The range is not the response
Most product pages mention 20 Hz to 20 kHz because that loosely matches the common limits of human hearing. The missing detail is level. A headphone may technically produce 30 Hz, but if it does so very quietly compared with the midrange, you will not experience that bass as full or authoritative.
Why the middle matters most
Vocals, guitars, dialogue, and many instruments live in the midrange. That is why a narrow focus on extreme bass and treble can be misleading. A headphone with smooth mids often feels more natural than one with a dramatic spec sheet. The frequency generator helps you hear how evenly your pair moves through the range.
Treble extension is only useful when it stays controlled
Extended highs can add air and separation, but they can also create sharpness if the tuning spikes in the presence region. When people say a headphone is detailed, they often mean the upper mids and low treble are elevated, not that it reaches higher than other models.
Use the number as a floor, not a verdict
Treat printed frequency range as a basic compatibility statement, not a promise of sound quality. Fit, pad seal, driver design, and tuning shape usually matter more than the number on the box.