Manifestations of Task-Induced Stress in the Acoustic Speech Signal

In order to induce stress in an experimental subject, a task involving the addition of numbers under time pressure was developed. The subject was required to read six meters and to announce the sum of his readings, together with a test phrase. By controlling the duration of the meter display, the experimenter could vary the level of stress induced in the subject. For each of 10 subjects, numerous verbal responses were obtained while the subject was under stress and while he was relaxed. Contrasting responses containing the same test phrase were assembled into paired-comparison listening tests. Listeners could identify the stressful responses of some subjects with better than 90% accuracy and of others only at chance level. The test phrases from contrasting responses were analyzed with respect to level and fundamental frequency, and spectrograms of these test phrases were examined. The results indicate that task-induced stress can produce a number of characteristic changes in the acoustic speech signal. Most of these changes are attributable to modifications in the amplitude, frequency, and detailed waveform of the glottal pulses. Other changes result from differences in articulation. Although the manifestations of stress varied considerably from subject to subject, the test phrases of most subjects exhibited some consistent effects.