By space
How auditorium acoustics work
Large rooms where the room is part of the instrument: shaping reflections to carry sound to every seat, and setting reverberation for speech, music, or both.
8 min read6 sourcesChapter 22 of 24
Schematic
An auditorium in section. The ceiling and side walls are shaped to send useful early reflections towards the rear seats, the rear wall is absorbed or diffused so nothing returns to the stage as an echo, and the raked seating keeps every listener in the direct sound.
Animated illustration of the principle. Not measured data.
An auditorium reverses most of the small-room instincts. The room is large enough that reverberation is a real, diffuse field; early reflections from the walls and ceiling are wanted, because they carry sound to the back rows; and the design challenge is geometry as much as material. The same physics, applied to a very different end.
What the room is for
Everything follows from the use. Speech — lectures, drama, corporate — wants clarity: a short reverberation, strong early reflections, and no late energy to blur consonants. Music wants warmth and envelopment: a longer reverberation and lateral reflections that wrap around the listener. A multi-purpose hall has to do both, which is why such halls have adjustable elements: curtains that draw over reflective walls, banners that drop from the ceiling, or an electronic enhancement system.
Published guidance puts lecture rooms and speech auditoriums around 0.7–1.0 s, multi-purpose halls around 0.8–1.2 s, and the best concert halls at about 1.7–2.0 s occupied, mid-frequency. A hall designed to one of these and used for another is a hall people complain about.
Geometry: getting sound to the back
Direct sound weakens with distance, and in a large hall the rear seats would hear too little of it. The room makes up the difference with early reflections: a ceiling angled or sculpted to reflect stage sound towards the rear, side walls that splay outward to spread lateral reflections across the seating, and a stage enclosure that projects performers’ sound forward. Raked seating lifts each row above the one in front so the direct sound is not absorbed by the audience it passes over.
The same geometry has to avoid the failures: concave curved walls that focus sound into hot spots, a flat rear wall that returns a distinct echo to the stage, parallel side walls that flutter.
A reflection arriving within about 50 ms of the direct sound reinforces speech. Later than about 80 ms it starts to be heard as an echo. Hall geometry is designed around those numbers.
Material: setting the reverberation
The audience is the largest absorber in the hall, and it comes and goes. Upholstered seats are chosen to absorb about as much empty as a person does occupied, so the hall sounds the same at a rehearsal and a full house. Beyond that, absorption is placed deliberately — the rear wall, sometimes the upper side walls — and the remaining surfaces are left hard and shaped, or made diffusive, so the reverberation is even across the range.
Low-frequency absorption is the usual gap in halls that sound muddy: large surfaces of thin panelling over cavities absorb bass unintentionally, or nothing absorbs it at all. The design accounts for the whole curve.
Noise
A hall is only as good as its background noise. Air-conditioning is the usual culprit — the plant, the duct velocity, the grilles — followed by traffic, rain on a lightweight roof, and adjacent spaces. Design targets are set as a noise criterion curve and achieved through plant location, duct sizing and the building envelope. A quiet hall with modest acoustics outperforms a well-shaped hall you can hear the air handling in.
For integrators and architectsThe numbers, the standards, and what to specify
- Parameters
- Reverberation time (T30) per band, early decay time, clarity C50 (speech) or C80 (music), definition D50, strength G, lateral energy fraction, and background noise as an NC / NR curve. ISO 3382-1 for performance spaces.
- Reflection timing
- Design ceiling and side-wall reflectors so first reflections at rear seats arrive within ~50 ms of direct sound for speech; check for any single strong reflection beyond ~80 ms (echo risk), particularly from the rear wall and balcony fronts.
- Seating
- Specify seat absorption occupied and unoccupied from test data; sightline rake also sets grazing-incidence attenuation over the audience.
- Variable acoustics
- Retractable absorption (curtains, banners) or an active enhancement system for multi-purpose halls; state the reverberation range each setting achieves.
- Modelling
- Ray-tracing or hybrid room-acoustic modelling at design stage, calibrated to measurement after construction. The model justifies the geometry before it is built.
What to write into the brief
- Room-acoustic parameters by band for each intended use, background noise target as a criterion curve, and a modelling report before the shape is fixed.
Sources
- 01UK Department for Education — BB93: Acoustic design of schools, performance standards (2015)Mid-frequency RT limits for lecture rooms, drama studios and halls (Table 6).
- 02DIN 18041:2016 target formulas, as quoted in Germany’s federal BNB rating system (criterion 3.1.4)RT targets by room volume and use, and the tolerance band across frequency.
- 03Beranek, “Aspects of Concert Hall Acoustics” — AES Heyser Memorial LectureThe best halls: occupied RT between 1.7 and 2.0 s.
- 04Beranek, Concert Halls and Opera Houses, 2nd ed. (Springer, 2004)
- 05ISO 3382-1:2009 — Measurement of room acoustic parameters, Part 1: Performance spacesDefines EDT, clarity (C50, C80), definition and strength for halls.
- 06Long, Architectural Acoustics, 2nd ed. (Academic Press, 2014)Textbook covering halls, isolation and building acoustics.
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
Keep reading