# Discrimination Accuracy of Sequential Versus Simultaneous Vibrotactile Stimulation on the Forearm
## 2 Voice Coil Actuator and Vibrotactile Sleeve  
We used voice coil actuators (VCAs; model: Lofelt L5) for generating vibrotactile stimulation. VCAs offer a range of benefits over alternative actuator types. They are known for fast response times, high force density, and low power consumption. Furthermore, they are versatile and can be integrated into various devices and systems. These actuators are designed to deliver rich vibrotactile stimulation and, importantly, allow for the separate manipulation of both frequency and amplitude (details provided in Table 1).  
For mounting, the actuators were affixed to a soft and thin fabric, ensuring a snug fit and efficient transfer of tactile information from the actuators to the skin. The six voice coil actuators were arranged longitudinally in a 3 \(\times\) 2 array to produce back-and-forth vibrational movement perpendicular to the length of the forearm (Figure 1).  
The experiment utilized a bespoke vibrotactile sleeve crafted from a Power Mesh fabric consisting of 90% nylon and 10% spandex. This choice of materials ensured that the sleeve was both comfortable and user-friendly. To accommodate various forearm sizes, stretch straps equipped with Velcro were used to secure the sleeve, allowing easy adjustment for individual participants. The L5 actuator enclosures were 3D-printed directly onto the fabric, ensuring that the actuators could vibrate without any hindrance. The printing was executed using a Prusa MK3s 3D printer and Prusa Slicer, with PLA as the chosen  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Performance Characteristic** & **Technical Specification** \\ \hline Operating voltage (V) & 0–3.3 \\ \hline Frequency range (Hz) & 0–15,000 \\ \hline Force (G-p.p.) & 0–15 \\ \hline Rising time (ms) & 5–15 \\ \hline Operation current (mA) & 200–500+ \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of the Lofelt L5 actuator [35].  
Figure 1: (**a**) Overview of the design of the sleeve used to apply the Lofelt L5 actuators along the forearm; (**b**) an example of a wearable vibrotactile sleeve and the configuration of the actuators.  
material. Importantly, there were no fabric layers obstructing the point of contact between the enclosure and the skin.  
The L5 actuators had dimensions of W: 17.0 \(\times\) D: 20.5 \(\times\) H: 6.2 mm. The custom-made enclosures were slightly larger, measuring W: 17.0 \(\times\) D: 28 \(\times\) H: 6.2 mm. One significant advantage of this enclosure design is its protective quality, safeguarding both the participants' skin and the elastic material from potential abrasion or damage inflicted by the moving actuator. Within the enclosure, the actuator was oriented so that it produced vibrations parallel to the skin and perpendicular to the forearm's length.  
The enclosure was vital for delivering vibrotactile stimulation safely. Its tight fit with the actuator ensured that it functioned seamlessly as a single unit. With a smooth surface, the enclosure was lightweight and thin, at an approximate thickness of 1.6 mm.  
For stimulus presentation, the system included a digital audio interface (RME MADI face XT), digital-to-analog converters (Ferrofish A32), multi-channel amplifiers, and the Lofelt L5 voice coil actuators. The tactile sleeve interfaced with the audio hardware through wired connections.