Dip at 138 Hz, HELP

**leifl** · **10-23-07, 02:25 PM**

Hi there,

a small problem: there is a huge dip at 138 Hz in my otherwise not so bad curve. The dip is room-dependent and I haven't been able to figure out what to do with it. The dip is very audible when listening to sweeps when measuring. Frequency sweeps are not really my favourite music, but even with 'real' music there seems to be something missing around 138 Hz.

Any suggestions?

The attached graph shows the subs, the mains and the subs + mains combined. I haven't tweaked the subs responce all that much yet, getting a flat curve is not too hard in my room. The dip causes me more headache.

BTW: the room is very lightweight, all walls, the ceiling and even the floor ( the room is in the attic) is made of tree. I still suspect (or rather know) that the dip is room dependent, but what to do about it?

Regards

Leif/Finland

**brucek** · **10-23-07, 03:09 PM**

Yeah, that's a fair size dip for sure. I guess I can only suggest moving the mains around, or perhaps the listening position. Have you done a mains only sweep with each main separately to see if it's one or the other or both in concert that causes it?

brucek

**10-23-07, 04:01 PM**

you can change the room dimensions, the listening position, or get an audio control DIVA to account for that as best you can...

**10-23-07, 04:26 PM**

I recently flattened a persistent peak/valley in my room response by starting with the speakers against the front wall, spread to the width required for good stereo imaging. (subwoofer was disabled)
Measure.
Moved them 2 inches forward.
measure.
etc... to a point as far into the room as WAF would allow.

I found a juxtaposition between approx. 60hz and 100hz, wherein as the response dropped at one it would rise at the other. I simply placed the speaker in the zone that produced balanced output at both frequencies.
The resulting response with the subwoofer dialed-in is now an almost perfectly smooth "house curve" (about 2-3db/octave clockwise) from 200hz down to 40hz, with a 6db(relative to reference) peak at 30hz and a rolloff to reference level at 20hz.

The bass sounds very natural and transparent, but can still shudder the air in the room very nicely if called for.

**leifl** · **10-24-07, 12:42 AM**

Hi there,

thanks for the advice. I can't change the room, in fact it's a dedicated home theatre room! The room is symmetrical and thus the responce from the left and right speaker is more or less identical. I'll try moving the dipoles around a bit. I'm a bit reluctant to move them a whole lot, they give me a very good depth at their present location.

Some pictures (not 100% up-to-date) of the room can be found here.

regards
Leif

**JohnM** · **10-24-07, 05:00 AM**

Dips in that region can often be caused by cancellation from the floor bounce, or from a ceiling bounce. If measuring a foot or 2 further forward or backward gives a similar response (perhaps with a small shift up or down in the frequency of the dip) that may be the problem, in which case a thick rug on the floor may help.

**clubfoot** · **10-24-07, 01:05 PM**

Nice setup. You seem to have enough "breathing" room for the dipoles, but you do have a llot of hard reflective surface. Try moving your seating location in small increments, measure.... add a larger thick rug or carpet as John suggested,...heavy thick drapes behind the screen and/or listening position too.

**leifl** · **10-24-07, 02:17 PM**

Dip cured!

The problem was a cancallation of the rear wave from the dipoles. Moving the dipoles only changed the frequency of the dip. Same thing when moving the listening position.

Adding a total of 4 acoustic panels (thickness 2") the dipoles did the trick. The first panel is 4" from the front wall in the corner behind the dipoles, the second panel is 4" in front of the first panel.

This is the effect of the panels, red = with panels, blue without panels. The audiable impact is less noticable that you might expect, but I imagine that there is a bit more body in the sound now.

**brucek** · **10-24-07, 02:36 PM**

Impressive what the panels did..........

I have to agree with clubfoot - nice setup.......

Are the panels where the DVD rack is located?

brucek

**leifl** · **10-24-07, 03:05 PM**

Hi,

the panels are located in the corners behind the dipoles, actually right in front of the IB subs. The subs don't mind, they're not affected by the panels at all. The panel's WAF is not high at the moment, have to see what can be done about that....

**clubfoot** · **10-24-07, 08:12 PM**

Nicely done! A good fabric store is your friend,...check for speaker cloth, that's what I used to cover my DIY panels on my front wall.

**cynical2** · **10-24-07, 09:01 PM**

I don't know anything about fixing your "dip" (which it appears you've taken care of), but I did want to say that you have a beautiful HT!

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