Peaks and dips/nulls. - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 8 Old 08-22-2017, 10:34 AM - Thread Starter
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Peaks and dips/nulls.

My room is now sealed off from the rest of the house and i wanted to see how my subs measure.

For the most part it was very managable, but one interesting thing was this....

I have one futon until i get a new couch. It has room for about three people. Across the entire three seating area's there's a peak at 27hz, no matter which seat you measure from , no mater if you drag the mic across its as steady as can be, never higher never lower, always about +9db, about a half octave wide. There is tons of room gain, no rolloff , all the way down to 10hz.....

There is also a 60hz dip, that actually goes from a subtle dip at one seating position to a true Null at certain points, and every variation inbetween.

It seems that the 27hz peak is everywhere and solid, very easy to control with my MiniDSP, i built a filter and lowered the peak and it was taken care of. The Null is a different story, i think its a positional issue of the subs and the seating area. Your thoughts.
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post #2 of 8 Old 08-22-2017, 11:18 AM
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Hey KG, I am wondering if the 60hz dip is a reflection issue off of your glass patio door since the opposite side is solid wall (with some treatments if I remember correctly?). Can you play with front to back movement of the couch and see if anything changes? I had concerns about where your couch was placed in relation to the front and back boundaries of the room since I struggled with a similar null in my last house.
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post #3 of 8 Old 08-22-2017, 12:11 PM
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Could be a lot of things...

Room/placement.

Cancelation if you have more that one sub playing the same range, again a room/placement/phase kind of thing.

Cancelation at crossover between sub/s and mains.

Natural response anomaly of the sub/s.

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post #4 of 8 Old 08-23-2017, 11:55 AM - Thread Starter
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Quote:
Originally Posted by dtsdig View Post
Hey KG, I am wondering if the 60hz dip is a reflection issue off of your glass patio door since the opposite side is solid wall (with some treatments if I remember correctly?). Can you play with front to back movement of the couch and see if anything changes? I had concerns about where your couch was placed in relation to the front and back boundaries of the room since I struggled with a similar null in my last house.
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Originally Posted by emilime75 View Post
Could be a lot of things...

Room/placement.

Cancelation if you have more that one sub playing the same range, again a room/placement/phase kind of thing.

Cancelation at crossover between sub/s and mains.

Natural response anomaly of the sub/s.
It was interesting that one mode, the peak, was everywhere, and constant. Very easy to fix, and the fix was just as constant.

OTOH, the null, is very inconsistant. There are locations that a +6db filter raises it up where the response is "fixed". Two feet to the right the null is just that, a total blackhole. I did not leave that filter inplace, it was too inconsistant. I'll be play'n around with placement this weekend. Incase you are interested the center seat had a shallow dip, the seats to the left and right, were narrow blackhole nulls......

It seems like the peaks are easy, consistant from position to position, the nulls or dips are very different.

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Last edited by kgveteran; 08-23-2017 at 12:42 PM.
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post #5 of 8 Old 08-24-2017, 05:56 AM - Thread Starter
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My room is 13x19 with a vaulted ceiling. I sit about a quarter of the way in from the back of the room ( front and back width is the 13' walls ), i can move a little forward but not back, my (4) subs are all up front, a quarter of the way in from the front, they can come forward, but not back either.

So far this is what i obseved while using my OmniMic. Below 40hz, the FR does not change at all inregards to my three seat LP, even if i hold the mic and move through the seating position. Low frequency measuring is much more forgiving that above, so static measuring using the mic stand is out for now. Its almost like the mic isnt even moving, like a real steady pressure at the entire LP, but not flat, it has a +7db rise which is very consistant too, and responds to me EQing with my MiniDSP.....We all know nulls are nulls, cant be EQ'd out.....

Above 40hz to 100hz, the response changes foot by foot, center of the LP flatter than the two outside positions. It ranges from a dip to a true null, that cant be fixed other than sub location.

What is this dividing point im seeing

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post #6 of 8 Old 08-25-2017, 11:44 AM
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Well, if the 27 Hz peak is constant left to right, it follows that it's caused by either a length mode (front to back) or a height mode, since those distances remain the same while moving the microphone along the seating position. For the same reason, the null is most likely caused by a width mode.

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post #7 of 8 Old 08-26-2017, 08:45 AM - Thread Starter
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Because i am fortunate enough to have a dedicated room, four subs, four midway points on all four walls, i tryed that config this morning. There was no real null, couple of dips, and still that 27hz peak, the response looks great, i'll post a graph when im done.

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post #8 of 8 Old 08-26-2017, 11:21 AM
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Extremely common. 27hz is a room mode as mentioned above. 60hz is a placement issue. I would guess the sub is by the mains and often theres a null somewhere bn 60 and 80. Move the sub or your futon to fix 60....pull down 27hz that will cause no issues
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