PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

How to present pure-tone infrasound to the ear
Holger JOOST1; Björn FRIEDRICH2; Jesko L. VERHEY2; Thomas FEDTKE1
1

Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
2

Otto von Guericke University Magdeburg, Germany

ABSTRACTAn increasing number of individuals are being exposed to infrasound. It is, however, still not clear, howinfrasound is perceived by human beings. Regarding auditory perception, one hypothesis is that non-linear
processes within the ear generate distortion products that make infrasound audible. To investigate thishypothesis, it is crucial to rule out that such distortions were produced technically, i.e., by the stimulus
presentation system rather than the auditory system. This paper introduces a compact, low-distortion
infrasound system, based on a commercially available audiometric earphone, for simultaneous presentationof pure-tone infrasound and audio sound to the ear via an ear insert. This system also allows for the recordingof sound in the ear canal during signal presentation. Performance characteristics of the system for the
stimulus presentation and the sound recording units are presented. It is shown that the interaction of
infrasound and audio sound, such as modulation of the audio sound by the infrasound, is negligible withrespect to auditory perception.
Keywords: infrasound stimulus system, ear insert, acoustic distortions, sound recording in the ear canal

1. INTRODUCTION
Infrasound is commonly defined as sound with frequencies below 20 Hz (1). Although the term
infrasound insinuates that these frequencies are outside the audible range, t here is some evidence thatthe human auditory system is sensitive to infrasound down to 2 Hz (2, for a review; 3). Yet, theknowledge about the underlying mechanisms is still vague. One hypothesis is that (per se inaudible)
internal sounds in the audio-frequency range produced by physiological processes, such as blood flow,
are modulated by infrasound, making the latter audible (4). An alternative hypothesis is that non-linear
processes within the cochlea produce audible distortion products in the audio-frequency range (5). In
order to investigate whether non-linear processes within the ear play a role in infrasound perception, it
is crucial that the system used for the presentation of the infrasound generates as little harmonic
distortions as possible. Thus, the main aim of the present study was to develop a sound reproduction
system that can generate sufficiently high sound pressure levels and, at the same time, has distortion
component levels well below the threshold in quiet at these frequencies. The second aim was toincorporate into the system a microphone that monitors the sound in the ear and detects possible
distortions. To this end, it was investigated if a system that was initially developed to measure
otoacoustic emissions (OAE, 6) in the ear canal can be used when infrasound is presented.

2. INFRASOUND REPRODUCTION AND ANALYSIS SYSTEM
2.1 Requirements for the sound reproduction and the in-ear sound measuring system
In order to investigate the infrasound perception by the auditory system it is mandatory that thesound is only delivered to the ear. The system should allow for presenting (sinusoidal) infrasound (IS)signals as well as sounds with frequencies in spectra in the audio sound (AS) range from 20 Hz to
4 kHz. The system should be able to deliver sounds up to sound pressure levels (SPL) that areequivalent to a loudness level of 40 phon (2). Note that, at low frequencies, these levels are quite high.
11470holger.joost@ptb.deFrequencies down to 4 Hz should be presented at this loudness level . At 4 Hz, this is equivalent to an
SPL of 121.3 dB. It should also be possible to present combinations of IS+IS and IS+AS stimuli to theear. This simultaneous presentation should not generate distortions. Since a simultaneous presentationof sound combinations by a single transducer might produce distortion from its non-linear behavior,
the system should incorporate two separate low-distortion transducers for infrasound and audio sound.
Many researchers working on the effect of infrasound on the human auditory system used theaudiometric earphone transducer Beyerdynamic DT48 (Beyerdynamic, Germany) as an infrasoundreproduction system. However, the DT48 is no longer commercially available. Thus, a new infrasoundsource with at least equivalent performance is required.
In addition to a high-quality sound reproduction system, a low-noise ear canal sound measuringsystem with a high sensitivity is necessary, which is not affected by high infrasound pressure levels tomonitor the sound in the ear canal.
2.2 Sound reproduction system
Figure 1 shows a photograph of the sound reproduction system. It includes two RadioEar DD45
(RadioEar, USA) audiometric earphone transducers. It is an electrodynamic sound transducer
specified by the manufacturer to have a nominal impedance of 10 Ω and to be linear up to an input
power of 500 mW. The harmonic distortions are below 1% at 120 dB SPL. The two DD45 earphonetransducers were mounted in air-sealed aluminum housings, each with a sound outlet in the front plate.
Sound tubes with a length of 25 cm connect the sound outlets to the Etymotic ER-10B+ low-noise
microphone system (Etymotic Research Inc., USA), which was initially developed for measurementsof distortion product otoacoustic emissions (DPOAE). It was chosen for this study, because it allows
for coupling external transducers as sound sources (see Figure 1). In addition, it ensures that the soundis only fed into the ear canal.

Figure 1 – Photograph of the sound reproduction system consisting of two DD45 earphone transducers (topleft) coupled with the Etymotic ER-10B+ system (bottom right) via two tubes.
The volume that is enclosed by the membrane of the earphone transducers, the tubes, and the ear issmall. It acts as a pressure chamber for low frequencies, enabling acoustic stimulation with the DD45
earphone transducers down to the infrasound frequency range. As requested (see Section 2.1), the two
DD45 transducers offer the possibility of separated generation of audio sound and infrasound or of two
independent infrasound stimuli.
Another essential part of the sound reproduction system is the headphone amplifier. To drive theDD45 transducer, a dual-channel headphone amplifier was built, based on an LPA-2S board (FunkTonstudiotechnik, Germany), that the manufacturer modified for our special purpose of driving thislow-impedance (10 Ω) headphone transducer down to 2 Hz with a maximum power of 500 mW. This
headphone amplifier offers high output power combined with very low noise floor and very low
harmonic distortion (THD+N: 1-kHz signal 0.0013 %; 10-Hz signal 0.0006 %; both at 500mW/10 Ω).
2.3 Setup for analyzing the system performance
The system performance of the sound reproduction system and the Etymotic ER-10B+ low-noise
microphone probe was analyzed within the setup shown in Figure 2. The sound emitted from the DD45
earphone transducers is guided through the sound tubes to the Etymotic ER-10B+ ear insert. The ear
insert is connected to an occluded-ear simulator B&K 4157 (Brüel & Kjær, Denmark) by using the1471

adapter DB-2012, which simulates the whole external ear canal. The signal of the ear simulator
microphone is amplified by the preamplifier B&K 2669, forwarded to the microphone conditioning
amplifier B&K Nexus 2690, and, finally, to one channel of the APx555 audio analyzer (Audio
Precision, USA). The Etymotic ER-10B+ microphone signal is amplified and conditioned by theprovided microphone preamplifier, the signal of which is forwarded to the other channel of theAPx555 audio analyzer. The APx555 audio analyzer serves not only as a signal analyzer but is also
used as a high-precision generator of the pure-tone test signals, which are fed into the dual-channel
earphone amplifier Funk LPA-2S driving the two DD45 transducers.

Figure 2 – Block diagram of the sound reproduction and performance analysis system.
Note that IEC 60318-4 specifies the occluded-ear simulator B&K 4157 only in the frequency range
from 100 Hz to 10 kHz. For analyzing the system performance down to infrasound frequencies,
however, the ear simulator had to be validated for frequencies below 100 Hz. In particular, alow-frequency level drop-off caused by the pressure equalization needed to be ruled out. The system
was compared to a closed coupler (no vent) with identical effective volume, where a B&K 4193
infrasound microphone is used. This comparison revealed that differences are less than 0.7 dB in thefrequency range from 2 Hz to 100 Hz (not shown). Hence, the B&K 4157 occluded-ear simulator canbe used down to 2 Hz.
For all measurements, the acoustic parts of the setup (from the two DD45 earphone transducers to
the ear-simulator B&K 4157) were placed in an anechoic test chamber (B&K 4222) to reduce
background noise.

3. SYSTEM PERFORMANCE - RESULTS AND DISCUSSION
3.1 Single pure-tone infrasoundThe top panel of Figure 3 shows the measured sound pressure level magnitude spectrum of a 4-Hz
pure tone acoustically presented through the RadioEar DD45 transducer at 40 phon (red curve). In
addition, a spectrum of a recording without pure-tone stimulus is shown in blue. In both spectra, thesame peaks can be observed at 3.7 Hz and harmonics of this frequency. These are caused by the display
refresh process of the B&K Nexus 2690 microphone-conditioning amplifier. In addition, peaks at
150 Hz, 250 Hz, and 350 Hz are observed that are resulting from electromagnetic interference of the
50-Hz power line frequency. The peak at about 84 Hz is of unknown origin. Since it is observed withand without stimulus, it is probably an environmental background signal. The spectrum of the soundpresented through the DD45 earphone transducer reveals two prominent upper harmonics. The level ofthe first harmonic is 65 dB lower than that of the 4-Hz component. This component should be inaudible
since it is 45 dB lower than the infrasound threshold reported by Moeller and Pedersen (2). The levelof the second harmonic is even 20 dB lower than that of the first one and is, thus, also inaudible, as well
as all other higher harmonics.
For comparison, the spectrum was also recorded when the sound is presented through a
Beyerdynamic DT48 earphone transducer under the same conditions as for the DD45. For that purpose,1472

the two DD45 transducers in our measuring setup were replaced by two DT48 transducers, modified
with air-sealed mounted front plates with sound outlets for the sound tubes. The LPA-2S headphone
amplifier was able to drive the DT48 transducer with similar low noise and distortion as the DD45,
despite its even lower impedance of 5 Ω. This was proven by comparing the frequency spectra of thetransducer terminal voltages of the two earphone types. In the bottom panel of Figure 3, the measured
spectrum is shown (black curve). Also the DT48 earphone shows two prominent upper harmonics.
However, the level of the first harmonic is only 45 dB lower than the level of the 4 -Hz component andonly 25 dB lower than the infrasound threshold. These values are about 20 dB higher than those of theDD45. Moreover, the DT48 produces pronounced higher harmonics up to the audio frequency range,
with levels close to the hearing threshold. Since this threshold represents a median value and someindividuals can detect signals that are several decibels lower than normal threshold, these higher
harmonics produced by the DT48 may be detected by some of the listeners.
The RadioEar DD45 shows remarkably low harmonic distortions, with levels of more than 45 dB
below the hearing threshold. This is sufficient to rule out any notable influence of distortions producedby the sound reproduction system on signal detection. Furthermore, the DD45 shows cons iderably
fewer distortions than the often used Beyerdynamic DT48.

Figure 3 – Spectrum of a 4-Hz pure tone acoustically presented at a loudness level of 40 phon through aRadioEar DD45 transducer (top panel, red curve) and through a Beyerdynamic DT48 transducer (bottom
panel, black curve). In addition, the threshold in quiet and the 40-phon equal-loudness-level contours (ELLC)
are shown (green lines). The top panel also includes a spectrum when no stimulus was presented (blue curve).

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3.2 Combination of infrasound and audio soundFigure 4 shows spectra of a 4-Hz pure tone combined with a 31.5-Hz pure tone, both at a loudness
level of 40 phon. The top panels show the spectra when they are presented with the DD45 transducers(red curves). The bottom panels show the spectra when the DT48 transducers are used for stimulus
presentation (black curves). In order to test the necessity of separate transducers, one for each puretone (as commonly used), the combined sound was also presented through a single transducer. The left
panels show the spectra when each of the two sounds is presented through a different earphone, theright panels show the spectra when the same earphone is used for simultaneous presentation of the twosounds. As in Figure 3, thresholds and 40-phon equal-loudness-level contours are shown (2; 7).

Figure 4 – Spectrum of a 4-Hz pure tone and a 31.5-Hz pure tone, both acoustically presented at a loudness
level of 40 phon through RadioEar DD45 transducers (top panels, red curve) and through Beyerdynamic
DT48 transducers (bottom panels, black curve). The left panels show the spectra when each pure tone ispresented through a different transducer, the right panels show the spectra when the same transducer is used
for the simultaneous presentation of both pure tones. Thresholds and equal-loudness-level contours are also
shown (green lines, see Figure 3).

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When the 4-Hz infrasound and the 31.5-Hz audio sound are presented with different DD45
transducers (left top panel), the audio signal is modulated by the infrasound signal : Side lines with a
4-Hz spacing around the 31.5-Hz line appear. Note that the levels of the side lines are more than 65 dB
lower than the level of the 31.5-Hz component.
The DT48 transducers produce side lines that are 20 dB higher than those of the DD45 (left bottom
panel). In addition, side lines at multiples of 4 Hz occur around 31.5 Hz. Nevertheless, in both cases
the modulation of the audio-signal should not be audible, since at this modulation rate, the levels of theside lines at modulation detection threshold (8) are at least several dB higher than those produced by
the non-linearity of the sound reproduction system. Thus, the modulation produced by the systemshould be inaudible.
If the two sounds are presented with the same transducer, the DT48 shows an increase in the
distortions, as expected (bottom right panel). For the DD45 (upper panels), the level differences
between the 31.5-Hz component and its side lines were larger when only one DD45 transducer wasused (Δ𝐿 = 75 dB) as compared to when two DD45 transducers were used (Δ𝐿 = 65 dB).
In summary, for the presentation of two tones, the DD45 is more suitable than the DT48. In the caseof using two earphone transducers to present two tones separately from each other, the DD45 emitting
the audio signal is less sensitive to the infrasound signal, emitted by the second transducer of the same
type, than the DT48. Unexpectedly and in contrast to the DT48, a single DD45 transducer, playing
both tones together, results in fewer distortions than playing the tones using separate DD45
transducers. That offers the possibility of using a single DD45 transducer for the presentation of
stimuli that are more complex than a single sinusoid.
3.3 Characterization of the Etymotic ER-10B+ microphone system
Figure 5 shows the transfer characteristics of the sound reproduction system in conjunction with theEtymotic ER-10B+ microphone probe (blue curve) and the occluded-ear simulator B&K 4157 (redcurve), recorded with the setup described in Section 2.3. Both curves were measured simultaneously assound pressure level against frequency at a constant earphone transducer driving voltage of 0.8 V. Boththe ear simulator and the probe microphone have been calibrated by means of a sound calibrator B&K
4131 at 1000 Hz to provide a reading in dB re 20 µPa.
From 150 Hz to 1000 Hz, the curves are almost identical, above 1000 Hz they begin to slowly
diverge, but are still very similar. The ripple results from sound tube resonances and does not have any
undue effect for our purposes. Below 150 Hz, the sound pressure level measured by the Etymotic
ER-10B+ microphone probe shows a considerable drop-off towards lower frequencies. The difference
to the ‘true’ value measured by the ear simulator microphone reaches a value of about 40 dB at 3 Hz.
By using the difference of both curves, however, the output of the Etymotic ER-10B+ microphonesystem can be easily equalized to get the ‘true’ SPL values.

Figure 5 – Sound pressure level produced by the sound reproduction system in the occluded-ear simulator
B&K 4157 (red) and measured by the microphone probe of the Etymotic ER-10B+ (blue) when driven by aconstant driving voltage of 0.8 V.
Figure 6 shows spectra of a 4-Hz 40-phon pure tone produced by the sound reproduction system inthe occluded-ear simulator B&K 4157. The red curve shows the spectrum measured with the ear
simulator microphone. The blue curve shows that measured with the Etymotic ER-10B+ microphone
probe that has been equalized as described above. The ER-10B+ has slightly lower noise levels ascompared to the ear simulator microphone from about 400 Hz to 4000 Hz, but the noise has essentially1475

the same order of magnitude in both devices. Below 150 Hz, the spectrum measured with the ER-10B+
microphone probe shows higher background noise levels than that recorded with the ear simulator
microphone. At 3 Hz, it is about 20 dB higher for the ear simulator microphone. This results from theequalization gain needed for compensating the low-frequency drop-off of the probe microphone.
Nevertheless, the 4-Hz pure tone and its higher harmonics are clearly visible, and no additional
components are added due to any non-linearities of the ER-10B+ microphone probe. The microphone
is not overloaded by the high infrasound pressure level, i.e. it is possible to measure audio sound while
high-level infrasound is present in the ear canal. Although the background noise level of the ER-10B+
microphone probe in the lower frequency range is not as low as that of the ear simulator microphone,the ER-10B+ sensitivity seems to be high enough to detect signals with low sound pressure levels, forexample infrasound-generated distortion products. Bearing in mind that measurements seeking for thelatter in the ear canal will be made in real human ears with their inherent physiological noise, themicrophone probe performs really promising.

Figure 6 – Spectrum of a 4-Hz pure tone presented at a loudness level of 40 phon measured with the ER-10B+
microphone probe (equalized, blue) and with the occluded-ear simulator B&K 4157 (red).

4. SUMMARY AND CONCLUSIONS
A sound reproduction system was presented that consisted of two RadioEar DD45 audiometricearphone transducers connected to an Etymotic ER-10B+ system with integrated low-noise
microphone. It was shown that the system (a) generates sufficiently high sound pressure levels, (b) has
distortion product levels well below the respective hearing threshold levels, and (c) enables one, bymeans of an incorporated low-noise microphone, to monitor the sound and to detect possible
distortions directly within the ear. The system can be used to present sound signals with frequencies
ranging from 4 Hz to 5 kHz with loudness levels of up to 40 phon. The levels of the harmonic
distortions were more than 45 dB below the hearing threshold. It was also shown that the RadioEar
DD45 outperformed the often used Beyerdynamic DT48, no matter if stimuli comprising a single
infrasound, two infrasounds, or an infrasound and an audio sound are played. Noticeably, a single
DD45 transducer, for a signal combining infrasound with audio sound, performs better than two DD45
transducers, which play the two sounds separately. This opens up more possibilities of playing
combined signals with the two-transducer system. The ER-10B+ microphone probe could be used tomeasure sound in the ear canal even while high-level infrasound as low as 4 Hz at a loudness level of
up to 40 phon was presented. In the spectrum, the infrasound component and its upper harmonics wereclearly visible without distorting components of the microphone probe itself. Altogether, the system
can be used in psychophysical experiments on infrasound perception.

ACKNOWLEDGEMENTS
This work was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft,
DFG) (FE 1192/3-1 and VE373/4-1).

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