NTi Audio Acoustilyzer AL1 User Manual
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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.