Morten Lydolf

A questionnaire survey of complaints of infrasound and low-frequency noise

A survey of complaints about infrasound and low frequency noise has been carried out. 198 persons reported their troubles in a questionnaire. Their verbal reports often described the sound as deep and humming or rumbling, as if coming from the distant idling engine of a truck or pump. Nearly all respondents reported a sensory perception of a sound. In general they reported that they perceived the sound with their ears, but many mention also the perception of vibration, either in their body or in external objects. The sound disturbs and irritates during most activities, and many consider its mere presence as a torment to them. Many of the respondents reported secondary effects, such as insomnia, headache and palpitation, which they associtated with the sound mainly because it occured at the same place as the sound. In a majority of the cases, only one or a few persons can hear the sound, but there are also examples, where it is claimed to be audible to everybody. Typically, measurements have shown that existing limits (and hearing thresholds) are not exceeded. The investigation leaves the key question: Are the troubles induced by an external sound or not, and if they are, which frequencies and levels are involved? The feasibility of a study of this is supported by the results.

Audiometric tests with various earphones, signals of short duration, and signals of high frequencies

New audiometric standards on tests with signals of short duration and tests with signals of high frequencies are in the course of preparation by the international working group, ISO/TC 43/WG 1. In this paper, first a selection of five different audiometric earphones is compared by using threshold measurements in the frequency range 125 Hz–8 kHz. These data are meant for validation of the reference threshold for regular pure‐tone audiometry given in ISO 389‐1. Signals of short duration consist of either rectangular pulses with a duration of 0.1 ms or pure‐tone samples including only seven full periods at the octave band frequencies 0.5–8 kHz. Both single events and bursts with 20 signals presented during 1 s were used as stimuli. Finally, audiometric tests were made with high‐frequency pure‐tone stimuli of 1‐s duration at: 8, 9, 10, 11.2, 12.5, 14, and 16 kHz. For all experiments the chosen psychophysical method was the ascending method, implemented nearly as described in ISO 8253‐1. Twenty‐five young, oto...

Human localization of sound signals with reduced bandwidth

In multimedia systems, the sound is often presented with reduced bandwith. As 3‐D sound is introduced, this might give rise to localization problems, a consequence which is partly confirmed by this investigation. A listening experiment was made, in which 12 subjects participated. The subjects were placed in an anechoic room in a setup with 17 loudspeakers placed in different directions. As a reference experiment the subjects listened to a pink noise signal played from one loudspeaker at a time. The subjects were asked to point out the loudspeaker from which they perceived the sound. In eight other experiments the sound was low‐pass filtered at frequencies of 1, 2, 4, or 8 kHz, or high‐pass filtered at 0.5, 1, 2, or 4 kHz. The procedure was the same as in the reference experiment. For each subject the order of the tests was randomized. The experiment showed that all four low‐pass filtered signals gave an increase in localization errors. This was not the case with the high‐pass filtered signals.

Equal‐loudness level contours below 1 kHz

Binaural equal‐loudness level contours and the threshold of hearing have been measured on 25 otological normal‐hearing subjects. In the frequency range from 50 Hz to 1 kHz the measurements were made in the free field for frontal sound incidence. From 20 to 100 Hz the experiments were made in a pressure field chamber with an inner volume for the test subjects of approximately 1 m3. The experiment is made with pure tones at each of the third octave frequencies in the frequency range. Loudness levels between 20 and 100 phon are measured. The data are going to be used for the standardization work in the Working Group ISO/TC43/WG1 for a revision of the international standard ISO 226.