This article is divided into six parts: Between Time (how early reflections are distributed in time and the possibility of comb filter coloration), Between Musicians (how musicians on stage influence the acoustics, and how distinct, strong reflections on stage create comb filters), Between Mouth and Ears (how persons who are blind see with their ears: echolocation in both time domain and frequency domain), Between Measurements (how musicians and actors perceive reverberation and room acoustic details not covered by the standardized criteria, and how one can investigate this by one's own handclaps and tongue drops recorded with in-ear microphones), Between Echoes (how An echo is not an echo, but is highly dependent on the type and duration of the signal, masking by other sounds [music and noise], and masking by the signal itself), and finally, Between Walls (how details like room resonances and shimmering treble are more important for the perceived acoustics of [too] small rehearsal rooms than common reverberation time and SPL criteria).Between TimeSound travels slowly, just 343 m/s at common room temperature, only 1235 km/h. This is actually extremely slow compared to other wave phenomena like light and makes sound much more interesting. When you turn on the light, the whole room is illuminated immediately. Your clap, however, sneaks slowly around: searching, tasting the walls, making advanced combinations with its own delayed reflections. Because sound is so slow, and because we can hear such a huge span of frequencies, we must treat bass sounds and high frequency treble in totally different ways. The lowest bass frequency we perceive as sound (some 20 Hz) has a wavelength of 17.2 m. That is longer than typical room dimensions. On the other side, high pitched brilliance at 15,000 Hz has a wavelength of just a couple of centimeters, and is easily disturbed/scattered by just small objects and irregularities.When the sound radiates from a source, the simplest obstruction it can meet on its way is a single surface, which will give a reflection (if the surface is hard and smooth and has a dimension on the order of or larger than the wavelength of the sound). When a sound is combined with a reflection of itself, the result depends on the excess distance the reflection travels, compared with the direct sound path: the time delay. We all know that if a single reflecting surface is far away, we perceive the reflection as a distinct echo. When the time delay is shorter than some 50 ms (the sound has traveled less than some 17 m), we might not perceive the reflection as an echo for speech because the direct sound and the reflection more or less combines into one sound; but, the frequency content (timbre/klangfarbe) of the sound will change. Figure 1 a-c shows typical frequency responses when a (broadband) direct sound is combined with a reflection. The short reflection in Figure 1a gives a very broad comb filter with large spacing between the teeth of the comb (a large comb-between-teeth bandwidth [CBTB], almost like a simple bass control on a Hi-Fi-system). In Figure 1b, the long delay between the direct sound and the reflected sound gives very narrow spacing between comb teeth, no noticeable coloration, and the reflection is perceived as a distinct echo in the time domain. Figure 1c shows something in between: The extra sound path for the reflected sound is 3.43 m, which gives a delay of 10 ms and a CBTB of 1000/10 100 Hz. This could be the reflection coming back to a person standing 1.715 m in front of a wall.This means that the delay between the direct and the reflected sound from surfaces at distances typical in a medium-sized room might produce an audible comb filter. The average person perhaps does not notice such coloration of the sound because often there are several surfaces at different distances that more or less randomize the comb filters, or perhaps the person has not experienced the original/direct sound by itself, unmodified by the reflection. …