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Sound source for the objective investigation of the auditory perception of infrasound by means of magnetoencephalography (MEG) and magnetic resonance imaging (MRI)

31.07.2014

Contrary to general opinion, the hearing of infrasound (f < 16 Hz) is possible and leads to distress which differs from person to person. However, the mechanisms of this auditory perception by human beings are still largely unexplained. Within the scope of the European research project "EARS", these mechanisms are to be investigated in detail by means of objective procedures of medical engineering, e.g. magnetic resonance imaging (MRI) and magnetoencephalography (MEG), and objective thresholds are to be determined.

The investigation of neuronal brain activity triggered by an acoustic stimulus by means of magnetic resonance imaging (MRI) and magnetoencephalography (MEG) represents a challenge for sound engineering. Due to the high sensitivity of the MEG sensors to the smallest magnetic field variations, and due to the strong static magnetic field of the MRI (> 3 tesla), conventional audiometric earphones cannot be used owing to the metal components contained therein. Furthermore, these earphones do not generate stimuli with low distortion and a sufficiently high sound pressure level. The sound source developed at PTB is based on the idea of guaranteeing sound transmission via a tube, similar to a hearing aid. The source consists of an electrodynamic loudspeaker with a strong magnet and warp-resistant chassis and is able to generate very high sound pressure levels with very low harmonic distortions in the frequency range to be examined (2.5 Hz < f < 125 Hz). The loudspeaker is mounted hermetically into a damped wooden box, and the coupling to the ear is carried out via a polyethylene tube and an audiometric ear plug (see Figure 1). By using this sound tube, the active part of the source can be placed outside the detection area of the imaging sensors. The closed volume between the loudspeaker membrane, the tube and the ear of a test person represents a pressure chamber for low frequencies, thus enabling a low-loss acoustic excitation in the infrasound area. The sound pressure level will be monitored via an optical microphone which does not represent a magnetic source of interference and can therefore be fitted close to the ear.

Figure 1: Schematic set-up of the infrasound source. Harmonic distortions in the higher frequency range are minimized by a second-order low pass and acoustic muffling material inside the tube.

In order to validate the infrasound source, the subjective thresholds of hearing were determined for 14 pure tones in the frequency range from 2.5 Hz to 125 Hz of 18 otologically normal persons aged 18-25 each time, for the better ear. Figure 2 shows the thresholds which have been determined for the infrasound source developed at PTB. The mean threshold of hearing for a pure sound with 125 Hz was attained at a value of 26.4 dB SPL. At the lowest frequency (2.5 Hz), a mean sound pressure level of 120.7 dB SPL was required for the triggering of an auditory sensation.

Figure 2: Monaural thresholds of hearing for pure tones between 2.5 Hz and 125 Hz determined by means of the infrasound source for 18 otologically normal test persons aged 18 to 25. For comparison, the standardized thresholds of hearing for an insert earphone (Etymotic Research ER-3A, ISO389-2:1994) and for the free field (ISO226:2003) are shown. In addition, the binaural threshold of hearing is shown below 20 Hz in accordance with [1].

The thresholds of hearing determined via the newly constructed source show a good agreement with standardized thresholds of hearing and infrasound thresholds (blue in Figure 2) taken from literature [1, 2] and, for the first time, link threshold values above 20 Hz with thresholds of hearing in the infrasound range. When looking at the individual thresholds of hearing (grey in Figure 2) in the infrasound range (f < 20 Hz), the scattering of the values around the median is larger than in the "normal" frequency range of audible sound. Furthermore, it is to be seen that individual test persons show a clearly lower threshold for frequencies in the infrasound range when compared to the mean threshold of hearing, i.e. these persons detect infrasound at certain frequencies, even if the sound pressure level is still far below the mean threshold of hearing (black in Figure 2). The individual thresholds of hearing represent orientation values for subsequent investigations in the MEG and MRI at PTB in Berlin.

References:

[1] Moeller H., Pedersen C. S.: Hearing at low and infrasonic frequencies. Noise Health 2004; 6:37-57

[2] DIN ISO 226:2006-04: Akustik – Normalkurven gleicher Lautstärkepegel (ISO 226:3003)

Contact persons:

Robert Kühler, Dept. 1.6, WG 1.61, e-mail: robert.kuehler@ptb.de
Johannes Hensel, Dept. 1.6, WG 1.61, e-mail: johannes.hensel@ptb.de