Room Acoustics

Reverberation, reflections, and room modes

Lorenz Schwarz
Karlsruhe University of Arts and Design (HfG)

Winter Semester 2024/25
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Room acoustics

Room acoustics shapes every sound we hear and create. Understanding how spaces interact with sound enables:

• Microphone placement decisions
• Informed choice of performance venues
• Creative use of acoustic environments

Historical development of acoustics

Antiquity:

• Pythagoras & Hippasus: Mathematical basis of sound (monochord)
• Archytas: Physical vibrations as source of sound
• Vitruvius: Acoustic design in amphitheaters (echea/resonators)

Modern era:

• Wallace Clement Sabine (1868–1919): Quantitative architectural acoustics
• Developed reverberation time measurement (RT₆₀)
• First scientifically designed concert hall (Boston Symphony Hall)

Amphitheater

Acoustic Fields

Understanding sound propagation requires considering:

• The acoustic environment (open or enclosed space)
• The listening position relative to the sound source

These factors determine how sound energy behaves and is perceived.

Free field vs. diffuse field

Wave propagation and Huygens’ principle

Huygens’ principle states that every point on a wavefront acts as a source of secondary spherical wavelets. The superposition of these wavelets forms the new wavefront as the wave propagates.

This principle explains:

• Diffraction: bending and spreading of waves around obstacles or through openings
• Refraction: change in wave direction when passing between media

Wave behaviour at acoustic barriers

Acoustic barriers and boundaries

Surface shapes and materials have a profound effect on the behavior of sound waves:

• Reflection
• Diffusion
• Absorption
• Diffraction
• Refraction

Reflection

Diffusion

Absorption

Room corners

Diffraction

Acoustic shadow

Refraction

Sound in enclosed spaces

When sound propagates within an enclosed space, it interacts repeatedly with multiple surfaces.
These interactions give rise to characteristic acoustic effects that shape how a room sounds.

Typical phenomena include:

• Reverberation – persistence of sound due to repeated reflections
• Standing waves – resonances caused by boundary conditions

Reverberation

Reverberation

• No reverberation (e.g., in an anechoic chamber) sounds unnatural and artificial.
• Excessive reverberation reduces speech intelligibility (e.g., measured by Alcons).
• Can make certain sound structures muddy and unclear.
• Can mask imprecise playing by a musician.
• Enhances and amplifies sound, making it richer and more resonant.

Temporal structure of reverberation

Initial time delay gap (ITDG)

The Initial Time Delay Gap (ITDG) is the time interval between the direct sound and the first early reflection at the listener's position.

→ Close sound sources result in a longer ITDG, while distant sound sources result in a shorter ITDG.

Echo

An echo is a distinct, repeated sound reflection heard after the original direct sound.

→ Whether a sound is perceived as an echo depends on the nature of the sound and the number of reflecting surfaces.

Precedence effect (Haas effect)

The precedence effect describes how spatial localization of a sound is dominated by the "first wavefront", even if subsequent copies of the sound (reflections) arrive within a short delay window (a few ms to ~30 ms).

• Law of the first wavefront

▶ Effects of different delays 0, 1, 20, 45, 100 ms

Critical distance

Reverberation time (RT₆₀) measurement

▶ RT60 measurement of noise decay

Room modes

Room modes

Anechoic chamber

A controlled environment for acoustic measurements under free-field conditions.

• Insulated against structure-borne sound.
• Constructed using sound-absorbing materials.
• Provides an approximation of free-field conditions.

RT60 guidelines for different spaces

A general guideline for roughly evaluating the quality of auditory conditions in a typical multi-purpose auditorium:

• Below 1 second (lecture hall): Good for speech, too dry for most music
• 1 to 1.5 seconds (concert hall): Good for speech and chamber music
• 1.5 to 2 seconds: Fair for speech, good for orchestral, choral, church music
• Over 2 seconds (church): Poor for speech, good for large organ, liturgical choir

Artificial reverberation

Artificial reverberation simulates the natural persistence of sound in a space, adding richness and spatial depth.

• Physical approaches:

  • Echo chambers: Utilizing physical spaces to record natural reverberation
  • Convolution reverb: Recreating real acoustics by applying impulse response recordings
  • Acoustic raytracing / finite element method: Computational modeling of sound propagation and reflections

• Synthetic approaches:

  • Plate and spring reverb: Vibrating metal plates or coiled springs to emulate reflection patterns
  • Digital delay lines: Creating reverb through digital signal processing algorithms

Copyright and Licensing

Original content: © 2025 Lorenz Schwarz
Licensed under CC BY 4.0. Attribution required for all reuse.

Includes: text, diagrams, illustrations, photos, videos, and audio.

Third-party materials: Copyright respective owners, educational use.

Contact: lschwarz@hfg-karlsruhe.de

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