On the acoustics of alleyways

Alleyways bounded by flat, reflective, parallel walls and smooth concrete floors can produce impulse responses that are surprisingly rich in texture, featuring a long-lasting modulated tone and a changing timbre, much like the sound of a didgeridoo. This work explores alleyway acoustics with acoustic measurements and presents a computational model based on the image method. Alleyway response spectrograms show spectral zeros rising in frequency with time, and a modulated tone lasting noticeably longer than the harmonic series associated with the distance between the walls. With slight canting of the walls and floors to produce the long lasting modulated tone, the image method model captures much of this behavior.