Sound with frequencies below and above the audible frequency range are known…

Concept — The human ear responds only to a limited band of the frequency scale, roughly 20 Hz to 20,000 Hz, called the audible range. Sound is classified into regions by where its frequency sits relative to that band: below its lower limit, inside it, or above its upper limit. Frequency alone fixes these regions. Amplitude, the intensity it produces (the power a wave carries per unit area) and the loudness a listener perceives are distinct quantities on an entirely different axis, and a name borrowed from survey instrumentation labels a device rather than a region of the frequency scale.

Application — Placing the three regions of the frequency scale side by side gives:

Contrast — Now compare the four names offered in the stem, one by one:

• Feeble sound classifies a sound by intensity, not by frequency: it simply means a faint, low-amplitude sound, which can occur at any frequency, including well inside the audible range.

• Infrasound classifies a sound by frequency and names the region below the lower hearing limit of about 20 Hz.

• Parasound is not a region of the frequency scale at all: the name belongs to parametric, non-linear echo-sounding equipment used to profile layers beneath the sea floor.

• Ultrasound classifies a sound by frequency and names the region above the upper hearing limit of about 20,000 Hz.

Result — The stem asks for the two names that lie outside the audible band, one below it and one above it. Only infrasound and ultrasound are defined that way, so the correct pairing is infrasound with ultrasound, listed in the stem as items B and D.