# 50 Hz null is gone, what now



## Luca61 (Feb 16, 2012)

Now that I finally managed to get the sound card calibration out of the way, I would like to ask for some guidance on how to interpret these new REW measurements.

The main problem in my room used to be a large null around 50Hz. The FR graph below marked "initial" show this.

First I moved the subs closer to the corners of the room (they used to be aligned behind the mains). After moving the subs I re-run YPAO: the situation had improved a little, but the null was still there.

What seems to have made the most difference, was inverting the phase of the left sub (-180°). The right sub phase has not been touched (0°). The 'distance' set by YPAO has not been changed either. The null is almost gone. 

The LFs sound more clear than before, though over the years I came to realize how powerful self-suggestion is.  From there, I pushed the mains a little closer to the front wall to gain same space, which seems to have helped a little with smoothing mid high frequencies (after a third YPAO run).

Is there anything else that I could have affected by inverting one sub's phase? Is this an accepted practice? I'm not complaining, after that was kind of the point. I am wandering what should I look at next. 

A layout of my living room is attached, along with the original mdat files:

Audio gear:
Speakers: Monitor Audio gx200 mains
Subwoofers: SVS SB12-NSD
Receiver: Yamaha RX-A1000 (pre), Emotiva XPA-3

Measurement gear:
Art USB dual pre
EMM-6 mic (s/n calibrated)
REW 5.0.1b13, Java drivers

Thanks,
Luca


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## AudiocRaver (Jun 6, 2012)

We need to see two separate plots, one for left main + subs and the other for right main + subs.

Thanks.


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## Luca61 (Feb 16, 2012)

AudiocRaver said:


> We need to see two separate plots, one for left main + subs and the other for right main + subs.
> 
> Thanks.


Here they are...


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## Skylinestar (Oct 19, 2010)

I've asked the same question to Brian (maker of Rythmik subwoofer) before. He said changing the phase of one subwoofer (although the subs are equidistant to the listener) is a wrong thing to do. He said if the subs are equidistant, the phase settings must be same. He didn't give me the answer to correct the null though. Some people do agree with him as phase alignment and timing is more important than just a smooth flat frequency response.

Other forumers said the opposite. They said one must vary the phase controls to fix the nulls.

Anyone can clarify this?


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## AudiocRaver (Jun 6, 2012)

First a comment, your left and right channels match beautifully above 300 Hz, nice work on your room to get there. I assume the 10 DB falloff from 100 Hz to 10 kHz is your target. Was it achieved totally with room plus speakers? Are you happy with it? Just curious.

RT 60 looks good at about .3 seconds above 100 Hz.

Back to the subs. Is it correct that the two plots you have provided are with the two subs at the same phase setting? That is what it looks like.

Inverting the phase of one sub may seem to help the bass but is probably also causing a null at the crossover frequency with the main on that side. Plus what seems like better bass that way would be very position dependent at best.

Sub experts will probably recommend that you try a combination of separating the two subs to different parts of the room, like maybe front center and rear center, along with matching their time delays, not phase settings, for best response at the listener position.


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## Luca61 (Feb 16, 2012)

AudiocRaver said:


> First a comment, your left and right channels match beautifully above 300 Hz, nice work on your room to get there. I assume the 10 DB falloff from 100 Hz to 10 kHz is your target. Was it achieved totally with room plus speakers? Are you happy with it? Just curious.


Thanks for taking the time to check this out, AudiocRaver. The room has no acoustic _treatment_ whatsoever at this time. I was thinking to optimize things as far as possible before adding anything else. The two areas where I could realistically add some panels are the ceiling and the middle area of the front wall. 

On the gear side, I am also planning to move to a receiver with Audyssey XT32 (most likely Onkyo 818, 1010 or one of their successors). I have been reading that XT32 should have the upper edge vs. the YPAO room eq. I have now, especially in the LF range.



> RT 60 looks good at about .3 seconds above 100 Hz.


I assume this is a good thing. I'll have to look up RT60... 



> Back to the subs. Is it correct that the two plots you have provided are with the two subs at the same phase setting? That is what it looks like.
> 
> Inverting the phase of one sub may seem to help the bass but is probably also causing a null at the crossover frequency with the main on that side. Plus what seems like better bass that way would be very position dependent at best.


If I remember correctly, the mdats I posted were taken with the -180° phase for the L sub. I distinctly recall that the dip around 50Hz was still very pronounced after just re-positioning the subs. I am positive that it took the L sub phase flip to "fill" the dip.

Incidentally, I also remember playing by setting 0° phase for the left sub and -180° for the right one, but the FR did not seem to improve at all, so went back to L = -180°, R = 0°.

Luckily I kept the measurement for the L channel with the sub at 0° phase: as you can see the 50Hz null is still there. I would not mind taking more measurements in the coming days, if helpful.



> Sub experts will probably recommend that you try a combination of separating the two subs to different parts of the room, like maybe front center and rear center, along with matching their time delays, not phase settings, for best response at the listener position.


I don't really have a good spot for the subs to move to the side walls... In my limited understanding, I thought that time delay and phase were directly related (like to sides of the same coin, so to speak). From your comment I gather that they are actually distinct. 

I suspect that the opening close to the left sub is mainly responsible for the 50Hz null. One thing I could do is install a door - maybe I could try installing some plywood to 'seal' the opening temporarily, and measure again?

Would you mind commenting on the waterfall graph? I see a long remnant around 20 and, to a lesser degree, 30Hz, not sure how worth is this worrying about. The rest does not look so bad? The noise floor was quite high when these measurements were taken around 50, at time 55dB.


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## AudiocRaver (Jun 6, 2012)

Time delay and phase are directly related, for sub setup they allow you different kinds of control. A phase switch helps you match the phase of a sub to a main speaker at the crossover frequency so they are working together and not canceling at that frequency. As subs get moved to different locations in a room, and distance from sub to the sitting position varies, delay adjustment allows the distance changed to be compensated for.

There is a fair amount of low-frequency noise in your measurements, not sure what from, causing those long remnants and making it difficult to see what is going on with the sub itself. It is more obvious in the spectrogram. It could be the rumble from a refrigerator compressor or furnace motor or something like that.

The room seems fairly symmetrical except for that door on the left. It does seem logical that it can be the cause of the difference between left and right sub behavior. Try this: With the left sub running alone, run a sweep to verify the null is still there (it looks like 47 Hz to be precise), then have REW supply a sine wave at 47 Hz. At the listener position you should hear the null (or NOT hear it). See how it sounds in other parts of the room, particularly in the doorway and in the adjoining room/hallway. If it sounds really boomy down the hallway, that tells you it is a contributor. Just for the purpose of seeing what makes a difference, try moving the left sub small amounts until the null goes away at the LP. REW can be monitoring the SPL at the listener position for you while you're doing this. Another idea, just for kicks, gather all the pillows and cushions in the house and pile them in the doorway, block it off the best you can with absorptive material, see what difference that makes. With a little creative detective work, you can figure out the cause of the null and at least have an idea how to attack it.


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