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Table 1. Summary of standard room acoustics parameters
and divided temporally, although the time periods most relevant to each percept are the subject of ongoing research. To understand the spatial distribution of reflec- tions around the listener, a multi- channel impulse response can be measured with a directional mi- crophone array (Figure 2, right). Several numerical parameters standardized by ISO 3382-1:2009 (International Organization for Standardization [ISO], 2009; summarized in Table 1), can be derived from impulse responses measured with an omnidirec- tional sound source.
There is growing consensus among acousticians that al- though many of these param- eters are useful, they do not provide a complete representa- tion of concert hall acoustics. In an interview with the author of
Parameter
Reverberation time (RT, T20, T30)
Early decay time (EDT)
Strength (G)
Clarity (C80)
Early lateral energy fraction (JLF)
Late lateral sound level (LJ)
Interaural cross- correlation coefficient (IACCEarly, IACCLate)
Stage support (STEarly, STLate)
Description
Time for sound to decrease by 60 dB, based on linear fit to energy decay (excluding direct sound and earliest reflections)
Similar to RT, but based on initial 10 dB of decay (including direct sound and earliest reflections)
Logarithmic ratio between sound energy in room vs. free field 10-m away from the same source
Logarithmic ratio between early (0-80 ms) and late (after 80 ms) energy
Ratio between lateral and total energy within first 80 ms
Logarithmic ratio between late lateral energy (after 80 ms) and total energy
Binaural measure of similarity between sound at left and right ears, reported separately for early (0-80 ms) and late (after 80 ms) energy
Ratios between reflected and direct sound measured on stage, reported separately for early (20-100 ms) and late (100-1,000 ms) energy
Intended Perceptual Correlate N/A
Reverberance Loudness
Clarity
Apparent source width
Listener envelopment
Early: apparent source width Late: listener envelopment
Early: ensemble hearing Late: reverberance on stage
Data from the International Organization for Standardization (ISO, 2009).
Eckhard Kahle (personal communication, 2018) discussed how the early design process differs outside the United States, where project owners often hire independent acousticians to develop an “acoustic brief,” and design teams compete against each other to develop a conceptual design that most effec- tively responds to the acoustical goals outlined by the brief. With such a variety of perceptual factors and approaches to prioritizing them, it is no surprise that concert halls around the world sound as different from each other as they do.
How Can Perceptual Goals be Translated into Quantitative, Measurable Data? Considering a concert hall as a linear, time-invariant sys- tem, an impulse-response measurement can be used to understand how the room modifies the sounds of musical instruments. Figure 2 shows impulse responses measured with an omnidirectional loudspeaker and two different mi- crophones. The impulse response measured with an omni- directional microphone (Figure 2, left), illustrates the direct sound path from the loudspeaker to the microphone, early reflections that are strong and distinct from each other and weaker late reflections that occur closely spaced in time and decay smoothly. Measurements are analyzed by octave band
this article, Pfeiffer (personal communication, 2018) noted “there’s a range of performance in any of the given parameters that’s acceptable, and a range that’s not” but that the “mythi- cal holy grail of perfect acoustics” does not exist. Acoustician Paul Scarbrough (personal communication, 2018) noted that standard parameters are particularly limited in describing the spatial distribution of sound, saying “we’re not measur- ing the right things yet.” Bassuet (personal communication, 2018) suggested that “we should not be afraid to make con- nections between emotional value and [new] acoustical met- rics.” Recent article titles are similarly critical and illustrative: “In Search of a New Paradigm: How Do our Parameters and Measurement Techniques Constrain Approaches to Concert Hall Design” (Kirkegaard and Gulsrund, 2011) and “Throw Away that Standard and Listen: Your Two Ears Work Better” (Lokki, 2013).
Deficiencies of Existing Objective Parameters
In summary, the limitations are largely attributable to dif- ferences between an omnidirectional sound source and an orchestra and between omnidirectional microphones and the human hearing system. As described in detail by Meyer (2009), each musical instrument has unique and frequency- dependent radiation characteristics. As musicians vary their
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