Application note – Calculation of STI in rooms
5
Loudspeakers - Input signal
For a loudspeaker system the acoustic chain starts with speech that reaches a microphone, followed by
amplifiers and equalizers and one or more loudspeakers.
In relation to the STI parameter the input signal is supposed to represent speech and the input to the
electro-acoustic system may be from a microphone or playback. The input signal may then depend on the
gender (male or female speech), vocal effort, and distance from the mouth to the microphone. Speech
spectra for male and female speech that should be used for the STI calculations are given in Table 3.
Table 3 – Speech spectra in octave bands relative to the A-weighted SPL for different gender. This data are
from IEC 60268-16 [1], except the 125 Hz value for females, which has been suggested in ISO 9921 [2].NB,
these speech spectra should only be applied to electro-acoustic systems.
Comparing the IEC speech spectra in Table 3 with the ANSI speech spectra in Table 1, there are obvious
differences. While the ANSI spectra apply to the far field (1 m or longer distances), the relative IEC spectra
in Table 3 apply to close distances, typically a microphone distance of only 5 cm from the mouth. The origin
of the IEC spectra is dull, but may go back to very early measurements at the Bell Laboratories in the
1930’ies or 1940’ies. The spectra show unexpected high values at the low frequencies (125 – 250 Hz), which
may be explained by the so-called proximity effect
2
.
The male speech is generally recommended to assess speech transmission channels [1, A.3.4]. The reason is
that female speech is considered to be more intelligible than male speech, so the latter is a kind of “worst
case”. In addition to different speech spectra, there also applies gender-specific weighting factors, so e.g.
the 2 and 4 kHz octave bands are given more weight for female speech compared to male speech. The
parameters STI (male) and STI (female) are calculated by ODEON.
Loudspeakers for sound reinforcement - Output signal
The important acoustical parameters for a sound source representing a loudspeaker are the sensitivity, the
frequency response and the directivity in octave bands. Loudspeakers for sound reinforcement are low
impedance speaker systems (8-ohm speaker systems).
Loudspeaker types
Low impedance loudspeakers are divided into different types: Passive loudspeakers, for direct connection
to a power amplifier; active loudspeakers, with equalization/processor/cross-over required before
amplification, and powered loudspeakers, with built-in equalization/processor/amplification.
2
Proximity effect is the increase in the low-frequency sensitivity of a microphone when the sound source is close to it.
The proximity effect may be responsible for speech spectra showing emphasis in the low-frequency range.
Frequency, Hz 63 125 250 500 1000 2000 4000 8000 A-weighted
Males, dB - 2,9 2,9 -0,8 -6,8 -12,8 -18,8 -24,8 0,0
Females, dB - -4,4 5,3 -1,9 -9,1 -15,8 -16,7 -18,0 0,0