NTi Audio Acoustilyzer AL1 User Manual

Page 92

Modulation Transfer Function (MTF)

For high speech intelligibility it is mandatory that the integrity of the

modulations of the transmitted voice signal be preserved. Therefore, the

three core intelligibility measurement methods, STI, RASTI und STI-PA are

based on measuring the MTF (Modulation Transfer Function). This function

quantifies the degree to which the voice modulations are preserved in the

individual frequency bands.

The envelope shown in Figure 3 is divided into 1/3rd octave band

frequencies, thus resulting in 14 frequency bands between 0.63 and 12.5

Hz. The modulation transfer function determines for each of the 7 voice

bands (refer to Figure 1) how well the modulations are preserved.

Based on the MTF results as well as further parameters such as the

sound pressure level, hearing threshold, frequency response or psycho

acoustic effects (masking effects) it becomes possible to reliably determine

the speech intelligibility. The corresponding calculations, which are

optimized continuously, are based on extensive and profound evaluations

and comparisons with subjective methods. Measuring the complete

MTF – as required for STI – can become rather complex. For instance,

14 * 7 = 98 individual measurements must be executed, thus resulting in

a total acquisition duration of 15 minutes. Therefore, different approaches

have been developed to reduce the test period and to enable speech

intelligibility measurements with portable instruments.