Unlike its sibling the (which is linear) or the LM3916 (which mimics a VU meter), the LM3915 has a 3dB/step logarithmic response.
The LM3915 reacts to DC voltage. For audio applications, an updated design often includes a precision rectifier (using an Op-Amp like the TL072) to convert AC audio signals into a smooth DC envelope, ensuring the "bounce" of the LEDs matches the music’s rhythm. lm3915 calculator updated