THE ART AND SCIENCE OF CONCERT HALL ACOUSTICS

II. DEFINITION OF TERMS

It is generally agreed that good acoustics are a balance of several variables, and that different values for these variables suit different types of music.

The Reverberation Time, also called RT, is defined as the amount of time necessary for a sound impulse to decay to inaudibility. It is also sometimes called T30, because it is measured by amount of time for the sound to decay 30dB multiplied by 2. A high RT value is favored by most religious music: imagine a Gregorian chant that needs a long “ringing” sound to sustain it. Music of the Romantic period also needs a longer RT (Beranek 569). The ringing part of the note is known as the tail, and is a factor that supports and improves musicality by giving the musician a feeling of depth (Kahn 2).

The downside of a high RT, however, is decreased clarity and definition. The longer a note sustains, the less likely the attack on next note will be heard well. The music of the Baroque and early Classical composers is best suited to low RT values because the intricate musical patterns need a high degree of clarity to communicate, and are only muddled by reverberation. Clarity is technically a measure of the degree to which the individual sounds stand apart from one another. Measured in decibels, it is the ratio of the sound energy in the first 80msec to the sound energy following the first 80msec. Definition is a more decisive measurement: the percentage of sound energy that falls in the first 50msec versus the total sound energy. This is the energy of the attack, and for proper rhythm and musicality, it must be stronger than the tail (Beranek 32-33).

The implication, therefore, is that the Early Decay Time, or the amount of time in which the first 10dB fade away (multiplied by 6 for comparability), must be short (Figure II.1).

Fig. II.1. Three decay curves with varying EDT’s, demonstrating how a short EDT will keep the early sound strong and clear.

If the EDT is long, it indicates that the early sound itself is weak, and the initial attack of the notes will be lost into a tail that is too powerful (Figure II.2).

Figure II.2. Two examples of how the length of RT and EDT can affect the sound of a series of notes.

Furthermore, a long EDT can indicate a low uniformity of sound; a pleasant feeling of the note hanging in the air will instead sound more like an echo or a hum (Beranek 32-33).

Another important impression that assists any style of music is that of envelopment: being surrounded by the music. Envelopment is often quantified by the value of Lateral Fraction; the amount of sound that did not come directly from the source but came on the horizontal plane over the total amount of sound (including that which came from the source). It can also be graphed in a soundrose – a circular representation of the direction from which a listener receives sound impulses (Figure II.3).

Figure II.3. Two Sound Roses demonstrating good and bad lateral reflections.

This is the great difference between halls with good and bad acoustics. In good halls, the sound reflects off of walls at several angles before meeting the receiver. Because a listener’s ears are picking up signals from more positions than straight ahead, the ears’ use of delay to measure direction reads the lateral reflections as music emanating from all around the listener. A hall with good lateral reflections usually has a feeling of intimacy; it feels smaller. All halls have the general goal of sounding as small as possible, because that way the audience feels up close and personal with the orchestra. The best way to achieve this is to build a small hall (Kahn 3-4).

There are a few other variables that should be mentioned. The value of Sound Pressure Level is directly related to loudness, and how much a hall helps or hinders volume in general. A quiet hall would not be conducive to the extraordinary loudness of Wagnerian Opera. Speech Transmission Index is a good measure of how the hall relates to a speaker, and is literally the percentage of syllables spoken that are understood (Lynge 53-56).

Abbreviation	Name	Range	Definition
RT or T30	Reverberation Time	1.8 - 2.2	Time necessary for the signal to decay from –5 to –35 dB multiplied by 2
EDT	Early Decay Time	1.6 - 2.2	Time necessary for the signal to decay from 0 to –10 dB multiplied by 6
C80	Clarity Factor	> 3	Ratio in dB of energy in first 80msec / energy after 80msec
D50	Deutlichkeit (definition)	> 65%	Ratio in dB of energy in first 50msec / total energy
SPL	Sound Pressure Level	Close to orig. level	Loudness
LF	Lateral Energy Fractions	30% - 45%	Ratio of energy that does not come directly from source / all energy
STI	Speech Transmission Index	65% - 100%	Percentage of words/sentences/phonemes understood by the listener

Figure II.4. Chart of acoustical variables, their definitions and an ideal range in which a good hall’s values would fall.