Sound source system for investigating the perception of infrasound combined with audible sound

Within the scope of the EU joint project titled Ears II (EMPIR project No. 15HLT03), hearing tests were planned at PTB in order to investigate how humans perceive infrasound (frequencies f below 20 Hz) combined with sound in the "conventional" acoustic frequency range (i.e."audible sound" in the frequency range 20 Hz ≤ f ≤ 20 kHz). The first step consisted in developing a sound source system able to transmit infrasound and audible sound to the ear canal. One of the challenges of this first step was that extremely high sound pressure levels are required in the infrasonic range for humans to be able to perceive them, while at the same time, no audible distortions may occur.

The way humans perceive infrasound is a field that is currently being researched. Research projects have already shown that humans can perceive infrasound down to 2 Hz with their ears if the sound pressure level is sufficient (see e.g. [1] and [2]). Humans, however, are usually exposed to a mixture of infrasound and audible sound. The question that therefore arises is whether the presence of audible sound has an influence on the perception of infrasound, and vice versa. PTB therefore carried out hearing tests as part of a study which aimed to investigate how signals that consist of a mixture of infrasound and audible sound are perceived by humans. Developing a sound source system (Fig.  1) that allows the ear to be stimulated with infrasound and audible sound was an essential precondition for these hearing tests.

Figure 1: Sound source system in PTB's anechoic room (figure taken from [3]).

This sound source system consists of two loudspeakers, with one of them generating the infrasonic signals and the other one the audible sound signals. The loudspeakers were mounted in separate hermetic wooden boxes. The sound from the loudspeakers was transmitted to an eartip via tubes that connected the wooden boxes with each other. During the hearing tests, the eartip was located in the ear canal of the test person in order to transmit the sound generated to their ear.

It was ensured that infrasound above the perception threshold was generated without audible distortions. For example, infrasound of 5 Hz frequency can be provided up to a sound pressure level of 123 dB (re 20µPa) [3]. One of the challenges was the audible sound signals possibly being involuntarily distorted due to the high pressure of the infrasound. However, the distortions that occurred were successfully compensated for by using a third loudspeaker that also generates infrasound [3]. This third loudspeaker was mounted below the loudspeaker generating audible sound.

For further information concerning the technical setup and the characterization of the sound source system, please consult publication [3] (Link). The sound source system has already been successfully used in hearing tests to determine the detection thresholds for a combination of infrasound and audible sound and the unpleasantness it causes (results of pilot studies in [4, 5]).

This study is part of the Ears II project “Metrology for modern hearing assessment and protecting public health from emerging noise source". This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme.

Literature:

[1]    H. Møller, C. S. Pedersen: Hearing at low and infrasonic frequencies. Noise and Health 6 (2004) 37–57. Link
[2]    R. Kühler, T. Fedtke, J. Hensel: Infrasonic and low-frequency insert earphone hearing threshold. The Journal of the Acoustical Society of America 137 (2015) EL347–EL353. Link
[3]    E. Burke, J. Hensel: Sound source system for investigating the auditory perception of infrasound accompanied by audio sound. Acta Acustica united with Acustica 105 (2019) 869-874. Link
[4]    E. Burke, J. Hensel, T. Fedtke: Hearing threshold measurements of infrasound combined with audio frequency sound. Paper presented at 12th ICBEN Congress on Noise as a Public Health Problem, Zurich, Switzerland, 2017.
[5]    E. Burke, E. P. Stederi, S. Uppenkamp, C. Koch: Investigation of the unpleasantness of infrasound combined with audio sound using psychoacoustic scaling methods. Paper presented at the 23rd International Congress on Acoustics, Aachen, Germany, 2019.

Contact:

Elisa Burke, FB 1.6, AG 1.61, E-Mail: elisa.burke(at)ptb.de