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Sonoflex: Embroidered Speakers Without Permanent Magnets


Abstract and Figures

We present Sonoflex, a thin-form, embroidered dynamic speaker made without using a permanent magnet. Our design consists of two flat spiral coils, stacked on top of each other, and is based on an isolated, thin (0.15 mm) enameled copper wire. Our approach allows for thin, lightweight, and textile speakers and does not require high voltage as in electrostatic speakers. We show how the speaker can be designed and fabricated and evaluate its acoustic properties as a function of manufacturing parameters (size, turn counts, turn spacing, and substrate materials). The experiment results revealed that we can produce audible sound with a broad frequency range (1.5 kHz - 20 kHz) with the embroidered speaker with a diameter of 50 mm. We conclude the paper by presenting several applications such as audible notifications and near-ultrasound communication.
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Sonoflex: Embroidered Speakers
Without Permanent Magnets
Thomas Preindl1, Cedric Honnet2, Andreas Pointner1, Roland Aigner1,
Joseph A. Paradiso3, Michael Haller1
1Media Interaction Lab, University of Applied Sciences Upper Austria
2, 3MIT Media Lab, Cambridge, Massachusetts, United States,,
Figure 1. We propose a flexible embroidered speaker, which does not require a permanent magnet. By using a 0,15 mm Cu/Ag20 yarn (A), we
embroider two flat coils (B) which are placed on top of each other (C). One possible demo application could be an embroidered jacket speaker which
can be controlled with a pocket synthesizer (D).
We present Sonoflex, a thin-form, embroidered dynamic
speaker made without using a permanent magnet. Our de-
sign consists of two flat spiral coils, stacked on top of each
other, and is based on an isolated, thin (0.15 mm) enameled
copper wire. Our approach allows for thin, lightweight, and
textile speakers and does not require high voltage as in electro-
static speakers. We show how the speaker can be designed and
fabricated and evaluate its acoustic properties as a function
of manufacturing parameters (size, turn counts, turn spacing,
and substrate materials). The experiment results revealed that
we can produce audible sound with a broad frequency range
(1.5 kHz - 20 kHz) with the embroidered speaker with a diam-
eter of 50 mm. We conclude the paper by presenting several
applications such as audible notifications and near-ultrasound
Author Keywords
Embroidery, embroidered speaker, lightweight speaker, smart
textile, textile loudspeaker, textile transducer.
Textile based input and output interfaces allow for seamless
integration of technology into our everyday environments.
Consequently, there have been numerous efforts in wearable
computing research over the last decades specifically focusing
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on interactive textiles [33, 42, 36]. As electronic components
are becoming more compact, flexible, and efficient at the same
time, evidently, both academic and maker communities as
well as artists embrace new potentials in augmenting existing
textiles with diverse input modalities [39, 16, 55, 25, 38, 4, 17].
Also, a large body of work is committed to using e-textiles as
output medium, be it visual [26], auditory [27] or for actuation
[6, 1].
In this paper, we present Sonoflex, a design for a textile-based,
embroidered speaker (see Figure 1), capable of lo-fi audible as
well as near-ultrasound output without requiring a permanent
magnet (C). We focus on the design and fabrication aspects
of our implementation using flat air-core coils (B), made of
enameled wire (A). As a consequence, we greatly reduce the
device’s weight and rigidity, as well as fabrication complexity,
while keeping it harmless to the wearer by using low operating
voltages, when compared to flexible electrostatic speakers (D).
We first introduce and discuss the basic concept of our embroi-
dered speaker and show several options for implementation.
Subsequently, we present a range of speaker prototypes, vary-
ing geometric and fabrication parameters (e.g. coil size, turn
count, turn spacing, fabric substrate), which were evaluated in
order to maximize performance.
Our results show that we are able to emit audible sound with a
broad frequency range (1.5 kHz - 20 kHz) with a mean value
of 29.34 dB
4.88 dB SPL, using a coil diameter of 50 mm.
Furthermore, we present a set of application scenarios arising
from our speaker design, including a beanie notifier, a jacket
speaker that is fed with music from a handheld synthesizer, as
well as close-range data communication. Finally, we conclude
with discussion, limitations, and future work.
In summary, the contributions of this paper are:
We demonstrate a textile-based embroidered speaker, which
is operational without permanent magnets and potentially
dangerous high voltages.
Details on the functionality and lessons learned regarding
design and fabrication.
Insights with respect to relevant design and fabrication pa-
rameters, including results of our experiments.
Finally, we present a set of applications and discuss both
advantages and limitations of our approach.
Our work relates to many areas of HCI research, including
embroidered smart textiles, coils in textiles, and light-weight
speakers. In this section, we address relevant work of each
field, starting with a review of smart textiles, mostly focusing
on embroidery. We then discuss work concerned with the
implementation of textile coils. Finally, we discuss light-
weight speakers and contrast them to Sonoflex.
Embroidered smart textiles
Due to the properties of textiles being lightweight, highly
flexible, and often also stretchable they are applied in a wide
range of use cases, e.g. for wireless monitoring suits [3],
music control [7], multi-touch input [49, 15] and motion cap-
turing systems [54, 11]. Machine embroidery has been one
of the first fabrication techniques used in the field of smart
textiles [37, 41], as it is highly useful for rapid prototyping
[28, 48]. Mecnika et al. provide an excellent overview of
numerous embroidered smart textile applications [31], includ-
ing embroidered circuits, electroconductive interconnections,
embroidered antennas, embroidered heating, and embroidered
keypads. With Tessutivo [14], Gong et al. demonstrate an
inductive sensing device based on embroidered textiles, which
inspired our coil parameters and shapes, as well as the usage
of different fabric substrates for evaluation.
Coils in textiles
Researchers use coils in combination with textiles for vastly
different applications, ranging from inductive sensors for heart
rate monitoring [24] to magnetic resonance sensors [34], fur-
thermore for wireless charging of wearable electronics [50],
and for inductive wireless power transfer [51, 29, 56]. In
contrast to the mentioned applications, a higher turn count is
necessary to implement an electromagnetic coil for a speaker,
as the strength of the electromagnetic field is crucial.
Light-weight speakers
When implementing light-weight speakers many researchers
focus on "paper-based" solutions Ishiguro et al. presented Umi-
nari, an interactive electrostatic loudspeaker [19], which was
also combined with 3D printed objects [20]. Similarly, Kato
et al. presented OrigamiSpeaker, a hand-crafted paper electro-
static speaker, which is low-cost and can easily be fabricated
with inkjet-printers [23]. Beyond electrostatic loudspeakers,
researchers implemented a piezoelectric version of a paper-
speaker [18].
A variety of textile speakers has already been built by re-
searchers and artists. A starting point for our research was
the work of the Kobakant collective [47, 40] and Leclerc and
Berzowska from the XS Labs [27]. They used a textile as base
for an embroidered coil as well as the speakers diaphragm.
In contrast to our approach, they utilized permanent magnets
fixed to the speaker for an opposing magnetic field.
The main inspiration for our work is drawn from Dinh [5],
who presented a concept of a speaker lacking a permanent
magnet which is based on two coils. Their main objective
was implementing a lightweight speaker. While the concept
and motivation are similar, our solution uses flat air-core coils
fixed onto textiles which act as a diaphragm, omitting a heavy
ferromagnetic core and consequently further reducing weight.
Essentially, a loudspeaker generates sound by pushing air via
a moving membrane. In a dynamic speaker, which is the most
common type, an electromagnet (also called voice coil) is
mounted onto the membrane, which interacts with the mag-
netic field of a fixated permanent magnet. The voice coil
is fed with the audio signal, which causes corresponding re-
pelling and attracting forces, ultimately moving the membrane
in correspondence to the signal’s amplitude. In contrast, as we
are dealing with textiles and inherently are bound to surfaces,
we make use of two flat spiral coils, mutually attracting and
repelling, thus simultaneously acting as our speaker’s mem-
branes (see Figure 2).
Amplied AC signal
Gated DC signal
Line level audio signal
0 t
0 t
  
  
Voice coil
Bias coil
Figure 2. The function principle of the embroidered speaker. The voice
coil is powered by the amplified source signal, the bias coil is powered by
a gated DC signal. A full-wave rectifier with a smoothing capacitor was
used in our implementation to power the bias coil.
Flat air-core coils are widespread in PCBs design, although
generally with a comparable low turn count. While they are
usually meant for inductive sensing, we want to generate ade-
quate magnetic flux density and resulting Lorentz Forces to
cause sufficient physical motion of our speaker membranes.
Furthermore, we are trying to maximize pressure, thus force
by area, to achieve the highest performance in terms of sound
pressure level. Consequently, our objective is to maximize the
magnetic flux density, with a given length of wire or conduc-
tive yarn. From a coil-shaped as an Archimedean spiral
r(φ) = a
, the magnetic flux density
on a point
can be
derived from the Biot-Savart law as
B(x) = µ0I
is the current flowing through the coil,
is the mag-
netic constant,
is an infinitesimal segment of
as a vector
on curve position
with the direction along the flow of
, and
is the distance vector from
. Since our speaker is
composed of two identical coils, sandwiched coplanar and
centered, we can estimate mutual Lorentz forces, resulting
from opposing magnetic flux densities. According to the Am-
pere’s force law, the Lorentz force exerted on a coil
, as
resulting from the magnetic flux density B1of a coil C1is
and vice versa for
acting on
, where
denotes the nor-
malized vector of
. Assuming identical coils, both forces are
equal in magnitude and opposed in direction. Consequently,
with mentioned simplifications, we can approximate the abso-
lute value of the total force between C1and C2as
tot al |µ0I2
. When assuming identical current magni-
, we see that
scales proportionally to
. From that, it is apparent that we can make maximum use
of both coils, when we power both coils with our input signal.
However, according to the Ampere’s force law, currents
are then always equidirectional, producing magnetic
flux with equal directions, and consequently Lorentz forces
in same directions with magnitudes depending on the input
signal, thus the coils would be always attracting themselves
but never repulse. Furthermore, this implies a doubling of
output frequency, since during a single sine wave period, the
membranes would attract (or repulse) twice instead of attract-
ing and then repulsing. Hence, we need to rectify the input
signal of the bias coil, so force vectors go both ways. Initially,
we implemented a full wave bridge rectifier to produce the
desired output [53]. The required gated DC signal can also
be achieved with different approaches, e.g. with an envelope
follower or a digitally controlled high-current transistor.
Coil shape
In related literature, non-circular shapes can be found, includ-
ing squares, octagons, hexagons, among others [35, 52, 14].
Use cases are numerous, ranging from inductive sensors to
oscillators. As mentioned earlier, our objective is to maximize
the magnetic field and inherently, we require to maximize the
number of turns of the resulting coil, which can be formulated
as maximizing the shape outline’s roundness
is the shape outline length (i.e. perimeter) and
is the enclosed area, which for circular shapes needs to be
approximated, as we are tackling spirals, we do not constitute
closed shapes.
is maximized for circular outlines, which we
deem our justification to focus on those.
Conductive wire
Instead of using conductive yarns to embroider coils, as pre-
sented by Gong et al. [14], in Sonoflex, we use thin and insu-
lated copper wires, enabling us to stitch dense coils. A special
requirement to the wire is that it has to be usable with em-
broidery, thus it needs to be flexible and tear-resistant, while
featuring high conductivity. Table 1 provides an overview
of four wires we chose for closer examination. All of them
exhibit high electrical and thermal conductivity and are well
Name Material Thickness Resistance
Textile Wire 31604143 Cu/Ag20 0.06 mm 6.05 /m
Textile Wire 30901494 Cu/Ag20 0.07 mm 4.44 /m
Textile Wire 31605093 Cu/Ag20 0.15 mm 0.97 /m
Verowire Cu 0.20 0.86 /m
Table 1. Enameled wires tested for their embroidery suitability. The
thicker the wire, the smaller the resistance /m.
Figure 3. The thin versions of the Textile Wire tended to break (A,B),
while a thicker wire does not allow tight stitches (C). We settled on the
Textile Wire with a diameter of 0.15 mm as the most useful for our ap-
proach (D).
We conducted an informal experiment to find the thinnest
wire from this set, still feasible for the fabrication of a dense
Archimedean spiral with an outer diameter of
= 50 mm. The
turn spacing was chosen 1.5 times bigger then the diameter of
the wire, so that the needle does not break the wire of neighbor-
ing turns. All samples were embroidered with a Tajima SAI
in combination with digitizing software Creator from Pulse
Microsystems Ltd. and a custom Python script for generating
spiral traces from the given spiral parameters 2.
The results are shown in Figure 3. The thinnest wires (e.g.
0.06 mm, 0.07 mm) proved difficult to embroider as they
are very prone to wire breakage during embroidery. Also
reducing the embroidery speed from a maximum of 800 to
300 stitches/min to reduce stress on the wire, did not yield
better results. The Verowire with a diameter of 0.2 mm could
be embroidered without problems, yet limited the turn count
at equal diameters. We settled on a 0.15 mm diameter wire
which is composed of a copper core covered with concentric
silver plating for corrosion resistance. The wire is insulated
with a polyurethane enamel coating, which makes it washable
up to 60°C, weakly dyeable and easily dryable3.
An embroidery machine commonly utilizes a rotary hook to
create lock-stitches, thereby fixing upper and lower thread onto
the base fabric. Starting from the spool, upper thread passes
through thread guides, take-up lever, and the needle before it
is entwined with the bobbin thread on the textile. When using
a wire as an upper thread instead of a conventional embroidery
yarn, the wire is bent at many positions during the embroidery
process while experiencing high tensile forces. This results
in many wire breaks and prevents consistent embroidery. To
avoid this, we use the wire as a bobbin thread, which is un-
spooled from within the bobbin case and does not have to pass
steep angled turns before it is entwined with the upper thread.
This significantly reduced the risk of wire breaks and allows
for a consistent stitch quality.
As an upper thread, we used a 270 dtex polyester multi-
filament yarn from Amann, which is optimized for embroidery.
We prefer to use a thin upper thread to minimize the distance
between coil turns.
After approximately 30 stitches, the process is paused to move
residue wire, left by the embroidery machine during the initial
stitch. This is required to prevent the beginning of the wire of
being stitched over, and consequently not being accessible for
connecting it to the circuit. This is challenging when starting
embroidering at the coil’s center, therefore we start stitching
from the outside and progress towards the center.
By embroidering insulated wires we can fabricate coils with
high turn density. E.g., with a thread of thickness of 0.15
mm, a coil with 300 turns would result in a diameter of 90
mm. However, we found that a minimum distance of the wires
diameter must be kept between turns, as too closely placed
stitches can result in wire breaks. The coil’s turn spacing is
limited by the diameter of the embroidered wire. To limit the
stress applied to the wire and to avoid wire breakage, the turn
spacing has to be chosen carefully. We empirically found a
value of wire diameter times 1.5 as a viable minimum.
Furthermore, we conducted an evaluation regarding the effect
and applicability of different fabric substrates, including twill
weave (BadgeTex 2900 twill weave with 330 g/m
), plain
weave (SEFAR Acoustic, 280-12), and leather (Lecapell) -
cf. Figure 4. In embroidery, denser base materials allow
for more accurate stitches. Based on prior experience with
embroidery, we primarily used the twill weave, as it allows
Figure 4. Three different fabric substrates and their influence to the
embroidered coil shape have been evaluated.
for sub-millimeter precision in positioning of stitches and
shows little tendency to frail when trimmed. It comes with
the top cloth pressed onto a polyester fleece stabilizer, to
prevent warping. This base material in combination with
the embroidered coil forms the diaphragm of our speaker
design. Diaphragms are generally optimized for operating at
certain frequency ranges using a variety of different materials.
Therefore, to ascertain the impact of the fabric substrate on the
performance of our sonic transducers, we further manufactured
coils using fine weave and leather as substrates.
coil turn turn wire electrical inductance fabric
diameter count spacing length resistance substrate
25 mm 44 0.25 mm 1.93 m 2 40 µH Twill Weave
50 mm 94 0.25 mm 7.82 m 8 125 µH Twill Weave
100 mm 194 0.25 mm 31.37 m 32 1078 µH Twill Weave
50 mm 47 0.50 mm 3.91 m 4 64 µH Twill Weave
50 mm 94 0.25 mm 7.82 m 8 125 µH Leather
50 mm 94 0.25 mm 7.82 m 8 125 µH Plain Weave
Table 2. Parameters of the speaker coils which have been evaluated.
The performance of these fabric substrates in combination with
different additional parameters have been evaluated, which we
describe in the following sections, cf. Table 2. The diameters
have been chosen to both correspond to the common sizes of
treble speakers and so that the resistance is not lower than the
usual speaker impedance ratings. Through that we ensure that
the voice coil can be driven by an off-the-shelf audio amplifier.
The turn count resulted from the turn spacing we defined.
Figure 5. Different coil patterns have been embroidered varying the dif-
ferent parameters.
To evaluate the performance of our approach, we conducted
experiments with six different embroidered speakers. For each
speaker we used two identical coils on top of each other.
For each sample, we investigated impedance and frequency
response. Additionally we examined the wire length and re-
spectively the electrical resistance. We evaluated different coil
sizes (25 mm, 50 mm, 100 mm), turn spacings (0.25 mm, 0.50
mm) and turn counts
(44, 94, 194), cf. Figure 5. The coils
were embroidered onto different fabric substrates as discussed
The textile speaker under test was clamped to a 10
10 cm
frame, allowing for an exact, repeatable, and plain positioning
of the two sandwiched coils. The frame was then suspended
between two aluminum beams using nylon yarns to minimize
resonances of the fixture. To shield the recording of the stim-
ulated textile speaker from unwanted reflections, we built a
small box for sound insulation (80
80 cm) with sound-
absorbing foam, see Figure 6.
Figure 6. The opened sound-absorbing test chamber with a suspended
50 mm diameter textile speaker about to being tested.
The test signals were generated by a Steinberg UR12 audio in-
terface and amplified via the class-D audio amplifier TDA7498,
driven by a 24 V power supply. The voice coil was directly fed
with the amplified signal. The bias coil was driven with a recti-
fied signal smoothed with a 1000
F capacitor. Before record-
ing the frequency response of the speaker being tested, the
amplitude (dBFS) and respectively the voltage was adjusted
in software, so that 0.5 Ampere were consumed by the ampli-
fication circuit when driving the speaker with a frequency of 1
kHz. This was done to achieve comparable powers consumed
by the tested speakers. Driving all the speakers with the same
voltage would result in considerable differences in amplitude,
and would even destroy the coils with lower resistances.
The sound emitted by the speakers was measured using a
Behringer ECM8000 high precision condenser measurement
microphone that was positioned at 30 cm distance to the em-
broidered speaker and connected to the Steinberg UR22 MK2
audio interface. We stimulated the embroidered speakers us-
ing frequency sweeps from 200 Hz to 20 kHz. The duration
of each measurement was 43.7 s including 8 sweeps, 256k
samples each. For generating and recording the frequency
responses we used the audio measurement software Room EQ
Wizard (REW).
Figure 7 (A) shows the frequency response of our embroidered
coils (diameter sizes of 25 mm, 50 mm, and 100 mm).
Figure 7. Frequency response of our coils using different sizes (A) as
well as different fabric substrates (B).
As the chart clearly shows, all of them can reproduce high
frequencies very well, the reproduction of frequencies below
2 kHz was comparable weak, especially with the smallest size
of 25 mm. This chart also shows a significant increase of the
SPL (dB) in the frequency range of 1,500 Hz - 20 kHz with a
mean value of 29.34 dB
4.88 dB for the coil with a size of
50 mm, compared to a mean value of 14.63 dB
3.01 dB (25
mm) and to a mean value of of 27.71 dB
4.84 dB (100 mm).
Figure 7 (B) shows the frequency response of the embroidered
coils with size of 50 mm, using different substrate materials.
While the chart clearly shows a very weak reproduction for the
plain weave material, we can reproduce again high frequencies
well for the samples, embroidered on the twill weave, and on
leather. Again, it also shows a significant increase of the SPL
(dB) in the frequency range of 1,500 Hz - 20 kHz with a mean
value of 21.19 dB
5.92 dB for the coil embroidered on
the twill weave, compared to a mean value of 25.46 dB
3.49 dB for the coil embroidered on leather, which leads to
the conclusion that it is good if the substrate textile is a little
stiffer and less air permeable as it provides a better response.
Figure 8 shows the results for the impedance measurement
varying different parameters. The evaluation is based on the
Figure 8. Impedance measurement.
measurement design as described in REW
, with a sensing
resistor of 1 k
. The calibration and measurement was done
using the line output of the Steinberg UR22 MK2 audio inter-
face. The impedance measurement shows that the 100 mm
coil has a high impedance rise at frequencies beyond 4 kHz.
Concluding it is preferable to use smaller coils or lower turn
count for high-frequency applications.
Summarizing, we would suggest using the Textile Wire with
a diameter size of 0.15 mm and a resistance of 0.97
/m in
combination with a coil size of 50 mm or 100 mm, depending
on the use case and a turn space of 0.25 mm on a substrate
fabrics, which has low air permeability.
In this section and the supplementary video, we present four
example applications (see Figure 9) that show our embroidered
coil in action. The demonstrated applications combine the
advantages resulting from the speaker’s flexibility and thin
form factor in real-world scenarios.
Wearable speaker
In the first application (see Figure 9 (A)), we demonstrate a
textile-based mobile speaker for a handheld synthesizer. We
used the Pocket Operator PO-20, with a textile-speaker which
we embroidered into the front pocket of a Jeans Jacket. The
diameter size was 50 mm.
Beanie notifier
Figure 9 (B, C) shows a beanie with an integrated embroidered
speaker. In comparison to existing approaches, the speaker is
directly integrated into the textile while preserving the flexi-
bility of the garment. The speaker is placed directly at the ear
of the user and, just as headphones, can only be heard by the
user if the amplitude is adjusted accordingly. It is well suited
for unintrusive notifications.
Figure 9. Three different demonstrator applications have been designed
and implemented, including a wearable speaker for a handheld synthe-
sizer (A), a beanie notifier (B, C), and an ultrasonic communicator (D).
Inaudible communication
Building on the wide frequency range capabilities of our
speaker, we demonstrate its dynamic possibilities by using
it as a transducer for audible and inaudible near-ultrasonic
communications. We validated our system with two different
modem protocols: quiet.js
which can communi-
cate with a frequency around 19 kHz. Figure 9 (D) illustrates
the successful communication by visualizing the spectrogram
of the audio signal and the resulting message "UIST 2020"
In this paper, we have focused on the design, implementation
and characterization of a flexible textile speaker as an output
device for smart textile applications. Additionally, our im-
plementation of the embroidered coils allows for additional
properties and functionalities, which will be shortly discussed.
Although they are out of the scope of this paper, we judge
them viable areas that may be addressed in future work.
For investigating the stretchability of our speaker, we addi-
tionally embroidered a speaker onto a jersey fabric as seen in
Figure 10. The embroidered speaker itself wasn’t designed
to be stretched. Nevertheless, we achieved fairly good results
and the embroidered coil was robust enough not to break when
stretching it as much as possible by hand. In the scope of
this paper we did not evaluate the speakers performance while
being stretched. A circular zig-zag stitch could enable greater
deformation of the coil. Sendrolini et al. [46] and Patino et
al. [13] show, for instance, embroidered planar zig-zag in-
ductors, that are stretchable and therefore optimal for elastic
fabrics. Their results of the magnetic interference test did not
fail – even when the textile has been stretched up to 132%.
We, therefore, think that we could also achieve similar results
by applying a zig-zag shape to our coil design.
Figure 10. An embroidered coil design on a stretchable jersey fabric (A)
was possible and even once fully stretched, it did not destroy the speaker
Safety issues and tactile feedback
In contrast to electrostatic speakers [19, 20, 21, 22, 45], dy-
namic speakers as in our approach do not require high-voltages
and are therefore more safe to touch. Similarly to the vibro-
tactile widgets (cf. printed actuators and sensor), i.e. [12, 43],
the speakers can provide mild tactile feedback. When touched
by the user, they still function as a speaker.
The easiest way of transforming our speaker approach using
two flat embroidered coils into a microphone would be the
implementation of a dynamic microphone – without using a
permanent magnet. We simply apply a direct current to one
of the coils to generate a constant magnetic field. When the
other coil is vibrating through excitation by an acoustic wave,
voltage is induced in the moving conductor. The voltage can
then be amplified and converted to a digital signal for further
processing or recording.
Audible sound
Our design and implementation is not focusing on high-fidelity
sound reproduction. Instead, the embroidered speaker is de-
signed to produce audible sounds for notifications or to replay
sounds in scenarios where audio quality is not key but low
weight and high flexibility are a more important factor. Ad-
ditionally, we aimed for inaudible applications such as near-
ultrasonic communication.
Another option to get a better sound quality would be to use a
meshed membrane as proposed by Bradley et al. [2].
Power consumption
To improve power consumption, our circuit could be enhanced
by the usage of an envelope follower. It would allow detecting
a minimum threshold in the signal amplitude to dynamically
switch the power of the bias coil.
Magnetic field strength
As illustrated in Figure 11 (C), we compared the magnetic field
generated by the magnet (D) from a wireless headphone
the field generated by our embroidered coil powered by a 9V
8 (or
battery (at 346 mA), the field generated by our embroidered
coil which is about 6 times lower (1159 µT vs. 7024 µT).
To further increase the volume of the speaker, two or more
embroidered coils can be layered and connected in series to
achieve a higher turn count and thereby a stronger magnetic
field. The resulting coil should be used as the bias coil be-
cause of its resulting higher impedance. A lower impedance is
expected to have a better power efficiency at high frequency
AC, thereby a lower turn count for the voice coil is preferred.
Additionally, a ferromagnetic thread with high magnetic per-
meability can be embroidered in the middle of the coils as a
ferrite core to focus the magnetic field at the center.
One of the goals in our design was the avoidance of rigid per-
manent magnets, as they are not flexible, heavy, and difficult
to fix to the textile. In contrast to our method, magnetore-
sponsive polymer-coated copper substrates could be used as
permanent magnet, to avoid the mentioned downsides. These
materials are coated with magnetoresponsive PNIPAM-based
microgels and provide excellent magnetic properties [8, 57,
32]. We see potential in using these microgels in combination
with copper-based threads, which again could be embroidered
Heat development
We examined the heat development and dissipation of different
coils being powered by 6 V, 12 V, and 24 V depending on their
overall resistance. With increased temperature, the resistance
increased up to 25% at 100
C compared to the resistance at
Figure 11. Limitations of our approach. Measurements with a thermal
imaging camera show that the coil heats up significantly if it is powered
with a high constant current (A-B). We also compared the magnetic field
generated by the coil (C) to a magnet of a headphone speaker (D).
To avoid overheating, the current passing through the biasing
coil should be limited, while only being powered when an
audio signal is emitted. For short signals, as needed for audible
notification sounds or inaudible communication signals, the
current limit for the bias coil can be set higher (up to 1 A) as
it is powered only for a very short time.
Litz wire
Depending on the application, it might also be worth think-
ing about using different wires, as the used Textile Wire has
a fairly high resistance compared to that of more common
voice coil wires. Especially for use cases, where high fre-
quencies are more important, litz wires are a good alternative.
These wires – as they form a bundle of individually insulated
conductors – are designed to reduce the skin effects and are
therefore optimal at high frequencies from kilohertz to the
megahertz range. The litz wire must be chosen depending on
the frequency range, as the number of threads and their thick-
ness change their resistance and inductance. Various online
resources exist to help for this choice and the manufacturer
YDK provides a useful chart9for example.
In this paper, we demonstrated the design and implementation
of an embroidered speaker without using a permanent mag-
net. Our proof-of-concept prototype consists of two flat spiral
coils, stacked on top of each other. The two coils are fabri-
cated by embroidering an isolated, thin (0.15 mm) and highly
conductive wire. We carefully evaluated different designs and
performed several experiments, varying different properties of
the coil, including size, turn counts, turn spacing, and different
substrate fabrics. The experiment results revealed that we can
produce audible sound with a broad frequency range (1,500
Hz - 20 kHz) with with a mean value of 29.34 dB
4.88 dB
SPL for the coil with a diameter of 50 mm.
For future work, we plan to investigate in more detail alterna-
tive yarns, including the litz wires as discussed in the section
before. Especially, for the non-audible use cases these wires
seem to be an excellent alternative to our used Textile Wires.
For focusing the magnetic field to increase the effective vol-
ume we want to experiment with ferromagnetic thread as an
iron-core for the coils. Another promising idea is the use of
spacer fabrics between the coils to reduce distortion resulting
from collisions of the coils.
Additionally, we also want to integrate the coils into the textile
during the fabrication of the textile itself. The prototypes by
Fobelets et al. are highly motivating and show the possibilities
of developing a knitted version of a speaker [10, 9].
Finally, we also see great potential in combining multiple
coils into one array to achieve a more powerful speaker, cf.
Rowland’s flexible audio speaker array [44]. Alternatively, we
also would like to explore shapes that are beyond a simple
circle, as proposed by Chong Loo et al. [30].
9 product-list-in- stock
This research is part of the COMET project TextileUX (No.
865791) as well as of the project eMotion (No. 878111). Tex-
tileUX is funded within the framework of COMET – Compe-
tence Centers for Excellent Technologies by BMVIT, BMDW,
and the State of Upper Austria. Both programs are handled
by the FFG. The authors would like to thank Christoph Schaf-
fer for the measurement hardware setup as well as Rebecca
Kleinberger and Akito Van Troyer from the "Opera of the
Future" (MIT Media Lab) for their contribution in the initial
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... One key attribute of our process is that, equivalent to [34], we use our wire as a bobbin thread, in order to minimize mechanical stress and thereby preventing thread breakage. In contrary to conventional embroidery, where visual quality of the textile's front side is most valued, we require flawless finishing of the back side, i.e. an exact routing of the electrode wires is essential (cf. Figure 2B). ...
... For the approach presented in this paper, no expensive multineedle machine is required; settings as provided by entry level embroidery machines are sufficient. 1 As an electrode, we used an enameled silver-plated copper wire 2 with a diameter of 0.15 mm and a resistance of 0.95 Ω/m. Despite its labelling as "wire", it is specifically designed to be fit for textile manufacturing and also proved practical in previous work [34]. ...
... Regarding smaller body landmarks such as fingertips and knuckles on one's hands, smaller single-layer coils made of other fabrication methods such as flexible printed circuits ( Figure 15) can better fit in these smaller regions like a fingernail, but the Kapton substrate of FPC makes it uncomfortable for the body landmarks that requires flexion. Future work can consider combining the embroidery with very-thin enameled wire (e.g., Textile Wire 5 ) to make flexible and stretchable coils [46], because conventional vinyl cutter that cannot reliably cut thin (e.g., < 0.5mm-width) traces with copper foil limit the minimal size and density of coil winding. Regarding larger body landmarks such as back or hip, increasing a single relay coil's size may not be efficient because it may create blind spots in its middle. ...
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