Nielsen has been experimenting with using a form of audio frequencies to determine viewing.

Because all the bugs operated on the same very narrow audio frequency, And could only access one at a time.

Different pairs of audio frequencies were used by each station:

A hum sounded on an audio frequency, and the captain's voice broke through the static.

FM is also used at audio frequencies to synthesize sound.

Human hearing is predominantly less acute at perceiving the direction of certain audio frequencies.

In other words, when observing the audio frequencies only, the record appeared to have an ordinary stereo recording.

This "singing arc" was probably limited to audio frequencies.

I spoke for all my comrades in the 3d Battalion, though they remained silent on the audio frequencies.

A note on the design of iron-cored coils at audio frequencies.