Early Energy Fraction Research Articles

Developments in Concert Hall Acoustics in the 1960s: Theory and Practice

After the war, there was a general understanding of reverberation time (RT), including how to measure it and its significance, as well as its link to a state of diffusion. Reverberation refers to a property of late sound; there was an appreciation that early sound must be significant, but in what way? Research had begun in the 1950s using simulation systems in anechoic chambers, with the Haas effect of 1951 being the most prominent result. Thiele’s Deutlichkeit, or early energy fraction, was important from 1953 and indirectly found expression in Beranek’s initial time delay gap (ITDG) from 1962. The 1960s produced a possible explanation for RTs in halls being shorter than calculations predicted, the importance of early sound for the sense of reverberation (EDT), the nature of directional sensitivity, conditions for echo disturbance, and the importance of early lateral reflections. Much of the research in the 1960s laid the foundations for research investigating the relative importance of the various subjective effects for concert hall listening. Important concert halls built during the period include Philharmonic Hall, New York (1962); Fairfield Hall, Croydon, London (1962); the Philharmonie, Berlin (1963); and De Doelen Hall, Rotterdam (1966). The parallel-sided halls of the past were rarely copied, however, due to architectural fashion. These various halls will be discussed as they make a fascinating group.

A possible acoustic design approach for multi-purpose auditoria suitable for both speech and music

In several auditoria, it has been observed that the reverberation time is longer than expected and that the cause is a horizontal reverberant field established in the region near the ceiling, a field which is remote from the sound absorbing audience. This has been observed in the Boston Symphony Hall, Massachusetts, and the Stadthalle Göttingen, Germany. Subjective remarks on their acoustics suggest that there are no unfavourable comments linked to the secondary sound field. Two acoustic scale models are considered here. In a generic rectangular concert hall model, the walls and ceiling contained openings in which either plane or scattering panels could be placed. With plane panels, the model reverberation time (RT) was measured as 53% higher than the Sabine prediction (frequency 500/1000Hz), compared with 8% higher with scattering panels. The second model of a 300 seat lecture theatre with a 6m or 8m high ceiling had raked seating. In this case, the amount of absorption in the model was increased until the point was reached where speech had acceptable intelligibility, with the early energy fraction, D⩾0.5. For this acceptable speech condition with the 6m ceiling, the measured mid-frequency T15 was 1.47s, whereas the Sabine predicted RT was 1.06s. The sound decay was basically non-linear with T30>T15>EDT. Exploiting a high-level horizontal reverberant field offers the possibility of acoustics that are better adapted as suitable for both speech and unamplified music, without any physical change in the auditorium. Using secondary reverberation in an auditorium for a wide variety of music might also be beneficial.

Relationships between preference ratings, sensory profiles, and acoustical measurements in concert halls

Preferences of concert hall acoustics are explored with preference mapping. The investigation is performed on previously gathered data from individual vocabulary profiling of nine concert halls and three pieces of symphonic music, namely, excerpts of compositions by Beethoven, Bruckner, and Mozart. Individual preferences are regressed onto a latent three-dimensional sensory space obtained by multiple factor analysis of descriptive sensory data. Overlaying individually estimated preference surfaces onto one another produces preference maps which illustrates both the overall preference of the stimuli as well as differences between individual listeners. A comparison of the maps between music motifs illustrates how each music signal affects the weighting of different acoustical qualities in preference judgments. Differences in preferences between individuals are pronounced in the excerpts of Beethoven and Bruckner, while the responses are more homogeneous for Mozart music motif. Overall, proximity is identified as the main aspect associated with preference, but also loudness, envelopment, and bass are important. A correlation analysis of objective parameters and subjective perceptions substantiates the importance of lateral sound energy for good concert hall acoustics. Particularly, the lateral early energy fraction at high frequencies is found to be associated with the perception of proximity, and hence, also with preference.

The Use of Virtual Reflectors to Improve Speech Intelligibility in Open Stage Auditoria

A potential problem encountered with open stage auditoria is poor speech intelligibility arising from the directional nature of human speech sources. The conventional technique of employing large overhead reflecting surfaces to ensure a large early energy fraction to improve intelligibility is shown to be unsuitable in this situation. The use of Virtual Reflectors consisting of microphones and loudspeakers symmetrically located relative to the stage is proposed as a possible solution to the problems of open stage auditoria. It is suggested that the Speech Transmission Index is a suitable parameter for assessing the performance of Virtual Reflectors when in use and that computer modelling could provide a suitable method for investigating the action of Virtual Reflectors.

Numerical Analysis of the Acoustical Quality of a Multi-Purpose Hall

In this paper, the acoustical features are described of a multi-purpose auditorium of the Free University of Brussels which were investigated both with field measurements and computer simulation. The convergence of the algorithm of the simulation package RAYNOISE was investigated as a function of the influence on the calculated results of the choice of the number of rays and the reflection order. By comparing the numerical and experimental values of the sound pressure level and early decay time, it is demonstrated that sufficiently accurate acoustical models can be developed. Based on these models, acoustical quantities such as the early energy fraction, sound pressure level, early decay time and early lateral energy fraction were calculated and employed to evaluate the acoustical quality of this multi-purpose hall.

Measurement and in situ calculation of room acoustic parameters

Room acoustic parameters should describe the subjective evaluation of a room. The most important parameter for evaluation of acoustic comfort in a room is the reverberation time, but additional parameters are necessary for evaluation and design of concert halls and auditoria. Beside the reverberation time and EDT (early decay time) it is possible from the integrated impulse response to calculate acoustical parameters such as ‘‘early to late sound index’’ (clarity), ‘‘early energy fraction’’ (deutlichkeit), ‘‘center time,’’ and ‘‘total sound level.’’ These measurements and calculations are today possible to perform insitu by a sound level meter with application modules connected to a sound source and a lap-top computer; traditionally these measurements required a much bigger measurement system. Therefore, by means of this easy portable instrumentation, the results may be inspected in the field and additional measurements can be done if necessary. During the presentation, the above acoustical parameters are mentioned and measurement results, obtained by instrumentation from Bruel & Kjaer, from a European concert hall and from a hall in the USA are described.

Design considerations for good theatre acoustics

Despite continued efforts by many researchers, there is little information available for acoustical design of theatres. An investigation, aimed at identifying important design elements was carried out. Extensive objective measurements were made in 12 unoccupied British theatres. Of the objective measures for speech intelligibility, the 50‐ms early energy fraction is used in the analysis of the results. The significance of geometrical reflections through the design of ceiling, side walls, balconies, and through the provision of special overhead reflectors has been examined. The effects of directionality of the speaker source in relation to these design elements have also been studied. The results suggest that it would be possible to predict important reflections and thereby the early sound level as a function of design characteristics from architectural drawings. The results of late sound energy have been compared with theory and the agreement was found to be good. The study has found some of the recently completed theatres having low values of the 50‐ms early energy fraction. [Work supported by SERC.]