ISASE 2024

Relaxation Effects of Auricular Vibration Stimuli
Synchronized with MusicYuta GOTO * and Shogo OKAMOTO ** Tokyo Metropolitan University, 6-6, Asahigaoka, Hino, Tokyo 191-0065, Japan

Abstract: When people listen to music, vibratory stimuli that are synchronized with sounds enhance the affective effects of music.
We investigated the effects of vibratory stimuli on relaxation. Vibratory stimuli were delivered to the cymba concha, an auricular part,concentrating on the vagus nerves. Five vibration patterns were prepared based on music that evoked relaxation. A user study involving
nine participants found that they felt more relaxed under the condition in which a vibration stimulus was generated from sounds loweredby one octave than under the condition with no vibratory stimuli applied. This effect was more prominent for one of the two musicalsounds tested.
Keywords: Vibration, vagus nerve, cymba concha

1. INTRODUCTION
The emotional experience when viewing music orvideos is an antecedent to determining the value of thecontent. Thus far, various studies have been conducted toenhance the emotional experiences evoked by music andvideos by presenting haptic stimuli to the viewer’s torso[1–5]. For example, the presentation of vibration stimuli
to the upper body while viewing a horror movie increased
the sense of fear [1]. Emotional valence and arousalincrease when listening to music using a vibrationimparting chair [2]. In general, vibrational stimuli applied
to the torso are used as haptic stimuli. Most earlier studies
have reported that vibratory stimuli to the upper bodyincrease arousal status, such as fear and excitement;
however, few studies have reported an increase in nonarousing emotions, such as relaxation.
Some studies have shown that mechanical stimulationof the external ear, where the vagus nerve is located,activates the parasympathetic nervous system and induces
relaxation. Lee et al. [6] reported that air pressure stimuli
to the external ear increased parasympathetic nerveactivity. Boehmer et al. [7] reported that acupuncture of
the cymba concha, where the vagus nerve is densely
distributed, reduced heart rate. Vibration stimulation of
the cymba concha exhibited physiological effects similar
to those observed when the auricular vagus nerves wereactivated by electrical stimulation [8]. These studies
suggest that mechanical stimulation of the ear effectively
affects the parasympathetic nervous system, causing
relaxation.

Figure 1: Experimental apparatus. Left) Experimental
scene. Right) Resin rod with a vibrator.

In this study, we investigated the relaxation effects ofvibration stimulation on the cymba concha while listening
to relaxing, voiceless music. Vibratory stimuli weresynthesized by referring to music, and five types ofvibration patterns were tested. Through a questionnairebased experiment, we investigated the degree ofrelaxation experienced through the combination of music
and vibrations. This study extend the authors’ earlier
studies [9–11], where the affective effects of stroking
stimuli to the ear were demonstrated. The findings of thisstudy are expected to provide a basis for designing new
emotional haptic interfaces.
2. METHODS
2.1 Apparatus
Figure 1 shows the contactor that vibrates the cymba
concha. The main components are a 3D-printed resin rod
with a sphere of 8 mm in diameter at its tip and a recoil-1

type voice coil motor (Haptuator MM3C, Tactile Labs Inc.,
Canada). The motor is fixed 30 mm from the tip of theresin rod. It fits into the groove of the rod and is fixed bya string. The rod vibrates perpendicular to its longitudinal
direction. The shape and size of the tip are designed to fit
the cymba conchas.
The voice coil motor was controlled using MATLAB(MathWorks, USA) through a data acquisition board(USB-6212, National Instruments Co., USA) and an audio
amplifier (FX-2020A+ CUSTOM, North Flat Japan Co.,
Ltd., Japan). During the experiment, participants held thegrip of the contactor with their left hand. Vibrations wereadministered to the left ear. Bone-conduction earphones(Aeropex, Shokz, USA) were used to play the soundstimuli. Sound and vibratory stimuli were synchronizedusing MATLAB.

Figure 2: Methods of generating the vibration stimuli. a)
Vibration stimuli was the direct transmission of soundstimuli. b) The sound stimulus was lowered by one octave andused as the vibration. c) Amplitude-modulated vibration.

2.2 Sound stimuli
Two pieces of stereo music, Lune [12] and Samidare[13], were used to evoke relaxation. Lune is jazz musicwith guitar and bass as the main instruments. Samidare
primarily includes string instruments, such as violin andkoto. In a preliminary study, seven participants rated 11
pieces of music, including Lune and Samidare, on how
relaxing each piece was. Lune and Samidare received thehighest scores. The lengths of these pieces were truncatedto 30 seconds, and the sampling frequency was 44100 Hz.

calculated using MATLAB’s envelope function, andamplitude modulation was applied. Carrier-wave
frequencies were set at 60, 100, and 140 Hz. A lower limitof 60 Hz was determined considering the frequency
characteristics of the vibrator; vibrations at frequencies
lower than this were perceptually weak. Preliminary
experiments showed that carrier frequencies above 150 Hz
significantly diminished the music experience due to thesound generated by the vibration, setting the upper limit
at 140 Hz.
The subjective intensities of the vibrations wereequalized among all stimulation methods through apreliminary subjective evaluation test.

2.3 Vibration stimuli
The vibrator was actuated by applying voltage signals
based on sound. Three audio-to-vibration conversion
methods were used, as shown in Figure 2.
The first method transmitted sound signals directly tothe vibrator. Because the vibrations were presented to theleft ear, sound signals for the left channel were used.
The second method transmitted a sound source one
octave lower than the original sound to the vibrator.
Previous studies used a method to reduce the frequency ofsounds to convert them into vibrational stimuli [3,14]. Ina preliminary test, we used vibrations based on sounds
lowered by one and two octaves, as well as vibrations one
octave higher than the original sound stimuli. Amongthese, the one-octave-lowered vibration was found topotentially induce relaxation. In the main experiment, theleft channel of the original stereo sound was lowered by
one octave, and this audio signal was presented to thevibration actuator.
The third method involved amplitude modulation of theoriginal sound signal. First, the absolute value of thesound signal was obtained; then, its envelope was2.4 Site of vibration stimuli presented
The site for presenting the stimulus was the cymba
concha, where the vagus nerve is densely distributed [15].
Previous studies have shown that vibratory stimulation ofthis region elicited physiological reactions similar to thosecaused by electrical stimulation of the same area [6, 8].
Vibratory stimuli to the external ear spatially coincidewith sound stimuli. Moreover, noise generated during
vibration stimulus presentation is less likely to be heard,
potentially causing minimal disturbance to the music
listening experience due to vibration-related noise.
2.5 Participants
Nine participants in their early 20s participated in theexperiment after providing written informed consent.
None of the participants were aware of the experiment’s
purpose.

2

Figure 3: Experimental results. Means and standard errors for each stimulus condition. The value in parentheses is a p-value
without Bonferroni correction. 1-oct stands for one octave lower. AM-60, 100, and 140 stand for the amplitude modulated vibration
with the career frequency of 60, 100, and 140, respectively.

Figure 3 displays the relaxation scores for each soundand vibration condition. A significant increase inrelaxation was observed in Samidare under only one
condition. The vibration designed with the one-octave
lowered sound significantly increased subjective reportsof relaxation (𝑝 0.05 ) compared to the no-vibration
condition.

2.6 Ethical statement
The study protocol was approved by the Institutional
Review Board of the Hino Campus of Tokyo Metropolitan
University (approval number H23-11).
2.7 Procedures and tasks
Participants listened to music under six different
vibration conditions: in the first condition, the vibrationwas directly converted from the sound; in the second, thevibration was made by sound lowered by one octave; inthe third, fourth, and fifth conditions, amplitudemodulated vibrations with carrier frequencies of 60 Hz,
100 Hz, and 140 Hz, respectively, were used; in the sixth
condition, only sound was presented. The stimulus length
for all conditions was 30 seconds.
After experiencing each stimulus, participants answereda questionnaire. They rated the degree of relaxation theyfelt while listening to the music using a 9-point Likert
scale (1–9). A score of 1 indicated that they did not feel
relaxed during the stimulus period, while higher scoresindicated greater relaxation. This procedure was repeated
12 times in total (two musical pieces × six stimulusconditions) in random order. During the experiment,
participants were asked to close their eyes.

4. DISCUSSION
The one-octave-lower vibration was effective for only
one of the two musical sounds tested in this study. Thisfinding suggests that the optimal vibration synthesis
method may vary among different sounds. The twomusical sounds tested were distinctly different from eachother. A key difference between them was the frequencyof the main sound. Although research on the effects ofvibratory frequency in stimulating the cymba concha islimited, certain vibratory frequency bands may be more
effective. For Samidare, the one-octave lowered vibration
might have aligned with such effective bands.
Determining whether the vagus nerve mediated therelaxation effects demonstrated in this study ischallenging. Thus, future research should experimentallycompare different body sites with varying densities of thevagus nerve.

2.8 Data analysis
For each musical sound, the ratings between the novibration condition and each of the five with-vibration
conditions were compared using a one-sample t-test witha Bonferroni correction for multiple comparisons.

5. CONCLUSION
In this study, we investigated whether vibrational
stimulation of the cymba concha enhanced participants’relaxation while listening to two relaxing musical pieces.
Five vibration-generating algorithms were tested. The
results showed that, for one of the two sounds, the3. RESULTS

3induces pleasant feelings while listening to sounds,
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https://www.youtube.com/watch?v=ucRQph70hmc[13] Samidare, https://recochoku.jp/song/S1012400917/[14] R. Okazaki, H. Kuribayashi, and H. Kajimoto; The
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2002.

subjective report of relaxation increased with the vibration
generated by the sound lowered by one octave comparedto the audio-only condition.
ACKNOWLEDGMENTS
This study was in part supported by MEXT Kakenhi(23H04360 and 20H04263).
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