Evaluation of In-Car Communication Systems - Digital Signal Processing for In-Vehicle Systems and Safety - page 76

0°

30°

330°

0 dB

-5 dB

60° 300°

-10 dB

Front
passenger

270°

90°

1400 - 2000 Hz

240°

120°

125 - 250 Hz

150°

210°

180°

Left rear
passenger

Right rear
passenger

Middle rear
passenger

Fig. 5.6 Average directionality of a speaking human head with additional passengers inside the
vehicle (According to [ 8 ])

Hence, depending on the level of the background noise, different amplification values
are needed to improve speech intelligibility.
Fortunately, the ICC system does not have to compensate the whole degradation
of the SNR due to the increased background noise. Because of the Lombard effect,
any person speaking in a noisy environment automatically will raise his voice in
order to increase the efficiency of the communication [ 9 ]. This leads to an increased
overall speech level while increasing the noise level. In literature rates of about
0.3-0.7 dB, speech power increment per 1 dBA increase of the background noise
level (A-weighted) has been published [ 6 , 10 ]. Due to the increased background
noise as well as the higher speech level (Lombard effect), the system has to exceed
an amplification of about 6-12 dB higher (dependent on the vehicle) in comparison
to the standstill case [ 6 ].
All these boundary conditions enforce a trade-off between sufficient amplifica-
tion for increasing the speech intelligibility and disturbances for the talking and
listening passengers. This compromise has to be found independently for every type
of vehicle.