Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 155

(7-27)

One should thus strive for an intelligibility of syllables
of at least 85%, D=D 50 t0.5, or 50%.

7.2.2.7 Speech Transmission Index (STI) (Houtgast,
Steeneken)

The determination of the STI values is based on
measuring the reduction of the signal modulation
between the location of the sound source—e.g., on
stage—and the reception measuring position with octave
center frequencies of 125 Hz up to 8000 Hz. Here
Steeneken and Houtgast^15 have proposed to excite the
room or open space to be measured with a special modu-
lated noise and then to determine the reduced modula-
tion depth.
The authors proceeded on the assumption that not
only reverberation and noise reduce the intelligibility of
speech, but generally all external signals or signal
changes that occur on the path from source to listener.
For ascertaining this influence they employ the modula-
tion transmission function (MTF) for acoustical
purposes. The available useful signal S (signal) is put
into relation with the prevailing interfering signal N
(noise). The determined modulation reduction factor
m(F) is a factor that characterizes the interference with
speech intelligibility

(7-28)

where,
F is the modulation frequency in hertz,
RT 60 is the reverberation time in seconds,

SNR is the signal/noise ratio in dB.

To this effect one uses modulation frequencies from
0.63 Hz to 12.5 Hz in third octaves. In addition, the
modulation transmission function is subjected to a
frequency weighting (WMTF—weighted modulation
transmission function), in order to achieve a complete
correlation to speech intelligibility. In doing so, the
modulation transmission function is divided into
7 octave bands, which are each modulated with the
modulation frequency.^14 This results in a matrix of
7 × 14 = 98 modulation reduction factors, mi.
The (apparent) effective signal-noise ratio X can be
calculated from the modulation reduction factors mi

(7-29)

These values will be averaged and for the seven

octave bands the Modulation Transfer Indices

MTI=(Xaverage+ 15)/30, are calculated. After a

frequency weighting in the seven bands (partially sepa-

rated for male or female speech) you obtain the Speech

Transmission Index, STI.

The excitation of the sound field is done by means of

a sound source having the directivity behavior of a

human speaker’s mouth.

In order to render twenty years ago this relatively

time consuming procedure in real-time operation, the

RASTI-procedure (rapid speech transmission index)

was developed from it in cooperation with the company

Brüel & Kjaer.^16 The modulation transmission function

is calculated for only 2 octave bands (500 Hz and 2 kHz)

which are especially important for the intelligibility of

speech and for select modulation frequencies—i.e., in all

for only nine modulation reduction factors mi. However,

this measure is used increasingly less.

Note: Schroeder 42 could show that the 98 modula-

tion reduction factors m(F) may also be derived from a

measured impulse response

(7-30)

This is done now with modern computer-based

measurement routines like MLSSA, EASERA, or

Win-MLS.

A new method to estimate the speech intelligibility

measures an impulse response and derives STI values

with the excitation with a modulated noise. The

frequency spectrum of this excitation noise is shown in

Fig. 7-9.

You recognize ½octave band noise, radiated through

the sound system into the room. By means of a mobile

receiver at any receiver location the STIPa values can

be determined.8,9 Any layman may use this method and

no special knowledge is needed. It is used more and

more to verify the quality of emergency call systems

(EN 60849),^33 especially in airports, stations or large

malls.

According to the definition the STI-value is calcu-

lated by using the results of Eq. 7-29

C 50 10

D 50

1 – D 50

= log©¹§·----------------- dB

mF 1

1

2 SFRT

13.8

-----------------------

©¹

+§·^2

--------------------------------------------^1

110

SNR

©¹ 10 -------------dB

• §·

= u--------------------------------

Xi 10

mi

1 – mi

= log©¹§·-------------- dB

mF

h

2

te

• j 2 SFt
  td
  0

f

³

h^2 t td

0

f

³

=----------------------------------------