Bass rise in reflected sound is physically unavoidable with forward-firing speakers.

Evidence

In Figure 17.1(b) the predicted room curves tilt downward because the frequency-dependent directivity of forward-firing loudspeakers results in progressively more sound energy being delivered to the room at lower frequencies. Adding to this trend, reverberation time normally increases at lower frequencies and very high frequencies are reduced by air attenuation. With forward-firing loudspeakers, it is not possible for both the direct sound and the steady-state curves to be flat except in an anechoic space. The physical reality of bass rising in the reflected sounds following the direct sound, exists in most live listening experiences as well as in reproduced sound (see Figure 12. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1927

Categorization

• DEFINE: Clarifies why a tilted steady-state room curve is natural rather than a defect — directional-energy, reverberation, and air attenuation all conspire toward it.