Handbook for Sound Engineers

Computer Aided Sound System Design 1341

with ear simulator). The transfer function between
transmitting and receiving locations is calculated from
the obtained room impulse response. Very often a spark
discharge generator is used as a sound transmitter in air
(nowadays electronic MLS scale radiators are also
used). With an impulse width in the model of 80μs, it is
possible to resolve path differences equaling 60 cm in
the original room. The reproducibility of the maximum
sound pressure is below ±0.2dB.
Special model sound sources enable simulation of a
talker or a singer, of an orchestra as nondirectional
sound source in the center of the same, of orchestral
instrumental groups (see Section 35.1.2.2), and of loud-
speaker lines with variable directivity characteristics.
Preferably the registration of the room impulse
response is dual-channel by the microphones of the
dummy head at listener seats, which are representative
for determined seating groups so that the binaural head-
related listening parameters of the human auditory
organ are optimally reproduced. In a model of scale
1:20 the diameter of this miniature dummy head must
be about 11 mm, Fig. 35-2.

The investigated frequency range lies between 5 kHz

and 200 kHz in the scaled model, which corresponds to

250 Hz to 10 kHz in the original. Structures whose

linear dimensions fall below approximately 8 cm in the

original room are not reproduced in the model. Also

sound absorbers and wall impedances below the studied

frequency range are not considered in the model tests.

For better access to the models during the measure-

ments, these should be carried out in air under normal

pressure. Owing to the excessive atmospheric absorp-

tion occurring at the model frequencies there occurs a

faulty momentary value of the sound pressure that is

mathematically corrected by a real-time compensation.

Without these mathematical corrections the measure-

ments must be carried out in a nitrogen environment

instead of air, in which case the drawback consists of

bad access to the model.

All physical acoustic phenomena such as diffraction

and scattering are represented in a frequency-true

fashion.

The obtainable accuracy is presently still superior to

that of a computer simulation. The method is capable of

providing answers to questions concerning balance

investigations in rooms for music performances, see

Section 35.1.2.2, the influence of electroacoustical

components on room-acoustical parameters, and the

directional effect of wall and ceiling structures prepared

as scaled models.

By using original sound source simulations (talker,

singer, nondirectional sound source, orchestra instru-

mental groups, loudspeakers) and a dummy head as a

receiver unit, the described measuring procedure is

applicable also for original rooms.

35.1.1.2 Balance Investigations of Music Performances

The scaled-model simulation of an orchestra can in first

approximation be realized by a nondirectional sound

irradiation from the center of the same. A more detailed

simulation is necessary, however, if one wants to have

information about the influence of a room on the bal-

ance of the different instruments at the listener’s seat. A

useful approximation can be obtained by a simulation of

orchestra instrumental groups in which their sound spec-

tra are based on the frequency response chiefly repro-

duced in music presentations. Their directional

characteristics are derived from the usual playing pos-

ture.^2

The simulated orchestra is subdivided into four

instrumental groups, in which the percussion instru-

ments, in view of their considerable loudness and adapt-

able style of playing, may be left out of consideration:

• String instruments (St)


• Woodwind instruments (Wo)


• Brass instruments Bl (Bi)


• Bass instruments Ba (Ba)

To this may be added the electroacoustical model trans-

ducer of a singer/talker (S).

The scaled-model simulation comprises the impulse

excitation by a spark-gap generator provided with a

shading reflector of defined sound attenuation so as to

align it with the directional characteristic of the instru-

Figure 35-2. Model-size dummy head for measurements at
a scale of 1:20.