# Can I use REW to...



## elee532 (Aug 21, 2008)

Can I use REW to...

1. Determine best crossover frequency between sub and mains?

2. Set levels for sub and mains?

3. Determine best location to place sub & mains?

If so, can anyone point me to some info on how to do so? Sorry if i'm missing something obvious, I'n new to this acoustic measurement stuff.


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## thewire (Jun 28, 2007)

Do you have a CD burner and can you do a loopback recording? I made a CD the other day using the REW generator into my Windows recorder with a loopback of the left and right channels. There was some computer noise on the recording that I burned to the CD but it still worked rather well since I can't measure both mains and the sub not having a receiver. I used a -33dB FS level and each level measured alike to using another test CD. It was much easier to use than a disc that has a menu also since I have to walk to the next room right now to make adjustments. I actually can adjust levels on my XA2 without going into the next room to look at the menu, except to confirm my settings were correct. I checked the phase of each of my speakers and levels by turning off the subwoofers and channels one by one and writting down what I measured on my meter, checking the level of pink noise, sub calc noise, then sine waves from 800Hz to 100Hz in 100Hz increments, 100Hz to 20Hz in 10Hz increments. I discovered alot of my speakers were wired out of phase with my subwoofers, and I determined that some levels were off since moving my speakers which I corrected. 

You could also use the RTA in REW and do the same thing using the generator but look at the meter or the one in REW. To do all of this you would need to move the input into your AVR or amplifier to each channel when checking levels on them. Please let me know if I need to give better instruction.


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## elee532 (Aug 21, 2008)

Thanks Steven. Unfortunately, you're definitely talking above my head. More detailed instruction would be most appreciated!!


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## thewire (Jun 28, 2007)

I will make detailed instructions.


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## brucek (Apr 11, 2006)

> 1. Determine best crossover frequency between sub and mains?


Measure the sub and mains separately and ensure they track the crossover target shown in REW. Try for 80Hz as your goal, and then test above and below that crossover if 80Hz is unsuitable.



> 2. Set levels for sub and mains?


Initially set the sub and mains level with the receiver internal test tones. Measure sub and mains together with REW and tweak the levels to be even between the two (if needed).



> 3. Determine best location to place sub & mains?


Take measurements with REW and move the speakers until you get the best results.

brucek


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## thewire (Jun 28, 2007)

Much of this will be determined by the quality of drivers, size of drivers and number of drivers (speakers) you have. How well and how loud they are playing down to this crossover is improtant the quality resulting in the choosen crossover. Using software or an automatic setup cannot tell the difference between how many drivers there are, or how loud they can play exactly according to our taste so we must make some good judgements here, and take into account what the manufactor of our setup has informed to us is ideal. Looking at the manual of the speaker there will usually be a "-3 dB point. If this is 60Hz then 60Hz may very well be a good crossover, but a speaker manufator may not be publishing an accurate representation, so there will be some needed testing on our own. Much of this also will be dependent on how much bass can be produced by our amplifiers. How much bass our amplifiers can send to our speakers will be a result of sensitivity of the amp, how much power it can put out before clipping (running out of signal strength) and the quality of the signal to name a few. We want the crossover to have sent to an amplifier a strong signal with lots of power handling and low distortion to our speaker with respect to how well the amp has sent the signal to the speaker, and how well the speaker will handle the sound. The higher the crossover, the easier it is for the speaker to play the sound, and the less likelyhood that the amp will not be able to send the clear signal. This is true of the front speakers or surrounds, and in some cases an appropriate crossover for our front speakers, which may be larger, are not good for our surrounds. Then we will want to select a crossover which is best for both our main speakers, and surrounds. In some cases a component will allow multiple crossovers so if that is the case, consider that, but knowing that sending more bass to a speaker than the next, may result in just that. There would be more bass sent to one area than another with multiple crossovers. Assuming that your subwoofer is of good quality and is non localizable (unless you are sitting right on top of it) than studies show that 80Hz and bellow has no information which is dependent on the direction that the signal is coming from (our speakers or the sub). Smaller speakers such as bookshelves our sats will work best with the small setting on them as well which depending on the type of bass management on your gear, will send more of the bass information to the subwoofer. Sometimes a crossover is on with a large setting or it will send a full range signal to the speaker and only the LFE to the subwoofer. There are other possibilities that certain things will be occurring with the bass such as a different type of crossover slope or maybe some kind of DSP (signal proccessing) is redirecting bass or changing it some manner. It is first critical to familarize yourself as much possible with how your gear uses a crossover, what your speakers can do, and then lets assume that we have no extra bass management occuring and it just the crossover we have on, with speakers set to small. This is almost best 100% of the time. THX recommends an 80Hz crossover, speakers set to small, and all the subs in mono. :bigsmile:

In some circumstances there will be a possibilty that the room itself it plays a role in which crossover may be best. Now that we apply all this to what we will test with REW, we are looking for one that works well for our speakers, and our room. If we have a room where a crossover is creating a dip or a peak at certain frequencies this would be a problem. 


Step 1 - Calibration of the measurements

After you select the settings in REW, begin by taking a measurement of your soundcard if you have not done this already. Connections should be setup properly and the appropriate inputs for our card selected in the menu. This will be such and such card, line-in etc. If you have done this already go to step 2. First make a loopback of your card by connecting the right output, into the right input. Start to calibrate the soundcard by clicking the measure button in the menu, and carefully follow each of the steps in the captions. 










Be sure that you do not have mic calibration loaded in the other menu when you do this. When it has measured the loopback, it will show the result, and then you should save the measurement of the card. It is automatically loaded and ready. Now click on the measurement button and check that the response of the measurement is flat. Here is an example of mine.










This is depending on the card of course how flat this will look but it should look good and without dips or peaks in the response. Then something I would recommend to do personally is to check the waterfall of your loopback measurement. To do this go to the waterall tab and click generate waterfall. There should be a smooth looking waterall without any noise in it for best results. Here is an example of how mine should look. When I have setup incorrectly there will be peaks in the response of the loopback at 60Hz in the form of ringing so I recommend to check this, because they will only show as very small dips or peaks on the loopback measurement that may go without notice. This is what mine looks like without much ringing detected in the loopback. I only measured 10Hz - 200Hz for this one. For subwoofer measurements you will usually want to start at 0Hz and go no higher than 200Hz.










Now go to the settings and select the mic/meter tab. Browse to where you have saved your mic file and load that. 










If you are using a mic that is not available from download at the shack, and one was included with your mic use that. If you have a pre-amp for your mic, be sure that settings on it resulted in a flat response. There are excellent guides on how to do this and much of these first steps in the FAQ sticky at the top of the REW fourm. Just making sure we have full understanding of the basic setup here because I myself have gotten somewhat far into measurements without a correct setup of these steps.  Now go back to the first tab in the settings "soundcard" and hookup the mic to the right line-in of your soundcard. Next turn down the level of your REW output wave volume to a setting of around 5 to no lower than 2, or whatever level is comfortable for your subwoofers, and is high enough that the soundcard/computer and room background noise is not going to influence the response your measurements. Lets say for example if there is some noise coming from an air conditioner, we want our measurements to be more loud than this would be, or we should shut this off if at all possible. There might also be noise on our signal as a result of electrical devices such as the air conditioner running again, people in the next room doing whatever, people running electric leaf blowers, building a house next door, etc. It is best to inspect around the home before you start because it may not be apparant the noise is going to interfere. Wait for them to finish, ask them politely to be quiet, and shut off devices if these interfere. You will know if the level must be raised or lowered with respect to the sensitivity and background noise by trial and error. Whew..that was complicated. :sweat:

Be sure that the mains are off, or disconnected. Turn off the receiver or amp before disconnecting any wires or cables. I use a 18 level sweep for my measurements but one may use high as a 12 sweep. Click on the "Check Levels" and raise the output volume so that the subwoofer measures this level to get on your meter, and you are able to select the input level of REW to match in the right bar to match closely. 










If you want to use an external source such as your receiver (which is typically more accurate) that is fine too. You can't really go much wrong here with a level so as long as it is within the capablilites of your sub, and is not so high as to cause clipping on your mic.

If you are reading very sensitive measurements it may be a good idea to use a higher level of sound such as what might read 80dB, or 85dB on your meter. If you are comfortable using the 75dB setting that REW and your meter uses, then calibrate the output level so that your meter reads 75dB. Then go to the REW calibrate SPL meter and tell it that the output is also 75dB. If you were appying any equalization to your subwoofer, it would be best that the level you measure at is done at the same settings that you are using on your gear/subwoofer or subwoofers. This is not to be confussed however with simply calibrating your meter. REW will track the signal as it is told regardless.

Step 2 - Subwoofer Setup

There will be areas where can place the sub or not. Lets not get too carried away. We can't have the subs sitting on the floor for people to trip over, subs sitting in front of other speakers, firing directly into a wall or other objects, or out of the room. Ideally it should be in a place that we and others will be comfortable with, and it is aiming into the room. If there are ports on your subwoofer be sure these have room to breath also. If it is downfiring down worry about which way it is aiming. A good place to start with a single subwoofer is generally in the center of the room at the front wall, or slightly to the left between the center channel and left speaker. The best place for a subwoofer will usually be furthest from a corner or wall, or as close to it as possible, but we need to experement to find what works best for our rooms. We want at this time any special settings on the subwoofer to be off, and all available ports to be open. The crossover should be at the highest possible setting. The receiver or your source should be controlling the crossover if possible. You can enable any filters or tune the sub lower (if needed/possible) and use any special modes on the reciever later, but we need these to be off at this time. There should also be plenty of cable to move your subwoofer, and no obstructions that might prevent us from being able to move the subwoofer comfortably. If it is heavy get help, or bend at the nees. :flex: 

Now go to the spectrum tab in REW where the RTA is located. 










Make sure that the settings are set to the RTA mode, and you may select which resolution you prefer or if you would like averaging on or off here also. Using 1/24 octive or better is recommended. 











Click then on the red record button to the right of save in the box to activate the monitoring of the mic. Be sure that the Spectrum check box is checked over to the left side to see the level measured is shown in the graph. With the mic placed at your listening position, now select the generator in the tools menu at the top and select the Pink PN. 








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Now press on the green arrow to start playing the signal, and confirm that a response is shown on the graph as a result. Now your ready to start moving the subwoofer to improve any dips or peaks while you monitor the result in REW. To save a response click on the save button. You can use this to directly compare the result of your last position with the measurement selected. Try keeping in mind which areas will improve these dips or peaks as you progress. For example if moving your subwoofer in many areas does not improve a dip or peak, but a small area starts to show progress, I would recommend to place some tape on the floor to mark the location for reference later on so you know where to start again if you get lost. Once you have found an area that is resulting in the improvement of these peaks or dips (every room has them), try moving the subwoofer in smaller amounts in each direction, taking into account which frequencies will go up, or which will go down. Moving it one direction might cause more dips or peaks than moving in a different direction for example, so moving it in the direction that will you give you the most improvement without sacrificing the critical peaks or dips will need to be determined. Once you have gotten as flat as response as possible, or one with as little dips, or as little peaks possible your ready to check the results. It might be good to mark this location for later also.

With you new subwoofer location found, try listening to some familar material you are used to hearing and see if it sounds better or worse. It is important that you do not base or findings on the quality of the signal, but if it doesn't sound right then lets move on and try and find another location if there is time. If the signal is to loud to you then you may want to wear some ear protection, or if you start having listeners fatigue, try taking a break. The more carefully you consider your options and as troublesome this will be, the better the results. When you have found a nice location with the smoothest response, and that sounds good to you, then your ready to go onto the next step and check how the decay of the signal looks using the waterfall again. Take a measurement using the measurement tab and following instructions there. If this reads "clipping detected" than you will need to turn down the level of your subwoofer, or try adjusting back the level of your input and output. If there is a message that says signal level too low, simply ignore that. Looking at the results, If your location is resulting in some very long decay, you might consider trying another location again, or until you have found one resulting in the smoothest response and best looking decay. A corner will usually result in higher output and less smooth of a decay, and further from a corner will result in less output, but smoother decay. The decay we are looking at is how long the signal has last after the time the signal has left the subwoofer. If the signal has a long decay such as in a corner for an example, the sound is being prolonged by each surface where these very long wavelengths have more time until there energy has been exausted, and it might result in a boomy sound. What is improtant however is that your subwoofer is at least interacting with one of these room modes exciting the room at the frequency which will result in a bit of ringing. If there are too many of these room modes and to long of ringing, then the bass may result in more of a one note type of bass, as oppossed to having distinct sounds such as in a drum, chello, piano, etc. If this is all to much to think about, just place it where it is looking flat for now. If it is a corner, there may be no problems leaving it there.

Step 3 - Room Treatments

Now that we have as best possible a smooth response with the subwoofer, we might consider room treatments or using equalization to smooth out the decay or frequency response. If doing room treatments, you will want to start looking at the mains also while you are making changes to the room so that the mains response are being treated with respect to the treatment of the lower frequencies you are treating. Placing room treatments in one area may result in some changes to the response you are getting from speakers, and this will also be dependent on thier placement, so if treating the room, remember that you are also changing the way speakers will be interacting with your room. At that time it will certainly be helpful to get some calculations using a room mode calculator which while not 100% taking into account the amount of absorption, diffussion, or other variables into perspective, it will be a good guide to how the size of the room plays a role. Like using a map, this gives some general guidlines to avoiding mistakes. It is also a good idea to become as familiar as possible with different speaker placement options, and listen to the room with the most careful consideration possible. Using some broadband pink noise, and tracking down problem areas by moving into areas of the room, or out of areas of the room can help find some problem areas. Before treating the room I would recommend that you familarize yourself with acoustics in depth. Ethan has a very good source of articles on acoustics much of which I based most of my own room treatments. Also I have found a very good source of info is at GIK Acoustics. How one will treat a room will ultimately be the preference of the listener. Hiring an acoustician may be of an excellent value in that many cases it could cost more to fix later, than to get it done correctly the first time. You will also want to determine what kind of crossover may be best suitable before doing treatments.

http://www.realtraps.com/info.htm

http://www.gikacoustics.com/education.html

Once these are in place (every room needs them), it is time to examine the interaction between the mains and your subwoofer more in detail. If it is not an option at this time, moving on.. 

Step 4 - Setup of the crossover and the mains

First be sure you have set the distance of your speakers. Now go back to the spectrum window and monitor the same Pink PN. Watch what happens to the signal as you move the other speakers (mains or surrounds) this time and while you add each speaker, observe how this effects the response. Moving them closer to a wall will result in more sound interacting with the wall called "acoustical intereference". Aiming the speaker away from the wall will also cause less sound to be influenced by the walls. This may or may not be a problem. The front speakers (much more important the surrounds) should sound as those they are coming from the image on your screen. For movies, if this is for movies, this will be not so close together as to result in a mono sounding signal, and not to far apart so that the sound imaging is lost. If you are able to aim your speakers, or adjust the height of your listening position, do so in a way that results in the most best results. For movies somewhere close to 45 degrees for your left and right speakers will be ideal. If you aim directly to your right or left, then 45 degrees is half way between them. Your center channel will be as close possible to the screen so that dialogue is anchored to the screen, and there is a smooth transition from left to right panning across your mains. Do not place the speakers so that the drivers on them are at equal distance from the ceiling and the floor since this might cause problems. If you do so and it sounds fine with some material you listen to, that is alright and nothing to worry about. Placing them at ear level will be best, and sometimes the smoothest response may be at a level just between the midrange, and the tweeter. 

Checking the resulting dips or peaks on the RTA or by other means, it may be that your speakers phase will be interacting with your subwoofer in which case you will want to try adjusting your phase (if possible) or choose another crossover. Slight adjustments to the position to the mains or subwoofer may also minimize any problems that may be occuring. There several ways I can think to do this. 

To check that the speakers are interacting proper and in phase with the subwoofer, check each one at a time.

Method 1 

Check left, center, right, surround left, surround right, or rear surrounds each playing one a time by changing the input on the back of the receiver. In theory, you should be able to input a signal into the subwoofer and the speaker channel at the same time, and with a crossover engaged. Instead of placing the input into the left and right RCA inputs, place one at a time into the 5.1 or 7.1 inputs while the other is connected to the subwoofer input. To do this use the RTA window again but go to the generator and select the Pink Noise. Select the Custom button and enter into the window a 200Hz High Cut. 










Now watching the RTA you should be able to observe any interaction between your selected crossover and the speaker you are testing while turning on/off the subwoofer or if turning on/off the speaker. With this you should be able to get an idea of what channels are working for your crossover.

Method 2

Use the measurement in REW as you would normally and observe the response with, and without the left and right mains on. Observe whether or not a dip or peak is created that otherwise could be minimized by correcting what might be an incorrect speaker phase, or by being in need of changing the crossover. To do this you will need to simply try each crossover and a measurement with, or without the speaker on. Using the measured tab window, you can select or deselect each measurement (in the check boxes) to overlay the different responses for a more careful look at the difference changing them will make. You can make a notes of each measurement in the window above the details. You may also choose to use the RTA window again.


When you have found the one that works best, check both speakers and the subwoofer playing at the same time.

Another method (the one I used as mentioned before) was to record a loopback of the generator using my Windows media recorder. I connected the input and output of the soundcard together while there was no monitoring going on, and so that the input was directly recorded by the output. I took this file into Windows Media Player and dragged the file into the playlist on the right side of the player. Using the "Burn" tab, I clicked to start burning the CD and put into the CD drive my disk and made the CD. I then played this on my DVD player while connecting, or disconnecting the speaker inputs into my amplifiers, and by turning on/off my subwoofers. I wrote down each of the measurements and used this to base my results on.

Step 5 - Levels 

To set the levels is pretty simple. You can use the CD if you are using that, or use whatever method you prefer. Usually the receiver or your primary source will be correct, but if you want to use REW than simply use the generator pink noise level to set to your reference level. Set each of the speakers to the same level (or your surrounds maybe 2dB less) and set the subwoofer as it sounds best to you, or at the same level. You could also use a disc such as AVIA or DVE, which would be available in the shack store. These disc will have better instructions and more some other nice features to them as well.


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## jerome (Apr 24, 2007)

brucek said:


> Take measurements with REW and move the speakers until you get the best results.


I would add that you should use the Real-Time measurement function (RTA). Some more info can be found in some other places. One of them is this one for instance:
http://www.hometheatershack.com/forums/rew-forum/9872-spectrum-rta-feature.html
This one is also a very good explanation on how to set up the RTA:
http://www.hometheatershack.com/forums/rew-forum/13178-how-start-measurements-when-you-have-multiple-subs.html#post117869

The huge advantage with this method is that, once set up and started, you can move your speakers and/or subs around, change the gain/phase/etc and see in REAL-TIME the effects. No need to start a new measure every time you change something, you see the effect as you're modifying your setup.
You will maybe use some time to get started but this time will be saved multiple times later on :T

EDIT: Added new link. Congrat Steven. What a reply!!! :gulp: :yes:


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## elee532 (Aug 21, 2008)

Wow, Steven, I don't know what to say. This is quite the response! Thanks!!

I spent some time working on this tonight, but I still have a ways to go. A few (hopefully much simpler) questions so far...

1. I generated a waterfall of my soundcard calibration. Does this look generally as it supposed to?

2. I included some graphs after testing the sub in several different locations. Am I correct in understanding that the middle graph (side wall), is the better (flatter) of the three?

Thanks!


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## thewire (Jun 28, 2007)

Yes the waterfall looks clean of any noise, the curve is just a result of having the mic calc loaded, so no worries there. That middle one looks very good and is deffinitly the best looking in my opinion. The last one looks pretty good also.


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## elee532 (Aug 21, 2008)

thewire said:


> Yes the waterfall looks clean of any noise, the curve is just a result of having the mic calc loaded, so no worries there.


Steven, I did NOT have the mic calc loaded when I ran the waterfall posed above. However, I did select the "Make Cal..." option before I generated that waterfall. I re-ran the soundcard calibration, and generated a waterfall BEFORE i selected the "Make Cal..." option. I think this one looks more like your example. Did I do it right this time? 

Oh, also, I notice in one of your screen shots below that you have the "C weighted SPL meter" box unchecked. It looks like you are using the same SPL calibration file that I am (I have the Radio Shack 33-2055), which I thought was C weighted? Should I check or uncheck this box?

Thanks!


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## brucek (Apr 11, 2006)

> Should I check or uncheck this box?


C-Weight box is trumped by a meter calibration file over its limits..... so, it matters not for subwoofer measurements.

brucek


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## elee532 (Aug 21, 2008)

So, I spent some additional time with this today. :dizzy:

1. I tried using Step 4, Method 1 to check that the speakers are interacting proper and in phase with the subwoofer. However, I found myself a bit confused. I plugged the right output of my sound card into the RF and the left output SW of the 7.1 analog input on my pre-pro. However, isn't it true that most AVR's/Processors do NOT perform bass management on the 5.1/7.1 analog inputs? Is this a problem given what I am trying to accomplish here?

2. I took some readings of sub & mains together (red), sub only (blue), and mains only (green). However, I really can't tell what to make of them. Can you help me understand? Should this be helping me determine the best crossover frequency? 

3. I also have a 0/180 switch on my sub. Can I use REW to analyze which is likely the better setting?

Bottom line, I just can't seem to get my sub and mains to blend well. The sub seems way louder than the mains and the bass notes from the sub almost sound like they are not in time with the music coming from the mains. As a starting point, I calibrated levels using the SPL meter and the test tones of my AVR. I don't know if it's a levels issue, placement issue, crossover issue, or all of the above.

FYI, my mains have a frequency response of 42Hz to 20,000Hz +/- 3db (Salk SongTowers) and my sub is a HSU VTF-2.


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## brucek (Apr 11, 2006)

> I plugged the right output of my sound card into the RF and the left output SW of the 7.1 analog input on my pre-pro


The right channel of your soundcard from REW plugs into the AUX or CD input of your receiver. (Use a Y-splitter to feed both left and right input).
The receiver is set to stereo mode and the crossover is set to whatever you normally use.
That's it.



> Should this be helping me determine the best crossover frequency?


See above.



> I also have a 0/180 switch on my sub. Can I use REW to analyze which is likely the better setting?


Yes, if you plug into the correct input of your receiver as I mention above.

brucek


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## thewire (Jun 28, 2007)

If there was no way to test the other channels by means of switching inputs, there could also be the option to connect the surrounds or the center to the left or right output, then change the settings of them to match those of the speaker being tested, but thats alot of work. Some test using a 7 channel stereo mode and turn on/off speakers but I would question what occurs in the DSP doing so. Are you measuring anything other then the mains? If not, and you are simply using the same crossover for each channel, and do not seem to be experiencing any problems elsewhere, then using the standard REW connection will allow you to test with the mains playing one at a time, and the sub playing one at a time. It is a good idea to test each one a time to start with to avoid any combing effects that might occur masking any issues with the subwoofers and speaker phase. The speaker phase is simply if the speaker is connected proper with the red and black so if that is not an issue, no need to test switching the connection. The 0-180 setting is your phase setting where you may be able to make a difference in how flat the response is by adjusting that. What may cause the subwoofer to sound like it is a different timing could be related to a particular mode in the room or playing with the phase setting may help. I would check the waterfall to see if there is not some ringing occurring. The peaks and dips look spaced pretty well but there could be something lower in the signal telling otherwise. If there is a significant amount of ringing near many modes close together, it might cause the subwoofers to seem louder also. Another possibility is that there might be a null somewhere in the room in which case the sound from the subs may seem to come and go in a very short time which might sound like two sets of a bass sound instead of a continous sound. You might try standing at your back wall and see if there is not some sort of amount of bass building up back there. You could check by placing your hand on the wall with some ear protection also. Since with a crossover the bass is being redirected to the subs, the subs should sound louder with regards to the bass. Then there is also that the LFE track is played +10dB louder. I would take a tape measure and measure each of the speakers to your seat and enter that into the AVR to see if that doesn't help also. 


It does not look there is a crossover occurring in your graphs. I would check that the crossover is playing when testing with the mains since something does not look right. Testing without the crossover on is for testing the placement. The subwoofer only should have a gradual drop, and the mains should have a gradual rise to the response. Here are some examples I made. Please note my HT is not completed.

pink noise in phase (blue) out of phase (red) right no sub










left in phase (blue) or out of phase (red)










right in phase(green) left in phase (blue) or out of phase (red)










right in phase (blue) out of phase (red) sub calc signal no sub










left in phase










left out of phase










left in phase










Since I am not able to do Pink PN with my test CD is not near as accurate as an example, but when I measure using my meter, I measure the speakers to be in phase louder, and with less dips than out of phase. I determined this using sine waves.

This is what a crossover change looks like on my right speaker.

80hz (blue) or 100hz (purple) pink noise










80hz (blue) or 100hz (purple) sub calc noise


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## thewire (Jun 28, 2007)

Looks like you found a good sub location.


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