On the loudness of low-frequency sounds with fluctuating amplitudes.

Some environmental sounds have strong amplitude fluctuations that may affect their perceived loudness and annoyance. This study assessed the effect of beat rate (fb) and center frequency (fc) on the loudness of low-frequency beating tones. The loudness of two-tone complexes (TTCs) with fc = 40, 63, 80, and 1000 Hz was matched with that of unmodulated tones (UTs). Frequency differences between the TTC components, corresponding to fb = 1, 2, 5, and 12 Hz, were used. To compensate for the steep decline in hearing sensitivity below 100 Hz, prior to the loudness match, subjects adjusted the relative levels (ΔL) of the TTC components to give maximum beat perception. Twenty-four normal-hearing subjects were tested. The values of ΔL giving best beats were well predicted from the transfer function of the middle ear and the estimated shapes of the auditory filters, assuming that the auditory filter whose output dominated the beat percept was centered somewhat above fc. At the same root-mean-square level and independent of fc, TTCs were perceived as louder than UTs for fb ≤ 2 Hz, had roughly equal loudness to UTs for fb = 5 Hz, and were less loud than UTs for fb = 12 Hz.

The detailed shapes of equal-loudness-level contours at low frequencies.

High-resolution equal-loudness-level contours (ELCs) were measured over the frequency range 10-250 Hz using 19 normal-hearing subjects. Three levels of the 50-Hz reference sound were used, corresponding to the levels at 50 Hz of the 30-, 50-, and 70-phon standardized ELCs given in ISO-226:2003. The dynamic range of the contours generally decreased with increasing reference level, and the slope was shallow between 10 and 20 Hz, consistent with previous studies. For the lowest level, the ELCs were sometimes but not always smooth and on average followed the standardized 30-phon contour for frequencies above 40 Hz. For the two higher levels, the individual ELCs showed a distinct non-monotonic feature in a "transition region" between about 40 and 100 Hz, where the slope could reach near-zero or even positive values. The pattern of the non-monotonic feature was similar across levels for the subjects for whom it was observed, but the pattern varied across subjects. Below 40 Hz, the slopes of the ELCs increased markedly for all loudness levels, and the levels exceeded those of the standardized ELCs. Systematic deviations from the standardized ELCs were largest for frequencies below 40 Hz for all levels and within the transition region for the two higher levels.