We all know timing is important. We may not always appreciate the degree to which it applies to room acoustics, both large and small. Although the specific means of achieving speech intelligibility in a large hall may slightly differ from those used to obtain musical articulation in a small room, the difference is simply in the application and manipulation of timing. From knowing the big, know the small.
This week, use the physical events described below to try to better understand your listening or mixing space, so you may experience the same benefits – on a smaller, faster, more musical scale, of course!
Early reflections are those that bounce off “nearby” objects. Late reflections are those that are distinguishable as separate acoustic events from the direct signal. There can be just a few late reflections or many. Echoes are the most obvious late reflections, and multiple strings of echoes are another type of late reflection. Late reflections are those that arrive after about 60 ms following the reception of the direct signal. They can stretch out to around 250 ms (1/4 second) or so, at which point reverberation takes over.
If you concentrate you might be able to identify where some of the late reflections come from. When too many are coming, there is no hope to separate them in terms of direction or timing but you still hear them as being separate from the direct signal.
Timing Aspect #2: Understanding
In order to understand speech, it is important to hear the start, the sound and the ending of each syllable. Each syllable contains both loudness and tonal variations that add emphasis and even meaning to the spoken word. To understand speech, we must be able to hear and understand the rapid changing sonic variations within the syllables of speech.
When people are speaking, they produce about 4 complete sonic events per second which are typically interspersed with short quiet moments. We can imagine a model for speaking that generates a sequence of sounds, each lasting 1/8th second, followed by 1/8th second of silence. Any sound that fills in the 1/8th second of quiet between syllables is noise, and tends to mask and disrupt the understanding of the communicated signal.
Late reflections easily backfill those quiet 1/8th second periods, causing separated speech syllables to seem to slur together. The excessive presence of late reflections makes the difference between a room that yodels and a room that gargles.
Timing Aspect #3: Improvement
Late reflections are best converted into early reflections whenever possible because this helps with the understanding of the direct sound. There are two main ways to get rid of late reflections (including echoes): absorption and diffusion. If late reflections are absorbed, they are removed from the sonic space entirely and the overall loudness of the subsequent reverberation is markedly reduced. If late reflections are diffused or scattered about, they are not removed from the sonic space and the reverberation remains loud.
It is important to rearrange the late reflections in designing or re-voicing an auditorium. A good sounding speech hall has an “early time gap” in the sequence of ongoing reflections, a unique absence of late reflections. It sports a strong direct signal, accompanied by a distinct and flush group of early reflections. This is immediately followed by a distinct absence of late reflections (the early time gap) and after about ¼ second of relative quiet, the ambience of reverberation is noted.
Fix the Timing of Your System!
In the case of small-room musical playback or mixing, we generally want to minimize early reflections that interfere with the direct signal, so we see a different version of the early time gap than shown above. Following this, we want a smooth, diffuse fill that makes the space sound big and immersive, but does not color or change the timbre of the sound.
Know the applicable timing for your situation, pay attention to the relevant details, and an unparalleled listening experience will be your reward.