Equal annoyance contours for infrasonic frequencies

Eighteen subjects (age range: 18–25) rated the annoyance of 18 sound stimuli on a graphic scale (four infrasonic frequencies at different intensity levels and four levels of 1000 Hz octave-filtered pink noise for reference). The exposure time for each stimulus was 15 minutes. The order of exposures was determined from a latin square and each subject was exposed to only one stimulus per day. Equal annoyance contours were constructed to connect points that produced the same annoyance rating. The equal annoyance curves demonstrate that the lower the frequency the greater must be the sound pressure to cause a given amount of annoyance. Compared with 1000 Hz the curves lie much closer together in the infrasonic range. The closeness of the curves in the infrasonic region implies that small changes in sound pressure may cause relatively large changes in annoyance. Based on the experimental results a weighting curve with a slope of 12 dB per octave is suggested for the assessment of annoyance and loudness in the infrasonic range. A curve with the same slope and an attenuation of O dB at 10 Hz is at present under consideration in the International Standardization Organization. For environmental purposes a maximum permitted level of 95 dB is proposed for use with this curve. </jats:p

Loudness of pure tones at low and infrasonic frequencies

Contours of equal loudness were determined in the frequency range 2–63 Hz and the loudness range 20–100 phon. The loudness curves run almost parallel in the infrasonic frequency range and much closer than in the audio region. Infrasound only a few dB above the hearing threshold will therefore seem loud and possibly annoying. The subjects were 20 normal hearing students aged between 18 and 25, and the psychometric method was based on maximum-likelihood estimation of psychometric functions. </jats:p