# Best way to calibrate (EQ) two subs



## gperkins_1973

Hi there,

I will be build two single subs in a couple of weeks time. I have the BFD model DSP1100 and need to know what is the best way to EQ the subs to get the best from them. They will be located at the front of the from identical to each other either left and right sides of the front speakers or firing into the wall of each corner depending on what gets the best response. I know if you have both L + R green lights on the bdf on it puts it in coupled mode and copies one set of filters to the other. If when you run REW you get two very different measurements is it best to EQ them seperately and if so is it possible to enter different sets of filters for the left channel and right channel so that you EQ them seperately to get the same curve.

I can only guess that the different locations will defo give me different measurements unless a miracle was to happen.

Be good to get some advise on this so I don't end up going around in circles.

cheers as always.

Graham


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## bigwoof

I just equalized dual subs -- one on either side of my main speakers as well. What I did was this

a) measure each sub individually from my main listening positions. i.e., turn one sub off and run the REW measurement routine. then repeat with the other sub.
b) notice that the curves were very similar. the main difference was in the measured volume. 
c) fix the volume on the subs to make them equal
d) equalize both subs together

you can send different filters to the left and right channels of each filter. If you are using the Midi interface in REW, you will be given the option to choose which channel to send the filters to. Hence, you could send different filters to each sub. However, equalizing each sub individually and then making everything match when combined is really hard and you may not need to do it (especially since your placement of subs is fairly symmetric).

Raj


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## bigwoof

sorry. had to up my post count to five before I could send these.

These were some of the references I used when I was figuring out how to equalize two subs.

http://www.hometheatershack.com/forums/rew-forum/22876-balancing-equalizing-dual-subs.html
http://www.hometheatershack.com/for...ization-devices/22155-2-bfd-1124s-2-subs.html
http://www.avsforum.com/avs-vb/showthread.php?t=841814&highlight=


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## gperkins_1973

bigwoof,

Thanks for that, I will have a read tonight when I get home from work.

cheers

Graham


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## Drizt

Sorry, can I ask what crossover point on the subs you will be using? Will you use it for 2ch / HT or both?

I am going through a similar delmar right now. Stereo or mono subs for 80Hz and below. So far mono is proving to be good enough. I haven't tried stereo yet, but will once I have a really good handle on mono.


From what I have learnt so far. If you can get each sub to measure flat from the listening position then they will measure flat if run in mono.


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## gperkins_1973

Hi there,

I am using a crossover of 90hz at the moment but 80hz would probably be ok. I will be running them for HT and for music which I run as 5 channel stereo. I don't listen to music in stereo as it doesn't give a large enough sound field.

I kinda thought you would have to treat them as individual subs as they will have different set of EQ filters to get flat. And then running from each channel of the behringer. I have never had two subs before so I am not sure how they would be wired up from the BFD to the behringer. I presumed you would run the left input of the BFD to the left output of the behringer and the right ouptut of the BFD to the right output of the behringer. Once EQ'd just run Audyssey as normal.

cheers

Graham


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## Sonnie

bigwoof said:


> sorry. had to up my post count to five before I could send these.
> 
> These were some of the references I used when I was figuring out how to equalize two subs.
> 
> http://www.hometheatershack.com/forums/rew-forum/22876-balancing-equalizing-dual-subs.html
> http://www.hometheatershack.com/for...ization-devices/22155-2-bfd-1124s-2-subs.html
> http://www.avsforum.com/avs-vb/showthread.php?t=841814&highlight=


Forget about those links... I have yet to find anyone that can prove to me that there is any method better than equalizing all the subs in your room together. If someone does ever show me two or more graphs of their individually equalized subs with relatively smooth response... and then show me a combined response (what they are going to actually hear with all subs playing at one time) that is still showing the same relatively smooth response as each of the individually equalized subs have shown, I will call it purely incidental... a one is fifty-eleven thousand cases.

A couple of quotes I made in a couple of other threads:


Sonnie said:


> I have two plus two by two... two in front and two by two in back. I equalize them all together. Trying to eq them separately will never work. Think about it... when you eq them separately, then put them together, nothing is the same... you have one eq'd location canceling or enforcing the other location. The only way to eq multiple subs and not pull your hair out is going to be eq'ing them all together... been there and done that a bunch of times. Eq'ing them all at once is very easy and works well, no matter the location of them. Remember... you are eq'ing the overall response of the subs while seated at a particular location. :T





Sonnie said:


> For the record... level match the two subs and equalize them together. It is next to impossible to equalize two subs individually... it just does not work and I have yet to see anyone do it successfully. You listen to them both at the same time... if you measure them separately, you will not be measuring what you are really listening to and the response will not be the same - no way it can be.
> 
> I have three subs and when I did equalize them, I equalized them all together... works wonderfully. When I had two subs, whether symmetrical or not, I equalized them together (and yes I moved them religiously and equalized them - I was a testing fool to say the least)... it worked wonderfully and is very simple to do. No reason to make it complicated.
> 
> Level match, measure both, equalize both and it is flawless. I have done it over and over and over in numerous setups and it works every time. If you think about it, it makes perfect sense why it works. :T


The thread that the second quote comes from is probably the best thread on the subject, but there is really not much to discuss about it... equalize them all together and you will be fine. :T


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## gperkins_1973

Sonnie,

That does make sense really because if you run a REW measurement with both subs playing together at the same time it will give you a graph. The you EQ that one graph to get flat. So I take it when you do this the graph will be smoother than with just one or is that not the case. So do you run a splitter from one of the BFD outputs to then split to the two left and right of the behringer.

cheers

Graham


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## Sonnie

I am fortunate enough to not have to have a BFD. When I did have a BFD in the system with two subs, I ran my sub pre-out from my receiver to one input of the BFD and then split back out to my two subs. When I had three or more, I split the output from my sub pre-out on my receiver to both inputs of the BFD, then split the outputs of the BFD as necessary... and coupled the BFD channels using the same filters for both channels.


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## gperkins_1973

Sonnie,

I am defo not that lucky. The only good thing going for me is that both opposite corners are good sounding corners so the subs will be symmetrically the same facing both inwards into the wall so I am hoping that the responses would be very similar.

cheers

Graham


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## Drizt

I have found that two seem to add together, they don't seem to subtract or 'average' if you will.
This has not been exhaustive testing, but seems to be consistent from the testing I have done.

Here is the left and right subs have setup as they are now, with yellow being the combined response.


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## gperkins_1973

Drizt,

If the yellow was the same level as the two seperate ones it would look like an average of the two to get a combiined response to me. It looks ok to EQ them together based on that graph.

cheers

Graham


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## Drizt

gperkins_1973 said:


> Drizt,
> 
> If the yellow was the same level as the two seperate ones it would look like an average of the two to get a combiined response to me. It looks ok to EQ them together based on that graph.
> 
> cheers
> 
> Graham


By average I mean there is no subtraction... as in the troughs do not subtract from the peaks from one sub to the other. If you use the average function in REW you would get something different from the yellow line.


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## glaufman

We discussed this at some point. A trough in a single sub's response will not subtract from the combined response of the two subs when you have not recalibrated everything prior to taking the combined scan... but most of the time once you put the two subs together you turn down the sub level in the AVR to once again match the mains, in effect normalizing the new scan... when you do this, you see that the combined response is an average of the individual responses, i.e. dips in one subtract from the other and peaks from one add to the other to present the combined response.

The reason behind this is that prior to the normalization, the only way adding the second sub to the first could reduce level is if they're out of phase at a given frequency (at the mic position). But in such a case, you would not have seen this as a dip in the individual responses, as there was nothing for them to be out of phase with.


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## Drizt

Sorry if I missed the previous discussion on this matter. Got a link for me ?

Even after normalizing the graphs I still have not found that a combined response will have one response subtract from the other. For example a complete null in one, does not completely cancel out the response from the other. I've found that they add, but dont subtract. Of course more testing would be needed, but its happened consistently for me. Of course if the two subs were out of phase then cancellations would be expected.


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## glaufman

Drizt said:


> Sorry if I missed the previous discussion on this matter.


Sorry, I didn't mean to imply you had done anything wrong:innocent:


> Even after normalizing the graphs I still have not found that a combined response will have one response subtract from the other. For example a complete null in one, does not completely cancel out the response from the other.


Like I said, it shouldn't. What the combined graph should show is a dip proportional to the amount you turned down the master gain in the normalization process. Size/shape of the dip will vary somewhat depending on exact shape of the individual curves. The sub showing a dip is simply netting zero energy there, not negative energy. In fact, that sub would have to show the same POSITIVE energy, but inverse phase, to the other sub in order to completely cancel it out. 


> I've found that they add, but dont subtract. Of course more testing would be needed, but its happened consistently for me. Of course if the two subs were out of phase then cancellations would be expected.


Yes, it's inevitable that they will be out of phase at certain frequencies at certain locations. That being said, keep in mind that WRT to subtraction due solely to dips, even after normalization, you're looking for at MOST a 6dB dip ... more likely less than that. This is why multiple subs can improve smoothness. One fills in where the other drops out... you just introduce more phasing issues...


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## Ricci

Here's my take on this. Some of this is a bit more in depth than needed and tedious but bear with me. This is just the way I do it that makes sense to me BTW. Others have their methods.

I'm assuming that you have 2 subs that are identical and that you have a few different placement options BTW. 

Take one subwoofer and run it by itself, with the x over raised as high as possible,measuring the response at each possible placement looking for the best one. If it's a large sub even the orientation of the driver can affect things (firing into the corner, versus out, etc.). Look for the placement that gives the smoothest response at the main position without huge holes or peaks in the response. I concentrate on 25-120hz. Below 25hz is usually nearly the same everywhere and above 120 will be cut out by the crossover. Once you have that spot leave the sub there. It's home. 

At this point you'll try out the second sub in the same way as the first, but you'll be wanting to run both subs at the same drive level for the measurements. For DIY or passive subs this is easy, just set the amp to max for both channels, or drive both cabs from a bridged amp. If using 2 amps they need to be the same model. It'll be close enough. Use the receivers sub out to control the overall level for calibration. If you are using powered subs you'll need to take a close mic pink noise reading of each sub one at a time, at exactly the same distance from the cone and keeping all other levels the same, adjust the 2nd powered subs gain until the measured SPL level matches the 1st as closely as possible. This only applies with identical sub models. If you do it this way you assure that both subs are driven equally as hard and that both will contribute the maximal amount of clean output that it can to the overall sound. Both should start running into overload problems at the same time. 

You are looking for the placement of the second sub that best combines with the 1st subs output to give a smooth response at the listening position. Make sure that the 2 identical subs have the same drive level as above. Try all of your available spots for the second sub until you get the spot that combines for the smoothest response at the main position without huge holes or peaks in the response as per with the first sub. 25-120hz. 

From there throw the mains in with the 2 subs and select a crossover point. 80hz is a good start. With the meter at the listening position calibrate the sub level to match up with the mains at 75db. Use the receiver to control the sub level so as to maintain the drive level match of the subs. If you have an EQ, you are running a single bridged amp, or have other means to adjust the level to both subs with one control that is also fine. Just avoid independently messing with their levels after they've been matched. 

At this point check for proper polarity between the subs and mains just in case. Run a sweep and then flip the polarity of either the subs or mains and run another. It should be very obvious which way is correct due to a lot of cancellation on the out of phase one. 

Now you have the 2 optimum spots for the subs, they are level matched to each other and the mains and are in phase. At this point start trying different crossover points that will work for your set-up (usually 60-100hz) and pick the one that has the best transition from the mains to the subs. Next start playing with the distance setting in the reciever for the sub out and measuring the results. Fine tuning this can allow you to make further improvements to the crossover area. Look for the loudest and most filled in response in the octave centered at the x over. 

This is where EQ would come in if it is needed. Sometimes it is not even needed if you are lucky. I do not like to boost if it can be avoided. There's pages of arguments on that, but that's my stance. I'd rather concentrate on EQing down the 2 or 3 worst peaks if they are notable. Sometimes a bit of boost is needed though. Start by looking at the latest response measure after doing all of the above and generate the waterfall from it and pour over that too. Especially look for peaks that also ring out excessively long. Those are where I'd concentrate the cuts at. Don't get too crazy with the EQ. If you find yourself eqing a half an octave of freq's down more than 20db, something else needs to be done. *Make all EQ adjustment globally to both subs*. Otherwise you've just thrown out all of the hard work down previously. Not to mention that eqing identical subs seperately now introduces differences in drive level that may cause one to distort or limit well before the other and that in turn means that you are leaving at least some clean headroom on the table. You are looking at the response at the listening position which is being produced by a combination of both subs. EQ them as one. 

After you EQ go back and calibrate the subs and mains again. If you had a peak that you knocked back it was likely overly contributing to the level before and the level may now need raised a bit. Measure the response again. A further tweak of the EQ may be warranted. Calibrate again. Write down all of these settings somewhere. 

Start listening. Enjoy. 

Again that's just the way I've been doing it and I'm sure I'll learn some new techniques but it seems to work out well.

This is what I started out with in my original placement...:unbelievable:










Just terrible.


Here is what I now have after using the above method. That's the same system and listening position after just tweaking all of the above. Much better. I'm only using 2 moderate EQ cuts with a Q10 BTW. Both are for nasty room resonances. One at 46hz and the other at 70ish.


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## Sonnie

I would suggest that this is extremely good advice... nicely written... :T


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## gperkins_1973

Ricci,

Thanks for that brilliant explanation. One thing first is I won't be able to run bridged as I have dual 1 ohm FI Q18 subs and will have to run them two single 2 ohm channels from both outputs of the behringer. That should still be fine as I can just turn both left and right gains on the behringer to max. They will be in identical cabinets and have the same 1200 watts rms going to each driver. I am guessing here, but can put a splitter on the back of the bfd so it goes to both inputs on the behringer amp so it applys the filters to both left and right subs. The reason I say that is I don't think the left channel on my bfd works properly so I am not sure if I run both left and right outputs of the inputs on the behringer whether if I coupled the bfd, the right channel filters would copy to the left channel filters on the bfd.

I hope this makes sense. My present sub is in the left front corner facing the side wall where this is the best spot for the first one. The direct opposite corner facing the sidewall again was where I originally had my PB13 so I know that is a good spot as well although not quite as good as left front corner so hopefully it should work fine. I will try outside the left and front speakers facing in different directions to see what response I get.

The main reason for me doing two subs is because my dual sub had a high qtc of 0.973 mainly because the drivers demand a big box but with EQing I got it flat to 12hz with a little boost at 20hz but when I removed one driver and listened again it sounded alot cleaner but was a little lacking at the mid bass. So my single subs will have a qtc of 0.83 each so it should still be cleaner than before but hopefully still have some nice mid bass. I don't like too clean bass, it sounds too hollow to me.

It defo makes sense to EQ them together as if they were one sub. When you say flip the polarity do you mean swap the pos and neg over on the subs to check they are in phase. 

Once I have the subs in the best position, I take it I EQ them and then run Audyssey to finish off. I ask as I was told to do it the other way originally so that Audyssey does it stuff first and then apply the filters. I have found that I have to apply more filters if I do it before Audyssey as the graph is quite bad before Audyssey and the BFD. 

cheers

Graham


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## glaufman

Very nice writeup. Perhaps you'd like to elaborate on when, and when not, to adjust phasing (sub to sub) in this process. I assume you would do it while testing placements for the second sub?


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## Ricci

Hey Greg,

I admittedly have not spent much time on adjusting the sub to sub phasing relationship, just the subs to mains. Reason being that I don't have anymore than about a 3ft difference in distance between the 2 subs to the main position any of the systems using 2 or more subs. I don't think that a few feet difference in distances will make a big difference below 60hz with the 25+ft wavelengths that get involved, but I guess it could have more impact at the top in the crossover region. The other thing for me is that I try to get a good response over a large listening area not just at one sweet spot. One listening position is closer to one sub and the other may be much closer to a different one. How to account for this when independently adjusting each subs phase? I think it may be better off left alone with identical subs. With disimilar subs perhaps it'd be best to match the phase by measuring with them sided by side at the same distance from the mic? Then go about placing them.

I did try to do something like this with 2 sealed subs and a DTS10 but I just couldn't get it to work without having at least one large null in the bass. It moved around with the phase. The sealed subs are much different than the tapped horn which has a very long internal pathlength and different phase characteristics overall. 

Good question. I'd like to see more discussion on this. If you are a 1 or 2 listening position, "sweet spot" sort of listener, then mayhaps it'd be worthwhile to really dial in each subs phase for that small area.onder:


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## Ricci

gperkins_1973 said:


> Ricci,
> 
> Thanks for that brilliant explanation. One thing first is I won't be able to run bridged as I have dual 1 ohm FI Q18 subs and will have to run them two single 2 ohm channels from both outputs of the behringer. That should still be fine as I can just turn both left and right gains on the behringer to max. They will be in identical cabinets and have the same 1200 watts rms going to each driver. I am guessing here, but can put a splitter on the back of the bfd so it goes to both inputs on the behringer amp so it applys the filters to both left and right subs. The reason I say that is I don't think the left channel on my bfd works properly so I am not sure if I run both left and right outputs of the inputs on the behringer whether if I coupled the bfd, the right channel filters would copy to the left channel filters on the bfd.
> 
> I hope this makes sense. My present sub is in the left front corner facing the side wall where this is the best spot for the first one. The direct opposite corner facing the sidewall again was where I originally had my PB13 so I know that is a good spot as well although not quite as good as left front corner so hopefully it should work fine. I will try outside the left and front speakers facing in different directions to see what response I get.
> 
> The main reason for me doing two subs is because my dual sub had a high qtc of 0.973 mainly because the drivers demand a big box but with EQing I got it flat to 12hz with a little boost at 20hz but when I removed one driver and listened again it sounded alot cleaner but was a little lacking at the mid bass. So my single subs will have a qtc of 0.83 each so it should still be cleaner than before but hopefully still have some nice mid bass. I don't like too clean bass, it sounds too hollow to me.
> 
> It defo makes sense to EQ them together as if they were one sub. When you say flip the polarity do you mean swap the pos and neg over on the subs to check they are in phase.
> 
> Once I have the subs in the best position, I take it I EQ them and then run Audyssey to finish off. I ask as I was told to do it the other way originally so that Audyssey does it stuff first and then apply the filters. I have found that I have to apply more filters if I do it before Audyssey as the graph is quite bad before Audyssey and the BFD.
> 
> cheers
> 
> Graham


graham the way you are running your subs seems good and they'd qualify as identical. 

I think it'd be worth trying to split the BFD signal into both EP2500 inputs so adjust both subs at once if you are worried about one channel of the BFD not working right.

2 subs split up is better than both in one big cab because you have more placement flexibility and it allows you to smooth out the overall room response with placement.

Your polarity is probably fine on the 2 subs in relation to each other. I'm suggesting that you do a sweep with both the subs and mains on and then another with the subs polarity flipped (both of them). Just to make sure that your polarity of the subs and mains is correct in the crossover area. I've caught myself with inverted polarity of the subs before. :whistling:

Do Audyssey last.


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## gperkins_1973

Ricci,

Thanks for that matey. My subs will probably be about 16 feet apart so I am not too sure if that is too large a distance or is ok. Just to clarify when you say reverse polarity do you mean swapping the pos and neg on the speaker plate on the rear of the sub. Sorry if it's a dumb question. I kinda presumed that if you wired up the subs with the correct plus/minus from the amp to the sub it would be correct unless you mean something else. 

I wish I had the guts to do two slot ported subs but my diy skills/tools dont' stretch to that but a single sealed sub should get me about 99db at 10hz and 111db at 20hz without room gain but I am not too sure what it would be with two in opposite corners. 

I have to say I was surprised how much extra excursion there was with just one driver in a box rather than the two I had. 

I am really excited about getting back out in the garage to build them. With regards to the cable what will go from the bfd to the behringer, does it matter what the quality is like. At the moment I am using a QED sub cable and a Mart grant cable but I need to buy an extra one when I split the signal from the bfd.

cheers as always.

graham


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## Ricci

Nah cable quality doesn't mater IMO. 

Yes flip the pos and neg on the subs and leave the mains the same. Do another sweep. It'll either fill in your crossover region more or cause a huge drop out there. You want the one that is filled in more. 

It's not the distance between subs that matters but the difference between the 2 and also your mains to your head where you are listening.


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## gperkins_1973

Ricci,

I have seen some single 1/4 to 2 rca cables which I guess I can use to connect the bfd to the behringer amp. And then I have my rca to rca (with 1/4 adapter) from my receiver to the input on the behringer.

Is that correct. 

cheers

Graham


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## glaufman

Ricci said:


> Hey Greg,
> 
> I admittedly have not spent much time on adjusting the sub to sub phasing relationship, just the subs to mains. Reason being that I don't have anymore than about a 3ft difference in distance between the 2 subs to the main position any of the systems using 2 or more subs. I don't think that a few feet difference in distances will make a big difference below 60hz with the 25+ft wavelengths that get involved, but I guess it could have more impact at the top in the crossover region. The other thing for me is that I try to get a good response over a large listening area not just at one sweet spot. One listening position is closer to one sub and the other may be much closer to a different one. How to account for this when independently adjusting each subs phase? I think it may be better off left alone with identical subs. With disimilar subs perhaps it'd be best to match the phase by measuring with them sided by side at the same distance from the mic? Then go about placing them.


With identical subs that are equidistant from the LP, you may be right. I don't like the idea of using dissimilar subs together, unless they're both being bandpassed to cover separate regions. I'm most definitely not a single LP type of guy, although I do tend to sit in the same place when I'm doing "critical" listening, which I guess makes me a single LP type of guy...:huh:


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