Normal Incidence Sound Absorption Testing
Normal Incidence Sound Absorption testing is performed the ARL's 24 inch, 12 inch, 8 inch, and 2 inch impedance tubes. Unlike sound absorption testing performed in the reverberation room, which utilized a diffuse, random incidence field to determine the sound absorption properties of a specimen, the normal incidence method utilizes a long tube that is driven at one end by a broadband noise source (in this case pink noise). Planar waves, or sound waves with relatively flat wave fronts, are produced in the tube, and our data acquisition system is able to measure the difference between the incident wave (the sound wave traveling from the source to the specimen), and any reflected sound waves from the specimen to calculate the normal incidence sound absorption properties of the specimen.
From houses of worship to movie theaters sound absorption is utilized to control the sound quality in a space. Acousticians have several criteria that they strive to control in many different kinds of spaces. One of those criteria is reverberation time. Reverberation time, or RT60 for short, is a measure of the time it takes for a sound to reduce, or decay, by 60dB in a space. Typically the longer this time is the more difficult it becomes to discern details in the program audio that is being presented, whether that is speech, music, or a mixture of both. To be able to control the reverberation times within a space acousticians will generally add absorptive materials within the space. One of the challenges that acousticians face is the fact that reverberation times will not always be consistent over the audible frequency range, in fact most of the time they are not. Similar to random incidence absorption testing, normal incidence absorption testing allows the interested parties the ability to observe the absorption of a specimen throughout the frequency range to determine if the material would work well for an application. Random incidence absorption testing that is completed in a reverberation chamber will provide the user with data within 1/3rd octave band frequency ranges, while normal incidence testing provides data at discrete frequency steps allowing the user of the data to see the absorption of the specimen in greater detail than with random incidence absorption testing.