Lateral Energy Fraction Research Articles

The nervous spatial behavior of early-energy measures in auditoria

Most commonly used room acoustical measures are defined as the ratio between the energies in two time windows of the impulse response. In the Clarity Index and the Lateral Energy Fraction—supposed to be indicators for transparancy and spaciousness, respectively—the time window 0–80 ms plays a predominant role. When impulse responses are recorded along an array of closely spaced microphone positions and the above measures are calculated for each position, it appears that large fluctuations, up to a factor of 2, occur on a small spatial scale, e.g., in front of one and the same seat. These fluctuations are due to interference of early relections and can be physically well understood. It makes clear that the acoustic quality of a seat or row should not be based on one single measurement but on some spatial average, or that the measures should be redefined such that interference, to which our hearing mechanism apparently is not sensitive, is eliminated.

Late lateral energy fractions and the envelopment question in concert halls

In concert hall acoustics, spatial perception is a crucial element of the experience, yet several questions remain unresolved. In the 1960s, there was some work on what was then called ‘room impression’ caused by diffuse reverberation. The possible importance of early lateral reflections was proposed in 1967 by Marshall [Marshall AH. A note on the importance of room cross-section in concert halls. Journal of Sound and Vibration 1967;5:100–12] and until recently concern for the effect of early reflections has overshadowed study of the spatial effects linked to the later sound. Bradley and Soulodre [Bradley JS, Soulodre GA. The influence of late arriving energy on spatial impression. Journal of the Acoustical Society of America 1995;97:2263–71; Bradley JS, Soulodre GA. Objective measures of listener envelopment. Journal of the Acoustical Society of America 1995;98;2590–7] have now suggested that early reflections are predominantly responsible for creating a sense of source broadening [and apparent source width (ASW)], whereas a sense of envelopment, which had on occasions been linked to ASW, is almost solely produced by later lateral reflections. Bradley and Soulodre have proposed the late lateral energy level as a measure of listener envelopment (LEV). This paper considers some of the history of spatial perception in concert halls and reports on measured results made in 17 halls of the late lateral energy fraction (LLF) and the late lateral energy level (GLL). The spread of measured values of LLF turned out to be small and GLL was found to be predominantly determined by the total acoustic absorption of halls.

Subjective relevance of objective measures for spatial impression

Several objective measures have been proposed to describe the feeling of spatial impression in concert halls, including Lateral Energy Fraction (LF) and Interaural Cross-Correlation Coefficient (IACC). However, previous studies have shown that LF and IACC values did not highly correlate with each other at individual seat positions in real halls [J. S. Bradley, J. Acoust. Soc. Am. 96, 3525–3535 (1994)]. To investigate the listener envelopment aspect of spatial impression further, subjective paired-comparison tests have been run using signals which have various values for LF, early IACC (from 5–80 ms), late IACC (from 80 ms–1 s), and late lateral relative sound level (GLL). Another proposed measure, called Interaural Level Fluctuations (IALF), has also been included, which is based on the rate of change over time of the level difference between the ears. The binaural test signals were generated by the room acoustics simulation program ODEON, using four different musical motifs, and were presented to test subjects via headphones. Results from the experiments are presented. [Work supported by ASA Hunt Postdoctoral Research Fellowship.]

An assessment of the accuracy of a hybrid room acoustics model with surface diffusion facility

Computer modelling of acoustics in enclosures has developed into various forms, none of which have yet demonstrated 100% accuracy. This paper, therefore, details a study of room acoustic computer modelling that highlights weaknesses with existing modelling techniques. The study illustrates that for accurate prediction of many room acoustic parameters the modelling of surface reflections needs to be refined. Accounting for scattering in the modelling of all reflection orders would improve accuracy in the time domain and assist in the prediction of parameters such as reverberation time, early decay time and clarity index. Allowing the directivity of scattered energy to be modelled would improve spatial accuracy and prediction of lateral energy fraction. Definition of scattering parameters in the frequency domain would also improve predictive accuracy, especially at low frequencies, where geometrical modelling is particularly inappropriate. Improvements in accuracy may also be achieved by including the modelling of barrier edge-diffraction, especially for seating areas behind balcony fronts.

MEASURED EARLY LATERAL ENERGY FRACTIONS IN CONCERT HALLS AND OPERA HOUSES

In the 30 years since early lateral reflections were first suggested as important for concert halls, spatial impression and source broadening have become almost universally accepted as essential characteristics of halls with good acoustics. Two objective measures of source broadening have been proposed. Measured values of the best defined of these measures, the early lateral energy fraction (LF), are considered here. Results from two independent measurement surveys are discussed. Comparisons of LF values by hall show a significant link between hall mean LF and hall width. There is however considerable overlap between measured LF values in different halls so the relevance of describing halls by their mean early lateral energy fraction values is questionable. The behaviour of LF values within auditoria is discussed for different concert hall plan forms and within opera houses. A measure of source broadening including sound level is proposed and results considered in the context of auditorium design.

Reflection and diffraction in room-acoustic modeling

Many programs for room-acoustic modeling are based on the mirror image source concept. This way, only specular reflections are taken into account such that the resulting wave field is incomplete. Nonspecular reflections, or diffraction phenomena, can be added using an algorithm based on Pierce’s theory. Then, the true wave field is significantly better approximated. However, comparison of both results reveals that, averaged over time and space, the energy is equal in both cases: the added diffraction field so strongly interferes with (i.e., is so strongly correlated with) the specular reflections that no energy is added. This intuitively surprising result can be argued with a thought experiment. In this context, taking into account that acoustic perception is described with energetic criteria (clarity, definition, lateral energy fraction, etc.), it should be critically reconsidered as to how far the physically incomplete mirror image source programs are adequate for the prediction of the global acoustic quality of a hall.

Fluctuation of room acoustical parameters on small spatial intervals

The Delft Acoustics group has introduced array technology in room acoustic measurement practice: instead of at a limited number of ‘‘representative’’ places, impulse responses are measured along an array of closely sampled microphone positions. By displaying the responses as a visual entity, the wave character of the sound field is clearly revealed. The data form a base for further processing, enabling the separation and analyzing of the different wave-field components. The sound fields in several concert halls have been measured this way. From impulse responses recorded with 0.05-m spatial separation, the common room acoustical parameters have been determined. It appears that, even between the ten positions in front of one and the same seat, unexpected fluctuations occur, e.g., when measured in octave bands, the clarity index varies over 1 dB on this interval, the lateral energy fraction 0.2 (i.e., between 0.1 and 0.3). For the broadband versions, these variations are approximately halved, still being significant. These results explain why the predictive value of these parameters for the perceptual quality of the acoustics on a (group of) seat(s) is limited. Instead of at one point, the values at several adjacent points should be considered.

Scattering by a barrier in a room

In this paper, the prediction of the effect of a simple scattering element on the acoustics of a room is studied. For the sake of simplicity, the room is a rectangular one with hard walls, and the scattering element is a horizontal thin hard strip-like barrier placed at one or two side walls. The diffraction model adopted for the sharp edge of the scattering hard strip is an exact one based on geometrical optics assumptions. The impulse response of the room may be predicted through using the image sources method for the specular reflections at the hard walls and the barrier in combination with the diffraction model which is expressed in the time domain. From the impulse response many room acoustical parameters related to the subjective rating of the room can be processed. Among these, the inter-aural cross-correlation coefficient (IACC) between two space separated impulse responses (these are usually measured at the ear entrances of a listener), which is a measure of the diffuseness of the field, can thus be calculated at different positions in the room. Another factor, the early lateral energy fraction (ELEF) can also be calculated. Some experimental results on the measurement of the impulse response and of the IACC on a scale model are also presented to support the theoretical ones.

Subjective assessment of the uneven distribution of reflections within a concert hall

Most physical variables other than RT are known to vary significantly within a concert hall. Namely, early to late energy ratio, lateral energy fraction, and sound strength. Less well known are the subjective correlates to the above variations. A series of experiments were carried out in order to assess the number and meaning of subjective dimensions throughout a single hall. Testing environments were both an anechoic chamber array and an actual hall with artificial sources, where subjects were asked to judge classical music samples. In both cases the standard spatial distribution was modified by the addition of discrete reflections while keeping level and RT constant. At least four subjective dimensions were found, as many as found in hall-to-hall comparisons. Up to 59% of variance was related to the spatial image of the source. After comparing results between the anechoic chamber and the actual hall, the visual impression seems to partially compensate for the variations due to distance. These results raise some doubts on the validity of listening tests performed without a visual stimulus. It is also suggested that these effects should be considered in designing halls with active systems.

Spatial impression as measured in concert and opera auditoria

A coherent set of subjective experiments (published in 1973 and 1981) established the effect of spatial impression associated with early lateral reflections. The early lateral energy fraction was proposed as an objective measure. Some authors have preferred to use measures linked to cross correlation, presumably on the basis that this is probably closer to the mechanism used by the ear. The paper reviews the results of measurements of the early lateral energy fraction in 17 British concert spaces, as well as considering the measured results by Bradley in 13 North American concert halls. The discussion considers the relationship between early lateral energy fraction and hall width, the behavior of the measure close to the source and in both parallel-sided halls and fan-shaped halls. The design requirements for high spatial impression are also considered. The subjective effect of early lateral reflections is also influenced by sound level. The consequences for design of considering this additional variable in combination with the directional distribution will also be discussed. Spatial impression in concert halls and opera houses will be examined.

The auditorium measurement chain: A study of the use of auditorium synthesis in the design of concert halls

In the early 1960s, Paul Veneklasen began to develop an idea to synthesize an indoor acoustical space coming to fruition with the development and construction of an electroacoustic system called Auditorium Synthesis. This system allows the listener to preview various types of spaces prior to construction [Paul S. Veneklasen, Int. Symposium on Arch. Acoustics, 21–43 (1974)]. Listener preferential settings including source strength (G), early decay time (EDT), early/late energy (C), and lateral energy fraction from the Otis A. Singletary Center for the Arts (Lexington, Kentucky). A comparison of preferred orchestral settings and modeled seat locations will be performed. With impulse response data from the completed concert hall at the Singletary Center for the Arts, a comparison will be shown of newer parameters between the modeled seat locations from Auditorium Synthesis and concert hall data.

Acoustical characteristics of a 360-degree surround hall

Boettcher Hall in Denver is a large multipurpose hall with audience seating completely surrounding the stage and has been described as a 360-deg surround hall. Extensive acoustical measurements were carried out to evaluate the characteristics of this hall configuration and to compare it to other halls of more conventional design. Impulse response measurements were made for the 45 combinations of three on-stage source positions and 15 receiver positions distributed throughout the audience seating area. The setup of the hall was representative of a typical unoccupied concert configuration. Measured parameters included octave band values of: reverberation times, early decay times, relative sound levels, and early/late sound ratios. Various measures of spatial impression were also measured including: lateral energy fractions, inter-aural cross correlations, and the newly proposed measure of listener envelopment, the late lateral sound level. Hall average values as well as the spatial variation of acoustical measures are compared to data obtained from a number of other halls. [Work supported by the Concert Hall Research Group.]

Effects of an audience and variable architectural features on the acoustics of Boettcher Concert Hall

Acoustic parameters (including reverberation time, early decay time, relative sound level, early to late sound ratios, lateral energy fraction, and interaural cross correlation) have been measured in Boettcher Concert Hall. All parameters were measured using different settings of variable architectural features such as acoustic drapes, variable height overhead stage reflectors, and movable stage towers. The monaural parameters were also measured with and without an audience. The effects of the variable features on the acoustic parameters are assessed. The effects of the audience on some of the acoustic parameters have been estimated using standard calculation methods. Calculated differences are compared to those actually measured in the hall. [Work supported by the Concert Hall Research Group.]

An acoustical design criterion for balcony configuration in auditoria. I. Acoustical characteristics measured under the balcony and the effects on auditory envelopment

The object of this study is to make clear the relation between the shape of balcony and acoustical properties under the overhang, and to derive a design criterion for balcony configuration in auditoria. On the assumption that a physical change of the sound field caused by building a balcony would be primarily a reduction of the energy of early reflections from upside, directional characteristics of early reflections were measured in auditoria both with and without a balcony, and the subjective effect of reflections arriving from upside on auditory envelopment was psycho- acoustically examined. First, the results of measurement showed that the ratio of the vertical component to total early reflection energy ERV was very small under the overhang. That is a peculiarity of the sound field under the balcony. Next, the results of a series of psychoacoustical experiments showed that auditory envelopment became weaker as ERV decreased when lateral energy fraction was constant. They suggested that the lack of early reflections from the ceiling above the main orchestra causes the peculiarity of directional distribution of early reflections, and consequently disturbs auditory envelopment under the balcony.

Consistent physical measures of auditory source width for various frequency components of reflections

At this writing, there is no physical measure that can evaluate consistently the auditory source width (ASW) perceived for sound fields consisting of a direct sound and reflections with various frequency components. In the experiments, a music motif is used as a source signal. The frequency components of reflections are varied by changing the cutoff frequency of low-pass, high-pass, and band-elimination filters, and simultaneously their directions are varied from ±18 to ±90 deg by steps of 9 deg. ASW and some physical measures are obtained for all sound fields in the experiments. The results on ASW indicate that any frequency component of lateral reflections contributes to create ASW almost equally. The comparisons of ASW with many physical measures show that ICCavg and Lfavg can evaluate ASW for all sound fields consistently with high accuracy. They are the average of the degrees of interaural cross correlation and the lateral energy fractions for 1/1-oct bands for 125 Hz to 8 kHz, respectively. Furthermore, the average of the degrees of cross correlation between two points 30 cm apart without using a dummy head for 1/1-oct bands for 125 Hz to 8 kHz is also adequate, although it is less accurate than ICCavg and Lfavg.

An overview of the acoustical effects of an audience.

This paper reviews the various effects of an audience on the acoustical properties of an auditorium. At the most basic level, the total sound absorption of both occupied and unoccupied chairs can be predicted in terms of the perimeter/area ratio of audience seating blocks. Using this technique, reverberation chamber measurements of occupied chairs can be used to estimate the reverberation times of fully occupied halls. This technique has been extended to provide estimates of typical sound‐absorbing properties of occupied chairs for all sizes of seating blocks. Partially occupied seating areas present further problems. Audience effects on early‐ and late‐arriving sound levels and early/late sound ratios are related to the geometry of the halls as well as to changes in the total sound absorption. Interaural cross correlations and lateral energy fractions are not greatly influenced by the presence of an audience. Changes to the more detailed spectra of the direct and early arriving sound show that the effect of the audience on the seat dip attenuation can vary with the type of seating as well as other properties of the hall.

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.

Effect of early reflections from upside on auditory envelopment.

Auditory spaciousness is primarily dependent on the strength of early lateral energy. The difference between the degree of spaciousness caused by only lateral reflections and that caused by lateral and ceiling reflections, however, has not been definitively clarified yet. In this paper, a series of psycho-acoustical experiments is performed to examine the effect of early reflections from upside on spaciousness with lateral reflections. The results show that a single reflection from upside causes a vertical spread of sound image and auditory envelopment becomes stronger as vertical component of reflected energy increases when lateral energy fraction and listening level are constant.

Comparison of concert hall measurements of spatial impression

Two different types of measures of spatial impression, lateral energy fractions and interaural cross correlations, were measured at a number of locations in 14 different concert halls. The mean and range of the hall-average values of these quantities give an indication of typical conditions to be expected in other larger concert halls. Comparisons of hall-average values of the two types of quantities showed them to be significantly related at medium and lower frequencies in good agreement with theoretical expectations. Individual position measurements were less well related and showed that other factors had a major influence on the individual position measurements. In some halls the two types of quantities varied quite differently with increasing source–receiver distance. At a particular seat location, the two types of measures cannot always give similar indications of expected spatial impression. Calculating hall-average values from a small number of measurement locations is shown to introduce significant errors.

Spaciousness judgments of binaurally reproduced sound fields

Spaciousness is known to be a key subjective parameter in determining the acoustical quality of a concert hall. Spaciousness is described as the sense of being enveloped by the sound, or as an increase in the apparent width of the soundstage. Inter-aural cross correlations (IACC) and lateral energy fractions (LF) are two objective measures used to describe the relative spaciousness of concert halls. However, the two measures often do not correlate well with each other. In this study binaurally simulated sound fields were presented to listeners in paired comparison tests. Listeners were asked to identify the more spacious of the two sound fields. Binaural simulations of various North American concert halls were produced by convolving source material with impulse responses measured using a dummy head. The measured LF and IACC values are compared with the judged spaciousness. [Binaural impulse response data were obtained from a measurement tour partially supported by the Concert Hall Research Group.]