A BS TRA C T This article reviews the fundamental ideas of the binaural recording technique. A model is given that describes the sound transmission from a source in a free field, through the external ear to the eardrum. It is shown that sound pressures recorded at any point in the ear canals-possibly even a few millimeters outside and even with a blocked ear canal-can be used for binaural recordings, since the), include the full spatial information given to the ear. The sound transmission from a headphone is also described. It is shown how the correct total transmission in a binaural system can be guaranteed by means of an electronic equalizing filter between the recording head and the headphone. The advantage of an open headphone is stated. It is shown that a certain degree of loudspeaker compatibility can be achieved, if the equalizer is divided into a recording side and a playback side. A method for true reproduction of binaural signals through loudspeakers is also described. A number of topical andprospectedapplications of binaural technology are mentioned. Some of these utilize computer synthesis of binaural signals, a technique which is also described.

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.

As wind turbines get larger, worries have emerged that the turbine noise would move down in frequency and that the low-frequency noise would cause annoyance for the neighbors. The noise emission from 48 wind turbines with nominal electric power up to 3.6 MW is analyzed and discussed. The relative amount of low-frequency noise is higher for large turbines (2.3-3.6 MW) than for small turbines (≤ 2 MW), and the difference is statistically significant. The difference can also be expressed as a downward shift of the spectrum of approximately one-third of an octave. A further shift of similar size is suggested for future turbines in the 10-MW range. Due to the air absorption, the higher low-frequency content becomes even more pronounced, when sound pressure levels in relevant neighbor distances are considered. Even when A-weighted levels are considered, a substantial part of the noise is at low frequencies, and for several of the investigated large turbines, the one-third-octave band with the highest level is at or below 250 Hz. It is thus beyond any doubt that the low-frequency part of the spectrum plays an important role in the noise at the neighbors.

The risk of fistulae is especially high in patients initially treated with radiotherapy for nonglottic advanced stage tumors. A significant decrease in the number of performed salvage laryngectomies over the 10 years was seen. Over the same time period, the annual number of fistulae remained almost constant. The resulting more than doubling of fistulae rate could thus in part be explained by less surgical routine.

Sound transmission to the eardrum from various points in the external ear was measured by means of probe microphone technique. Twelve human subjects participated, and three directions of sound incidence were included. For the major part of the audio frequency range the transmission to the eardrum proved independent of direction from points at the centerline of the ear canal, including the entrance (open or blocked). The results further suggested that the region with independent transmission extends some millimeters outside the entrance plane. The transmission from the free field to the eardrum was divided into a directional-dependent part and two directional-independent parts: (1) the transmission from the free field to the blocked entrance, (2) a pressure division between the radiation impedance and the ear-canal input impedance, and (3) the transmission along the ear canal. All parts of the transmission were seen to be highly individual. The first part was shown to be uncorrelated with any of the other parts, whereas mutual dependence of parts (2) and (3) resulted in a smaller variation in the combined transmission than for the parts in separate. The standard deviation between subjects for head-related transfer functions (HRTFs) measured at the eardrum, the open entrance, and the blocked entrance was studied, and the lowest values were found for the blocked-entrance HRTFs. It is concluded, that the blocked entrance is the most suitable point for measurements of HRTFs and for binaural recordings, since sound at this point includes the complete spatial information, and in addition to that the minimum amount of individual information.

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Thresholds of hearing were determined in pressure field at frequencies from 4 Hz to 125 Hz. At the frequencies 4-25 Hz hearing thresholds were found that a re in the lower middle of the range already reported by other investigators. At frequencies from 25 Hz to I kHz thresholds have already been determined in free field by the same method and using the same subjects. The two investigations overlap at frequencies from 25 Hz to 125 Hz, and in this range the results were almost identical. The differences were below 1 dB, except at 63 Hz where the difference was 2.5 dB. None of the differences was significant in a t•test.

Contours of equal annoyance were determined for pure tones in the frequency range 4 – 31.5 Hz. The curves show a narrowing of the dynamic range of the ear at low frequencies. The same pattern is seen for equal loudness curves, and the results support the theory that the annoyance of infrasound is closely related to the loudness sensation. Annoyance ratings of 1/3 octave noise did not deviate from ratings of pure tones with the same sound pressure level. Combinations of audio and infrasonic noise were in general given a rating close to or slightly above the rating of the most annoying of the individual noise conditions. For infrasound the proposed G1-weighting curve is shown to give values that correlate well with subjective annoyance rating. Values obtained with the G2-curve do not correlate as well. Low audio frequencies are not covered by the G-curves, and it is shown that these are insufficiently covered by the A-curve. Further research is needed in this area.