Acoustic waves generated by a TA (ThermoAcoustic) laser pair

Sound waves and acoustic energy generated by two identical TA (ThermoAcoustic) lasers were analyzed and studied. SPL (Sound Pressure Level) meters and microphones were employed to measure and study the sound waves at different distances from the openings of the TA laser pair for different laser position arrangements. The sound waves of the two TA lasers at or near the focusing point were found to be almost 180° out of phase when the distance between the openings of the two lasers was within about two tube diameters and the angle between the laser axes did not exceed 135°. As the distance increased, it became difficult to control the two TA lasers in synchronized operation. For separation distances greater than three times tube diameter, the sound wave amplitudes and the phase difference between the two laser outputs varied periodically with time. With the openings of the two TA lasers touching each other, the frequency of the sound waves increased when the angle between the laser axes was very close to 180°. In this case, the glass tube opening was no longer a pressure anti-node and the wavelength of the fundamental mode reduced to approximately twice the tube length.