Architectural Acoustics: Echoes and Ambience

Architectural Acoustics

• Studies how sound propagates in spaces like concert halls and recording studios.
• Includes acoustic isolation.
• Spaces need appropriate acoustic qualities for their specific use.
• These qualities are early reflections, reverberation, echoes, resonances, and sound coverage.

Echoes

• An echo is a sound reflection that returns to the source about 100 ms after emission.
• Echoes happen when the closest reflective surface is about 17 m away.
• Repetitive echoes can occur between distant, parallel walls.

Early Reflections

• The first sound reflections to reach the listener after the direct sound.
• In small rooms, early reflections happen quickly and aren't heard as echoes.

Ambience

• The timing of early reflections creates a sense of ambience.
• Ambience helps people identify a space by how it sounds.
• Ambience is controlled through careful design, using software to map acoustic paths.

Sound Absorption

• Surfaces both reflect and absorb sound.
• The sound absorption coefficient (α) measures how much sound is absorbed.
• Hard materials like concrete reflect sound.
• Soft, porous materials like fiberglass absorb sound.
• Absorption usually increases with frequency because of shorter wavelengths at high frequencies.

Reverberation Time

• Reverberation is when sound lingers after the source stops because of multiple reflections.
• Reverberation time (T) is how long it takes for the sound to drop by 60 dB from its initial level.

Optimal Reverberation Time

• The ideal reverberation time depends on the space's purpose and volume.
• Speech needs less reverberation than music.
• Excess reverberation muddles consonants, reducing clarity.
• Music benefits from more reverberation, blending sounds and smoothing imperfections.

Direct and Reverberant Sound Fields

• The sound field divides into direct sound (original sound) and reverberant sound (after the first reflection).
• Direct sound decreases by 6 dB each time the distance doubles.
• Reverberant sound remains constant in enclosed spaces due to multiple reflections.
• Open spaces have only direct sound, which fades quickly with distance.
• Reverberation boosts loudness in enclosed spaces, especially with reflective surfaces.
• Reverberation can mask speech, causing people to raise their voices and increasing overall sound energy.

Resonances

• Resonances, or normal modes of vibration, affect acoustics in small rooms.
• Standing waves between parallel walls amplify certain frequencies and prolong reverberation.
• This prolongation is considered an acoustic defect.
• Solutions include avoiding parallel surfaces, adding sound absorption, or adjusting the sound system.
• Resonances impact sound diffusion in small spaces, causing noticeable differences in sound levels.
• Design considerations for small rooms include avoiding symmetry and parallelism, and using absorbent or diffusing materials.

Acoustic Absorption Materials

• Building materials have different sound absorption properties.
• Materials like fiberglass and polyurethane foam are often for acoustic optimization.
• Fiberglass requires protection to prevent harm to people and the material.
• Polyurethane foam, especially with a structured surface, enhances sound absorption.
• Sound-absorbing ceiling tiles are used in ceilings, and their absorption improves with spacing and added fiberglass.
• Materials like expanded polystyrene and egg cartons aren't very effective for sound absorption.
• Carpets and curtains can absorb sound, particularly when made with suitable materials and structures.

Sound Isolation

• Acoustically isolating a space means blocking sound from entering or leaving.
• A wall's effectiveness depends on its density and the sound's frequency.
• Double walls, separated by air and filled with absorbent material, offer better isolation.
• High-isolation windows use glass sheets of different thicknesses, sealed with silicone and absorbent material.
• Transmission Loss (TL) and Sound Transmission Class (STC or Rw) rate sound isolation.
• Effective isolation requires attention to details like gaps, connections, and structural sound transmission.
• Consulting a specialist is advisable for acoustic isolation projects.