Thermoacoustic engines are energy conversion devices that convert thermal energy into mechanical work in the form of sound wave (gas oscillation). The sound wave is generated by thermal interaction between working gas and porous medium (regenerator) that possesses a large axial temperature gradient. Waste heat or solar thermal energy can be used as the heat source and noble gases or other inert gases such as air can be employed as the working gas, so that the thermoacoustic engines act as environmentally benign machines. The sound energy output can then be harnessed to drive a linear alternator to generate electricity. This paper presents experimental characterization of a traveling-wave looped-tube thermoacoustic engine with air working gas at various pressures including the onset temperature difference (the temperature difference between the regenerator ends required to start producing the sound), the harmonic frequencies and the pressure amplitudes of the generated sound waves. In this experiment, an electric heater is used as the heat source which supplies heat into the regenerator hot end. The experiment is carried out by measuring the temperatures at regenerator ends and sound pressure amplitudes at several points along the looped-tube. The measurements are performed at different charged pressures of air inside the looped-tube in the range of 100 kPa - 400 kPa. It is found that the smallest onset temperature difference is 417 C which is obtained at charged pressure of 200 kPa. In addition, the second harmonic sound wave is dominantly generated at air pressure of 100 kPa, while the first harmonic (fundamental mode) sound wave dominates at 200 kPa, and the third harmonic shows up at other higher pressures. Moreover, the pressure amplitudes of the three harmonics are linearly getting higher along with the increasing charged pressure. The highest pressure amplitude of the first harmonic is 7 kPa, whereas those of the second and third harmonics are 4.8 kPa and 1.1 kPa, respectively, which are achieved at 400 kPa charged pressured. Furthermore, the sound frequencies are not significantly affected by the charged pressure variation, those are around 96 Hz, 192 Hz, and 289 Hz for the first, second, and third harmonics, respectively.