Frequency response is measured within a stated tolerance window, not a single curve

A frequency response specification states the range of frequencies a device handles plus a tolerance (e.g., 30 Hz–15 kHz, ±3 dB). The ±3 dB tolerance defines a 6 dB acceptance window — the output level at any frequency within the stated range must stay within that window relative to the nominal level. A device may physically operate somewhat outside the stated range, but the spec defines the ‘flat’ region. Comparing two specs is only valid when both use the same measurement conditions and tolerance. A common error is reading ‘frequency range’ (no tolerance stated) as if it were ‘frequency response’; the former is meaningless without the tolerance.

Examples

A loudspeaker rated 30 Hz–15 kHz ±3 dB produces a 6 dB acceptance window. A device rated ‘20 Hz–20 kHz’ with no tolerance stated tells you nothing useful about how flat the response actually is.

Assessment

Given two spec sheets — one stating ‘20 Hz–20 kHz’ and one stating ‘40 Hz–18 kHz, ±3 dB’ — explain which provides more information and why the first is potentially misleading.