How to tune a 4-string bass
The four strings of the bass are tuned like the four lowest guitar strings, one octave lower. The string table above gives the note of each one.
Pluck the string and let it ring: the attack pushes the pitch up for a moment. Tune on the sustained note, starting slightly below and tuning up.
5 and 6-string bass
A 5-string bass adds a low string below the 4th string; a 6-string also adds a high string. The lowest string vibrates around 31 Hz: hold your phone close to the neck or the amp, as small microphones struggle with such low frequencies.