# Noob to REW...have a few questions!



## Basshead81 (Apr 9, 2013)

Hi, I am new to REW and this forum, but have been doing a lot of reading. My main purpose right now is to take measurements of my PSA xv15's. I have a windows 7 laptop, Dayton Audio umm-6 usb mic, and a Yamaha rxv773 AVR. I did not do the sound card calibration as I am not quite clear how to interface between the laptop and AVR(loopback?). I did however download and install the mic calibration file from Dayton. I currently have the Laptop connected to my AVR via hdmi(v-aux input). I then proceeded to calibrate my mic at 60db, but the subs sound way louder than that, so am I not doing something correctly? TIA!

Here are my first measurements(green line I changed distance setting in avr)...again I don't think the spl is calibrated correctly?


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## JohnM (Apr 11, 2006)

No soundcard cal needed with a USB mic, so no problem there. When you did the SPL calibration what figure was your external SPL meter showing?


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## Basshead81 (Apr 9, 2013)

JohnM said:


> No soundcard cal needed with a USB mic, so no problem there. When you did the SPL calibration what figure was your external SPL meter showing?


thanks! i read for hours and could not find a legit answer about that!!

i dont have a spl meter...so thats probably my issue. do i use the meter then adjust rew per what the meter is showing? I changed the number in the rew to match what the mic was showing. im guessing thats incorrect?


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## JohnM (Apr 11, 2006)

Yes, usual method is to adjust the REW meter reading to match the reading on your external meter. If you don't have an SPL meter an alternative is to set a comfortable level (about the same as an AVR channel level test signal) and tell REW that is 75 dB.


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## Basshead81 (Apr 9, 2013)

Thanks for the info! I went and purchased a spl meter and calibrated rew to the mic. After running rew and making a few changes here are my new results.

My subs are Power Sound Audio xv15's. First graph no smoothing, second graph 1/3 octave smoothing.


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## mpompey (Jan 5, 2007)

Personally, I don't use smoothing in my graphs. I want to see what the room is doing to my sound.

The levels look high, did you set your AVR to play the tone at 75 dB?

From the graph it looks like you could probably turn your subs down a bit to get that curve to come down some.


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## Basshead81 (Apr 9, 2013)

i set the avr to play 100db...thats what i read to do if you want to see the true extension of your sub.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Someone gave you some bogus advice there, Joe. You can get a perfectly acceptable frequency response graph with levels in the 75-80 dB range. :T (Perhaps they were talking about a measurement for room acoustics.)

BTW, you might want to check your smoothing settings. I’ve never seen an unsmoothed graph as smooth as your top one. Same with the one smoothed for 1/3-octave; it looks more like 1-octave smoothing.

Regards, 
Wayne


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## Basshead81 (Apr 9, 2013)

Wayne A. Pflughaupt said:


> Someone gave you some bogus advice there, Joe. You can get a perfectly acceptable frequency response graph with levels in the 75-80 dB range. :T (Perhaps they were talking about a measurement for room acoustics.)
> 
> BTW, you might want to check your smoothing settings. I’ve never seen an unsmoothed graph as smooth as your top one. Same with the one smoothed for 1/3-octave; it looks more like 1-octave smoothing.
> 
> ...


ill check it but im pretty sure i removed all smoothing on the top graph.


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## mpompey (Jan 5, 2007)

Basshead81 said:


> i set the avr to play 100db...thats what i read to do if you want to see the true extension of your sub.


You may have been reading about sub/LFE channel playing up to 105dB when an AVR is set to play at theatre reference levels. The sub/LFE(.1) channel can have peaks 30 dB above the other channels. When playing at reference your sub has room to go up to 105dB, but it is level matched at 75dB with all of the other channels if you are trying to balance at reference levels.


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## Basshead81 (Apr 9, 2013)

mpompey said:


> You may have been reading about sub/LFE channel playing up to 105dB when an AVR is set to play at theatre reference levels. The sub/LFE(.1) channel can have peaks 30 dB above the other channels. When playing at reference your sub has room to go up to 105dB, but it is level matched at 75dB with all of the other channels if you are trying to balance at reference levels.


ahh ok thanks!


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## Basshead81 (Apr 9, 2013)

I know this graph has no smoothing...ill take a new measurement when I get a chance.


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## mpompey (Jan 5, 2007)

No problem. We're all learning and fine tuning here.


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## Basshead81 (Apr 9, 2013)

here is where I read about taking measurements at higher spl.


"TIP: WEAR EAR PLUGS WHEN TAKING MEASUREMENTS. Taking measurements at this level will do a few things. First, and most importantly, it will get you above the noise floor so you can get accurate decay times and secondly it will show you how flat your subwoofer truly is down at 15-20hz when played at this volume. Lot’s of people brag they have “Flat” sub response down to 10hz, but at what level? 60db? 70db? You cannot hear or feel 10hz at 70db but at 100db if you have a sub-woofer(s) that can perform with reasonable distortion at this level, you have something to be quite proud of, along with a literally shaking foundation. If you cannot run a measurement at 100db and you cannot get your noise floor below 40db your measurements aren’t totally invalid, but you may not be able to get the full picture or capture your true in room decay times to 60db."

http://www.avsforum.com/t/1449924/simplified-rew-setup-and-use-usb-mic-hdmi-connection-including-measurement-techniques-and-how-to-interpret-graphs#post_22790110


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## Wayne A. Pflughaupt (Apr 13, 2006)

As I suspected, he’s mainly talking about generating acoustics measurements (which is what he means when he mentions the signal decay times). I‘ve been told by an acoustics expert that I trust that useful decay measurements can be generated with the signal level 50 dB above the ambient noise floor, which is ~35 dB in most rooms. 

As far as “’Flat’ sub response down to 10hz, but at what level? 60db? 70db? You cannot hear or feel 10hz at 70db but at 100db,” you don’t have to be able to hear the sub-sonic frequencies in order to get a frequency response measurement. If the sub is putting out that low and the measurement mic is accurate that low, you can get a frequency response measurement at any reasonable level above the ambient noise floor – you can probably dig up graphs in our DIY Subwoofers Forum to show this. Sure, it’s great if your sub can put out 100 dB at 10 Hz, but that has nothing to do with acquiring a legitimate frequency response measurement. Make sense? :T

Regards, 
Wayne


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## Basshead81 (Apr 9, 2013)

Wayne A. Pflughaupt said:


> As I suspected, he’s mainly talking about generating acoustics measurements (which is what he means when he mentions the signal decay times). I‘ve been told by an acoustics expert that I trust that useful decay measurements can be generated with the signal level 50 dB above the ambient noise floor, which is ~35 dB in most rooms.
> 
> As far as “’Flat’ sub response down to 10hz, but at what level? 60db? 70db? You cannot hear or feel 10hz at 70db but at 100db,” you don’t have to be able to hear the sub-sonic frequencies in order to get a frequency response measurement. If the sub is putting out that low and the measurement mic is accurate that low, you can get a frequency response measurement at any reasonable level above the ambient noise floor – you can probably dig up graphs in our DIY Subwoofers Forum to show this. Sure, it’s great if your sub can put out 100 dB at 10 Hz, but that has nothing to do with acquiring a legitimate frequency response measurement. Make sense? :T
> 
> ...


sure does...thanks for the info!


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## JohnM (Apr 11, 2006)

That AVS post was well-intentioned, but wrong. I commented on it later in that thread:



> I would strongly advise people not to attempt measurements 60dB above the background noise level in their environments. Measuring at high levels risks damage to equipment, hearing and relations with your neighbours and pushes equipment out of its linear range, increasing distortion and degrading measurement quality.
> 
> Measurement software can see well below the acoustic noise floor of your room. The important signal-to-noise (S/N) ratio in measurements is the S/N of the recovered impulse response. To see that, open the Impulse plot with the vertical axis set to dB FS - by default this plot is normalised, which means the peak value is at 0 dB FS, and the noise floor of the impulse response will be apparent when zoomed out so the plot spans a second or so - if the noise floor is at -50 dB FS, for example, the measurement has 50 dB S/N. If the impulse response S/N is below 40 dB or so you may start to see noisier frequency response plots and the floors of waterfalls and spectrograms will start to show more noise, but the measurement still remains useful.
> 
> ...


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## Basshead81 (Apr 9, 2013)

here are my latest measurements...I have the subs in what I think is about the best location...getting a fairly flat response, but I have a 30hz peak I need to tame. I want to get a mini dsp 2x4 but I am not sure what input to go with?

first graph I had the subs set way hot doing some max spl testing. this was with left channel and sub only

the second graph is all 5 speakers and sub.


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