where performance is to be evaluated.)

CLEAR SOUND

(varies from clear or distinct to blurred or muddy)

BLURRED SOUND

LIVE REVERBERANCE

(liveness or persistence of mid-frequency sounds)

DEAD REVERBERANCE

WARM BASS

(relative liveness of bass or longer duration of reverberance at bass compared to mid-and treble frequencies)

COLD BASS

INTIMATE SOUND

(auditory impression of apparent closeness of orchestra)

REMOTE SOUND

SATISFACTORY LOUDNESS

(indicate early or direct sound (symbol D) and reverberant sound (R) on scale)

UNSATISFACTORY LOUDNESS (too weak or too loud)

RICH DIFFUSION (expansive sound)

(envelopment of sound which surrounds listener from many directions)

POOR DIFFUSION (constricted sound)

GOOD BALANCE

(observe between musicians and soloist or chorus, among sections of orchestra)

POOR BALANCE

SATISFACTORY BACKGROUND NOISE (very quiet)

(from HVAC system, or intruding noise from ancillary spaces or outdoors)

UNSATISFACTORY BACKGROUND NOISE (very noisy)

ECHOES (long-delayed reflections that are clearly heard)

No___ Yes____ Direction:___

Music Performance Space: ________________________ Date: ____________

Seating Capacity: _________________ Cubic Volume: _________________ ft^3

Orchestra / Conductor: _____________ Composer / Work: _________________

Seat Location: ____________________________________ Seat No.: ________

(Use space at right to sketch floor plan, or cut and paste seating layout from program booklet.)

OVERALL IMPRESSION (Refer to instructions on above.)

--------------------------

MUSIC CONDITIONS AFFECTED BY ROOM ACOUSTICS DESIGN

The table below presents music listening conditions along with room acoustics properties which influence the corresponding subjective judgments of music performance. If a hail is to be successful, its design must satisfy all these requirements.

 

Subjective Music Conditions

Acoustical Properties of Room

Clarity and intimacy

  • Initial-time-delay gap (< 20 ms)
  • Shape and proportion (e.g., length-to-width ratio < 2, or use suspended sound-reflecting panels)
  • Avoidance of deep under-balconies

 

Reverberance (“liveness”)

  • Volume (300 ft^3 for rectangular halls, 450 ft^3 for surround halls) to provide sufficient reverberance (1.6 to 2.4 sec at mid-frequencies)
  • Shape and proportion
  • Furnishings and finishes (sound-reflecting walls and ceiling)
  • Audience capacity and seat spacing (< 9 ft^2/person including aisles)

Warmth

  • Relationship of absorption at low frequencies to mid-frequencies (bass ratio > 1.2)
  • Thick, heavy enclosing surfaces
  • Width of room (height-to-width ratio > 0.7)
  • Size and shape of sound-reflecting side walls
  • Coupled spaces (stagehouses, understage moats)

Loudness

  • Volume (and other reverberance properties listed above)
  • Distribution of sound-absorbing finishes
  • Stage enclosure and sound-reflecting surfaces at front end of room

Diffusion

  • Large-scale wall and ceiling surface irregularities, quadratic-residue diffusers
  • Shape and proportion (e.g., narrow widths, large height-to-width ratios)
  • Finishes and furnishings

Balance and onstage hearing

  • Size of stage enclosure (and use of risers for musicians)
  • Shape of sound-reflecting panels near orchestra (stage
  • enclosure design)
  • Distribution of sound-absorbing finishes (and audience seating
  • in surround hall)
  • Adjustability of overhead sound-reflecting panels

SABINE AND THE CONCERT HALL

After Wallace Clement Sabine completed his pioneering work to improve the listening conditions in the 436-seat lecture room of the Fogg Art Museum at Harvard University, he continued his studies testing a wide variety of rooms and measuring the sound absorption properties of numerous common building materials. Because of this work on room acoustics, in 1898 Sabine was asked to serve as acoustical consultant on the new Boston Symphony Hall (McKim, Mead, and White, architects). It was to be the first hail designed using scientifically derived principles of room acoustics.

Sabine favored the rectangular “shoebox” shape of the Boston Music Hail, the Leipzig Neues Gewandhaus, and other successful nineteenth-century European halls. In addition to recommending the proportion of length to width, he attempted to achieve a long reverberation time for music (> 2 s for the fully- occupied hail). He also recommended splaying the sides of the stage (to better project sound toward the audience), narrowing the stage width (to pro vide intimacy for orchestra performance), coffering the ceiling and constructing deep side-wall niches and pilasters (to diffuse sound throughout the hall), and controlling ventilation system noise. Nearly a century after its completion in 1900, Boston Symphony Hall is still rated as one of the outstanding concert halls in the world.

Shown below are plan and section views of three traditional rectangular halls: Boston Symphony Hall, Leipzig Neues Gewandhaus, and Vienna Grosser Musikvereinssaal.

aa-152.jpg Symphony Hall, Boston, Massachusetts (Completed 1900, seating capacity 2631-persons): Plan, Section

aa-153-0.jpg Stage of Boston Symphony Hall

aa-153-1.jpg Neues Gewandhaus, Leipzig, German Democratic Republic (Completed 1886, seating capacity 1560-persons, destroyed during World War II)

aa-154.jpg Grosser Musikvereinssaal, Vienna, Austria (Completed 1870, seating capacity 1680-persons).

The table below compares important characteristics of the three halls:

height to width ratio H/W, length to width ratio L/W, ratio of volume (ft^3) to audience areas (ft^2) and reverberation time T at mid-frequencies (500/1000 Hz).

 

H/W

L/W

Volume to Seating Area Ratio

T (s)

Boston

0.8

1.7

40

1.8

Leipzig

0.7

1.7

34

1.6

Vienna

0.9

1.6

44

2.1

------------------------------------------

Modern criteria

> 0.7

< 2

> 45

1.6 to 2.4

References

  • L. L. Beranek, Music, Acoustics and Architecture, Wiley, New York, 1962 (case histories 3, 20, and 35).
  • L. L. Beranek, “Wallace Clement Sabine and Acoustics,” Physics Today, February 1985.
  • W. J. Cavanaugh, “Preserving the Acoustics of Mechanics Hall,” Technology and Conservation, Fall 1980.

CHECKLIST FOR CONCERT HALLS

When auditoriums are used primarily for music performances, the design goal is to achieve throughout the hall: satisfactory level of sound or loudness, referred to as dynamic range by musicians; definition of music perception, i.e., clarity, reverberance, and intimacy; and appropriate tonal balance and texture. The list below summarizes important acoustical properties of rooms needed to achieve satisfactory music perception.

1. Reverberation time at mid-frequencies (i.e., average of reverberation at 500 and 1000 Hz) when hall is occupied should be 1.6 to 2.4 s for opera, sym phonic, organ, and choral music. An empty hall should have longer reverberation times. for preferred ranges of mid-frequency reverberation times for a wide variety of activities. Music in rooms with appropriate reverberation times sounds full-toned, live, and blended. In rooms with too much reverberance, music sounds “muddy” and indistinct.

2. For music performances, the bass ratio, a measure of the low-frequency responsiveness of a room, should be greater than 1 .2. The bass ratio is the reverberation time at low frequencies (i.e., average of reverberation at 125 and 250 Hz) divided by the mid-frequency reverberation time. For example, if the mid-frequency reverberation time is 2 s and the low-frequency reverberation time is 2.4 s, the bass ratio will be (2.4 ÷ 2) = 1.2. Higher values of bass ratio, indicating fullness of bass tone or “warmth,” may be acceptable in especially large halls. Reverberation times should increase about 10 percent per octave below 500 Hz to allow fundamental frequencies of musical instruments to persist sufficiently and to avoid masking from low-frequency background noise. Avoid thin (e.g., < 3/4-in-thick wood) or lightweight materials (not attached to a rigid backup surface), which absorb low-frequency sound energy by panel action. When coupled to the hall, a reverberant volume under the stage platform can be used to enhance low-frequency reverberation for the audience near the stage (see example below).

aa-155.jpg Acoustical Moat

3. Intimacy can be achieved by providing an initial-time-delay gap ITDG of less than 20 ms for reflected sound energy. For symphonic music, rectangular halls should have length-to-width ratio L/W of less than 2 to produce strong lateral reflections. Listeners prefer conditions under which sounds are different in each ear. An important goal, therefore, is to achieve strong lateral reflections from the side walls. Ray-diagram analyses should be used to assure ITDGs from side walls are less than 23 ft (i.e., 20 ms X 1/1000 X 1130 ft/s). In wide halls, the first reflection will be from the ceiling, producing similar sounds in both ears. The ratio of early lateral sound energy to total early energy in creases as the height-to-width ratio HI W of the hall increases (i.e., less wide halls). Several highly regarded traditional rectangular concert halls in Europe have H/ W ratios greater than 0.7. Ray-diagram analyses also can be used to design suspended sound reflectors (convex or flat panel elements sloped about 45 degree to the horizontal) so early reflected lateral sound will be provided. Area of suspended panel arrays typically should be 40 to 50 percent of the stage area. To aim sound to desired locations, individual panels should be adjustable in height and orientation. This feature also allows “tuning” of the hall.

aa-156-0.jpg Suspended Sound-Reflecting Panels

4. Loudness is determined by cubic volume, sound absorption, and shape of front end of hail. It contributes to the definition of music. For rectangular con cert halls, volume per seat ratio should be 300 ft per person; for surround halls, 450 ft per person. Loudness can be measured by placing a standard fan sound source onstage. For concert and opera halls, mid-frequency sound levels (from a standard reference source: ILG blower driven by a 1/4-hp electric motor) should be 52 to 58 dB. For music perception during performances, peak levels often are 90 dBA or more at fortissimo playing (very loud), de pending on dynamic range of instruments and passage being performed. How ever, preferred listening levels are normally less than 80 dBA.

aa-156-1.jpg Rectangular Hall; Surround Hall

5. Audience absorption can be a significant factor in concert hall design. Limit seating density to 6.5 to 9 ft per person because the more the audience is spread out, the more sound it absorbs. Halls with fewer than 2000 seats can be designed to provide optimum intimacy and loudness for symphonic music.

6. Halls should have diffusing surfaces, such as deep coffers, carved decorations, large-scale pilasters, or projecting piers at 1/4 X (i.e., > 6 1/2 in deep for 500 Hz and above) on side walls, balcony faces, ceilings (e.g., exposed beams, boxed-in air ducts), and stage walls, so that listeners perceive reflected sound from many directions. Even uniform coffers can enhance diffusion because the angle of incidence for sound waves from the stage will differ from front to rear coffers. It is especially important that musicians hear each other to play well. Sound-diffusing surfaces near musicians can provide useful early interreflection of sound energy. Avoid flat surfaces, which can cause “harsh” or “glaring” listening conditions for music. The quadratic-residue dif fuser was invented in the 1970s by Dr. Manfred Schroeder to provide con trolled diffusion in concert halls (cf., M. R. Schroeder, Number Theory in Science and Communication, Springer-Verlag, Berlin, 1986). These diffusers, constructed to have wells of varying depths, have found wide application in re cording studios, where they are called Schroeder boxes. A prime number (i.e., no whole number larger than 1 goes into it evenly) divided into each of the squares of the series of all numbers less than the prime number yields a remainder (or residue) sequence. This sequence can be used to establish pro portion for varying depths in a diffuser. The table below shows how residues are determined for the prime number 11. For example, 5 squared is 25, the prime number 11 goes into 25 twice (2 X 11 = 22), and the remainder is 3 (25 - 22).

Number n:

Number squared n^2

 

Remainder sequence (n^2 divided by prime number):

1 2 3 4 5 6 7 8 9 10

1 4 9 16 25 36 49 64 81 100

1 4 9 5 3 3 5 9 4 1

Example sound-diffusing shapes are shown below. Quadratic-residue diffusers QRDs for the prime number 7 are used at Segerstrom Concert Hall, Costa Mesa, Calif. (QRDs designed by J. R. Hyde, acoustical consultant).

aa-157.jpg Folded-Panel Surface

Audience absorption is not directly related to the number of occupants. It is proportional to the total floor area of seating, including part of the floor area of the aisles ( see definition of the edge effect ). The ratio of volume (ft^3) to audience area (ft^2) including orchestra seating, should be more than 45 degrees for concert halls.

aa-158-0.jpg Convex Modulations

aa-158-1.jpg “Quadratic-Residue” Diffuser (Prime no. 7) Isometric view of QRD wall panel

7. Sound-reflecting surfaces near the stage and orchestra should contribute to satisfactory tonal texture (i.e., subjective impression due to change or sequence of signal delays and levels of the sound reflections). These critical surfaces should be shaped so sound energy will reflect back toward the stage, providing performers with the essential sensation of responsiveness of the hail. Tonal texture is strongly affected by the acoustical properties of the hall, the composition and placement of the orchestra members, and the program selection and style of the conductor.

8. Avoid echo-producing surfaces onstage and in the hall. Discrete, flutter, creep, and other echo phenomena must be avoided. Nevertheless, some sound energy should return from the rear of rectangular halls to the stage so that the performers can gauge the responsiveness of the hall to their efforts. The balance between these conflicting goals is very delicate and requires careful tuning.

9. Background noise levels must be near the threshold of audibility (NC-15 or RC- 15 for concert hails) to achieve high signal-to-noise ratios for good listening conditions and to allow musicians to produce the greatest possible audible dynamic ranges. (See Section. 4 for preferred noise criteria NC levels for a variety of listening spaces. Do not exceed NC-20 for most auditoriums.) Lobbies and circulation corridors, when carefully designed, can act as buffers to noise.

10. Other important factors affecting the acoustical success of concert halls include psychological aspects of design such as: color (e.g., most orchestra conductors prefer white and gold interiors to blue interiors); use of wood (most musicians believe wood is essential in concert halls even though other materials reflect sound equally well); seating arrangement (provide good sight lines and comfortable seats); reference of listeners to other halls (which establishes personal norms); ancillary spaces (e.g., “dead” foyers or lobbies, which tend to create the impression of solemnity when entering a more reverberant environment, instead of “live” foyers or lobbies, which tend to create the impression of dryness by contrast); amenities such as spacious “green” rooms to enhance morale of performers; etc.

Note: A special orchestral composition called “Catacoustical Measures,” by T. J. Schultz and D. Pinkham, can be used to help evaluate listening conditions by measuring reverberance during pauses in both empty and occupied concert halls. This composition was scored by D. Pinkham to have musical interest while at the same time producing the entire frequency range of an orchestra with even distribution of frequencies from individual instruments (cf., T. J. Schultz, “Problems in the Measurement of Reverberation Time,” Journal of the Audio Engineering Society, October 1963, pp. 313-315). The composition can also be used to facilitate the tuning process.

References

  • V. Ando, “Calculation of Subjective Preference at Each Seat in a Concert Hall,” Journal of the Acoustical Society of America, September 1983.
  • L. L. Beranek, “Acoustics and the Concert Hall,” Journal of the Acoustical Society of America, June 1975.
  • M. Forsyth, Buildings for Music, Cambridge University Press, Cambridge, England, 1985. (Also available from MIT Press, Cambridge, Mass.)
  • C. Jaffe, “Acoustics of Concert Halls,” Architectural Record, March 1979, pp. 106-108.
  • V. L. Jordan, Acoustical Design of Conceit Halls and Theatres, Applied Science Publishers, Barking, England, 1980.
  • R. Mackenzie (ed.), Auditorium Acoustics, Applied Science Publishers, Barking, England, 1975.
  • R. B. Newman et al., “Adjustable Acoustics for the Multiple-Use Auditorium” in G. C. Izenour, Theater Design, McGraw-Hill, New York, 1977, pp. 479-493.
  • R. S. Shankland, “Acoustical Designing for Performers,” Journal of the Acoustical Society of America, January 1979.

CASE STUDY 1: WOODRUFF CENTER, EMORY UNIVERSITY

Woodruff Health Sciences Center Administration Building; Emory University; Atlanta, Georgia

The section view of the Woodruff building at Emory University shows the classrooms and auditorium below the plaza level and the medical school ad ministration offices above. The auditorium was designed to accommodate 500 people on two levels, primarily for lectures, convocations, and related speech activities. The balcony soffit was sloped to increase the size of the opening to the audience seated underneath (D < 2H), and to provide line of sight to the central loudspeaker cluster for persons in the last row (see Section. 7). The sound-reflecting oak ceiling panels were oriented to evenly distribute sound energy throughout the auditorium. Concave balcony face and rear walls were finished with glass fiberboard covered with Belgian linen (i.e., deep absorption to prevent echoes and control reverberation). The ceiling above the suspended panels was covered with glass fiberboard to further control reverberance. Mid- frequency reverberation time is 0.8 s. Heavy enclosing concrete constructions and duct treatment reduce mechanical equipment noise to noise criteria NC-25.

Architect: Henry Architects and Engineers, Inc. ( Atlanta, Georgia) Acoustical Consultant Office of M. David Egan ( Anderson, South Carolina)

aa-160.jpg Building Section

aa-161-0.jpg Auditorium Section

aa-161-1.jpg Auditorium Reflected-Ceiling Plan

aa-162-0.jpg Administration Building and Exterior of Auditorium Wing from Plaza Level

aa-162-1.jpg Auditorium from Balcony

Reference

R. Yee, “ Woodruff Medical Administration Building,” Contract Interiors, October 1977, pp. 74-77.

CASE STUDY 2: BOETTCHER CONCERT HALL

Boettcher Concert Hall; Denver, Colorado

Boettcher Concert Hall, completed in 1978, is a 360-degree surround music hall designed to place the audience of 2750 close to the performers. More than 80 percent of audience is less than 65 ft from performers. Audience seating is organized into several “terrace block” balconies rather than a single circular amphitheater pattern. This increases the area of reflective surfaces near the audience. Balcony faces are thick plaster shaped in convex ribbon patterns (to diffuse sound), balconies are steeply sloped (to reduce audience attenuation), and seats have sound-reflecting high wood backs (to increase intimacy). This 1,315,000-ft hall has an overhead canopy (panel array) consisting of 106 adjustable, suspended, sound-reflecting circular-convex 1/2-in-thick acrylic plastic saucers (to improve onstage listening conditions and reflect sound to audience seated near stage). Mid-frequency reverberation time is 2 s and volume-to-audience-area ratio is 51 (to provide sufficient liveness at mid- frequencies and warmth at low frequencies).

  • Architect: Hardy, Holzman, Pfeiffer Associates ( New York, New York)
  • Acoustical Consultant: Jaffe Acoustics, Inc. ( Norwalk, Connecticut)
  • Sound System Designer: David H. Kaye ( Boston, Massachusetts)

aa-163-0.jpg Hall Section

aa-164-0.jpg Hall Plan

aa-164-1.jpg Hall from Upper-Level Balcony

References

  • C. Jaffe, “360-degree Symphony Halls and Music Pavilions,” Audio Engineering Society, New York, Preprint No. 1092, November 1975.
  • R. H. Talaske et al., Halls for Music Performance, American Institute of Physics, New York, 1982, pp. 16-17.

CASE STUDY 3: DUNHAM AUDITORIUM, BREVARDCOLLEGE

Dunham Auditorium

Brevard College

Brevard, North Carolina

The 500-seat auditorium, originally completed in 1957, had an extremely short mid-frequency reverberation time for music (<0.6 s), poor diffusion, and poor coupling from stage to audience. To increase reverberance, the original sound-absorbing ceiling was removed, opening the entire auditorium to the attic volume above. In addition, the proscenium walls were removed to further increase the total effective cubic volume of the room by more than 40 percent. Windows along the side walls were filled in with masonry (to improve sound isolation from outdoor noise) and finished with large-scale gypsum board ‘bumps” (to provide diffusion and enhance lateral sound). The renovated auditorium reopened in 1981 with significant improvements in room acoustics, which greatly enhanced conditions for performance and perception of music. Mid-frequency reverberation time now is 1.4 s and improved volume-to- audience-area ratio is 44 (to increase liveness and warmth). Length-to-width ratio L/W is 1.5.

Architect: Daniels-Worley, Architects ( Brevard, North Carolina) Acoustical Consultant: Office of M. David Egan ( Anderson, South Carolina)

aa-165.jpg Plan

aa-166.jpg View toward Stage (before renovation); View toward Rear Wall (before renovation)

aa-167.jpg Sections

aa-168.jpg View toward Stage (after renovation); View toward Rear Wall (after renovation)

CASE STUDY 4: STUDIO B, TODD-AO FILMS

Studio B, Todd-AO Films; Hollywood, California

The film production studio shown below contains electronic control equipment for mixing and editing of prerecorded sound tracks. Loudspeakers are located behind a large perforated projection screen and surround” loudspeakers are located on the side and rear walls. The walls are treated with thick glass fiberboard and mineral fiber behind a transondent protective facing of wood slats . Three thicknesses of sound-absorbing material were randomly distributed on the walls to help achieve a relatively dead space with the desired reverberation times of 0.5 s at 125 Hz, 0.4 s at 500 Hz, and 0.4 s at 2000 Hz.

Acoustical Consultant: Charles M. Salter Associates, Inc. ( San Francisco, California)

aa-169.jpg Studio from Control Room; Bird’s-Eye View Perspective

CHECKLIST FOR ROOM ACOUSTICS DESIGN

1. The level of the background noise (e.g., HVAC, intruding environmental sources) must be sufficiently low to avoid interfering with the intended activities (see Sections. 4 and 5).

2. Sound energy must be evenly distributed throughout the listening space.

3. Avoid echoes and any focusing effects. In small rooms for conferences or music practice, with relatively little sound absorption, avoid parallel surfaces and shapes which might emphasize certain frequencies (e.g., ratio of any two of the length, width, and height dimensions should not be a whole number).

4. The desired sounds must be sufficiently loud. Shape room surfaces to provide useful sound reflections toward the audience. If size of auditorium and its use require an electronic sound-reinforcing system, carefully integrate the system with the room acoustics design (see Section. 7).

5. Provide the proper reverberation time characteristics. The reverberation time must be long enough to properly blend sounds, and yet short enough so there will be sufficient separation of successive sounds necessary for intelligibility. In rooms for both speech and music, there is a natural conflict. A long reverberation time is desirable for music so that successive notes blend together, giving richness in bass frequencies. However, for speech the reverberation time should be short so that the persistence of one syllable does not blur or mask subsequent syllables.

6. Provide short enough initial-time-delay gaps for the early sound reflections in concert halls and similar spaces. Initial-time-delay gaps should be less than 30 ms (i.e., a sound path difference < 34 ft) to provide useful reinforcement of direct speech sounds.

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