I have done demonstrations of audible beats with tone bars, with function generators and speakers, and with resonating columns, and also visible beats with combs.1 In looking for a new beats demonstration, I regretted never having taken a picture of an overpass with wire fencing on both sides: the otherwise identical fencing shows visual beats because the two repeating patterns are at different distances from the observer.

1.
Peter
Melzer
, “
Beat production analogy using pocket combs
,”
Phys. Teach.
14
,
120
(
1976
).
2.
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3.
A similar pattern may be found in Paul Tipler, Physics for Scientists and Engineers, 3rd ed. (Worth, New York, 1990), p. 453.
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