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Discussion Starter #1 (Edited)
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?
 

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Bill, the mdat file only has an average graph, there aren't any actual measurements so its difficult to say much about it. Please post a measurement.
 

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Nothing of concern there, measurement looks OK. There's no need for multiple sweeps or long sweeps unless there is a requirement for a particularly low noise floor, I'd use single 256k sweeps. If that is a measurement of both speakers best measure each individually to rule out the possibility of one speaker or one LF driver being out of phase.
 

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Discussion Starter #8
Thanks John. I have run both speakers individually and obtained essentially identical graphs. Perhaps I don't know how to read the graph. I would take the dB reading to be an absolute dB reading, in which case I am perplexed by a reading of -20 db, which would be a partial vacuum. Further, when I observe the the graph taken using the realtrap test tones, I don't see the radical decline from 85 db to -20 db below 150 Hz. I see only a 5-10 db decline.
 

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dB SPL are pressure fluctuations, not absolute pressures. 0 dB (for a calibrated reading) is a (somewhat nominal) threshold of audibility, levels below that are correspondingly below that threshold.

The lack of response below 150 Hz is a bit of a puzzle. You haven't selected an input or output in REW's soundcard preferences so the default audio devices will be used, which I'm guessing are the onboard audio input and output. Worth checking how the Mac's speaker outputs are configured in case it is using a setting which restricts LF.

As an alternative measurement method you can play REW's Pink PN test signal and look at the result on the RTA using the settings below, I'd expect it to be the same shape as the sweep measurement though.

rtasettings.jpg
 

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Discussion Starter #10
John, The iMac was set to use its internal microphone, which not doubt explains the poor low frequency response. I set the mac for line input, but now when I attempt to calibrate the meter I get a response that says the input level is very low (-93 dBFS). That is the most I can get even with the pink noise at 78 dB.
 

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Discussion Starter #11
John, also, my sound card is seeing the input from the spl meter. Playing music, even at low levels, gives a reasonable deflection of the iMac sound level meter. Switching back and forth between the mac mike and the spl meter inputs shows a difference in the sound levels in the iMac sounds input display, so I know it is reading the spl input when set that way and not the internal mike. So despite the fact that my sound card is now getting signal from the spl meter, it seems like REW is not seeing it at a sufficient level to permit generating a meaningful curve from a sweep. It looks like random noise.
 

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What have you selected as the input device and input in REW's soundcard preferences? Is the input channel selected in REW the same one the RS meter output is connected to?
 

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Discussion Starter #13
John, I have default audio device selected for output device and default output selected for output. With this setting the output volume selector is greyed out. I have tried all of the input device settings. The only one that allows REW to receive usable signal is input microphone, with microphone selected as input. This apparently reverts back to the internal mac microphone. All of the other settings for input give the -93 dBFS error message. I only have one microphone input port on the mac.
 

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Please use the Generate debug file button on the soundcard preferences and attach the file it produces. What cables and adaptors are you using from the RS meter to the line input?
 

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You should be using Built-in input as the input device, the input it has should be called LINE_IN. If you don't see any levels from that check the input volume setting and try each RCA plug in turn to connect to the meter.
 

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Discussion Starter #17
OK, the setting you specified is what I have. The REW SPL meter shows 76.0 dB(C) S in the big window and the preferences sound card window shows -20dBFS out, -20 dbFS out as Ref In and -92.7 as In. The input volume setting is 1.0. I'm not sure what you mean by try each RCA plug in turn. I only have one RCA plug on the input, and I have disconnected and reconnected both ends multiple times. I attached a screen shot of the iMac sound window showing that the soundcard is seeing the input from the Radio Shack meter.
 

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The REW meter is showing -20 dBFS on Ref In? Ref In is the other channel of the input, so if you have set REW to use the Right input for measurement then Left is Ref and vice versa. The Line In is a stereo input, so if you only have a single RCA on your cable it may be connected to a mono 1/8" jack rather than stereo, meaning the input would only be present on one input channel. If REW is set to listen on the Left input in the soundcard preferences switch it to Right or vice versa.
 

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Discussion Starter #19
Well, that is the ticket! See the attached graph. Thank you so much for all of the time you spent helping me get started with this outstanding tool!
 

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Glad to see the input is sorted out! That measurement has strangely high levels of distortion though, make sure the range on the RS meter is set so that the meter doesn't get overloaded during a sweep.
 
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