# "Persuasive Vibrations": Studying the influence of vibration parameters on speech persuasion

Sabrina Toofany, Anatole Lécuyer, Ferran Argelaguet, Justine Saint-Aubert

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Sabrina Toofany, Anatole Lécuyer, Ferran Argelaguet, Justine Saint-Aubert. "Persuasive Vibrations": Studying the influence of vibration parameters on speech persuasion. IEEE Transactions on Haptics (ToH), In press, pp.1 - 12. 10.1109/toh.2025.3600579. hal-05372456

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# “Persuasive Vibrations”: Studying the influence of vibration parameters on speech persuasion

Sabrina Toofany, Anatole Lécuyer, Ferran Argelaguet, and Justine Saint-Aubert

***Abstract***—This paper investigates the notion of “Persuasive Vibrations”, which showed that augmenting a person’s speech with vibrotactile feedback could artificially increase persuasion. However, while the initial paper has shown the effect, the underlying reasons why vibrations enhance persuasion remain unknown. Through two different user studies, this paper aims to study how the underlying parameters of the vibratory feedback (e.g., frequency, amplitude, or audio-vibration synchronization) influence persuasion. The first study aimed to identify the parameters of vibrotactile feedback that can positively influence persuasion. The second study evaluated vibrotactile feedback that might impair the persuasive effect. In a nutshell, the first experiment suggests that the isolation of different properties of the vibratory signal could tend to provide higher persuasion compared to no vibratory feedback. A lower frequency at 100 Hz seems the most efficient way to generate a persuasive effect. In contrast, the second experiment suggests that some alteration of the vibratory signal (e.g., latency) does not decrease the levels of persuasion compared to the no-vibration condition. All in all, the results suggest that using lower frequencies could have a better effect on persuasion. These results could serve as a basis for haptic design in applications like videoconferencing, virtual meetings, and training systems where supporting user speech is essential.

***Index Terms***—Audio, Haptic, Vibrotactile feedback, Persuasion, Social Communication, Speech

## I. INTRODUCTION

With the rise of social media and the widespread use of remote communication, human interactions are increasingly taking place at a distance. It raises challenges related to the sharing of emotions and intentions between individuals. Studies have shown that remote communication, primarily through text or video, often results in a loss of emotional nuance and clarity of intentions [1]–[3]. In particular, persuasion and influence during social interactions tend to decrease in remote settings [4], [5].

Touch is a fundamental sense that plays a crucial role in social bonding [6], and the integration of haptic technologies has emerged as a promising way to enhance interactions [7].

A relevant approach in this area is the concept of “Persuasive Vibrations” [8]. It suggests that adding vibrotactile feedback that corresponds to a speaker’s voice can enhance the persuasion of their message. Vibrations help emphasize the speech, making the message more engaging. In virtual meetings where individuals with varying levels of leadership participate, integrating the vibrotactile feedback could help

balance power dynamics. However, while the initial results are promising, the underlying reasons why vibrations support persuasion still need to be explored.

In this paper, our objectives were to identify which parameters of the vibrotactile feedback (e.g., frequencies, envelope, or synchronization) positively or negatively influence persuasion. For instance, audio research has shown that speech characteristics such as rate, intonation, or pitch can significantly affect persuasion [9], [10]. We aimed to evaluate if similar effects occur with tactile feedback.

To achieve these goals, we conducted two user studies. The first study aimed to identify the parameters of vibrotactile feedback that can positively influence persuasion. The second study evaluated vibrotactile feedback that might impair the persuasive effect.

## II. RELATED WORK

The following section discusses previous work related to how haptic feedback, particularly vibrations, can influence social interactions. We explore the impact of haptic on components related to persuasion, such as emotions, speech comprehension, and persuasion itself.

### A. Influence of Haptic Feedback on Speech Perception

The combination of haptic and audio have shown some benefits in augmenting speech perception and focus. Bernstein et al. [11] conducted experiments showing that in environments with high background noise, haptic cues helped listeners differentiate important auditory signals from noise, allowing them to stay engaged with the audio content. Similarly, Hnath-Chisolm et al. [12] found that adding vibrotactile feedback to audio in noisy settings enhanced the listener’s ability to focus on speech, helping them stay emotionally connected to the conversation. Reed’s work on the Tadoma method, in which individuals perceive speech by feeling the vibrations and movements of the speaker’s face, confirms the role of tactile input in communication. This method illustrates the effectiveness of touch in conveying complex information and influencing perception [13].

Vibrotactile feedback can improve speech perception in practical applications by providing tactile cues that supplement missing or unclear auditory information. Some studies [14]–[16] demonstrated that introducing haptic feedback to support auditory signals enhances speech comprehension. Vibrotactile cues help fill perceptual gaps, especially when crucial audio information is missing, improving overall understanding in challenging auditory environments.

In summary, past studies suggest that vibrotactile feedback

Sabrina Toofany, Anatole Lécuyer and Ferran Argelaguet are with Inria, Univ Rennes, IRISA, 35000 Rennes, France (e-mail: sabrina.toofany@inria.fr).

Justine Saint-Aubert is with CNRS, Univ Rennes, Inria, IRISA, 35000 Rennes, France.
can enhance audio clarity in noisy settings, supporting speech
comprehension and focus.

B. Influence of Haptic Feedback on Emotions

Understanding the link between haptics and emotions is
essential when studying haptic-based persuasion since studies
indicate that emotionally engaging communications can en-
hance trust and compliance, thereby increasing the effective-
ness of persuasion [17]. Haptic feedback plays a significant
role in conveying and amplifying emotions [6], [18]. Touch
communicates distinct emotions like anger, happiness, and
sadness, effectively complementing verbal cues [19], [20].
Sawabe et al. [21] emphasize that combining robotic touch
with speech boosts positive emotions in human-robot inter-
actions, suggesting that tactile feedback reinforces emotional
responses.

In the context of vibrotactile feedback, slower vibrotactile
stimuli tend to evoke positive emotional responses, while faster
stimuli are linked to negative valence [22]. Therefore, a lower
frequency might receive a more positive response from the
user. Additionally, vibrotactile feedback replicating real-world
perceptions can elicit emotions, with specific tactile patterns
intensifying feelings by mimicking physical experiences like
a steady, heartbeat-like pulse or cat purring [23].

Research has shown that haptic or vibrotactile feedback, combined with other cues or modalities, results in a more complete experience. For instance, when combined with thermal haptic feedback, vibrations still enhance persuasion, but warm feedback increases the friendliness of voice messages [24]. Integrating haptic feedback with audio offers a multisensory experience that enhances perception and emotional engagement. For example, in a study by Perrotta et al. [25], participants exposed to synchronized haptic and audio stimuli reported heightened emotional experiences compared to audio-only conditions. Specifically, tactile cues were synchronized with audio elements, such as rhythmic beats or voice inflections, allowing participants to experience the audio dynamics physically. These interactions enabled participants to feel sound intensity and rhythm variations, enhancing immersion and emotional engagement. Similarly, Gunther et al. [26] explored how vibrations synchronized with musical beats can create a deeper immersion for listeners, making the auditory experience feel more personal and physical.

Collectively, these studies underscore how haptic feedback can
shape emotional experiences and, when combined with audio,
enrich perception and intensify emotional engagement.

C. Influence of Haptic Feedback on Persuasion

1) *Potential of Haptic Feedback:* Some studies reveal the potential of haptic feedback to enhance persuasion and trust in both face-to-face and remote interactions. Concerning trust, mediated touch, such as vibrotactile feedback, enhances emotional closeness and trust between individuals even in remote settings [27]. Similarly, haptic feedback strengthens users' confidence in the robot's actions in human-robot interactions, increasing perceived reliability and safety [28].

A line of research, the “Midas Touch” effect, highlights

how physical contact, such as a light touch on the arm,
can significantly enhance persuasion by providing positive
feelings [7], [29]. Similar behavior can be found with a robot
as the touch origin [30], or in a virtual environment [31],
highlighting the potential of haptic feedback to impact remote
social interaction.

Based on these findings, some studies suggest that people
are more likely to be persuaded by persuasive messages that
include a tactile element [32], [33]. Further exploring this area,
Saint-Aubert et al. [8] show that vibrations combined with
speech can subtly and effectively increase the persuasion of
a virtual agent speaking or of some people when speaking.
This concept is known as "Persuasive Vibrations". Al Taha et
al. [34] suggested reducing decision-making biases favoring
lower-pitched voices by haptically enhancing higher-pitched
voices with a vibrotactile signal that mirrors the voice pitch.
However, a pilot study shows no significant improvement with
the vibrotactile feedback. Altogether, these studies suggest that
the type of vibrotactile parameters could influence persuasion.

2) *Influence of Vibrotactile Parameters:* In this paper, we
wonder how vibrotactile parameters affect persuasion when
combined with speech. Some works focused on the link
between audio and vibrotactile feedback on leadership and
trust. A haptic device called the BarryWhaptics [16], [35]
was introduced to address voice pitch biases by modulating
the intensity of the haptic response based on the pitch of the
speaker's voice. Specifically, high-pitched voices, which are
generally perceived as less dominant, were augmented with
higher vibration amplitude to balance perceived dominance.
A user experiment showed that female participants tended
to perceive voices as stronger when the haptic feedback was
displayed, making them appear more dominant. However, no
significant effect was found for enhancing trust or was noted
for male participants though.

While studies indicate that haptic feedback can influence
persuasion, the effects of vibrotactile parameters are poorly
understood.

III. USER STUDY 1: VIBROTACTILE PARAMETERS THAT CAN ENHANCE PERSUASION

A. Overview

The objective of the first study was to understand which
parameters of the vibrotactile signals cause the “persuasive”
effect when combined with speech. Each parameter was
isolated under different conditions. Participants were seated
in front of a screen that displayed a remote communication
interface (Fig. 1). They received audio feedback through
headphones and held an actuator in their hand, which pro-
vided vibrotactile feedback related to the speech signal. The
study consisted of two phases: a forced-choice phase and a
questionnaire phase. In the forced-choice phase, two virtual
speakers labeled “1” and “2” were presented to participants
. Both speakers had the same voice and delivered the same
sentence consecutively. However, the vibrotactile feedback
differed between the two speakers, and participants had to
choose which speaker they found more convincing. During
the questionnaire phase, participants listened to one speaker
and then completed a questionnaire for each condition. The study protocol was approved by Inria's ethical committee (COERLE). The experiment took approximately 60 minutes for a participant.

<figure><img src="image_2.png" /><figcaption>Fig. 1. The setup of the user study, including a screen (displaying the videoconference-like), an audio headset (for audio feedback) and a vibrotactile actuator (held in the right hand).</figcaption></figure>

## B. Hypotheses

Previous research on "Persuasive Vibrations" indicates that vibrotactile feedback, when correlated with audio feedback, may positively influence persuasion [8]. Therefore, we expected this feedback to enhance persuasion compared to no vibrotactile feedback. Thus, our first hypothesis is: (H1a) Vibrotactile feedback corresponding to the speech (all frequencies and envelope) positively impacts persuasion compared to no vibrotactile feedback.

Some studies suggest that a single-frequency vibrotactile signal could better enhance focus, hence persuasion, than a complex-frequency vibrotactile signal. Indeed, findings suggest that single-frequency signals allow for focused sensory engagement without straining cognitive processing, making them ideal for clarity and focus [36], [37]. Therefore, we set up the second hypothesis: (H2a) Vibrotactile feedback with only the fundamental frequency of the speech has the same impact on persuasion as vibrotactile feedback with all frequencies. The pitch of an audio speech seems to impact its persuasion: a voice with a low pitch will tend to be perceived as more persuasive than a voice with a higher pitch [38], [39]. The pitch is directly linked with the fundamental frequency of the voice [40], so tactile feedback composed of low frequency could positively impact persuasion. (H3a) Vibrotactile feedback with a low frequency positively impacts persuasion compared to vibrotactile feedback with a higher frequency.

Other studies suggest that the envelope of a vibrotactile signal is better perceived than frequency variations [41]. A better perception of the signal can increase the focus [42]–[44], which in turn could enhance the persuasion [45], [46]. Therefore, the signal envelope could play an important role in persuasion. Based on that, we define the last hypothesis: (H4a) Vibrotactile feedback with a variable envelope positively impacts persuasion compared to vibrotactile feedback with a constant envelope.

## C. Vibrotactile Conditions

A total of five conditions were investigated based on the previous hypotheses. Fig. 2 provides a schematic summary of the different vibrotactile conditions.

* **"No haptic"**: The audio was not augmented by vibrotactile feedback. This condition was the control condition.

* **"Audio like"**: The audio was enhanced with vibrotactile feedback replicating the same signal as the audio, including its frequency and envelope. The construction of the vibrotactile signal follows the same principle outlined in "Persuasive Vibrations". In other words, the audio is directly used as the vibrotactile signal without any modification [8].

* **"Mean frequency"**: The vibrotactile feedback was based on a sinusoidal signal at the same frequency as the mean value of the fundamental frequency of the audio speech (167Hz). The audio envelope modulated the signal, which was determined with a Hilbert transform.

* **"Low frequency"**: The vibrotactile feedback was based on a sinusoidal signal at a lower frequency (100Hz) than the mean value of the fundamental frequency of the audio speech. The choice of frequency was determined such that the difference between the mean value of the fundamental frequency and the lower frequency was higher than the just noticeable difference and respects the audio gap in the literature, that is to say, a frequency difference superior at five semitones (so frequency ratio of approximately 1.3 times the original frequency) [39], [47], [48]. The audio envelope modulated the signal, which was determined with a Hilbert transform.

* **"On-off"**: The vibrotactile feedback was based on a sinusoidal signal at the same frequency as the mean value of the fundamental frequency of the audio voice (167Hz). The envelope was constant.

The signal processing to create these vibrotactile feedback from the audio signal can be found in the appendixes (Tab. III). When vibrotactile feedback was rendered, it was synchronized with the speech: there was no feedback when the agent did not speak.

During the forced choice phase, two conditions were compared at each trial. The order of the combinations was randomized. Each participant tested all 20 possible comparisons of the conditions 5 times, so a participant performed 100 comparisons.

## D. Experimental Setup

The videoconferencing application ZOOM inspired the meeting interface. It was displayed on a screen (2560 x 1440 pixels) and implemented in Unity 3D (version 2023.2.3f1). Participants were seated at 1 meter in front of the screen. Virtual icons represented virtual speakers to avoid bias from physical appearance (Fig. 3). The icon was placed in the center of the screen to avoid bias from lateralization between visual and tactile feedback. Speakers spoke in a row and were identified by a label on the screen ("Listening 1" or "Listening 2").

An audio headphone provided the audio (HyperX Cloud II).
<figure><img src="image_3.png" /><figcaption>Fig. 2. Schematic of the temporal shape of haptic signal conditions for the first user study: "Audio like", "Mean frequency", "Low frequency", "On-off". More information about the signal frequencies and their temporal representation are available in the supplemental materials. The signal intensity attenuation can be found in the section D.2.</figcaption></figure>

An LRA actuator provided the vibrotactile feedback (Vybronics VG2230001H; cylinder of 22*30mm; resonance frequency at 70Hz; acceleration 2.5g) that allows the control of amplitude and frequency independently to some extent. Participants held it in their right hand. Depending on the actuator, the acceleration can vary according to the frequencies. We choose the vibrotactile actuator due to its constant acceleration value in an extensive range of frequencies. It was chosen so the resonance frequency is inferior to the fundamental frequency of the chosen voice and the frequency of the parameters chosen for the conditions. Therefore, the acceleration bandwidth on this range is linear so the frequency has less impact on the amplitude. The temporal shapes of the signals were measured with an accelerometer (LSM6DSV16BX 6-axis IMU model) and were also represented in frequency terms. The graphical representations obtained correspond to what was expected (see supplemental materials). Audio and vibrotactile feedback were synchronized using the audio stereo channels (left for tactile signal and right for audio signal) of an external audio interface (Focusrite Scarlett Solo 2nd Generation).

<figure><img src="image_4.png" /><figcaption>Fig. 3. Experimental videoconferencing interface. The agent's image is framed in yellow when audio is launched. The label varies ("Listening 1, Listening 2...") depending on whether it corresponds to the first or the second signal in the comparison pair.</figcaption></figure>

1) **Audio feedback:** Based on the literature, speakers always said the same sentence: "I urge you to vote for me" [35], [49]. We choose a male voice with a mean fundamental frequency (called here "Mean frequency") at 167Hz. We adjusted the audio volume to ensure the message was clear to users without being excessively loud. We chose a voice with a

mean frequency far enough from the resonance frequency of the vibrotactile actuator. During the signal creation, the audio signal was attenuated by -20 dB. The mean decibel level was at 52.3 dBA. The maximum value was at 57.0 dBA. These values were measured with a sonometer (01dB-Stell sonometer, SIP95S model) in A-weighting sound level.

2) **Vibrotactile feedback calibration:** The frequency perception of a signal is inherently linked to its amplitude perception and can vary among participants. Consequently, a calibration phase was conducted at the beginning of the study to determine the perceived amplitude of each signal relative to the reference signal, "Audio-like". Each signal was compared to the "Audio-like" signal, which was presented either first or second in a randomized order. During each comparison, participants were asked to adjust the amplitude of the signal, either by decreasing or increasing it, until they perceived intensity matched that of the reference. The direction of adjustment (increase or decrease) was also randomized. Each participant performed both type of adjustments for all signals, and the final perceived amplitude was computed as the average of the values obtained across four calibration trials. Participants calibrated 4 times per signal.

The calibration was performed using a scale from 100% to 0% of the signal amplitude, a global volume at 100%, and a resolution of 1%. The calibration was made on a sample of 7 volunteers (7 males, between 20 and 40 years). They all gave written informed consent before testing and were not compensated for their participation. The calibration lasted 30 minutes. The decibel attenuation of each signal has the followed values: Audio like: 0 dB, Mean frequency: -16.5 dB, Low frequency: -24.4 dB, On-off: -27.9 dB.

## E. Data Collected

During the forced choice phase, participants had to choose between the two speakers by choosing "Which agent was the more convincing?". They answered on a keyboard: "1" for the first agent and "2" for the second. Each participant did 100 forced choices. A break was proposed every 20 forced-choice questions to reduce fatigue-related biases.

During the questionnaire phase, participants completed a questionnaire for each condition about their haptic perception, based on questions proposed by Anwar et al. [50], and the
persuasion of the speaker based on a question suggested by
Hanus et al. [51]:

* The haptic feedback felt satisfying.

* I felt that the agent was convincing.

Participants answered these questions on 7-point Likert scales for each condition. After the first question, they were asked to develop their answer in a comment section.

## F. Participants

A sample of 25 volunteers (16 males, mean age= 27.6 ± 8 years) participated in the first study. They all gave written informed consent before testing and were not compensated for their participation. All participants tested all the possible combinations between each condition.

## G. Results

1) **Results of the Forced Choice Phase:** The results were first analyzed to see the global effect on all the conditions (Fig. 4). The normality assumption was met for the data. For more accuracy, we applied a repeated-measures Friedman test to analyze differences across conditions ($\chi^2 = 16.47$, *df* = 4, *p* < 0.01). This first test provides a compact overview of the results. We perform pairwise Wilcoxon signed-rank test post-hoc comparisons using Bonferroni correction to control for multiple comparisons.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Global results of the first user study. Percentage of choices made for each condition in pairwise comparisons to the question “Which one of the two agents seems more convincing?”. The symbol “*” marks the significance for a *p* < 0.05.</figcaption></figure>

The “No haptic” condition was significantly less chosen than the “Low frequency” condition (*p* < 0.05). Then, we analyzed each condition’s influence on the participant’s binary outcome choice (Tab. I).

For each comparison, we used a generalised linear mixed model (GLMM) to predict binary outcomes based on conditions while accounting for individual differences; and calculated probabilities of selecting one condition over another. The GLMM serves as a complementary model that estimates

probabilities and confidence intervals for each pairwise comparison. We used a binomial family with a logit link function, based on R function (lme4 package, version 2024.12.1) as: `glmer(Choice ~ Condition + (1 | Participant), family = binomial)`. The independent variable was the choice (binary) and the dependent variable was the comparison (method 1 vs method 2, 10 comparisons in total). We also included the participant as a random factor, and used a binomial function. If the probability and confidence intervals are < 50%, the condition is less chosen; if they are > 50%, the condition is more chosen. Regarding all the different pairwise conditions, two main results were observed. First, the “No haptic” has a lower level of persuasion than all the other conditions, as all the confidence intervals fall below 50% (first line of table I). Second, we observed the opposite effect for the “low-frequency” condition. For all pairwise conditions, it has a higher level of persuasion (fourth line of table I). For the remaining pairwise comparisons, as the confidence intervals contain 50%, we cannot conclude that there is any difference.

2) **Results for the Questionnaire Phase:** The questionnaire results are displayed in Figs. 5 and 6. Most of the data did not meet the normality assumption. We applied a Friedman test for repeated measures and adjusted it with a Bonferroni correction. No significant difference between each condition was found.

## H. Summary of the User Study 1

The objective of user study 1 was to define which vibrotactile parameters can enhance persuasion.

The **(H1a)** hypothesis (Vibrotactile feedback corresponding to the speech (all frequencies and envelope) positively impacts persuasion compared to no vibrotactile feedback) is supported by the fact that all haptic signals proposed show a positive significant difference compared to “No haptic” condition.

The **(H2a)** hypothesis (Vibrotactile feedback with only the fundamental frequency of the speech has the same impact on persuasion as vibrotactile feedback with all frequencies) is supported. The two conditions have no significant difference, and both are better at persuasion than speech without vibrotactile feedback.

The **(H3a)** hypothesis (Vibrotactile feedback with a low frequency positively impacts persuasion compared to vibrotactile feedback with a higher frequency) is also supported. The data aligns with the **(H3a)** hypothesis in pairwise and global analyses, with low frequency outperforming all other effects. These results suggest that frequency may be a key factor in enhancing persuasion.

The **(H4a)** (Vibrotactile feedback with a variable envelope positively impacts persuasion compared to vibrotactile feedback with a constant envelope) is not supported here. This result suggests that the effect of amplitude variations on persuasion is small.

Overall, the results of user study 1 suggest that vibrotactile feedback can positively influence persuasion, with lower frequencies appearing more effective, and vibrotactile persuasion could come from the fundamental frequency of the vibrotactile signal. However, these results should be further studied, to
<table><caption>TABLE I
PAIRWISE PROBABILITIES BETWEEN CONDITIONS BASED ON GLMM ANALYSIS FOR THE FIRST USER STUDY. EACH CELL REPRESENTS THE PROBABILITY OF SELECTING THE CONDITION IN THE ROW COMPARED TO THE CONDITION IN THE COLUMN</caption><thead><tr><th></th><th>No haptic</th><th>Audio like</th><th>Mean frequency</th><th>Low frequency</th><th>On-off</th></tr></thead><tbody><tr><th>No haptic</th><td></td><td>38 [29, 45] %</td><td>26 [19, 35] %</td><td>23 [17, 32] %</td><td>38 [29, 48] %</td></tr><tr><th>Audio like</th><td>62 [55, 71] %</td><td></td><td>42 [33, 53] %</td><td>32 [24, 42] %</td><td>41 [32, 51] %</td></tr><tr><th>Mean frequency</th><td>74 [65, 81] %</td><td>58 [47, 67] %</td><td></td><td>38 [29, 49] %</td><td>48 [38, 58] %</td></tr><tr><th>Low frequency</th><td>77 [68, 83] %</td><td>68 [58, 76] %</td><td>62 [51, 71] %</td><td></td><td>64 [54, 73] %</td></tr><tr><th>On-off</th><td>62 [52, 71] %</td><td>59 [49, 68] %</td><td></td><td>36 [27, 46] %</td><td></td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Fig. 5. Results for the first experiment, based on the Likert scale 7-point responses for the first question of the questionnaire phase: “The haptic feedback felt satisfying”.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. Results for the first experiment, based on the Likert scale 7-point responses for the second question of the questionnaire phase: “I felt that the agent was convincing”.</figcaption></figure>

explore the role of specific vibrotactile parameters such as frequency and envelope in persuasive vibration.

## IV. USER STUDY 2: VIBROTACTILE PARAMETERS THAT CAN IMPAIR PERSUASION

### A. Overview

The objective of the second user study was to determine whether the vibrotactile signals provided to participants might decrease persuasion compared to speech without any tactile feedback or other vibrotactile feedback. Vibrotactile conditions different from those in user study 1 were tested. These vibrotactile conditions were supposed to disturb the user's speech. The experiment setup, protocol, and data collected were similar to those of the first user study. For the forced choice phase, participants had to choose the speaker that was the most of the two. The protocol was approved by Inria's ethical committee (COERLE). The experiment took approximately 60 minutes.

### B. Hypotheses

Research in audio suggests that speech with a low pitch will tend to be perceived as more persuasive than a speech with a higher pitch [38], [39]. Our first user study shows that speech with a low pitch will tend to be perceived as more persuasive than speech with a higher pitch, here the Mean frequency. We can then test this assumption with higher frequency so that vibrotactile feedback with higher frequency could be less

persuasive than mean frequency. Thus, our first hypothesis is: (H1b) A vibrotactile feedback with high frequency is less persuasive than a vibrotactile feedback with low frequency. Previous research suggests that audio-haptic delays can be detected at intervals between 25 and 31 ms, though this threshold may extend up to 179.4 ms when using pulse feedback [52]–[55]. Humans are sensitive to the temporal congruency between feedback, so the perception of a delay could divert focus and, therefore, reduce persuasion. Therefore, we set up the second hypothesis: (H2b) A vibrotactile feedback delayed with the audio speech feedback is less persuasive than speech without vibrotactile feedback.

Audio research investigating the impact of environmental noise on focus finds that it tends to reduce focus, hence persuasion [45], [56]. Based on that, we define the last hypothesis: (H3b) A vibrotactile feedback consisting of random noise is less persuasive than speech without tactile feedback. We can then assume that a similar effect could be elicited with a tactile signal consisting of random noise. We did not use white noise, as it is perceived much weaker through the actuator than the reference signal (Mean frequency condition).

### C. Vibrotactile Conditions

A total of five conditions were investigated based on the previous hypotheses. Fig. 7 provides a schematic summary of the different vibrotactile conditions.

* “No haptic”: The audio was not augmented by any vibrotactile feedback. This condition was the control
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

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<figure><img src="image_15.png" /><figcaption>Fig. 7. Schematic of the temporal shape of haptic signal conditions for the second user study: "Mean frequency", "High frequency", "Delay", "Pink noise". More information about the signal frequencies and their temporal representation are available in the supplemental materials. The signal intensity attenuation can be found in the section D.</figcaption></figure>

condition.

* **"Mean frequency"**: Similar to the first experiment.

* **"High frequency"**: The vibrotactile feedback was based on a sinusoidal signal at a higher frequency than the fundamental frequency of the speech (231Hz). The choice of frequency was determined such that the difference between the fundamental frequency and the higher frequency was higher than the just noticeable difference and respect the audio gap in the literature, that is to say, a frequency difference superior at five semitones (so frequency ratio of approximately 1.33 times the original frequency) [39], [47], [48]. The audio envelope modulated the signal and was determined with a Hilbert transform.

* **"Delay"**: The vibrotactile feedback was similar to the "Mean frequency" condition but with a positive delay (+220ms) superior to the minimum delay perceptible between sound and vibrotactile feedback to ensure the user can perceive the delay without ambiguity.

* **"Pink noise"**: The vibrotactile feedback was based on a signal with constant amplitude and frequencies following a pink noise frequency spectrum.

Except for the "Pink noise" condition, when a vibrotactile feedback was displayed, it was synchronized with the speech: there was no feedback when the agent did not speak. The signal processing to create these vibrotactile feedback from the audio signal can be found in the appendixes (Tab. III). During the forced choice phase, a combination of conditions was compared. The order of the combinations was randomized. Each participant tested all 20 possible comparisons of the conditions 5 times, so a participant performed 100 comparisons. The signal processing to create these vibrotactile feedback can be found in the appendixes (Tab. III).

## D. Experimental Setup

The same protocol was used for User Study 2, and the same voice was chosen for the audio part. Moreover, a calibration part similar to the first user study was also made. The calibration was made on a sample of 7 volunteers (6 males, between 20 and 30 years). They all gave written informed consent before testing and were not compensated for their participation. The calibration lasted 30 minutes. The experiment itself took approximately 60 minutes. The decibel

attenuation of each signal has the followed values: Mean frequency: -16.5 dB, High frequency: -0.4 dB, Delay: -16.5 dB, Pink noise: -30.4 dB.

## E. Participants

A sample of 25 volunteers (17 males, mean age: 29 +/- 6 years) participated in the second experiment. Participants were not the same as in the first user study. All participants gave written informed consent before testing and were not compensated for their participation. They all tested all the possible combinations between each conditions.

## F. Results

1) **Results for the Forced Choice Phase:** The results were first analyzed to see the global effect on all the conditions (Fig. 8). The normality assumption was met for the data. For more accuracy, we applied a repeated-measures Friedman test to analyze differences across conditions ($\chi^2 = 26.06, df = 4, p < 0.01$). This first test provides a compact overview of the results. We perform pairwise Wilcoxon signed-rank test post-hoc comparisons using Bonferroni correction to control for multiple comparisons.

<figure><img src="image_16.png" /><figcaption>Fig. 8. Friedman Test for second experiment conditions. Bonferroni correction applied. "*" marks the significance for a $p < 0.05$. "**" marks the significance for a $p < 0.001$.</figcaption></figure>
<table><caption>TABLE II
PAIRWISE PROBABILITIES BETWEEN CONDITIONS BASED ON GLMM ANALYSIS FOR THE SECOND USER STUDY. EACH CELL REPRESENTS THE PROBABILITY OF SELECTING THE CONDITION IN THE ROW COMPARED TO THE CONDITION IN THE COLUMN</caption><thead><tr><th></th><th>No haptic</th><th>Mean frequency</th><th>High frequency</th><th>Delay</th><th>Pink noise</th></tr></thead><tbody><tr><th>No haptic</th><td></td><td>18 [12, 28] %</td><td>37 [26, 49] %</td><td>30 [20, 41] %</td><td>62 [50, 73] %</td></tr><tr><th>Mean frequency</th><td>82 [72, 88] %</td><td></td><td>79 [69, 87] %</td><td>71 [59, 81] %</td><td>76 [65, 85] %</td></tr><tr><th>High frequency</th><td>63 [51, 74] %</td><td>21 [13, 31] %</td><td></td><td>30 [20, 41] %</td><td>59 [47, 71] %</td></tr><tr><th>Delay</th><td>70 [59, 80] %</td><td>29 [19, 41] %</td><td>70 [59, 80] %</td><td></td><td>67 [55, 77] %</td></tr><tr><th>Pink noise</th><td>38 [27, 50] %</td><td>24 [15, 35] %</td><td>41 [29, 53] %</td><td>33 [23, 45] %</td><td></td></tr></tbody></table>

<figure><img src="image_17.png" /><figcaption>Fig. 9. Results for the second experiment, based on the Likert scale 7-point responses for the first question of the questionnaire phase: "The haptic feedback felt satisfying". "*" marks the significance for a p value inferior at 0.05. "**" marks the significance for a p value inferior at 0.001.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 10. Results for the second experiment, based on the Likert scale 7-point responses for the second question of the questionnaire phase: "I felt that the agent was convincing". "*" marks the significance for a p value inferior at 0.05. "**" marks the significance for a p value inferior at 0.001.</figcaption></figure>

Whereas in Experiment 1, the difference between "No haptic" and "Mean frequency" was not significant, this time the difference is ($p < 0.05$). This result can be explained by a bias caused by multiple comparisons with other persuasive conditions. In addition, the "Mean frequency" was significantly more chosen than other conditions ($p < 0.05$).

Then, we analyzed each condition's influence on the participant's binary outcome choice (Tab. II).

For each comparison, we used the same GLMM model as the first user study. First, compared to other vibrotactile included conditions, the "No haptic" condition seems less chosen except for the "Pink noise" condition (first line of table II). However, the probability difference between the median of the "No haptic" condition and the "Pink noise" condition is over 60%. Second, the "Mean frequency" has a higher level of persuasion compared to all the other conditions, as all the confidence intervals are superior to 50% (second line of table II). Third, the "High frequency" and the "Pink noise" conditions have lower level of persuasion compared to the "Delay" condition. For the remaining pairwise comparisons, as the confidence intervals contain 50%, we cannot conclude that there is any difference.

2) *Results for the Questionnaire:* The normality assumption was not met for most of the data. We applied a Friedman for repeated measures test adjusted with a Bonferroni method to see the global effect on all the conditions (Figs. 9 and 10).

There is a preference for the "Mean frequency" condition compared to other vibrotactile conditions (Fig. 9). It follows

the results seen in the global Friedman Test (Fig. 8). Moreover, the "Mean frequency" condition is seen as more persuasive than other conditions except the "High frequency" condition.

## G. Summary of the User Study 2

The objective of user study 2 was to define which vibrotactile parameters might impair persuasion.

The results support the hypothesis (H1b) (A vibrotactile feedback with high frequency is less persuasive than vibrotactile feedback with a low frequency), indicating that some vibrotactile feedback parameters might affect persuasion compared to speech not synchronized with haptic. It goes along with the hypothesis that there is a congruence between vibrotactile feedback persuasion and vibrotactile frequency. That is to say, when the frequency is higher than the mean fundamental frequency of the voice, persuasion decreases. Conversely, persuasion increases when the vibrotactile signal's frequency is lower than the mean fundamental frequency of the voice. (H1b) is also supported by the fact that "Mean frequency" condition is more persuasive than other conditions.

(H2b) (A vibrotactile feedback delayed with the audio speech feedback is less persuasive than speech without vibrotactile feedback) is not supported. However, a difference seems to appear in the pairwise analysis. So, this suggests that even with a delay, a vibrotactile signal could enhance the persuasion of a speech more effectively than speech not haptically augmented.

(H3b) (A vibrotactile feedback consisting of random noise is
less persuasive than speech without vibrotactile feedback) is not supported. The pink noise vibrations did not increase or decrease persuasion, obtaining comparable results as the no-haptic condition. These results suggest that coherent vibrotactile feedback concerning the speech is required. Nevertheless, although the results are non-significant, the pairwise probabilities and the subjective perception of persuasion suggest that the pink noise condition could even invert the persuasion effect and lower persuasion for the pink noise condition compared to the no-haptic condition. However, additional experiments are required to confirm this.

Comments from participants suggest that “Mean frequency” is mainly found satisfactory due to synchronization with the voice (18 out of 25 participants). “Synchronization” is chosen here to refer to the temporal synchronization for some participants and the fact that the vibrotactile signal effectively mimics the voice. However, the “High frequency” is not as well appreciated and is even seen by a small number as being out of sync with the voice (4 out of 25 participants). In addition, some perceive the “Pink noise” condition as overly aggressive (6 out of 25 participants) and “too strong” (7 out of 25 participants).

Overall, the results of user study 2 suggest that high frequency and delay on the vibrotactile signal could positively affect persuasion. However, these results should be further studied, to explore the role of specific vibrotactile parameters.

# V. DISCUSSION

## A. Influence of Vibrotactile Parameters on Persuasion

The first experiment investigated which vibrotactile parameters support persuasion when synchronized with speech. First, our results goes along with the idea that augmenting speech with vibrotactile feedback could positively impact persuasion compared to speech without vibrotactile feedback (**supporting H1a**). As such, our results align with previous findings on “Persuasive Vibrations” [8].

Then, our results do not show any significant difference between the entire audio frequency range and the mean fundamental frequency only (**supporting H2a**). It is in line with previous scientific literature on auditory perception, which found that mixed frequencies are perceived as having the same pitch as a single sine wave with the same fundamental frequency due to the auditory system's difficulty in integrating complex stimuli [57]. Here, a similar effect seems to arise for vibrotactile feedback, suggesting that a single and well-defined frequency could suffice for increasing persuasion, with no need for a complex range of frequencies.

Moreover, the results suggest that a low vibration frequency could increase the persuasive effect compared to a higher frequency (**supporting H3a**). It seems to follow again the same trend as in previous research on auditory perception, which showed that a lower pitch voice is generally perceived as more persuasive [38], [39]. Finally, the results do not show any significant impact of the speech envelope on persuasion compared to a dynamic or constant envelope (**not supporting H4a**). Therefore, the vibration frequency seems to be the primary factor influencing “Persuasive vibrations” more than

the signal envelope. However, this result does not fully align with related work suggesting that the envelope of a vibrotactile signal is more perceived than frequency variations [41]. It could be because persuasion involves complex cognitive processes such as argument evaluation, biases, and beliefs, which might diminish or overshadow the direct impact of vibrotactile envelope variations [58].

Overall, the findings of our first experiment suggest that a simple vibrator, such as an LRA actuator, without independent amplitude control, could be sufficient to produce an impacting persuasive effect, i.e., greater than speech alone, and should leverage a low vibration frequency.

On the other hand, the second experiment explored the vibrotactile parameters, which might impair or degrade the persuasion effect. Indeed, the results suggest that a higher vibration frequency decreases persuasion compared to the mean frequency. It supports the correlation between vibrotactile feedback frequency and persuasion (**supporting H1b**). This result aligns with past studies showing that voice pitch, and thus voice fundamental frequency, is a main factor in leadership and, as a result, in persuasion [38], [39]. It seems here that the fundamental frequency of the vibrotactile signal could also be an important factor in persuasion.

The results also suggest that a delay between the vibratory feedback and the speech of 220ms did not degrade persuasion (**not supporting H2b**). Moreover, on the contrary, it could still enhance persuasion compared to no haptic feedback at all. This result could be promising for real-time applications of persuasive vibrations in which temporal delays might occur. However, future work is still needed to investigate this aspect further, notably with longer delays or more prolonged or different kinds of speech. Finally and interestingly, using a pink noise did not bring any added value or impairment in terms of persuasion compared to the no haptic condition (**not supporting H3b**). It shows that any vibrotactile signal can obtain a persuasive effect from vibration and that a random signal is not expected to bring a clear benefit. We suppose that persuasive vibrations probably require to rely on a frequency concordance between the auditory and vibratory signals.

## B. Limits and Perspectives

The findings of our user studies open up several directions for future work. First, previous research indicates that leadership, and thus persuasion, can vary depending on the gender of the listener or speaker [16], [38]. It has also been observed that trust can diminish if a touch is deemed inappropriate, which may depend on several socio-cultural factors such as geographical region and gender [59], [60]. In our experiments, the level of persuasion was overall high for the subjective notation (questionnaire), even without vibrotactile feedback, and this could be due to the use of a male voice. Moreover, the sensitivity to vibrotactile stimulation also depends on the body position or the age of the participants [61]. Thus, future works could focus on some or all of these factors and study, for instance, the influence of gender, personality, tactile sensitivity, or age on persuasive vibrations.
Second, we noticed some differences between the first and the second experiments regarding the results of the "mean frequency" versus the "no haptic" conditions. Our forced-choice method, involving successive comparisons of all conditions, might have introduced unintended biases. Participants could have compared conditions within the same pair and across other conditions, potentially affecting the overall results. Moreover, some differences were observed between the results of the forced-choice phase and the participant's responses to the questionnaire regarding persuasion effects. The possibility for participants to "replay" the speech multiple times might have encouraged overthinking, contributing to discrepancies between the results of the two phases. It would, therefore, be interesting to reproduce our studies by changing and using different experimental protocols.

Our results are based on specific parameter values. Therefore, it would be interesting to test if different parameters lead to similar results. We also observed that the output frequency spectrum, particularly in the pink noise condition, differs noticeably from the input signal, likely due to the sharp resonant characteristics of LRAs. This suggests that the delivered stimuli may not have fully reflected the intended signal, which is important to consider when interpreting participant responses.

We decided to use a mean value for the fundamental frequency. However, the fundamental frequency can vary over time. In future work, it could be interesting to compare the fundamental frequency at each instance, using, for example, the YIN method [62] to the entire frequency range of the voice. Furthermore, certain parameters, such as the global signal amplitude, were not investigated to limit the number of conditions. Similarly, it might also be interesting to study the influence of using other actuator locations that on the hand. Moreover, the actuator's physical limits have to be considered. Due to its frequency behavior, not all the frequencies can be accurately reproduced, particularly for frequency values inferior to its resonance frequency or superior to its linear frequency range behavior. For future studies, it could be valuable to explore actuators with a broader and flatter frequency response, such as wideband voice-coil actuators or piezoelectric transducers, to more faithfully reproduce complex signals and isolate frequency-specific effects with greater precision.

Finally, studying the effect of persuasive vibrations in ecological scenarios such as during real videoconferences or virtual meetings in extended reality would also be interesting since visual feedback can affect persuasion [51].

We think that using persuasive vibrations holds potential across various domains to influence and guide behavior positively or to enhance communication by promoting greater cohesion and engagement in social interactions. For instance, during meetings, particularly remote ones, it could help balance dominance dynamics and foster equitable participation in discussions or collaborative groups, but also in extended reality, where communication can sometimes be difficult. In healthcare, this approach could be used to enhance trust in

healthcare professionals and encourage commitment to treatment plans. Vibrotactile feedback could guide players toward specific narrative directions within the entertainment industry, such as scripted video games.

However, these usages of persuasive vibrations also raise several ethical questions, particularly regarding the user's conscious choice and informed consent. It seems essential that users remain always fully aware of the presence and purpose of such effects and their potential impact on decision-making or behavior. This transparency ensures that users retain autonomy and are not unknowingly influenced in ways that conflict with their interests or values. Equally important is establishing clear frameworks governing these effects' use. Such frameworks should define acceptable contexts for their application and set boundaries to prevent misuse. These guidelines should prioritize the user's well-being and safeguard against exploitative or manipulative practices.

## VI. CONCLUSION

In this paper, we studied the influence of various vibrotactile parameters on persuasion when speech is augmented with vibrations. We conducted two user studies in which the participants faced a videoconference-like interface and listened to pairs of successive speeches, augmented or not, with vibrotactile feedback. The first study aimed to distinguish which vibrotactile parameters are the most important and can enhance the persuasion effect of vibrations. The results suggest that speech persuasion could increase with a vibrotactile signal with a lower frequency. Furthermore, the results could support the hypothesis for a larger range of parameter values. In that case, they suggest that a basic vibrotactile actuator could be sufficient to achieve a better persuasion effect, than speech alone.

The second study aimed to test if some vibrotactile parameters might impair persuasion. The results suggest first that even with a high frequency at 231 Hz or a temporal delay of 220ms, the persuasion effect remains effective compared to speech alone. However, the persuasion seems to be lower for a vibrotactile feedback of mean frequency without delay. Finally, using just a noise signal as a vibration was not found to be more persuasive than just the speech. Further experiments are necessary to confirm these results.

Overall, our results qualify the vibrotactile feedback to be used when aiming at persuasive vibrations. They could serve as a basis for future developments of haptic designers in a wide range of applications in which supporting users' speech is valuable, such as for videoconferencing, virtual meetings, or training systems.

## ACKNOWLEDGMENTS

The research leading to these results has been partially funded from the European Union Horizon 2020 research and innovation program under grant agreement No. 101017884 - GuestXR project. This work was also supported by a French government grant managed by the Agence Nationale de la Recherche as part of the France 2030 program, reference
APPENDIX
APPLIED SIGNAL PROCESSING

<table><thead><tr><th>Condition</th><th>Mathematical expression</th></tr></thead><tbody><tr><td>Audio like</td><td>h(t) = s(t)</td></tr><tr><td>Mean Frequency</td><td>h(t) = 0.15* h<sub>env</sub>(t) sin(2π·167·t) with the signal envelope h<sub>env</sub>(t) = |H(s(t))| where H is the Hilbert transform:<br>H{s(t)} = 1/π ∫<sub>-∞</sub><sup>∞</sup> s(τ) dτ<br>with P.V. the Cauchy Principal Value,<br>s(t) the audio signal, and t the audio time (s)</td></tr><tr><td>Low Frequency</td><td>h(t) = 0.06* h<sub>env</sub>(t) sin(2π·100·t) with the signal envelope h<sub>env</sub>(t) defined in the Mean Frequency part.</td></tr><tr><td>On off</td><td>h(t) = 0.04* m(t) sin(2π·167·t), with<br>m(t) = {1, if |s(t)| > ε and Δt > T<sub>min</sub><br>0, otherwise<br>where ε = 1 × 10<sup>-5</sup>, T<sub>min</sub> = 0.02 s, and<br>Δt = (t<sub>i+1</sub> - t<sub>i</sub>) / (frame rate)<br>(where t<sub>i</sub>, t<sub>i+1</sub> are instants such that<br>|s(t)| < ε)</td></tr><tr><td>High Frequency</td><td>h(t) = 0.95* h<sub>env</sub>(t) sin(2π·231·t) with the signal envelope h<sub>env</sub>(t) defined in the Mean Frequency part.</td></tr><tr><td>Delay</td><td>h(t) = 0.15* h<sub>env</sub>(t + 0.220) sin(2π·167·t + 0.220) with the signal envelope h<sub>env</sub>(t) defined in the Mean Frequency part.</td></tr><tr><td>Pink noise</td><td>h(t) = 0.03* p(t) where p(t) is pink noise with power spectral density:<br>S(f) ∝ 1/√f for f > 0<br>and S(f) is defined as:<br>S(f) = lim<sub>T→∞</sub> 1/T |F{p(t)}(f)|<sup>2</sup><br>with f the frequency (Hz)</td></tr></tbody></table>

* These gain values were defined during the calibration procedure (see Section D).

<caption>TABLE III
PROCESSING SIGNAL FOR EACH VIBROTACTILE SIGNAL NAMED h(t)</caption>

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BIOGRAPHY SECTION

**Sabrina Toofany** is working toward a PhD in Inria Seamless Team in Rennes, France. Her research interests include haptic interaction, virtual reality, and user experience.

**Anatole Lécuyer** is Senior Researcher and former head of Hybrid research team, at Inria, the French National Institute for Research in Computer Science and Control, in Rennes, France. His research interests include virtual reality, 3D interaction, haptic interfaces, and brain-computer interfaces.

**Ferran Argelaguet** is a Research Scientist and head of Seamless research team, at Inria, the French National Institute for Research in Computer Science and Control, in Rennes, France. His research interests include 3D user interfaces, virtual reality and human perception.

**Justine Saint-Aubert** is a CNRS researcher at IRISA, Rennes, France. She graduated with a PhD in Robotics at Sorbonne University in 2019. Her main research interests include human computer interaction, multisensory integration, haptic interfaces, and Virtual Reality.