Frequency-Following Responses (FFRs) Study to Infrasound and Low-Frequency Tones

With the increasing noise pollution, especially in the low-frequency (LF) (f<200) Hz) and infrasound (IS) (f<20)Hz), public health concern arises and IS had been a source of annoyance and complaints, affecting the quality of life negatively. Moreover, while natural IS origins are harmless, human-made sources of IS such as heavy machinery, aeroplanes, and wind turbines, emit significantly more abundant energy. Also, IS is ubiquitous, and audible to all healthy human hearing organs providing sufficient energy levels.

The current study investigates the question – is the infrasound special? By examining the human brain responses and providing an objective measure rather than subjective or political opinion, to establish substantial scientific evidence.

The study involves using comparison between frequency following responses (FFRs) to IS with an 11 Hz pure tone, and LF with 88 Hz pure tone, and envelope following response (EFR) to IS modulation with LF carrier, using a sinusoidal amplitude modulation (SAM) tone consisting 88 Hz carrier with 11 Hz modulation.

The results showed two subjects with valid responses, and spurious hardware effects prevented the recording of valid FFRs on others. The valid results indicated that both IS and LF do evoke FFR, and the response grows with higher stimulus levels. Furthermore, the FFR is higher for IS relative to LF, at equal loudness points.

The data analysis demonstrated that IS FFR grows faster and have higher overall brain response than LF. Additionally, the EFR of the SAM tone reached similar growth level as IS FFR, but for higher stimulus levels and added delay of growth above the signal noise.

In conclusion, the study shows a different brain response to IS compared with LF, and the IS EFR of the SAM tone responses behave more like the IS response rather than the LF response, in term of brain response, but with a growth delay relative to loudness and noise floor.