Echoes arise only when reflected sound exceeds a 30-millisecond delay

Evidence

A little time later, the listener will then hear sounds that have been reflected off one or more surfaces (walls, floor etc.), as shown in Figure 6.3. These sounds are called early reflections, and they are separated in both time and direction from the direct sound. These sounds will vary as the source or the listener moves within the space. We use these changes to give us information about both the size of the space and the position of the source in the space. If any of these reflections are very delayed, that is, total path length difference longer than about 30 milliseconds (33 feet), then they will be perceived as echoes. Early reflections can cause interference effects, as discussed in Chapter 1, and these can both reduce the intelligibility of speech and cause unwanted timbre changes in music in the space. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1693

Categorization

Category: Factual Claims

• DEFINE: Establishes the technical threshold distinguishing early reflections from echoes, a core psychoacoustics definition.
• QUESTION: The 30 ms/33 foot threshold for echo perception is worth pinning down further — does it scale with source distance or source level?

Related

• Early reflections must arrive within 20 ms or they are perceived as echoes. (similarity 0.89)
• The direct-to-reflection gap must stay under 30ms or become audible echoes. (similarity 0.88)
• First reflections must arrive within 30 ms to avoid being heard as echoes (similarity 0.88)
• A 0–30 ms gap before first reflections cues spatial size while avoiding echoes (similarity 0.85)
• Reflections arriving near the 50 ms echo threshold can severely disrupt performers (similarity 0.84)
• Early reflections within 30-40 ms improve speech intelligibility (similarity 0.84)
• The 30 ms fusion limit applies only to equal-level speech in hemi-anechoic space (similarity 0.81)
• Delays under 100 ms are heard as timbre change rather than distinct echo (similarity 0.80)
• Music listeners prefer reflections far stronger and later than small rooms provide. (similarity 0.80)
• The first arriving sound wave dominates perceived direction of a source (similarity 0.80)
• Early reflections boost perceived loudness while loud late reverberation degrades intelligibility (similarity 0.80)
• Echo timing is used as an auditory distance cue by the visually impaired (similarity 0.80)
• A 30-40 ms delay separates fusion from echo in the precedence effect (similarity 0.79)
• Room reverberation barely affects early reflection audibility but strongly elevates late-reflection thresholds (similarity 0.79)
• Stair reflections diffract off stair edges, producing a descending chirped echo (similarity 0.79)