Application of equal loudness contour in assessing the impact of random noise: Case study in continuous positive airway pressure systems with helmet

Empirical evidence is presented on the application of equal-loudness contour for assessing the exposure to noise to which patient are subjected when receiving continuous positive airway pressure respiratory support via a helmet. Sound pressure levels are compared in one third-octave bands with respect to the isophon lines and the influence of the heat and moisture exchanger filters coupled to the helmets on the sound pressure levels during the supply of air–oxygen is analyzed. This study has demonstrated that an appropriate change of scale enables a more objective assessment of the whole auditory spectrum and the levels of noise produced by the continuous positive airway pressure respiratory support via a helmet exceed the 40-phon line and levels of 45 dB(A) recommended by various organizations.     

An investigation into the noise generation of CPAP devices: Experimental and numerical study

Continuous positive airway pressure (CPAP) devices are popularly used for obstructive sleep apnoea (OSA) treatment. However, the noise level emission from these devices has been identified as a potential factor for patient’s discomfort and rejection. There is a need to obtain information on the noise characteristics and source locations in order to tackle the most serious noise source within these devices. A typical CPAP device was used for the investigation and its sound characteristics and sound power levels were determined. The noise generated from a centrifugal fan was also independently investigated to address its contribution to the overall noise of the device. Frequency analysis suggested that the noise generated from both the CPAP device and the fan is broadband in nature with discrete peaks containing rotational and non-rotational components. The broadband components were then studied in detail using numerical simulation approach. Computational aeroacoustics (CAA) method with hybrid approach was used to a three-dimensional (3-D) CPAP fluid model to predict the aerodynamic and aeroacoustics behaviours of the device. This showed a complicated flow structure involving flow separation, rotation, and vortices in several locations which resulted in high level of flow turbulence inside the device. The turbulent components were used to estimate the broadband noise level at source using the broadband noise source (BNS) models. It shows the most critical location is at the fan region and at the fan inlet.