Measurement of acoustic properties of sound-absorbing materials has been the source of much investigation that has produced practical measuring methods. In particular, the measurement of the normal incidence sound absorption coefficient is commonly done using a well-known configuration of a tube carrying a plane wave. The sound-absorbing coefficient is calculated from the surface impedance measured on a sample of material. Therefore, a direct measurement of the impedance requires knowing the ratio between the sound pressure and the volume velocity. However, the measurement of volume velocity is not straightforward in practice and many methods have been proposed including complex transducers, laser vibrometry, accelerometers and calibrated volume velocity sources. In this paper, a device to directly measure the acoustic impedance of a sample of sound-absorbing material is presented. The device uses an internal microphone in a small cavity sealed by a loudspeaker and a second microphone mounted in front of this source. The calibration process of the device and the limitations of the method are also discussed and measurement examples are presented. The accuracy of the device was assessed by direct comparison with the standardized method. The proposed measurement method was tested successfully with various types of commercial acoustic porous materials.

Helmholtz resonators are an interesting, widespread phenomenon, which can spark students’ interest in acoustics and more generally in applying physics to the phenomena of everyday life. Using a smartphone as an experimental tool to measure and analyze acoustic data, this work presents an investigation of the sizzling noise of something frying in a pan, and also of the change of this sound when the lid of the pan is opened and closed. We adapt the general resonance frequency result of the Helmholtz resonator (1) to the case of a real lid-pan system (2) and to a better controlled case using an artificial lid with a rectangular opening and a white noise source (also provided by a smartphone) inside the pan. Experimental spectra obtained for both cases show satisfactory agreement with the theoretical prediction. We conclude with a discussion of the limitations and perspectives of the experiment.

The standing vertical jump (SVJ) is a classical topic in Newtonian mechanics (Fig. 1). Although the topic has also been treated by others (other terms used are “standing high jump” or “squat jump”), the present paper shows how a smartphone can be used to capture video of a jump and determine the jump height. A crucial assumption often made in analysis of the SVJ is the constancy of force or acceleration during the stand-up phase. This is, however, not a trivial assumption in view of the changing geometrical configuration of the legs during standing up. We show that indeed the acceleration of the center of mass is nearly constant during the stand-up phase of the jump, a finding that has so far only been possible by much more expensive measurements (force plates).

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