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Holographic Interferometry Helps Make Beautiful Music

Photonics Spectra
Dec 1996
Kathleen G. Tatterson

This is the time of year when one hears the pealing of holiday bells. We take the sounds for granted, but how do bell makers know that their musical instruments are producing the best possible sound? One manufacturer teamed up with Northern Illinois University to measure its handbells using holographic interferometry.

When Malmark Inc. of Plumsteadville, Pa., started making its large bells with aluminum instead of the traditional bronze, it turned to the university's physics department to find out how vibrations traveled through the different metals. Demand has increased for larger, lower-frequency bass bells, but since the speed of the bending sound waves traveling through the larger instruments is much slower than the speed of sound in air, a larger bell radiates these waves inefficiently, particularly if it is made of bronze.

Malmark President J.H. Malta said that although the bronze handbells produce a richer, brighter tone, his company uses aluminum for its larger models. Otherwise, the weight would make them nearly impossible for musicians to handle.

Northern Illinois Professor Thomas D. Rossing and graduate student Deepak Gangadharan used time-averaged holographic interferometry operating a Jodon HeNe laser (612 nm) at 20 mW to record the vibrational mode shapes of bass bells made of each metal.

"It's the best way to observe what frequencies are being radiated off the bell," explained Malta. "With the interferogram, you can see how the frequency is distributed instead of having to rely solely on the ear."

For example, the researchers noticed an unusual set of modes with three unevenly spaced nodal circles in interferograms of the bronze bell -- indicating unevenness in the thickness of the bell wall. Such variations lead to a "muddier" bell tone.

When one hears a clock tower bell, there is often a sustained vibration -- which Malta described as a "wow-wow" effect -- caused by the uneven distribution of the metal mass throughout the bell's wall. The effect is more of a problem with handbells, because the frequency distribution has to be the same throughout the entire set of bells, which can run from five to seven octaves -- or 40 to 56 individual bells.

In time-averaged holographic interferometry, researchers expose film for several seconds while a bell completes many cycles of vibration, Rossing said. "Essentially, we are superimposing millions of holograms so that an interference pattern is created, which is an accurate contour map of the bell's vibrational motion," he explained.

Rossing and his students have used the technique to study a variety of musical instruments, including snare drums, steel drums, hammer dulcimers, guitars, violins and cymbals.


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