# complete noob: can REW be used to measure stereo response?



## Guest (Jun 23, 2006)

Hi, I recently came across REW which appears to be a great package. I am very interested in measuring the response of my 2 channel only setup. From what I understand though, REW can only be used to test 1 channel at a time? I am hoping the test tones generated by REW is in stereo. 

Thank you..


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## MikeOates (Jun 2, 2006)

Hi Tommy, welcome to the Shack (even though I have only been here 2-3 weeks myself!).

I suggest you take a look at my question Help needed understanding graph because this is basically what I am doing, looking at a two speaker set-up and may help.

You need to position your measurement microphone / RS SPL meter at your listening position and then run the measurements with only one speaker at a time on, make sure you measure the left speaker while having the left tab selected in REW in the bottom right section of the window, then repeat for the right.

It is important not switch off each speaker using a volume control as this will ruin and sound levels you have set up because you will not be able to set the volume control back to the exact position each time, you need to be able to switch off the audio to each speaker or turn off the power on the speaker if they are active speakers. I have done this by making a box that interrupts the audio signal to each speaker with a switch. I did this because I did not like the idea of switching off the power on the speaker each time I made a measurement. Believe me, you will do this many times.

A note to anyone that does make such a switch box, use the highest quality gold contact switches you can. I found that you get different impedances with each throw of the switch with some switches, causing the volume to change, when I replaced them with gold contact ones, I then got repeatable measurements.

Hope that helps,

Mike


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## Guest (Jun 24, 2006)

Hi Mike. Thanks for the reply. Can I ask why you measured 1 channel at a time? It makes sense to me (but I am a total noobie) to test both channels at the same time. Can you please explain the concept behind doing one ch at a time?


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## MikeOates (Jun 2, 2006)

Tommy,



> Can I ask why you measured 1 channel at a time?


Well, when you do a frequency sweep with REW, it outputs the same sound to all channels at the same time. So in order to see how each channel (Speaker) performs in your room you can only have the one speaker active at any one time. Otherwise the microphone is going to pick up the sounds from both speaks (for a two speaker setup). It will still produce a graph, but the results will not be as meaningful.

Does that make sense?

If you look at the graphs I have produced (Sorry REW has produced), you can see each speaker has a different response depending on its position in the room and any other reflections, absorptions that happen to the sound from leaving the speaker and arriving at the mic.

Mike


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## Guest (Jun 24, 2006)

Mike, forgive me...I must be completely dense. Since the music we listen to is recorded in stereo, doesnt it make sense to test our systems also in stereo? Dont we want the microphone to pick up the sounds from both speakers? Isnt that more indicative of our actual listening environment? I do not see the benefit of testing one speaker at a time since it does not take into account speaker-speaker interactions and the interaction of the system as a whole. Again, my apologies if I am missing something obvious...


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## JohnM (Apr 11, 2006)

You need to test speakers individually to assess what their contribution is, otherwise you risk applying EQ (or changing speaker position or applying room treatment to address a reflection) that is not properly targetted - when you see a peak in the combined response of the speakers, which one needs correction? You also need to test with both speakers active, particularly for low frequencies as much bass content is mono and having both speakers running drives the room differently; some resonances which are excited by an individual speaker are not excited when both are active, so they can often be left untreated.


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## Guest (Jun 25, 2006)

Joh, so are you saying I should have both speakers on for some freqs and not others? What do you mean by active? I think I am just getting more confused....


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## JohnM (Apr 11, 2006)

I am saying make measurements with each speaker individually and with both speakers together.


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## brucek (Apr 11, 2006)

> What do you mean by active?


Turned on or off...



> , so are you saying I should have both speakers on for some freqs and not others?


No, he's saying with both speakers turned on you will have trouble figuring out which speaker needs moving or equalization to correct a problem. He's saying you should test each speaker individually first and make all the adjustments necessary to smooth their response. Then after that's finished, turn on both speakers and test again to reveal any interaction that might be occuring and make any further required corrections.

This is the same process we usually go through when equalizing a subwoofer and mains speakers. First the subwoofer is tested alone and equalized and then both mains and subwoofer are tested together to check for interaction.

brucek


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