A distinctive characteristic of many traditional wind instruments is the ability to shape the sound by "shading" toneholes with your fingers. WARBL's infrared optical sensors detect how far a finger is from each tonehole, and WARBL uses the sensor information to send MIDI pitch-bend messages that allow for realistic slides and vibrato. Algorithms for cancelling the affects of ambient light mean that the sensors are accurate in nearly all lighting conditions.

WARBL uses breath pressure to allow "overblowing" to jump to the second octave, making playing easy and intuitive. In fact, WARBL is in many ways easier to play than actual wind instruments. Constant monitoring of breath pressure and sophisticated modeling detects the player's intentions by responding to breath or a bagpipe bag in a natural way. Settings can be changed to simulate the way that a variety of instruments respond to overblowing, or you can choose to be limited to one octave (like with a bagpipe chanter). Alternatively, you can play without using the pressure sensor, instead using a button click to start and stop the sound.

The new vented mouthpiece (November 2020) transforms WARBL, making it feel more familiar to a tin whistle player. It isolates WARBL from the air stream (and all moisture) and improves the breath response, increasing register control and allowing techniques like staccato playing and even flutter-tonguing. It has slightly higher back-pressure than a real whistle, so it requires less breath, while still feeling quite similar to the real thing. There is still a small closed "tube" mouthpiece supplied with WARBl for those that prefer the compact size or prefer a closed system where no air escapes at all.

WARBL was designed to be affordable by using a simple, universal design. It uses a sturdy 8 1/2" by 5/8" acrylic enclosure. There is a vented mouthpiece that can be removed and replaced with a bagpipe-bag attachment. The hole spacing is the same as on a standard great highland bagpipes practice chanter, is also very similar to a D tin whistle, and has a left-hand thumb hole and a right-hand "pinky" hole, enabling many fingering pattern options. There's even an optional "bell" sensor that plugs into the bottom end to simulate playing uilleann bagpipes "on the knee".

WARBL draws power from a host device like an iPad or PC, so no batteries are needed. Every attempt was made to reduce power consumption, and WARBL only uses 20mA of current, even while reading all ten sensors at a rate of 1000 times per second. This rate provides a high degree of responsiveness and no noticeable latency.

WARBL produces no sound on its own, instead sending MIDI messages to apps that produce the sound. This allows more control over customizing sounds, and also helps reduce complexity, cost, and power consumption. The WARBL firmware can be easily upgraded, and the entire design is open source, for those who are interested in tinkering with it.

In the interest of keeping the hardware design simple but functional, WARBL has limited onboard controls (three buttons and an LED indicator). Instead, the Configuration Tool web app (also available as the WARBL iOS app) can be used to calibrate the sensors and customize the device by reprogramming the buttons, controlling pitch bend, and changing the response to air pressure. You can store settings for up to three different "instruments" and recall them at any time.