Atmos up-firing speakers exploit ceiling bounce because elevation cues are high-frequency.

Evidence

It is fortunate that the elevation cues are at high frequencies. It means that bouncing some high frequencies off the ceiling can assist the illusion, as has been shown with small loudspeakers aimed upwards in Atmos systems. The effect is not as persuasive and the localization not as durable as with 208. loudspeakers in ceiling locations, but such installations are not always possible or practical. The ceiling-located loudspeakers must be able to deliver the necessary high frequencies to all listeners, which is a factor to consider in installations—off-axis output degrades rapidly at frequencies above 6 kHz. — Toole, Floyd E, Sound reproduction- the acoustics and…, 750

Categorization

• XREF: Connects to practical loudspeaker design tradeoffs (off-axis response above 6 kHz) relevant to speaker evaluation.