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 effects of ambient light mean that the sensors are accurate in nearly all lighting conditions (their effectiveness may be impaired somewhat outdoors in extremely bright sunlight).

WARBL optionally 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 vented mouthpiece 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.

WARBL2 also contains an Intertial Measurement Unit (IMU) consisting of an accelerometer and gyroscope. Acceleration and rotation information can be mapped to MIDI messages, used for pitch vibrato, and even used to control the register.

WARBL was designed to be affordable by using a simple, universal design. It uses a sturdy 8 3/4" 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 integrated "bell" sensor to simulate playing uilleann bagpipes "on the knee".

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.

The battery in the WARBL2 is replaceable by the user so that the device will enjoy a long life. This is possible because of the NiMH battery chemistry, which also allows shipping batteries anywhere in the world and traveling with the WARBL without concerns about the flammability of lithium batteries. When plugged into USB, WARBL draws power from a host device like an iPad or PC. Every attempt was made to reduce power consumption, and WARBL2 only uses 8.5mA of current, less than half that of the original WARBL.

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 and the intertial measurement unit (IMU). You can store settings for up to three different "instruments" and recall them at any time.