Low adjacent harmony voices need wide spacing to avoid a muddy sound

Evidence

As a rule when writing four-part harmony such as SATB (soprano, alto, tenor, bass) hymns, the bass and tenor parts are usually no closer together than a fourth except when they are above the bass staff, because the result would otherwise sound “muddy” or “harsh.” — David Howard, Jamie Angus, coustics and Psychoacoustics, 1028

Categorization

• XREF: Connects to voice-leading craft in composition and to the general principle that low-frequency intervals are harder to perceive clearly.
• CANDY: A neat, shareable rule of thumb about why bass parts stay sparse in choral writing.