# AVR's sub crossover - graphs look strange...



## PTAaron (Feb 16, 2012)

I have been playing with REW to try doing some room measurements. I have a HK 3600 as my AVR and the crossover is set to 80hz - but this is what the graph of my subwoofer response looks like:









I was expecting to see a cutoff above 80hz... Which is clearly not happening. 

I was surprised so I ran just the front left speaker with the sub turned off:









The peak around 60hz doesn't seem like it should be there if there...

Any thoughts?


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## ALMFamily (Oct 19, 2011)

I am by no means knowledageable about REW and measurements (at all really), but I think I have read a few times where a 60 Hz hump is caused by something electrical like a dryer, refrigerator, etc. That may be your hump.

As far as the sub reading, hopefully someone else knows what is going on there.


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## PTAaron (Feb 16, 2012)

ALMFamily said:


> I am by no means knowledageable about REW and measurements (at all really), but I think I have read a few times where a 60 Hz hump is caused by something electrical like a dryer, refrigerator, etc. That may be your hump.
> 
> As far as the sub reading, hopefully someone else knows what is going on there.


Oh ok, that could be since it isn't a dedicated circuit...


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## selden (Nov 15, 2009)

You might check to make sure your audio environment is really silent when you're calibrating. Often we get so used to background noise that we don't notice it.

Subs need to be able to produce audible sounds well above 80 Hz. If a preamp output is available, you might try plugging the sub into the center channel output to see how much of the higher frequencies you can hear. LFE audio channels are supposed to go up to about 120Hz. Apparently most AVRs send the LFE channel directly to the sub output with no filtering, so a sub's low pass filter has to be set at least that high, or disabled. Some small speakers require crossovers higher than 120Hz.

FWIW -- the LFE channels of both Dolby and DTS lossless BD soundtracks are full bandwidth, unlike the soundtracks on DVDs. This supposedly has revealed problems in some movie soundtracks, where the sound editor forgot to enable the LFE low-pass filter and spurious high frequency noise was included in the LFE channel. This is a reason for using a 150Hz (or so) low pass filter in the sub itself. Sorry, I don't know of any examples myself, but see http://documentation.apple.com/en/compressor/usermanual/chapter_10_section_1.html which includes a description of the LFE controls for one particular DD encoder, which has an explicit on/off control.


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## PTAaron (Feb 16, 2012)

Thanks for the reply. 
The only other sound in the room was the fan on my laptop computer that was running the test signals.


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## PTAaron (Feb 16, 2012)

Alright... I figured out what is going on with my subwoofer graph. Apparently my AVR doesn't ACTUALLY cross over at 80hz when it is set at 80hz!! The dip at 80hz with the peak following it is a result of the crossover I set in the BFD's filters - and the fact that the AVR didn't also cross it over there creates a peak.

I took a couple of readings 1 foot from the subwoofer to remove any room effects and this is what it looked like:


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## vann_d (Apr 7, 2009)

A couple comments, 

1) I have serious doubts that your AVR does not implement the crossover correctly. I suspect some error with your speaker settings on the AVR. Maybe we can go over those...

2) It appears that you are using the BFD incorrectly. The BFD should not be used as a crossover as it does not have the proper filters. The BFD can be used as a parametric equalizer.

3) Electrical or appliance noise would absolutely not be the cause of the 60Hz peak at 75dB unless you are sitting right next to a very old noisy unit while making the measurement. This is likely the result of a room mode. Even with a proper crossover, there can be significant energy below the cutoff frequency.

4) Your frequency response is really poor on your left front speaker. Can you post a picture of your setup? There may be something really wrong there. If not, I'd suspect there is something wrong with the measurement. What is your measurement system?

5) First graph looks like sub + front. Are you sure your speaker was unplugged?

Maybe we can help you figure this out. I've had lots of strange things happen when taking REW measurements and all of them turned out to be user error...:doh:


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## PTAaron (Feb 16, 2012)

vann_d said:


> A couple comments,
> 
> 1) I have serious doubts that your AVR does not implement the crossover correctly. I suspect some error with your speaker settings on the AVR. Maybe we can go over those...
> 
> ...


Thanks for the reply!

1) it states in the owners manual that the sub crossover is at a fixed 100hz - so that explains why there is a peak after the 80hz drop from the BFD filter. So my setting it to 80hz in the speaker menu doesnt actually change anything on the sub. This was confirmed by HK. 

2) it is being used as an EQ. In the filter settings for REW I set the crossover point at 80hz in the options assuming that the sun was already being crossed at 80hz. I wasn't intentionally setting up the BFD to function as the crossover 

3) excellent information, thank you. 

4) I didn't adjust the level on the speaker for that measure and didn't run a full range sweep as I was only looking to see what happened at the level it was supposed to crossover (80hz)... So maybe that explains it?
Measurement system - I assume you mean what equipment/software? REW with a Dayton EMM-6 mic and ART USB dual pre plugged into my laptop. 
Picture of the setup - this is before the sub was moved:









5) I may have labeled my graphs wrong now that I take a closer look at them. 
Regardless - the measurements I posted last show that the AVR is definitely not crossing the sub at 80. 

I'm reasonably sure that adjusting my filters to cover the range up to the actual crossover instead of stopping just above 80 will help that strange hump I was wondering about. I guess I will need to use the crossover settings on the sun itself to get it to truly crossover at 80hz though 

Measurements were done with it set up like this:


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## vann_d (Apr 7, 2009)

Nice looking room! I think your measurement setup looks good. I don't have time to discuss everything right now but

1) yeah, I would either set speaker crossover to 100Hz or use an external filter. With your large mains, you are still going to get a lot of content below the crossover frequency. The speaker crossover has a shallower slope than the sub crossover. This was designed to work with speakers that naturally roll off below the cutoff. With yours likely extending much lower, the crossover isn't going to work correctly. I had this problem with my towers and my AVR too.

2) I understand now...

good luck I'll try to post more later


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## PTAaron (Feb 16, 2012)

Thanks!

I was thinking the same thing on setting the crossover at 100 - I'll give it a try!


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## PTAaron (Feb 16, 2012)

Ugh... I just noticed that my iPhone keeps changing the word SUB into SUN ... So anything that doesn't make sense in the preceding posts - I blame on my phone!


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## vann_d (Apr 7, 2009)

One thing that I just realised is messing with me is that your abscissa (horizontal axis) is on a linear scale. Change it to a log scale, that is more appropriate and will help with comparison to other graphs.

You are down about 20dB from 100 Hz to 200Hz in your sub only plot. Everything might be ok. Just re-plot the graphs you previously posted and things might look a whole lot better....


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## vann_d (Apr 7, 2009)

Here's one of my recent measurements for reference...

http://www.hometheatershack.com/for...b-w6-1139sif-0-35-cu-ft-dual-mini-me-subs.png


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## PTAaron (Feb 16, 2012)

I will make that change. I thought - for some reason - that the linear graphs were more useful to people 

I'll repost them tonight. I don't know why I haven't just uploaded the full file yet so I can make changes to it when I'm not at home...


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## PTAaron (Feb 16, 2012)

Here is the sub from listening position - scale adjusted:









Here are the measurements taken directly in front of the sub with the scale changed with filters is green, yellow is no filters:


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## vann_d (Apr 7, 2009)

from what I've read, our hearing is logarithmic so it makes sense to plot on a log scale. Imagine what the 20 to 2kHz plot would look like on a linear scale...you would never see the subwoofer response!


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## vann_d (Apr 7, 2009)

You don't happen to have a +10dB filter at 150 Hz or so do you? Are you sure you didn't set REW to do flat to 200 Hz? Why would the filtered plot be so much higher above 150 Hz? Check your filter settings first.

If not, I would do a 20-2000Hz sweep with sub only (and unplug the rest of the speakers). If you can hear content from the sub (and measure it) at higher frequencies, you will definitely know the crossover is not working correctly.

In general, your graphs look a lot better on log scale (the more appropriate scale). You have a pretty nice flat response from the sub at nearfield so it must be a quality unit.


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## PTAaron (Feb 16, 2012)

vann_d said:


> You don't happen to have a +10dB filter at 150 Hz or so do you? Are you sure you didn't set REW to do flat to 200 Hz? Why would the filtered plot be so much higher above 150 Hz? Check your filter settings first.
> 
> If not, I would do a 20-2000Hz sweep with sub only (and unplug the rest of the speakers). If you can hear content from the sub (and measure it) at higher frequencies, you will definitely know the crossover is not working correctly.
> 
> In general, your graphs look a lot better on log scale (the more appropriate scale). You have a pretty nice flat response from the sub at nearfield so it must be a quality unit.


I don't have any filters at 150hz, so I don't know why there is a boost there when the BFD is on. 
My filters are (only listing the gain): 20hz -5; 23.25hz -7; 29.22hz -10; 44.50hz +2; 60.40hz -10; 73.20 +5; 82hz -11; 92hz -20.
I will have to try running the sweep like you suggested on Saturday when I have some time...

The sub is just a A2-300 from Elemental Designs... it gets the job done for the time being.


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## vann_d (Apr 7, 2009)

Hmm, yeah weird. I would turn the BFD off until you figure out what is going on.


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## PTAaron (Feb 16, 2012)

vann_d said:


> Hmm, yeah weird. I would turn the BFD off until you figure out what is going on.


Already took it out of the signal chain and re-leveled the speakers/sub without it. 

Once I get some time to really mess with it I'll get things figured out - hopefully.


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