the check levels function and the sweep level

[Drew Neilson]

I have my AVR's subwoofer pre-outs connected to my MiniDSP 2x4 HD, and am currently learning how to use Multi Sub Optimizer to optimize my subwoofers using the MiniDSP. To measure my subwoofers, in REW I set the output to one or both of my MiniDSP's two input channels (bypassing my AVR, by going directly to the MiniDSP via USB). I still use my front left speaker via my AVR as a timing reference. To measure my non-subwoofer speakers, I set the output in REW to one of my AVR's channels: the appropriate one for whatever speaker I want to measure, and I keep the front left speaker as an acoustic timing reference.
Regarding the "check levels" function in settings:
Why is the sweep level -12.0 dBFS by default? Is there some significance to that number?
Why is it "preferred" to change the AV receiver volume instead of the sweep level?
Does a MiniDSP count as an "equalizer"? I assume that it does, and that when I measure my subwoofers directly via my MiniDSP, I should change the sweep level instead of the MiniDSP input gain. Please correct me if I am incorrect.

 

[Drew Neilson]

Since I measure my speakers at 75.0 dB, then for measuring my subwoofers through my MiniDSP that would mean setting my MiniDSP's input gain to zero and changing the sweep level to -40.0 dBFS, instead of leaving the sweep level at -12.0 dBFS and setting the MiniDSP's input gain to -28.0 dB (-12.0 + -28.0 = -40.0). I've been doing the latter, not the former, when measuring my subwoofers, and I need to know if I should continue to do so. For measuring my speakers, since I have to go through my AVR for that, and since my AVR's reference level is 82.0 dB instead of 85.0 dB, then to measure my speakers at 75.0 dB (-7.0 dBFS on my AVR), if it is "preferred" to change my AVR's volume instead of REW's sweep level, I guess that I should set an AVR volume of +5.0 dB (because 5.0 + -12.0 = -7.0, and -7.0 dBFS = 75.0 dB when the reference is 82.0 dB). I guess that that is what I'll have to do from now on if it is important to keep the -12.0 dBFS sweep level. If it isn't, then I guess I'll set the sweep level to 0.0 dBFS, and set my AVR's volume to -7.0 dB., since 82.0 - 7.0 = 75.0.

 

[jschwender]

The absolute level should not matter in theory. I think the intention of a -12 dB preset is to avoid overload of your speakers or your ears when you start the measurement. The standard would be to measure parameters at 1 W power, which is (hopefully) more than 75 dB. But as long as you stay above the noise floor that limits your lower end and stay away from the power compression of your speakers and clipping of signal path, it should be ok. If it is better to adjust on the output level or the AVR setting depends. I would say it makes sense to go into the amp with a reasonable level, like it comes from audio sources you connect to, and adjust the rest on your amp. so for a consumer grade sound card, the -12 dBFS is not so bad, i would say. If you have a professional device, that may be much too high, as these have a FS of +22dBV.

 

[Drew Neilson]

for a consumer grade sound card, the -12 dBFS is not so bad, i would say. If you have a professional device, that may be much too high, as these have a FS of +22dBV.

I guess that you're talking about avoiding the noise floor of analog connections by having the sweep level high enough to avoid it, and also having it low enough to avoid the upper limit of analog connections, whatever that limit is. What about the fact that the connection between my PC and my AVR is digital (HDMI), and the fact that the connection between my PC and my MiniDSP is also digital (USB)? For me personally, when it comes to my home theater sources, my modern sources connect to my AVR via HDMI and my retro sources connect to my AVR via analog connections. My AVR's subwoofer pre-outs that go to my MiniDSP are obviously analog.

I think the intention of a -12 dB preset is to avoid overload of your speakers or your ears when you start the measurement.

I try to be very precise when I measure and calibrate my equipment, so if the default sweep level of -12.0 dBFS is only to prevent damage to my equipment and/or to my hearing and bad measurements due to the volume accidentally being too loud during the sweep, then you should know that I will always make sure to setup everything (the volume of my AVR and the input gain on my MiniDSP, for example) as precisely as I can before I measure by setting my MiniDSP's input gain, my AVR's volume, and REW's sweep level to given levels before taking measurements.

The absolute level should not matter in theory. [...] The standard would be to measure parameters at 1 W power, which is (hopefully) more than 75 dB. But as long as you stay above the noise floor that limits your lower end and stay away from the power compression of your speakers and clipping of signal path, it should be ok.

I thought that REW recommends measuring at 75.0 dB. Maybe it doesn't, and it only says that most people usually measure at 75.0 dB. It definitely recommends measuring at the user's preferred listening level, and my preferred listening level is -13.0 dB, which is 69.0 dB, since my AVR's reference level is 82.0 dB instead of 85.0 dB; 82-13=69. I've still been measuring at 75.0 dB, not 69.0 dB, since that's what my AVR measures my speakers at when I run its built-in AccuEQ Advanced calibration routine.
If I want to continue to measure at 75.0 dB (-40.0 dBFS for LFE, and -30.0 dBFS for other channels), then if I am measuring a subwoofer, can I set the sweep level to -40.0 dBFS and leave my MiniDSP's input gain at 0.0 dB, and if I am measuring a normal speaker, can I set the sweep level to -30.0 dBFS and set my AVR's volume to 0.0 dB (reference/unity)?

 

[Drew Neilson]

It would be easier for me to set the sweep level to -30 dBFS or -40 dBFS based on whether I'm measuring a speaker or a subwoofer, and then set the volume of the MiniDSP or AVR (or whatever other device I might use in the future) to 0.0 dB (reference/unity), instead of setting the sweep level to -12.0 dBFS and setting my MiniDSP's input gain to -28.0 dB (when I'm measuring a subwoofer) and my AVR's volume to +5.0 dB (when I'm measuring a speaker). I'd rather set the input gain on my MiniDSP and the volume on my AVR to 0.0 (reference/unity) and set the sweep level to -30 dBFS for a speaker measurement, or -40.0 dBFS for a subwoofer measurement, and when I'm done taking measurements, make sure to reset the gain or volume to whatever it was at previously so that an accident doesn't happen.

I understand that professional speaker measurements that are published by manufacturers and by reviewers use a 1 W input to the speakers and that measurements are taken with the microphone 1 foot (or is it 1 meter?) from the speaker, but that doesn't reflect the speaker's performance in my room when the mic is 1' away from the speaker nor when the mic is in my MLP, and doesn't help me to set the output level per speaker so they have the same SPL at my MLP regardless of their sensitivity. I'm trying to duplicate AccuEQ Advanced's methodology when it comes to how to take measurements.

If there is a disadvantage to taking subwoofer measurements with a sweep level of -40.0 dBFS and with my MiniDSP set to reference/unity gain, and taking speaker measurements with a sweep level of -30.0 dBFS and with my AVR set to reference/unity volume, please let me know.

 

[jschwender]

I'm trying to duplicate AccuEQ Advanced's methodology when it comes to how to take measurements.

ok, sorry, your intention was not clear to me. Nevertheless, if it is easier for you to set sweep level to -40 dBFS, that should still work. The down side is that your measuring input signal precision is reduced by 7 bits, but there should still be enough bits left for a proper sweep measurement.

 

[Drew Neilson]

ok, sorry, your intention was not clear to me. Nevertheless, if it is easier for you to set sweep level to -40 dBFS, that should still work. The down side is that your measuring input signal precision is reduced by 7 bits, but there should still be enough bits left for a proper sweep measurement.

In that case, I'll leave REW's sweep level at -12.0 dBFS and when I need to measure a subwoofer, I'll continue to change my MiniDSP's input gain to -28.0 dB, and when I need to measure a speaker, I'll change my AVR's volume to -18.0 dB; -18.0 + -12.0 = -30.0. I've already created a MiniDSP profile for measuring, which I can switch to in the MiniDSP app whenever I want to measure a subwoofer. That profile is already setup with a -28.0 dB input gain; that's how I've been doing this so far. Then, when I'm done measuring, I can switch my MiniDSP back to one of my other profiles. As for my AVR, I can attempt to program it to change its volume to -18.0 dB when I press an on-screen button in an app that I've downloaded from the Microsoft Store, and to change its volume to something else when I press a different button in that app. Having the preconfigured MiniDSP profile and the preconfigured button for my AVR in the app will make this easier, since I'll leave REW's sweep volume at -12.0 dBFS.
I still don't understand why REW's sweep level is at -12.0 dBFS by default, but that doesn't mean that I can't use that sweep level setting, and accomodate it by changing my MiniDSP's input gain and my AVR's volume when I want to measure a speaker or a subwoofer.

 

[Drew Neilson]

Is REW's -12.0 dBFS default sweep level 93.0 dB or 103.0 dB? Is the 12 dB subtracted from 105 dB, which is full-scale for normal speaker channels, or from 115 dB, which is full-scale for the LFE channel? To this point, I've assumed that it is subtracted from 115.0 dB, and that when I click on "check levels" before taking a measurement, if REW gives me a result of ~ -40.0 dBFS, then that means that it is at about 75.0 dB. If I am wrong, then my speakers and subwoofers might not have been measured at the absolute level that I wanted to measure them at: 75.0 dB. EDIT: This fails to take into account the fact that my AVR calibrates to 82.0 dB, not 85.0 dB, but my question is still valid.

 

[jschwender]

When you take a calibrated microphone you can adjust SPL level by using the SPL meter and tuning either sweep level or your other adjuster in the chain.

 

[Drew Neilson]

Yes, but that doesn't really answer my most recent question. My most recent question boils down to this: when I am about to measure a subwoofer, and I click on "check levels", and REW reports a signal of about -40.0 dBFS, is that 115-40 = 75, or is it 105-40 = 65? Since the LFE channel has a full-scale value that is 10.0 dB higher than the full-scale value of the full-range channels, I am uncertain which full-scale value is being used. I suppose that if I'm measuring a subwoofer that is connected to an AVR, and REW's test is going through that AVR and, from there, to the subwoofer, then the AVR is automatically adding 10 dB; but in my measurement setup, my PC bypasses my AVR and goes directly to my MiniDSP and, from there, to whichever subwoofer I'm measuring. The MiniDSP doesn't add 10.0 dB unless I've manually added 10.0 dB to its input gain. In that case, when REW reports that the incoming signal is -40.0 dBFS, is it 75.0 dB or 65.0 dB?

 

[sam_adams]

Your confusion is arising from the view that is being presented by the measurement units that are being displayed in the Preferences dialog. @John Mulcahy added an excellent feature in the latest release of REW that allows one to change the displayed unit of measurement in the preferences dialog when setting the level. Remember, a microphone does not measure in dBFS—a measurement of digital signal level—but in SPL, a measurement of the sound pressure level impinging on the microphone. When you are using a microphone as input to set the SPL measurement level from the preferences dialog, you must change the unit of measurement to SPL.

To change the unit of measurement, right-click, two-finger tap or ctrl-click, on any of the meters in the Preferences dialog and the pop-up menu will allow you to change the unit of measurement for the input to whatever is displayed in the list:

Mac:

Windows:

In the case that you are using a microphone as the input, you would select SPL as the unit of measurement for the input. You can then proceed to adjust the level accordingly to whatever level you choose.