How do acoustic shadows form in an auditorium?

In an auditorium, acoustic shadows are regions where specific frequency ranges of sound are attenuated. These shadows form primarily due to diffraction effects that occur when sound waves encounter large obstacles within the space.1

When sound waves pass around large structural elements—such as pillars, corners, or the underside of a low balcony—the waves do not simply stop; they spread around the obstacle. However, this process is frequency-dependent. Because diffraction is most pronounced when the wavelength is comparable to the size of the obstacle, lower-frequency sounds (longer wavelengths) are better able to bend around these barriers than higher-frequency sounds. Consequently, the areas behind these obstacles experience a reduction in the intensity of higher-frequency components, resulting in an acoustic shadow where the sound quality is altered or muffled.12

Additionally, large suspended reflectors, often called clouds, can be designed to reflect certain frequencies while allowing others to pass, which also influences the sound mixture and can create localized variations in acoustic clarity throughout the hall.1