Building Acoustics – Name for sound persisting after the source stops In room acoustics, the sound that continues in an enclosure even after the source is switched off is called:

Introduction / Context:
Acoustic quality of rooms depends on how sound decays after a source ceases. The lingering sound field inside a hall affects speech intelligibility and musical clarity. Proper terminology helps in applying design metrics like reverberation time.

Given Data / Assumptions:

• Enclosed space with reflective surfaces.
• Sound persists for a short duration after the source stops.
• No focus on outdoor long-path reflections.

Concept / Approach:
Reverberation is the persistence of sound due to multiple reflections within a room after the source has stopped. It decays exponentially and is quantified by T60 (time to decay by 60 dB). An echo is a distinct, delayed repetition due to a single long-path reflection, typically perceived when the delay exceeds about 50–80 ms.

Step-by-Step Solution:
Identify phenomenon: continuous, decaying sound field after switch-off → reverberation.Differentiate from echo: isolated repeat versus dense reflection decay.Exclude unrelated terms: intensity (a measure), interference (wave superposition), resonance (frequency-dependent amplification).

Verification / Alternative check:
Sabine’s formula relates reverberation time to room volume and absorption: T60 ≈ 0.161 * V / A (SI units), reinforcing design control via materials and geometry.

Why Other Options Are Wrong:

• Echo: single, delayed reflection; not the diffuse decay.
• Intensity: a magnitude measure, not a phenomenon name.
• Interference/resonance: different acoustic effects.

Common Pitfalls:
Confusing long flutter echoes with reverberation; both may occur but have distinct causes and remedies.

Final Answer:
Reverberation