Implementing electronic speckle pattern interferometry for a variety of activities in a general education acoustics class

A familiar acoustics laboratory experiment is finding Chladni patterns using flat plates of various geometries. Another method used in musical acoustics to find mode shapes of vibrating objects is electronic speckle-pattern interferometry (ESPI). We introduced students in a general education acoustics class to ESPI in addition to the traditional Chladni plate laboratory activity. A variety of ways to implement ESPI in the class were developed including: comparing traditional Chladni patterns to ESPI, examining traditional musical instruments with ESPI, and an advanced investigation of a well-known acoustics demonstration. A classic acoustics demonstration is rubbing the rim of a wine glass to create a musical tone. A coffee mug, unlike a wine glass, has an asymmetry due to the mug handle. ESPI can be used to study the effects of filling the mug with water in terms of the mug's natural frequency, amplitude, and mode shapes. Results from ESPI showed that the mode shapes do not change with the addition of water. Filling the coffee mug with water lowered the natural frequencies of the lowest modes in a manner similar to that of a wine glass.