Powerful Onset of Reverberation
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2.1 Introduction and Definitions of Terms When investigating the acoustics of a concert hall it is common practise to excite the room by a shot from the stage. A microphone placed in the room receives the “impulse response”, which is usually stored for later evaluations. Its oscillogram shows a large number of separate pulses that grow in density over time. To explain these pulses, we may consider sound propagation in the form of rays. The first pulse is caused by the “direct sound” which travels along a straight line from the stage to the microphone. All the later pulses are called “reflections”, as the sound path includes a detour with at least one reflection. The actual reflections of the sonic rays take place at the walls or the ceiling. The audience may be imagined to be sound-absorbing. The “onset of reverberation” is caused by the direct sound and a few early reflections. The genuine reverberation consists of the large multitude of later reflections, most of which have travelled over long zigzag paths. Because of the reflections’ large number and high density in time, the reverberation appears to decay smoothly. The limit between the onset of reverberation and the genuine reverberation is variable, and it depends on the style of music presented. However, these details are unimportant in this chapter. We may assume that the first 50 ms of the impulse response make up the onset of reverberation, immediately followed by the genuine reverberation. The duration and also the sound colour of the genuine reverberation are important quality factors in any concert hall, and there are many scientific publications in this field. An obvious and important property of the genuine reverberation is its smoothing effect, comparable to a piano pedal. However, the experiments described in the following concern the onset of reverberation. This topic is fascinating because the early reflections can open up strikingly
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2 Powerful Onset of Reverberation
wide sound impressions in the front while still maintaining high definition. Good halls can be distinguished from bad halls just by these details. Therefore we shall investigate how these subjective effects are generated by a few specific early reflections and their interaction. In this field of acoustic research only some tentative approaches are known to the author. One of the first approaches was meant to simplify the task. It is based on the fact that the analyzing power of the ear is limited. If, for example, only one reflection is eliminated from the great multitude forming the reverberation, the ear might be unable to notice the small difference. The same idea was applied to the onset of reverberation, and the Absolute Threshold Of Detectability was measured with early reflections (in German this is called absolute Wahrnehmbarkeits schwelle, or aWs: [1]). For this purpose, simple sound fields with a direct sound and only two reflections are simulated in an anechoic chamber. A subject has to analyze a given sound, continuous speech, for example. In relation to the direct
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