A new toy, and a call for help, in room freq sweeps

[johnsonad]

Well I'm the proud owner of a Dayton Omni Mic v2 and I put it to use right away today.  I'm not sure how to use it correctly but man is it ever easy to use.  I set it to 1/12th octave and made various sweeps on each speaker (center of the drivers at 1 meter) and in my listening position.  This thing is nearly effortless to use and gives real time response.


Now for the help.... In the photo below are two sweeps of the room.  The one in red was before the first speaker move and the blue sweep is after moving the speakers 3 inches forward.  I'm going to work form the blue sweep.  You can see there is a HUGE dip from 50 Hz to 62 Hz with the low point 56.3 Hz (67.5dB).  I set reference of 1 K at 83.2 dB.  At one meter from each of the speaker this dip is not there.  We moved the mic a full 6 feet back from the position the blue sweep was taken (listening position) and the node was still there but to a lesser extent (5 dB).  The rest of the sweep is pretty flat, plus or minus 5 dB which doesn't seem bad to me given a less than perfect room. We moved the speakers an additional 3 inches forward and this didn't help.

My options are moving the speakers further into the room (not ideal), or moving them closer together which I will try tomorrow and post new sweeps.  I would appreciate any expertise you may have in this area (I know there are some engineers around here).

I apologize for the resolution of the photo. I'm having a hard time getting it close to the 512KB limit with the iPhone.

Thanks,

Aaron

 

[Paul Birkeland]

How about toe in?

Are your running these with a powered sub? 

Is the room roughly enclosed?

 

[Grainger49]

I think you are on to something.  The suck out at about 58 Hz. can be made much less with proper speaker positioning.  You are going to have to try it with the speakers more into the room to find out if that is the answer.  I have an old program that shows me the theoretical bass response (only) for standing waves.  It has been right about all speaker positions for the last 10 years.

I don't think closer is going to help, moving backward will probably deepen the dip. 

 

[Doc B]

A few thoughts -

Measure one speaker at a time. That will help to determine which speaker might be the worst offender, particularly if the room isn't symmetrical.

Move the speaker more like a foot each time and remeasure - from the listening position. This can help you spot a trend in the position vs. room modes.

If you can't move the speakers any farther, try moving the listening position.

 

[johnsonad]

How about toe in?

Are your running these with a powered sub? 

Is the room roughly enclosed?

1. Massive toe in, the speakers are pointed at the listening position (i'll post some numbers later)
2. No sub, corner loaded thus that low extension (impressive to me)
3. Yes and no? Speakers near the rear wall of what should be a dinning room facing into the living room, with two side openings on one wall into the kitchen and entry way.

 

[johnsonad]

Alright, an update after a few hours of fiddling with it.  First off, the mic is an amazing tool and it helps but really it gave me the courage to get in there and really move these speakers around.  Before owning this tool I wasn't sure if what I was hearing was right or wrong as I'm not acoustically trained.  With the mic I can move, measure, listen and repeat without fear of loosing a sweet spot I may have had before.

Below you will see two photos.  One with a blue only sweep; this is what I ended up with at the end of the day.  The second has a black sweep added; this was my last sweep from yesterday.  Today I moved the speakers from 3" to 14" in from the side walls, 18.5" to 24" from the back wall and 6" to 13" of toe in.  The listening chair moved from around 90" to 130" from reference and I measured over and over and over..... For each I moved, measured and listened to the same music tracks at the same volume.  There were times when the mic said it was good but my head said no as it wouldn't image or the speakers were much too far apart.  What I ended up with doesn't solve my 50 Hz range node.  No position I moved the speakers to took care of this or moved it much at all (Dan, you are right, I need to move them much futher into the room).  I simply don't have enough room to move them in my limited listening space.  What I did find was a very happy medium as far as listening goes; this is the blue curve.  It's not ideal for everyone given the nearfield position but it really does sound great!  Horns can be painful and in this position I can listen stress free. 

Ideally I need to move these speakers much further into the room.  That isn't going to happen as it's a compromised space to begin with.  I thankfully have honest data that is repeatable and not many can say this.  I have a better understanding for my systems weaknesses and know that I can't compensate for them without DSP (digital signal processing).  This may be an option in the future but for now, I can live without that node of frequencies. 

I'm going to stick with this position for a few days and try it again.  I feel silly in a way waiting so long to purchase a tool like this.  It really should be available to every person who wants a good sounding system.  We spend silly amounts of money on gear trying to comp for signal changes and sound.  What I found was that a matter of a 1/2" can make a huge change in the signal response and my overall listening pleasure.  If it's within your budget to pick up a tool like this ($299 from Parts Express with a mic stand and free shipping, no affiliation of course) or something similar using free software I highly recommend it.  It's nice to compare gear but in the end it all comes down to the sound the end user is enjoying

I still would like help on that node but am limited on speaker movement.  Anyone have experience with DSP or a room corrected powered sub?

 

[2wo]

I don't know much about this stuff but all in all your room looks pretty good...John

 

[sl-15]

this is just a wild guess but have you tried moving other stuff around in your listening room other than your speakers or the listening chair? maybe there is a mirror, picture frame, curtains, coffee table etc. that reflects and or absorbs sound waves that would result in that dip around 50 hz. i mentioned this in another tread and there is no affiliation but i would recommend looking into jim smith get better sound. he has some good ideas about how to setup your room and gives some great tips and a good overview about what changes your sound. good luck, best, stefan

 

[ironbut]

Hey Aaron, just stumbled onto this thread but here's my theory/guess/daydream.

I was wondering what the curve looks like for each speaker?
You mentioned that the freq dip goes away when the mic is placed close to each speaker.
Considering what a huge dip it is (more like a notch filter than some room interaction), I started wondering if it could be something intentional?
The notch is, after all, right where someone might need to filter out hum (60Hz in the US and 50Hz and many other countries).
So,.. that led me to wonder if a matched pair of speakers might have a phase reversal notch on one of the two speakers?
And since it wasn't uncommon for these speakers to have been used in commercial installations (theaters etc), could it be that someone installed a circuit to head off any hum issues?
So, if the phase could be reversed right at the mean of 50 and 60Hz, any hum would be reduced as long as both speakers were used (which would explain why a mic very close to the speaker wouldn't exhibit the dip and each speaker alone wouldn't have a dip either).

Just my WAG of course but in the days when these were new, modifying gear in house was pretty common.

 

[johnsonad]

Hi Steve. They do flatten out the further you pull them into the room but I don't have enough space to get them flat. The curves in the factory manual look great. Interesting reading though and I wonder which speakers had notch filters such as you described?

Regards,

Aaron