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 The www center for search, research and free sharing in acoustics Reverberation Reverberation For a receiver in a room, reverberation is a decaying tail of reflected sound from a source. Impulse response The impulse response is the sound-pressure vs time data measured after a perfect impulse from a source. In a level vs time diagram, an initial impulse will be followed by peaks and dips corresponding to the time-varying energy density, with an overall decreasing tendency. Such a diagram is often called a reflectogram or an echogram, since it shows the arrival times of reflections and echoes. The diagram describes reverberation the way it can be perceived after e.g. percussive instruments and consonants of speech. Decay Curve The reverberation tail following the end of a sound of sufficient duration (long enough to create a steady state sound field) represented by an energy-time diagram is denoted the decay curve, which is generally frequency specific. The decay curve describes reverberation the way it can be perceived by listeners e.g. after the end of a musical phrase and in particular after the final note of a musical work. This is the aftersound. Technically, the decay curve can be obtained by backward (Schroeder) integration from an impulse response measurement, and is then referred to as the Schroeder Curve. In principle, the decay curve develops from t=0 (direct sound arrival) as follows (Figure 1): · At t=0, the level is equal to G, the total received (direct and reverberant) energy level · At t=0+, the level is equal to the total reflected energy level Grefl ; The more dominating the direct sound is, the larger is the step down from t=0 to t=0+, and vice versa; Less distance to the source and/or more sound absorption causes larger step down, and vice versa) · Between t=0+ and  t=ITDG (initial time delay gap), the decay curve is flat, since the interval contains no sound energy · At t>ITDG, a decay curve will deviate more or less from a falling slope, the deviations being caused by uneven temporal distribution (strong reflections, echoes, time-gaps) and/or room modes, esp. at frequencies lower than the Schroeder Frequency ~2000·(T/V)0.5 In its simplest form, the slope is a single straight line falling with a tendency close to 60dB / RT, typical for reverberant sound fields with even spatial distribution The double slope decay typically occurs if the sound absorption is unevenly distributed, e.g. excess absorption in the floor-ceiling axis, leaving a reverberant sound field remaining between a pair or a quadruple of hard walls; Another common example is the remaining energy slope from an undamped, coupled room. Reverberation Time A simple decay slope can be represented by a single number with unit dB/s. Although the impulse response has an overall decaying tendency, its level-time curve deviates significantly from a simple slope, making it difficult to determine a single number representing the reverberation properly. In contrast, such simple slopes are much more prominent in the down-step response, i.e. the decay curves of the aftersound (here: the reverberation after a sound source stops, not to be confused with the aftersound in our hearing). Therefore, it is to be expected that a single number representation like the Reverberation Time (RT) correlates better with the perception of the aftersound, than with the perception of an impulse response. This difference can be heard e.g. if comparing the aftersound following the final chord from a choir with the aftersound following the impulse from a percussive instrument in the same room. The latter may leave a complex auditive impression (including echoes), despite the simple character of the first. The ideal decay curve, occurring in a perfect diffuse sound field, is a sloping straight line. The time it takes for this slope to decay by 60dB is defined as the reverberation time RT60. More often, RT is calculated from the time interval from -5dB to -35dB on the decay curve, and denoted RT30 or T30. Early Decay Time (EDT) could be defined as 6 times the time interval from 0dB to -10dB on the decay curve, Figure 2.  However, in measurement standards and prediction tools EDT is often defined by the straight line that best fits the data set in the range of 0 to –10dB. The EDT measure was first suggested by V.L.Jordan in 1968. From being the one and only room acousical parameter, to being just one among many suggested parameters, RT could seem to have lost its importance. However, it can be shown that RT governs 4 out of 5 listener aspects in ISO 3382: Reverberation Time—the mother of all room acoustical parameters (paper);  (Presentation).
 Reverberation, decay curves, reverberation time RT, EDT, Reverberance   Figure 1. Decay curve
 Figure 2
 Figure 3. Decay curve and EDT typical for audience
 Figure 4. Decay curve and EDT typical for performers    