When we talk about how "loud" a sound is, most people think of decibels (dB). However, decibels measure physical sound pressure — not what your brain actually perceives. This is where phons and sones come in. They are units that measure perceived loudness . The Problem with Decibels A 50 dB tone at 100 Hz (low rumble) sounds much quieter than a 50 dB tone at 2000 Hz (mid-range) — even though their physical sound pressure is identical. Why? Human ears are not flat frequency sensors. We are most sensitive to frequencies between 2–5 kHz and less sensitive to very low or very high frequencies.