A baffle blocks sound-cancelling air flow and dramatically amplifies multipole sources

Evidence

It is possible, however, to stem the air flow, and with it the cancellation, by putting a stiff card along the nodal line, almost touching the tuning fork. The card acts as a baffle. The sound becomes much louder, and is louder still if both sides are baffled by using a card with a slit cut to accommodate one of the tines. The card has to be big enough that the air cannot short-circuit around the side and be heavy enough that it doesn’t move much with the pressure gradient it feels. Since air can respond to pressure fluctuations at the speed of sound, the card should ideally be about a wavelength of the sound across, preventing any sneaking around the card in under half a period. However, smaller baffles do help. This makes an impressive live demonstration. — Eric J. Heller, Why You Hear What You Hear- An…, 624

Categorization

• SEED: A counterintuitive demo: placing a card near a tuning fork makes it louder, not quieter — could anchor a piece on how horns and baffles shape instrument tone.
• XREF: Connects to loudspeaker enclosure design, which uses baffles to prevent dipole cancellation between front and back of a cone driver.