Mbiras as sensors

The frequencies of pitches produced by a musical instrument are determined by the physical properties of the instrument. Consequently, by measuring the frequency of a pitch, one can infer information about the instrument’s physical properties. By modifying a musical instrument to contain a sample and then analyzing the instrument’s pitch, one can make precision measurements of the physical properties of the sample.

Researchers used the mbira, a 3000-year-old African instrument that consists of metal tines attached to a wooden board; these tines are plucked to play musical pitches. By replacing the mbira’s tines with bent steel tubing, filling the tubing with a sample, using a smartphone to record the sound while plucking the tubing, and measuring the frequency of the sound using a free software tool available on their website, they could measure the density of the sample with a resolution of about 0.012 g/mL.

To demonstrate the mbira sensor’s capabilities, they used it to successfully distinguish diethylene glycol and glycerol, two similar chemicals that are sometimes mistaken for each other in pharmaceutical manufacturing (leading to hundreds of deaths).

Unlike existing tools for measuring density, the mbira sensor can be made and used by virtually anyone in the world with access to a smartphone and the free software tool posted on the Internet. Among many possible applications, consumers could use mbira sensors to detect counterfeit and adulterated medications (which represent around 10% of all medications in low- and middle-income countries).

This according to “Musical instruments as sensors” by Heran C. Bhakta, Vamsi K. Choday, and William H. Grover (ACS omega III [2018] 11026–32; RILM Abstracts of Music Literature 2018-95175). Many thanks to Improbable Research for bringing this article to our attention!

Above, (A, left to right) a conventional mbira, the same instrument with the tines replaced by a length of stainless steel tubing bent into a U shape, and an example easily made from scrap lumber and hardware; (B) the method of using an mbira sensor; and (C) a waveform plot of a sound recording of plucking an mbira sensor, obtained using a smartphone’s voice recorder app and the free online software tool.

Below, Stella Rambisai Chiweshe demonstrates traditional Zimbabwean mbira playing.

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