Christian Sejer Pedersen

Low-frequency noise from large wind turbines

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.

A detailed study of low-frequency noise complaints

From 203 cases of low-frequency complaints a random selection of twenty-one cases were investigated. The main aim was to answer the question whether the annoyance is caused by an external physical sound or by a perceived but physically non-existing sound, i.e. low-frequency tinnitus. Noise recordings were made in the homes of the complainants, and the complainants were exposed to these in blind test listening experiments. Furthermore, the low-frequency hearing function of the complainants was investigated, and characteristics of the annoying sound were matched. The results showed that some of the complainants are annoyed by a physical sound (20–180 Hz), while others suffer from low-frequency tinnitus (perceived frequency 40–100 Hz). Physical sound at frequencies below 20 Hz (infrasound) is not responsible for the annoyance -or at all audible – in any of the investigated cases, and none of the complainants has extraordinary hearing sensitivity at low frequencies. For comparable cases of low-frequency noise complaints in general, it is anticipated that physical sound is responsible in a substantial part of the cases, while low-frequency tinnitus is responsible in another substantial part of the cases.

Psychophysical tuning curves for frequencies below 100 Hz.

Psychophysical tuning curves (PTCs) were measured for sinusoidal signals with frequency f(s) = 31.5, 40, 50, 63, and 80 Hz, using sinusoidal and narrowband-noise maskers. For the former, conditions were included where a pair of beating tones was added to reduce the use of cues related to beats. Estimates of each subjects middle-ear transfer function (METF) were obtained from equal-loudness contours measured from 20 to 160 Hz. With decreasing f(s), the PTCs became progressively broadened and markedly asymmetrical, with shallow upper skirts and steep lower skirts. For the sinusoidal maskers, the tips were more irregular than for narrowband-noise maskers or when beating tones were added. For f(s) = 31.5 and 40 Hz, the tips of the PTCs always fell above f(s). Allowing for the METF so as to infer underlying filter shapes resulted in flatter lower skirts, especially below 40 Hz, and reduced the frequency at the tips for f(s) between 31.5 and 50 Hz; however, the tips did not fall below 40 to 50 Hz. The bandwidths of the PTCs increased with decreasing f(s) below 80 Hz. However, bandwidths remained roughly constant if the METF was included as part of auditory filtering for frequencies below 40 Hz.

The Influence of the Helicotrema on Low-Frequency Hearing

Below a certain stimulus frequency, the travelling wave reaches the apical end of the cochlea and differential pressure across the basilar membrane is shunted by the helicotrema. The effect on the forward-middle-ear-transfer function (fMETF) could be measured on both ears of five human subjects by a noninvasive technique, based on the suppression of otoacoustic emissions. All fMETFs show a pronounced resonance feature with a centre frequency that is similar between left and right ears, but differs individually between 40 and 65 Hz. Below this resonance, the shunting causes a 6-dB/octave-increase in the slope of the generally rising fMETF (20-250 Hz). The subject’s individual fMETF was then compared with their behaviourally obtained equal-loudness-contours (ELCs) using a 100-Hz reference tone at 20 dB SL. Surprisingly there, the resonance is only reflected in two of the five subjects. Nevertheless, the transition frequency of the slope appears to correlate between individual fMETF and ELC.

Comments to the Article “Sound Insulation of Dwellings at Low Frequencies” I

It is well known that, at low frequencies, the sound level normally varies substantially with position within a room. It is thus not straight-forward to measure and characterize indoor sound levels and to describe sound insulation from the outside to the inside of a building. It is therefore with significant interest and motivation that we have read this article. In the work reported, the sound insulation was measured for the same rooms using three different methods to measure the indoor sound level. Unfortunately, the authors forgot to report results from the first method, and even more unfortunately, a critical error was made in the second method. The comparisons between the second and third methods are thus flawed, and the conclusions as well as the main outcome of the work, a set of recommended sound insulation data for the second method, are wrong. To confuse matters further, the authors seem to be unaware that the measurement methods have different objectives and aim at different measures for the indoor sound level. With the present comments, we want to call attention to these detrimental errors, which – if unnoticed – may lead to serious misunderstandings and misuse of the data in the community. We will abstain from discussing in general the quality of the work, its use of statistics, and its presentation.

Methodological aspects in the determination of the auditory filters and critical band at low and mid-frequencies

In order to evaluate loudness or audibility of complex sounds, knowledge of the auditory filter characteristics is necessary. At low frequencies, where both the threshold of hearing and dynamic range become considerably frequency dependent, care must be taken to account for this both in the psycho‐acoustical model and the methodological approach. To account for variation in hearing sensitivity at low frequencies, equal loudness contours have been used to weight the stimuli accordingly. At mid and high frequencies, threshold of hearing curves have been used. These stimuli weightings can be applied before or after the experiment, normally being applied afterwards. Due to the non‐linear characteristics of the cochlear amplifier, it is arguable whether post‐experimental weighting is a proper approach, or whether at low frequencies there will be any difference between pre or post stimuli weighting. Listening experiments are then to be performed to test possible differences in pre or post filtering the stimuli....

A study of twenty-one cases of low-frequency noise complaints

From 203 cases of low‐frequency complaints a random selection of twenty‐one previously unsolved cases were investigated. The main aim of the investigation was to answer the question whether the annoyance is caused by an external physical sound or by a physically non‐existing sound, i.e. low‐frequency tinnitus. Noise recordings were made in the homes of the complainants, and the complainants were exposed to these in blind test listening experiments. Furthermore, the low‐frequency hearing function of the complainants was investigated, and characteristics of the annoying sound was matched. The results showed that some of the complainants are annoyed by a physical sound (20‐180 Hz), while others suffer from low‐frequency tinnitus (perceived frequency 40‐100 Hz). Physical sound at frequencies below 20 Hz (infrasound) is not responsible for the annoyance ‐ or at all audible ‐ in any of the investigated cases, and none of the complainants has extraordinary hearing sensitivity at low frequencies. For comparable c...