Consideration of Distance from Sample Surface to Reference Microphone in a Normal-Incidence Impedance Tube

A reference microphone can be used in a normal-incidence impedance tube to capture and properly set the appropriate total sound pressure level (SPL) for a given test condition. The standard goal is to achieve a near-constant SPL at the reference microphone across the entire frequency range of interest. This microphone is located in proximity to the sample surface, and its measured value is typically assumed to be the total SPL to which the sample surface is exposed. This paper documents the issues that arise from this assumption, particularly when spacers are used to increase the distance between sample surface and reference microphone. Five acoustic liners with various impedance sensitivities to source amplitude are tested in a normal-incidence impedance tube, excited by both tonal and broadband sources at various SPLs and frequencies. The distance from sample surface to reference microphone is altered during experiment through the use of spacers. Standing-wave patterns within the tube are predicted utilizing transfer functions of the measurement microphones. In doing so, SPL variations between the sample surface and reference microphone are assessed at a variety of distances for different samples, frequencies, and excitation intensities. A transmission-line impedance prediction model is used to assess the predictability of impedance spectra utilizing different spacers. It is found that SPL differences between the sample surface and reference microphone can be quite significant, particularly when spacers are used (up to 35 dB). This modifies the acoustic behavior of samples that are more sensitive to the source excitation, mainly impacting the resistance spectra. The impedance prediction model is also shown to be quite accurate for the selected samples and conditions assessed, as long as the computed surface SPL is used for excitation and not the reference microphone. It is recommended that future testing-standard documents include brief information and possible issues associated with the location of a reference microphone relative to the sample surface.