Listening Effort for Soft Speech in Quiet.

In addition to speech intelligibility, listening effort has emerged as a critical indicator of hearing performance. It can be defined as the effort experienced or invested in solving an auditory task. Subjective, behavioral, and physiological methods have been employed to assess listening effort. While previous studies have focused predominantly evaluated listening effort at clearly audible levels, such as in speech-in-noise conditions, we present findings from a study investigating listening effort for soft speech in quiet. Twenty young adults with normal hearing participated in speech intelligibility testing (OLSA), adaptive listening effort scaling (ACALES), and pupillometry. Experienced effort decreased with increasing speech level and "no effort" was reached at 40 dB sound pressure level (SPL). The difference between levels rated with "extreme effort" and "no effort" was, on average, 20.6 dB SPL. Thus, speech must be presented well above the speech-recognition threshold in quiet to achieve effortless listening. These results prompted a follow-up experiment involving 18 additional participants, who completed OLSA and ACALES tests with hearing threshold-simulating noise at conversational levels. Comparing the results of the main and follow-up experiments suggests that the observations in quiet cannot be fully attributed to the masking effects of internal noise but likely also reflect cognitive processes that are not yet fully understood. These findings have important implications, particularly regarding the benefits of amplification for soft sounds. We propose that the concept of a threshold for effortless listening has been overlooked and should be prioritized in future research, especially in the context of soft speech in quiet environments.