Influence of temperature and relative humidity on random incidence acoustic absorption measurements

摘要

Random incidence acoustic absorption measurements at frequencies above 5 kHz areparticularly prone to measurement error. Air absorption plays a major role in decay ratesat high frequencies and small variations in temperature and relative humidity can causesignificant changes in that decay rate. In spite of this, there is a great deal of interest inmeasurements up to 10kHz and some automotive companies require measurements to thatfrequency. The change in the air absorption due to temperature and relative humidity canbe accounted for in the absorption calculations, but precision measurements ofatmospheric conditions may be required when measuring high frequency absorption.When small samples (such as discrete components) are measured, all measurement errorsmust be minimized to insure accurate results. This is another case where small changes inair absorption will have a significant impact on the measurement results. This paper willexplore the effects of temperature and relative humidity on random incidence absorptionmeasurements at high frequencies and on the measurement of small samples. Themechanism by which temperature and humidity affects the decay rate will be consideredand simulated results will be compared. These will show the error in measured absorptionas a function of variation in temperature and relative humidity at given frequencies andsample sizes. The effects can be surprisingly large and indicate the need for precisemeasurements of these quantities.