Hello, my name is Thomas Preindl and on behalf of my co-authors,
I will present to you our paper Sonoflex: Embroidered Speakers without Permanent Magnets.
In our publication we show you how to design, fabricate and actuate embroidered dynamic
loudspeakers. In contrast to already established ways of manufacturing embroidered speakers
our approach omits a permanent magnet, maintaining the textile characteristics of our base material.
To evaluate the speaker's performance we analyze the effects of four manufacturing parameters
on their frequency response. Concluding we present three
potential applications for our speaker design, including near-ultrasound communication.
Sound output is one of the most important functions of human-computer interfaces. As
such it also has found its place in the field of electronic textiles. A variety of textile
and other flexible speakers has already been built and presented by researchers and artists.
The most prevalent approach is to translate the concept of a dynamic speaker
onto textile. Most notably this was done by the Kobakant collective.
In their approach the embroidered speaker consists of a voice coil and a permanent
magnet. The voice coil is fed with an audio signal, which causes corresponding repelling
and attracting forces moving the textile in correspondence to the audio signals amplitude.
Our approach operates without the permanent magnet by using a bias coil for generating an opposing
magnetic field. To alternate attracting and opposing forces the bias coil has to be powered
by direct current, thus unchanging polarity, while the voice coil receives an AC signal
from a conventional audio amplifier, to only use power when audio is being replayed. The DC signal
is gated by the audio signal. This approach allows us to embed sound output directly into the textile
while preserving textile qualities as flexibility and thinness,
preserving wearability, that is using safe voltage levels, and making it easy to
manufacture using broadly available materials providing an easy accessible technology.
We use machine embroidery as a manufacturing technique, which allows for high precision
and enabled us to produce effective and reproducible results when evaluating the speakers.
Instead of using conductive yarns to embroider coils, we use thin and insulated copper wires,
enabling us to stitch dense coils without creating shorts.
We evaluated four types of enameled wires for the suitability according to our criteria.
They had to be insulated, highly conductive, durable and sewable. The thinnest wires we
tested could not withstand the tensile forces, while the thicker wire could be embroidered,
yet limited turn density. We arrived at an enameled silver plated copper wire with a
diameter of 0.15 millimeter. It provided optimal conductivity, while still being thin and flexible.
To enable embroidering wire onto textile, we used the wire as a bobbin thread minimizing
the tensile stress and limiting wire breaks. We evaluated the performance of six different
embroidered speakers using different coil sizes, turn spacings, and turn counts. We also tested the
influence of different textile substrates on the sound quality, varying in weight and density. For
each evaluated speaker we investigated the coil impedance and frequency response. Additionally
we examined the inductance, wire length, and respectively the electrical resistance.
For the tests the bias coil was driven with a smoothed rectified audio signal.
Before testing the speakers the amplitude was adjusted so that the amplifier drew 0.5 ampere at
a sound frequency of 1 kilohertz. Here we see the results of our frequency response tests. We can
notice the stark difference in the sound pressure level when using the air permeable plane weave.
This makes sense as the fabric is less capable in pushing air when used as a membrane. The
twill weave with a polyester backing achieved the most balanced response from 2 kilohertz upwards.
In this chart we compare different coil sizes. We can see that in our
case, bigger coils do not automatically yield louder speakers. A small coil, as 25 millimeter,
on the other hand allows two little coil windings. A good middle we found to be
at 50 millimeter diameter with a turn spacing of 0.25 millimeters.
We implemented three different applications showing the embroidered speaker in action.
On the left we see an embroidered speaker on a jacket for a handheld synthesizer. In
the middle an embroidered speaker on a beanie for notifications only heard by the user. And on the
right, we see a speaker replaying near-ultrasound for inaudible device-to-device communication.
Informally comparing the magnetic field strength of the embroidered coil with a permanent magnet,
we see the limitations of our approach. The
flexibility has to be traded for a weaker magnetic field.
A high amount of current is needed to power the coils, which tends to heat up the textile.
To avoid overheating, the current passing through the biasing coil should be limited, only
being powered when an audio signal is emitted. Thank you for your time and enjoy embroidering.