Reflections delayed beyond about 30 milliseconds are perceived as distinct echoes.

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

• DEFINE: Defines ‘early reflections’ and quantifies the delay threshold (30 ms / 33 feet) at which they cross over into perceived echoes.
• XREF: Connects to the reader’s prior notes on discrete echoes, reverberation, and the precedence effect from earlier chapters.