Many modern box-style kalimbas feature two small holes on the back and one large hole on the front. These are not just for decoration or air circulation; they are used by the player to create a vibrato or 'wah-wah' effect. As the player plucks the tines with their thumbs, they can rhythmically cover and uncover the holes on the back with their middle fingers. This action changes the resonant frequency of the air inside the box, causing the volume and tone of the note to fluctuate. This technique adds a vocal-like quality to the instrument, making it more expressive. It is a simple but effective application of acoustic physics, where the player is essentially modulating the Helmholtz resonance of the wooden chamber. This feature is one of the main reasons the Hugh Tracey-style kalimba became popular in Western folk and pop music recordings.