aBio: Active Bi-Olfactory Display using Subwoofers
for Virtual Reality
You-Yang Hu1 Yao-Fu Jan1 Kuan-Wei Tseng1 You-Shin Tsai1
Hung-Ming Sung1 Jin-Yao Lin2 Yi-Ping Hung1
1 National Taiwan University, Taipei, Taiwan

2 Tainan National University of Arts, Tainan, Taiwan

hung@csie.ntu.edu.tw

Figure 1: Proposed active bi-olfactory display system (aBio). The left side shows the scenario of presenting a virtual flower’s
smell in front of a user’s nose. The right side is the configuration of the proposed olfactory display unit.

ABSTRACT
Including olfactory cues in virtual reality (VR) would enhance user
immersion in the virtual environment, and precise control of smell
would facilitate a more realistic experience for users. In this paper,
we present aBio, an active bi-olfactory display system that delivers
scents precisely to specific locations rather than diffusing scented
air into the atmosphere. aBio provides users with a natural olfactory
experience in free air by colliding two vortex rings launched fromdual speaker-based vortex generators, which also has the effect ofcushioning the force of air impact. According to the various requests
of different applications, the collision point of the vortex rings canbe positioned anywhere in front of the user’s nose. To verify theeffectiveness of our device and understand user sensations whenusing different parameters in our system, we conduct a series ofexperiments and user studies. The results show that the proposedsystem is effective in the sense that users perceive smell without
sensible haptic disturbance while the system consumes only a very
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MM ’21, October 20–24, 2021, Virtual Event, China© 2021 Association for Computing Machinery.
ACM ISBN 978-1-4503-8651-7/21/10. . . $15.00
https://doi.org/10.1145/3474085.3475678

small amount of fragrant essential oil. We believe that aBio has great
potential for increasing the level of presence in VR by delivering
smells with high efficiency.

CCS CONCEPTS• Hardware → Analysis and design of emerging devices andsystems; • Human-centered computing → Interaction devices.

KEYWORDS
olfactory experience, olfactory display, vortex generator, vortex
ring, virtual reality
ACM Reference Format:
You-Yang Hu, Yao-Fu Jan, Kuan-Wei Tseng, You-Shin Tsai, Hung-Ming Sung,
Jin-Yao Lin, and Yi-Ping Hung . 2021. aBio: Active Bi-Olfactory Display using
Subwoofers for Virtual Reality. In Proceedings of the 29th ACM International
Conference on Multimedia (MM ’21), October 20–24, 2021, Virtual Event, China.
ACM, New York, NY, USA, 9 pages. https://doi.org/10.1145/3474085.3475678

1

INTRODUCTION

Compared with the highly active development of VR techniquesfor visual, auditory, and even haptic sensations, the developmentof olfactory displays is slow but steady. Augmenting auditory andhaptic cues with olfactory cues would deepen viewers’ understanding and sense of reality [5, 9]. This is achievable via the use of an
olfactory display, a device which generates scents and stimulates
humans’ olfactory organs.

Many existing olfactory displays use wind to diffuse scents into
the air using fans [7, 38]; these are known as environmental-type
displays, and are effective at presenting ambient odors. However,such diffusion of scent molecules spreads odors throughout theentire space and it is difficult to eliminate the odor from the air.To
solve this problem, wearable olfactory displays have been developed
which present the odor directly to the user’s nose [16, 27, 41]. Thus
injecting the smell into the user’s nose makes it easier to control theolfactory information. Nonetheless, the user must bear the burdenof an olfactory device in addition to the head-mounted display,which tends to deteriorate the natural sensory experience in VR.
In light of these difficulties, there has been a growing interestin the vortex-type display. A vortex is a fluid-mechanical phenomenon in which a region of fluid rotates around an axis line. Scent
molecules are sealed in the vortex ring and delivered to the user via
airflow. The odor is released once the vortex ring has collapsed bycollision. To generate a vortex ring, scented air molecules must be
exhausted from a semi-enclosed chamber at high speed. Many approaches utilize an air vortex cannon to produce vortex rings. These
generally rely on a piston-driven slider crank to expel the odor fromthe chamber. Another solution is acoustically-driven vortex generator based on loudspeaker, in particular, subwoofer. Subwoofer convert electrical energy into mechanical vibration which compresses
and projects air molecules from the chamber. In comparison to
the piston mechanism, the subwoofer driven vortex generator has
more potential application, such as the sound system that capableof olfactory display and we can modify the input wave signal tocreate different launch configurations.
In this paper, we present an olfactory display system (aBiO) thatprojects smells of virtual objects and provides users with an interactive olfactory experience in VR. As shown in Fig. 1, scented
vortex rings launched from two subwoofer driven olfactory displays collide in mid-air to release smell. The airflow momentum
is eliminated by the collision of two vortex rings, thus reducing
unnatural and disturbing haptic sensation. The collision point isprecisely positioned in front of the user to maximize olfactory sensation while minimizing haptic sensation. In contrast to other typesof olfactory displays, the proposed system consumes a very small
amount of liquid odorant to provide the user with mid-air scents.
Experimental analysis shows that the proposed system achieves
precise localization of the smell in three-dimensional space. According to the design principle of the user-centered olfactory display,
the smell can be released anywhere in front of the user’s nose at any
time. Additionally, we conduct pilot and user studies to evaluate
the correlation between significant system parameters and humansense. This serves as a design guideline for implementations or
extensions of the proposed olfactory display system.

2 RELATED WORKS
2.1 Olfactory Displays for Multimedia
In the human-computer interface field, an olfactory display is defined as a computer-controlled system with hardware, software,and chemicals that presents olfactory sensations to humans [5].
Compared with other types of displays (e.g., visual and auditory),
although olfaction—that is, the sense of smell—is inherently complex as a media component, numerous olfactory displays have beendeveloped for multimedia. Existing olfactory displays can be largely
categorized into environmental displays, placed in the environment,
and wearable displays, equipped on the body or on the head [25].
2.1.1 Environmental Olfactory Displays.
Stationary environment olfactory displays typically rely on airflow to carry the scents to the user’s nose. Beginning with Sensorama [17], an immersive system that utilized an air conditioning
system to deliver smell released from vials for a multi-sensory experience, several techniques have been developed that use wind topresent smells [7, 10, 14, 30, 34, 38]. This approach’s primary advantage is in its unobtrusive nature: the user may even be unawareof the the presence of the olfactory display. With this advantage,
however, comes the the difficulty of precisely positioning an odor ata specific location. Although these approaches [15, 21, 23] facilitate
the spatial control of smell, they require a large amount of liquid
perfume to ensure continuing operation, which causes scents tolinger in the air. To solve these problems, [18, 29, 37] have developed olfactory displays based on inkjet technology. However, due
to the device’s range and fixed limitations, the user must sit in frontof it, which makes it difficult to provide an olfactory experience
within a movable area.
2.1.2 Wearable Olfactory Displays.
Wearable olfactory displays also provide users with an olfactory experience. These can be further divided into on-body andhead-mounted displays. For example, [6, 41] reported the backpackmounted devices that dictated the strength and timing of odor
presentation based on user’s location in the virtual environment.
More recently, several personal on-body olfactory devices [1, 2, 39]
are developed for VR therapy, psychotherapy, and social interaction. Following the development of VR head-mounted display(HMD) technology, many head-mounted olfactory displays havebeen proposed. For example, [16] have proposed an HMD olfactory display that uses a surface acoustic wave (SAW) device basedon previous research [3, 4]. This olfactory display provides theuser with scents vaporized from a SAW device with low liquid perfume consumption. [22] reflects scent-emitting sources intensities
based on environment fluid dynamic simulation. HMD olfactory
displays [8, 20, 28, 32] have also been developed to explore the connection between human olfaction and other senses. Compared withstationary environmental olfactory displays, the wearable approachfacilitates precise control of smell in terms of time, position, andintensity, as the scent is directly released to the user’s nose. However, users must wear an additional olfactory device, which manyusers find uncomfortable and intrusive, thus hindering immersion
in the virtual environment.
2.1.3 Vortex based Olfactory Display.
To address the limitations of these two types of olfactory displays, Watkins proposes a system that uses a vortex ring to present
scents [40]. Based on this concept, [45] proposed the "Scent Projector", composed of a vision-based nose tracker, a steerable accordion air cannon, and a commercially available scent diffuser. This
presents a smell in front of the user’s nose through a scented vortex
ring launched by the air cannon. However, intense air pressure
results when the vortex ring encounters the user’s face. Also, due
to tracking limitations, it cannot be applied to moving users. To

classical model of a vortex generator is a tube with a piston inside
and a circular aperture at the end (Fig. 2a). The stroke ratio, thatis, the ratio between the length of the slug L slug and the aperture
diameter D a , determines the stability of the vortex ring [11]:Figure 2: Fluid ejected from a nozzle forms into a vortex ring
reduce this haptic sensation, Nakaizumi et al. have two vortex rings
collide [26]: when the vortexes collide, the strong vortex airflow
decelerates into a scented breeze and releases an olfactory field
in front of the user’s face. [44] further use a pilot study to provethat the airflow direction after vortex collision can be controlled byadjusting the velocity of the vortex rings. To address the tracking
limitation, [24] proposed an olfactory display to deliver scented
vortex rings to a walking person. The system uses a time-of-flightcamera to track the moving user’s distance within a specific area.
Vortex based olfactory displays include a novel way to achieve
spatio-temporal control of olfactory stimuli, and does not require
the user to wear any physical devices.

2.2

Acoustically Driven Vortex Generators

Outside of the context of olfactory displays, our work draws fromapplications based on the acoustically driven vortex cannon [13,19, 31, 33]. This type of vortex generator simply uses the vibrations of a speaker diaphragm to squeeze the air out of a chamber
to generate the vortex ring. Compared with piston-driven vortex
generators used in most olfactory displays [24, 26, 43–45], speakerbased vortex generators has more potential application. For example, [33] developed robots that used speaker-based vortex generator
for communication. [19] used the collision of two vortex rings forvisual content projection. [13, 36] developed haptic devices using
the speaker-based vortex generator. In point of fact, each of thestudies described above used acoustically driven vortex generators
optimized for an specific application – communication, projection,and haptic feedback, respectively – except odor delivery, which motivating our design of a speaker based vortex generator specificallyfor olfactory display system.

3 PRELIMINARIES
3.1 Vortex Formulation
A vortex ring is an annular airflow, usually in the shape of toroid (ora “doughnut”). The vortex formation process is illustrated in in Fig. 2.
A slug of air is rapidly ejected from a circular aperture (Fig. 2a).
When the air slug leaves the aperture (Fig. 2b), its outer edge moves
slower than the inner edge due to the aperture’s friction, causing theouter edge to curl outwards (Fig. 2c) [35]. The air continues to rotate
until it accumulates into a vortex ring which pinches off from theaperture (Fig. 2d). This rotation allows the vortex ring to travel in astable shape for an extended distance. As the vortex is composed of
the air at the aperture, the odor can be encapsulated in the vortex
ring and then projected to the space beyond the aperture [42]. The
scent inside is released only when the vortex ring is collapsed. As
such, the vortex is an ideal medium for accurate odor delivery.
To make a vortex ring move to a target without deviating fromits trajectory or self-dissipating halfway, we must design a vortex
generator that can launch a stable vortex ring. In fluid dynamics, theR stroke =L slug(1)
Da
Assuming that the air pushed out of the aperture is incompressible,
we can calculate the length of the slug by the total air volume Vslug
leaving the aperture and the area S a of the circular aperture [12, 35].
L slug =

Vslug
Sa

=

4Vslug
π D a2(2)

From Eq.1 and Eq.2, we can express the stroke ratio as:
R stroke =4Vslug
π D a3(3)

which is the key to optimizing the design of the vortex generator.
For a stable vortex ring, the stroke ratio must be smaller than atheoretically defined threshold (called the formation number) which
is between 3.6 and 4.5 [11]. As previously mentioned, we have foundno prior study that theoretically analyzes the relationship between
the stroke ratio and the design of the olfactory vortex generator.
Hence in the next section, we experimentally calibrate the stroke
ratio parameters to optimize the design of the vortex generator.

4

PRECISION OLFACTORY DISPLAY

We developed a subwoofer-driven olfactory display that precisely
delivers the air vortex of the scents, both in terms of location andtime. Fig. 3 shows the workflow of the proposed olfactory display. A
tiny scent droplet is produced by a controllable scent generator andthen transmitted into an airtight chamber of the vortex generator.
The smell is not released until the shutter is opened, at which time
the subwoofer vibrates to compress the air, forming a vortex ringwhich carries the odor molecules to the target location.
Our olfactory display is composed of two main parts: a scentgenerator, and a vortex generator with a 2-DOF platform.

4.1

Scent Generator

In our olfactory display, the scent generator atomizes the odor
solution into tiny odor droplets and transmits them to the vortex
generator. It produces deliverable odor molecules from odor sources
with low consumption and high efficiency and achieves precise
control of the concentration during the generation process.
The scent generator configuration is shown in Fig. 3a. It is composed of a container, an air pump, and an atomizer. The system
uses a pump motor to generate a stable compressed airflow. Basedon Bernoulli’s principle, the vacuum suction generated by the compressed air flow draws the scented liquid from the container to theatomizer inlet at the top, after which the liquid is atomized into veryfine droplets by collision and then released to the outside througha tube. Many kinds of scented liquids can be used in this, including
essential oils or alcohol-soluble fragrance liquids. Note that if pureessential oils are used in this system, there is no need to add water,
heat, or use alcohol for dilution, which means the system maximizes the efficiency of essential oil-based scent production, andyields pure odors. With this approach, another advantage as that if

Figure 3: System Workflow
the compressed air flow generated by the pump motor remains unchanged, the production rate of the scented molecules is constant;thus, controlling the operation time of the pump motor allows for
precise control of the concentration of the scented droplets.

4.2

Vortex Generator with Platform

In this part, the vortex generator launches the vortex ring, whichcontains the odor droplets generated by the scent generator. As
shown in Fig. 1, this consists of a 0.1 L cylindrical chamber, a nozzle
with a Canon™ shutter, a 3-inch 20 W Ouxiang™ speaker, a 2-DOF
platform, and a Vive Tracker. All components except the shutter, thespeaker, the platform, and the Vive Tracker were 3D printed on an
Anycubic™ printer using 1.75 mm Esun™ polylactic acid material.
The speaker was installed opposite to the aperture to function asa piston, flexing the diaphragm to push air out of the chamber
to generate the vortex ring. To control the flight trajectory of thevortex ring, we mounted the vortex generator on a self-made electric
2-DOF platform equipped with two stepping motors, allowing for
precise control of the rotation of the vortex generator. In addition,
the shutter allowed us to control the opening and closing of theaperture to prevent scented air from leaking during filling.
Several cores of our vortex generator design are smaller in size,
produce less noise, and can deliver more odor in a single launch.
The most important feature is producing a stable vortex ring toachieve precise olfactory display. Equation 3 from the previous section shows that the stability of the launched vortex ring is related
to the aperture size and the air volume leaving the aperture. Furthermore, the literature indicates that the stability of the launchedvortex ring is also affected by the nozzle shape and the diaphragmvelocity [45], and is not significantly affected by the inner volumeof the enclosure [46]. Therefore, we first arbitrarily set the chamber
volume to 0.1 L to reduce the total size of the generator. Next, we
conducted three experiments to find the optimal design for ourolfactory vortex generator.
4.2.1 Air Volume Measurement.
To maximize the volume of scent delivered during each launch,
we first conducted an experiment to measure the maximum air slugvolume pushed by the diaphragm. As the surface of the speaker
diaphragm is not flat, it is not practical to calculate the pushed air
slug volume by measuring the diaphragm displacement. Thus webuilt a device consisting of the same chamber used for the vortex
generator and a needle tube with an inside diameter D tube of 30 mm.

Assuming that the air pushed out of the aperture is incompressible,
we obtain the total pushed air slug volume by measuring the displacement of the needle piston H piston . The speaker diaphragm’s
movement is triggered by a positive square pulse signal generatedusing Python. We control the diaphragm displacement by adjusting
the duration of the square wave. Notably, we found that the duration of 40 ms produced the greatest needle piston displacement;
increasing the amplitude beyond 40 ms can not increase the amountof the needle piston movement. We thus obtained 9 mm as the valueof Hn . For our vortex generator, the maximum volume of air slug
Vslug displaced by the speaker diaphragm can be computed as
D tube 2
Vslug = π× H piston = π × (15)2 × 9 = 110mm3(4)2
4.2.2 Aperture Selection.
We further conducted an experiment to determine the aperture
diameter and shape that produce the most stable and high concentration scented vortex ring. For the aperture diameter, from
Equation 3 and Vslug , we preliminarily calculated that the aperture diameters fall between 28 mm to 30 mm, which corresponds
to the formation number ranging from 3.6 to 4.5, which as mentioned above are the theoretically defined threshold to obtain astable vortex ring. As for the aperture shape, the literature indicates that a vortex generator with the aperture extruding from thebody launches a more stable vortex ring, and that a curved shapeproduces more haptic sensations than a flat one [36]. In contrast
to vortex-based haptic devices, olfactory displays should minimize
haptic sensations caused by the vortex ring. Thus we decided on aflat shape nozzle with the aperture extruding from the body. Notethat we found no studies detailing how the extruded length of theflat shape affects the stability of a vortex ring; nor did we find any
studies evaluating the relationship between the scent intensity ofthe vortex ring and the aperture parameters. Hence we evaluatednine different apertures with diameters ranging from 28 mm to
30 mm and nozzle lengths ranging from 20 mm to 40 mm.
To evaluate the accuracy and relative scent concentration carriedby the vortex rings launched using different apertures, we built
the electronic nose shown in Fig. 4a. Since it is difficult to find a
instrument that can sense arbitrary odor molecules, we use alcohol
instead. It consists of nine high-sensitivity alcohol sensors (MQ3)aligned in a 3x3 matrix. The distance between each sensor unit is
35 mm. The effective measuring range of this sensor is 0.05 mg/L

SquareClipped Sine

Clipped Sinc withHamming Window

Clipped Sinc

Duration
(ms)

Noise
(dB)

Accuracy(%)

Occ(mg/L)Noise(dB)

Accuracy(%)

Occ(mg/L)Noise(dB)

Accuracy(%)

Occ(mg/L)Noise(dB)

Accuracy(%)

Occ(mg/L)

50

75.1

100

1.72

64.8

100

1.6

63.3

100

1.63

61

100

1.63

60

74.8

100

1.69

63

100

1.62

62.5

100

1.51

59.6

100

1.54

70

75.2

95

1.76

61.5

90

1.49

61.3

90

1.41

59.2

95

1.46

Table 1: Signal waveform selection experiment results. (Occ: Odor carrying capacity). The ambient noise level was 42 dB.

Figure 4: (a) Electronic nose (b) Successful hit caseto 10 mg/L. The nine sensors’ detect concentrations were sent to adata visualization program running on PC by an ESP32 module. As
shown in Fig. 4b, the results of the center sensor were visualized
in a red line. When the vortex ring collides with the electronic
nose, the status of lines reflected the concentration distribution.
Hence, by analyzing the line that increased the most, we obtain theexact collision area. For detailed illustration of the electronic nose’s
data visualization, please refer to the supplementary. As shown in
Fig. 4b, a case was considered a successful hit only when the redline increased first and highest. And we can obtained the relative
carried concentration by calculating the difference from a stablestate to the peak(δ C).
The experiment was conducted in a windless environment. The
olfactory display was placed 70 cm away from the electronic nose.
We turned on the scent generator’s air pump for 3 s to fill the chamber with 99% alcohol droplets, after which we launched the vortex
ring to the electronic nose. Note that in this experiment, we used
a 60 ms duration square pulse signal to drive the subwoofer. Each
type of aperture was repeated 20 times using this process. Given thedetection data on the electronic nose, we calculated the successful
hit rate and the average concentration under the successful hits. As
shown in Fig. 5, the aperture with a 29 cm diameter and a 30 cm
length provided the most efficient and accuracy scented vortex ring.
Thus we used this aperture for further olfactory display design.
4.2.3 Signal Waveform Selection.
One of the limitations in acoustically driven vortex generator isthe bump sound produced by the speaker diaphragm, which interrupts the user’s sense of immersion in the VR experience. However,
we found no previous studies that attempted to mute this loudnoise. The electric impulse wave signal driving the movement of

Figure 5: (a) Accuracy result (b) Concentration result
the speaker diaphragm is the key to minimizing this noise. In contrast to previous works [13, 33] that used square wave to flex the
speaker diaphragm in and out, we tested a variety of different waveforms which have fewer high-frequency signals. Fourier analysis of
the square wave reveals high-frequency components, which could
cause audible noise. In contrast, the sine and sinc wave produces
fewer audible sounds, and thus could yield quieter operation during the launch of the vortex generator. Another issue is that whenusing waves to generate the vortex, negative amplitudes hinder
the creation of the vortex ring. Therefore, we adopted a Hamming
window on the sinc wave, taking the positive central part as ourpulse signal to drive the speaker movement.
To verify the noise reduction for this method and choose the best
pulse signal, we used the decibel meter (TM-101) and the electronicnose placed 70 cm away from the vortex generator to measure
the sound intensities, accuracy, and odor carrying capacity under four waveforms of pulse signals (square, sine, sinc, sinc withHamming window) in different durations. Notably, to keep the diaphragm at the maximum displacement long enough to launchinga stable vortex ring, we amplified all waveform signals by 6 dB to
become clipped signals. All signals were generated using Python
and exported as wav files for the subwoofer to play. Table 1 shows
the results produced with four waveforms in three different durations(50 ms, 60 ms, 70 ms), for more results in longer duration,please refer to the supplementary. The measurement results showthat clipped sine, clipped sinc, and clipped sinc with Hamming
window pulse signals were all effective in reducing noise, and theiraccuracy can reach 100% in the duration of 60 ms and below. In mostcases, a shorter duration meant that the vortex ring could carry
more odor, but it would produce a louder sound. Since accuracy andminimal noise were our primary concerns, we decided to use the
60 ms clipped sinc with Hamming window wave, which reduced

Figure 6: Vortex ring position at different time points.
Figure 7: (a)System concept (b)System top viewthe maximum launch noise by 15 dB compared with the squarewave, and it had 100% accuracy.
4.2.4 Vortex Ring Movement.
In the system with two olfactory display units, we want to makethe vortex rings collide at the target point: to do this we mustensure that the vortex rings reach the target collision point at
the same time. Since the distance between the collision point andeach generator is not always the same, we must determine therelationship between the reach distance and the elapsed time of
the launched vortex rings. Given this relation, we can control thelaunch time of each vortex ring to cause them to both reach thecollision point at the same time. To measure this relationship, we
first revealed the vortex ring using stage smoke (S-400W) and thencaptured the movement of the vortex ring with a camera (iPhone X
rear lens) at 240 fps. We analyzed this video using image processing,thresholding the greyscaled video clip to determine the position of
the largest contour of the vortex ring in each frame, as illustrated
in Fig. 6. We converted the image pixels to actual distances to yield
the real distance and time relationship. Finally, we used regression
to calculate the following time-distance formula of the vortex:
y = 3.629e − 07x 3 + 1.509e − 05x 2 + 0.008438x − 0.01392,(5)

based on which, given a certain distance, we can accurately control
the launch time of the two vortex generators.

4.3

Bi-Olfactory Display System

4.3.1 System Concept.
aBio can generate the smell at a specific target point via a midair collision of two vortex rings. Fig. 7b shows the concept of thissystem. It consists of two olfactory display units. Each olfactory
display unit launches the vortex ring at the target point. The local
high-speed airflow that forms the vortex ring is interrupted when
two vortex rings collide with each other, creating an . The 2-DOF
platform controls the rotation of the vortex generator, which allows
the vortex ring to be directed to a specific target point in a certain
range of 3D space. Through precise vortex control and Vive’s tracking technology, aBio can synchronize the presentation of scentswith the interactive plot of the moving user in VR, such as when theuser picks up a virtual flower, the fragrance of flowers is presented
around the user’s nose.
4.3.2

System Implementation.

As shown in Figure 8, the system involves four main components: (1) The sound card (AUD) drives the two subwoofer speakers through a 3.5 mm stereo audio cable. (2) The ESP32 (buzzer
firmware) controls air pumps that spread the odor into the chamber and switches the nozzle shutter. (3) The Arduino Uno (motor
firmware) controls the vortex generator’s horizontal and verticalrotation. These devices communicate with the VR system on the PC
through a simple custom protocol via Wi-Fi and a USB serial port,
respectively. (4) The interface script handles serial communication
with embedded devices. VR Logic uses the interface script to send
the rotation, launching, and charging commands to the firmware.
We combined the system with Vive tracking technology to enable
the vortex generator to track the specific target collision point. We
can use the tracker mounted on the vortex generator to track its
rotation in real time. Moreover, we can define the collision pointin the VR coordinate system and calculate the vector given the
tracker and collision point’s relative positions. In addition, we canuse the head-mounted display position to track the user’s nose inreal-time and use this as the target collision point. We can also
use the user-side controller’s position to obtain the user’s handcoordinate data to facilitate more interactive modes. For example,
a user can smell the virtual object while holding it close to theirnose. This approach helps aBio to significantly improve its spatial
control of the smell and enables the user to experience interactive
olfactory sensations without wearing additional olfactory devices.

5 SYSTEM EVALUATION
5.1 Collision Experiment
We first conducted an experiment to evaluate the collision accuracyof our system.
5.1.1 Experiment Configuration.
As shown in Fig. 7b, our experiments were conducted in a 1-m
by 1-m space. The olfactory display unit’s rotation range is ±45º
on Y-axis, and ±40º on Z-axis. The maximum delivery distance is
875 mm, which is the forward distance from vortex generators, andthe vertical direction is about ±770 mm. We chose five locationsin the reachable area as collision points. Points A, B, and C are at
the centerline between the two generators: their distances from thegenerators are 600 mm, 700 mm, and 1000 mm. Points D and E areat locations with different distances to each vortex generator; forthese we set the launch time depending on the relation between
the elapsed time and the reach distance determined in Section 4.2.4.

Figure 8: Overview of the architecture of aBio System.

Figure 9: Pilot study results

5.1.2 Experiment Procedure.
We again used stage smoke to make the vortex visible and thenlaunched the rings at target points. We repeated the experiment
50 times at each point to calculate the successful collision rate.
There are three cases, the first case is that the two vortex rings
successfully collapsed after collide, the second case is that the vortex
rings miss completely, and the third case is that the vortex rings
successfully collide but not collapsed. We only consider the first caseas a successful collision. To make the experiment more objective,
we recorded video of the collision process to check whether thevortex rings had collapsed after collide. For detailed illustration of
the collision cases, please refer to the supplementary.

experience. Also, we want to verify whether users can distinguishdifferent intensity of the olfactory field through this experiment.

5.1.3 Results.
The results are also illustrated in Fig. 7b. In general, The average
hit rate indicates that most of the vortex rings collided successfully
since all of the hit rates were higher than 80%. From the results weobserve two trends. First, the collision points with the same distanceto each vortex generator (points A, B, and C) have higher average hit
rates than the other collision points (points D, E). Second, collision
points at farther distances had lower collision rates, since the flightstability of the vortex ring decreases with increasing distance.

5.2

Pilot Study

In our system, there are two ways to deliver the scent: (1) Set thevortex collision point on the virtual object to generate its olfactory
field. (2) Set the vortex collision point in front of the user’s nose todeliver the scent to the user. In this research, our goal is to develop
an efficient olfactory display that the users can reliably smell theodor that the VR director would like them to perceive in a virtualenvironment. Therefore, we adopted the second approach which
deliver the scent vortex directly in front of the user’s nose.
We conduct some experiments to find the optimal parametersof the odor-filling duration and the vortex rings’ collision distancefrom the user’s nose. There are many controllable parameters that
influence the actual and perceived odor intensity. Fill duration is one
the most primary factors. Longer fill duration of odor can generally
increase the odor intensity. Yet, it sacrifices the temporal continuity,i.e. the number of launches in a unit time, which may degrade theuser experience. To this end, we first conduct a pilot study to findthe optimal fill duration that can provide users a better olfactory

5.2.1 Experiment Configuration.
Our experiments were conducted in a 1-m by 1-m space. To
ensure that no external airflow influenced the movement of thevortex rings’ or the distribution of smell, the air conditioners were
all turned off. Furthermore, to prevent participants from hearing
the environmental noise, we supplied them with noise-cancelling
headphones. We recruited 16 participants (7 males, 9 females) of amean age 23 years old (SD=4.18). All participants are non-smokers
with a normal sense of smell. Thirteen of these participants had
experienced VR, four participants are VR developers. We set the fill
duration to 0-2s, with 0.5s as the interval, and the collision point
distance was 5cm from the area in front of the participants’ nose. To
enable participants to rate the perceived odor intensity, we defined
four intensity levels: none (0), slight (1), noticeable (2), and strong(3) olfactory sensations, respectively.
5.2.2 Experiment Procedure.
At the experiment’s beginning, we had the participants sit atpoint B, which was defined in collision experiment. And they were
wearing the HMD with black scene displayed on it. In each round,
we delivered to them 10 scented vortex rings (10 launches) withthe same fill duration. For each launch, we guided the participants
to inhale and exhale at certain timing to ensure that they wereinhaling when the smell was released. There was a 20 seconds and
10 minutes rest between each launch and each round. Specificallyfor 10 minutes rest, they could smell the coffee beans or walk aroundto relieve their olfactory fatigue. The participants reported theiraverage perceived intensity after each round. To regularize theresult, there was a testing round with filling duration 2 seconds
before the formal rounds start. We asked the participants to set
the perceived intensity as strong (3). Afterwards, we conducted 5
rounds of experiments with 5 filling duration respectively.
5.2.3 Results.
The results are illustrated in Fig. 9. Theoretically, the delivered
odor concentration is positively correlated to the run time of thepumping motor. The longer the motor runs, the more odor droplets
are loaded into the chamber. We found that the average perceived
odor intensity increases with the fill duration. However, the growthbecomes insignificant beyond a fill duration of 1.5 seconds sincemore than half of the participants experienced strong feelings. Although increasing fill duration contributes to better olfactory sensation, it may compromise the number of launches per unit time,
which potentially reduces the temporal continuity of our device.
However, in real life, when we pick up a flower to smell it, we do
not always smell the flower’s full fragrance; it can be a fleeting,intermittent experience. we will not always feel the fragrance ofthe flower. This kind of intermittent smell can increase the authenticity, As such an intermittent smell can be said to increase theauthenticity of the experience, we chose a fill time of 2 seconds forthe device to ensure a better olfactory experience for the user.

5.3

User Study

Based on the pilot study, we conducted a user study to determine
the best collision distance. During the pilot study, many participantscommented that it was difficult for them to assign scores representing the smell’s intensity. Therefore, we changed the scoringquestion to a true/false question. To determine the best parameter, we asked participants at the end of the experiment whichsmell experience they felt to be most natural. We then used theseparticipants’ opinions to determine the best parameter value.
5.3.1 Experiment Configuration.
This experiment was conducted in the same setting as the pilot
study. In order to ensure that the user’s breathing rhythm would notaffect the experiment, we had the participants worn head-phones
with a voice guide played on it. We recruited 12 participants (5males, 7 females) with a mean age of 22.75(SD=2.7). All participants
were non-smokers with a normal sense of smell. Eight of theseparticipants had experienced VR; of these two were VR developers.
The test collision distance ranged from 5 cm to 17.5 cm—an increaseof 2.5 cm. The odor fill time was fixed at 2 seconds.
5.3.2 Experiment Procedure.
We had the participants sit at same position as pilot study andwear the HMD with a virtual scene displayed on it. Each collision
distance was tested 10 times, with 10 false launches as well todetermine whether it was an outlier. In each test, a virtual flower
floated towards the participants. If it was a true launch test, thedevices launched when the flower approached. We disrupted theorder of these 120 tests and asked after each launch if there was anodor, if the odor was too strong, and if there was an obvious haptic
sensation. After each launch, the participants took a 20 seconds
rest, and smelled the coffee beans to restore sense of smell. Aftercompleting the experiment, we asked them four questions: (1) How
do you feel about smell in the experiment? (2) How you feel about
the air columns hitting your face? (3) How long does the odor last?
(4) What smell experience do you find most natural? For the detailed
procedure flow of user study, please refer to the supplementary.
5.3.3 Results.
The results are shown in Fig. 10. We observe a logarithmic relationship between the probability of excessive odor and haptic
detection rate as the collision distance decreases. However, as thecollision distance increases, the probability of haptic detection andexcessive odors increases linearly. The chart also shows that forcollision distances greater than 17.5 cm, there is only a 50% chance

Figure 10: User study resultsof smelling in each launch; for distances less than 12.5 cm, theprobability increases to about 80%. Also, the probability of haptic
detection and excessive odor increases as the distance decreases.
However, in this chart we assume that every item of feedback isequally important. For a higher smell detection probability and reduced tactile sensation, the best parameter is closer to the upper-leftcorner, that is, a collision distance between 12.5 cm and 15 cm.
5.3.4 Discussion.
The above experimental results have helped us observe many
correlations between our design configurations and human sensory
perception. For the directors of VR works, this findings can be usefulfor those who want to use our system to deliver odors efficiently
to the VR viewers. If the director does not care much about theuser’s haptic sensations and wants to increase the probability of
the user’s smelling the odor and to save the scent essence, he canset the vortex collision point at 12.5 cm. If the director wants to
reduce the possibility of the user’s sensing the vortex impact on theface when smelling the odor, he had better set the collision point at
a farther away point, for example, at 15 cm.

6

CONCLUSION AND FUTURE WORKS

We propose aBio, an active bi-olfactory display system that demonstrates strong spatio-temporal control of scent delivery for virtual
reality. Scent molecules are carried by vortex rings and released at
designated collision points. Vortex collision significantly suppresses
undesired airflow impact, reducing unwanted haptic feedback. We
conduct pilot and user studies to determine the correlation between
important design configurations and human sense. These parameters can be adjusted to satisfy the requirements of various applications. We believe that these results should be taken into accountwhen designing virtual reality systems with olfactory experience.
In future works we would like to enabling the system to switch
between odors in a short interval of time. We also want to extend
aBio by adding more vortex cannons to create an ambi-olfactory
display system with no blind spots. And with the advantage of the
speaker’s characteristics, we believe it is possible to design a soundsystem capable of the precisely olfactory display, which can provide
users with a multi-sensory experience.

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