Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 167

values depend, of course, on the maximum possible
power of the sound source. By choosing Eq. 7-8 in level
representation and using the reverberation time formula
by Sabine,^6 one obtains the correlation between sound
power level of the sound source LW in dB and sound
pressure level Ldiff in dB in the diffuse sound field, as a
function of the room parameters, volume V in m³, and
reverberation time RT 60 in s.^6

(7-47)

The graphical representation of this mathematical

relation is shown in Fig. 7-19. For determining the

attainable sound pressure level in the diffuse sound field

one can proceed from the following sound power levels

LW .3,29,30

With musical performances, for instance in forte and
piano passages, perception of the dynamic range plays a
decisive role for the listening experience, indepen-
dently of the prevailing volume level. A sound passage
or a spoken text submerging in the surrounding noise
level is no longer acoustically registered and the perfor-
mance considered to be faulty. The mean dynamic range
of solo instruments lies with slowly played tones
between 25 dB and 30 dB.^30 With orchestra music it is
about 65 dB and with singers in a choir about 26 dB.

The dynamic range of a talker is about 40 dB and that of

soloist singers about 50 dB. Without taking into account

the timbre of a sound source, the SNR should generally

be at least 10 dB with pianissimo or whispering.

With large room volumes and high frequencies, the

increase of energy attenuation loss caused by the

medium air should not be neglected. This shall be illus-

trated by an example: in a concert hall with a room

volume of 20,000 m³ (700,000 ft^3 ), the unavoidable air

attenuation at 20°C and 40% relative humidity accounts

for an additional equivalent sound absorption area

which at 1000 Hz corresponds to an additional 110

persons and at 10 kHz to 5000 additional persons.

7.3.3.2 Room Shape

The shape of a room allows a wide margin of vari-

ability, since from the acoustical point of view it is not

possible to define an optimum. Depending on the

intended purpose, the shape implies acoustical advan-

tages and disadvantages, but even in the spherical room

of a large planetarium, it is by room-acoustical means

(full absorbing surfaces) possible to achieve good

speech intelligibility. Acoustically unfavorable,

however, are room shapes that do not ensure an unhin-

dered direct sound supply nor any omnidirectional inci-

dence of energy-rich initial reflections in the reception

area, as is the case, for instance, with coupled adjoining

rooms and low-level audience areas under balconies or

galleries of low room height.

When selecting different room shapes of equal

acoustically effective volume and equal seating capacity

there may result very distinct characteristics as regards

the overall room-acoustical impression. A more or less

pronounced inclination of the lateral boundary surfaces

may produce different reverberation times.^31 In

combination with a sound reflecting and not much

Music (Mean Sound Power Level with “Forte”)

Tail piano, open LW = 77–102 dB

String instruments LW = 77–90 dB

Woodwind instruments LW = 84–93 dB

Brass instruments LW = 94–102 dB

Chamber orchestra of 8 violins LW = 98 dB

Small orchestra with 31 string instruments, 8

woodwind instruments, and 4 brass instruments

(without percussion)

LW = 110 dB

Big orchestra with 58 string instruments, 16

woodwind instruments, and 11 brass instru-

ments (without

percussion)

LW = 114 dB

Singer LW = 80–105 dB

Choir LW = 90 dB

Speech (Mean Sound Pressure Level with Raised to Loud

Articulation)

Whispering LW = 40–45 dB

Speaking LW = 68–75 dB

Crying LW = 92–100 dB

Ldiff LW 10 V

T

= – log--- dB+14 dB*

* add 29.5 dB in U.S. system. Figure 7-19. Correlation between the sound pressure level

Ldiff in the diffuse field and the sound power level LW as a

function of room volume V and reverberation time RT 60.