Loudness of Speech and Speech-Like Signals

The influence of different properties of speech-like signals on loudness was investigated by measuring levels at equal loudness in normal-hearing listeners for 22 speech-like stimuli using categorical loudness scaling and an adaptive matching procedure. Eight of the stimuli, referred to as unprocessed stimuli, had the same speech-like long-term spectrum, but differed in other speech-related properties, ranging from a stationary noise over speech-modulated signals to real intelligible and unintelligible speech. The other stimuli were designed to investigate the influence of severe modifications of speech-related properties due to spectral filtering, reverberation, compression, and expansion on loudness. In general, results were similar for both measurement methods and revealed only small effects of temporal modulations on loudness perception, and no influence of intelligibility or reverberation. Even severe modifications of the stimuli by highpass-filtering and expansion did not affect levels at equal loudness. Only lowpass-filtering significantly decreased loudness and compression significantly increased loudness. The experimental results indicate that the long-term spectrum of speech has a major influence on loudness while other properties of speech only have a minor contribution. Some of the results could not be predicted by loudness models based on short-term loudness. Instead, longer time constants were required to not overestimate the loudness of time-varying speech-like sounds.