A signal processing device includes: a detection unit generating a digital detection signal corresponding to motion of a diaphragm of a speaker to output the digital detection signal; a gain adjustment unit generating a digital feedback signal by multiplying the outputted digital detection signal by again coefficient to output the generated digital feedback signal; a combining unit combining the outputted digital feedback signal with a digital audio signal; a storage unit storing plural gain coefficients; and a control unit performing control so that a given gain coefficient is selected from the plural gain coefficients and that the selected gain coefficient is used for the multiplication.