Greetings, I am a new user of REW software and am having difficulty with what looks to me to be faulty calibration. Unfortunately, I am too new to post attachments. I am using an 2010 vintage i Mac and its sound card and obtained a reasonable sound card calibration using the protocol specified by REW. I am setting up my room for 2 channel audio only, and do not use an equalizer or subs. The REW frequency sweep is output to my preamp, and I have separate left and right volume controls so that I can measure both channels together or each one separately. I am using a Radio Shack SPL meter (33-2055) set for C weighting and have used both fast and slow settings. I have tried both the C-weighted calibration option and also downloaded the appropriate file for my meter. Use of those calibration files improves the response somewhat but it is clear that there is still something wrong. I also have run sweeps using the 10-300 Hz test files available on the realtraps website, which shows reasonably flat response, generally averaging around 75 dB, with peaks and nulls of rational magnitude for a largely untreated room. In contrast, running a sweep using the REW software gives a believable graph from about 150 Hz to 15K Hz, but everything below 150 Hz slopes downward from about 80 dB to -20 dB at 20 Hz. Within that range, peaks and nulls are at the same frequencies as what I see with the Realtraps test signals, but the entire curve in that area is at a steep downwards slope. Any ideas what might be going on here?