57 A method and device for detecting a narrow acoustic spectral feature in a sample as described. The method uses an electrical frequency modulated (FM) signal that is obtained from a voltage controlled oscillator 58. The Fourier spectrum of this wave is a main frequency f c with two small sidebands at frequencies f c ±f s . This whole triplet is slowly swept in frequency by a slow sawtooth generator 52. The electrical signal is fed into a flat acoustic transducer 64; the acoustic wave is then transmitted through an acoustically active medium 66, i.e. the sample, and detected by another flat acoustic transducer 70 which produces an FM signal with a superposed amplitude modulated (AM) signal if one of the sideboards overlaps an acoustic feature. This electrical signal is then demodulated to give a pure AM signal which can be phase sensitive detected by using a lock-in amplifier 78. By suitable adjustment of the phase setting of the lock-in amplifier both the "in-phase" and "out-of-phase" signals of the detected acoustic signal can be detected.