Infrasound tones at sensation threshold level elicit measurable Frequency-Following responses.

Even barely detectable levels of infrasound are often reported to cause annoyance and complaints. We carefully measured the individual sensation threshold of a pure tone and recorded immediately after the brain's frequency-following response (FFR) at this intensity using the same stimulator. In contrast to 87-Hz tones, 8-Hz tones elicit an FFR already at sensation threshold. Control stimuli with trains of 1-kHz tone pips having the repetition rate of the infrasound tone frequency and sensation threshold intensities evoked no significant FFR. Thus, slow periodicity, causing synchronous activation of auditory nuclei, is not explaining the FFR to low-level infrasound alone.

Dependency of threshold and loudness on sound duration at low and infrasonic frequencies.

Many environmental sounds contain significant energy in the infrasonic and low-frequency (ISLF) ranges that have been associated with cases of annoyance and noise complaints. This study assessed the effect of sound duration on audibility and loudness of ISLF sounds. A first experiment evaluated detection thresholds for tones of 4, 16, and 32 Hz with durations up to 4000 ms. Furthermore, equal-loudness-level contours (ELCs) were obtained as function of duration up to 2000 ms. Tones of 1000 Hz were also included here. Results displayed the known pattern of general sound level decrease with increasing duration up to several hundred milliseconds. ELCs stabilized slightly earlier than thresholds, but after 1000 ms, levels remained roughly constant for both measures except for 4-Hz tones, where the decrease continued up to the longest durations tested. As 4-Hz cycles are perceptually resolved as separate pressure pulses, the authors hypothesized their duration dependence would resemble that of pulse trains. Hence, a second experiment evaluated pulse-train thresholds (1000-Hz carrier) for durations up to 4000 ms. For both pulse repetition rates of 4 and 32 Hz, threshold stabilized after 1000 ms as for tones ≥16 Hz, suggesting the continuing threshold decrease for a 4-Hz tone is specific to infrasound.

The Spectral Extent of Phasic Suppression of Loudness and Distortion-Product Otoacoustic Emissions by Infrasound and Low-Frequency Tones.

We investigated the effect of a biasing tone close to 5, 15, or 30 Hz on the response to higher-frequency probe tones, behaviorally, and by measuring distortion-product otoacoustic emissions (DPOAEs). The amplitude of the biasing tone was adjusted for criterion suppression of cubic DPOAE elicited by probe tones presented between 0.7 and 8 kHz, or criterion loudness suppression of a train of tone-pip probes in the range 0.125-8 kHz. For DPOAEs, the biasing-tone level for criterion suppression increased with probe-tone frequency by 8-9 dB/octave, consistent with an apex-to-base gradient of biasing-tone-induced basilar membrane displacement, as we verified by computational simulation. In contrast, the biasing-tone level for criterion loudness suppression increased with probe frequency by only 1-3 dB/octave, reminiscent of previously published data on low-side suppression of auditory nerve responses to characteristic frequency tones. These slopes were independent of biasing-tone frequency, but the biasing-tone sensation level required for criterion suppression was ~ 10 dB lower for the two infrasound biasing tones than for the 30-Hz biasing tone. On average, biasing-tone sensation levels as low as 5 dB were sufficient to modulate the perception of higher frequency sounds. Our results are relevant for recent debates on perceptual effects of environmental noise with very low-frequency content and might offer insight into the mechanism underlying low-side suppression.