Every speaker and headphone spec sheet includes a frequency response chart, and most shoppers skip right past it. That’s a shame, because it tells you more about how something will actually sound than any adjective on the box.
The chart plots loudness (in decibels) against frequency (in Hertz), usually from 20Hz up to 20,000Hz — the accepted range of human hearing. A flat, even line means balanced sound; a big rise around 60-150Hz means extra bass; a dip in the 2-4kHz range often makes vocals sound recessed.
What to actually look for: avoid charts with sharp spikes or valleys, since real-world music rarely sounds good through them. A gentle downward slope from bass to treble (sometimes called a ‘house curve’) is common in well-reviewed gear and tends to sound natural over long listening sessions.