The stereo effect is produced chiefly by the middle and upper frequencies.

In Auto Center phase, this is (generally) shown as the upper frequencies making a drastic change.

Tape head misalignment (azimuth) also will cause a phase change in upper frequencies.

A presence control boosts the upper mid-range frequencies.

Probably the upper frequency limit for such effects.

What sets the Linaeum speakers apart is a one-of-a-kind sound generator for the middle and upper frequencies.

Set at the back of the stage (or more forward during piano recitals), the screens were meant to absorb some of the upper frequencies.

Both middle and upper frequency ranges thus originate in the same place.

Consequently, in any given filter design, there is an upper frequency beyond which the filter will fail to carry out its function.

The upper frequencies, Kirk could hear.