Floor Question as it relates to Bass

[DJamesGoody]

Hey gents.

I've been setting up a working room with a friend/client..... after quite a bit of working and moving things, etc.... we've gotten to a decent spot. But I made a discovery I had never been aware of. I'll get to that shortly.

In short, the room is exactly 23' long, 12'6" wide, but the first 8ft. of the room is a bit wider, measuring 14' with a 2ft deep, 8ft long closet to the right. The ceilings are 8"1'.

The room is healthily trapped, the ceiling is almost fully trapped, with 4"
(spaced 4") clouds above mix position - all four corners have fully straddled floor to ceiling 6" traps. Reflection zones have 4" panels on the wall.

The front wall (behind monitors) has a large window (8' x 4'), and the back wall has a high window (6' x 3'). The left wall also has a large 8x4 window toward the back of the room.

There is no bass trapping on the back wall, aside from the corner traps.

There is trapping on the front wall behind monitors - 2) 6" panels and 2) 4" panels, as well as the full 6" corner traps.

The closet is not trapped at all. We measured the response at its flattest with the one closet door slid open.

Okay, so...... the biggest dip we have is -10db at around 53hz. The biggest peak we have is about +8 at 134hz. We've done our damnedest to get rid of both, to no avail. We've moved speakers a bunch, position, etc..... this is the best we could get it for the moment.

The discovery I made though, was that when I lowered the measurement mic near the floor at listening position, the bass flattened out considerably. In fact, it was damn near perfect, with maybe 5-6db variation between the previously mentioned null and peak.

Is that a normal phenomenon? I've never measured down there in any room, but I did it for giggles, and lo and behold, the thing looked good.

And finally, if this is normal (or abnormal), is there anything in particular that it indicates, in terms of what to do for additional treatment?

[SAC]

Just a general comment... Modal response will vary with measurement position.

You can evaluate a room from two perspectives (boy, am I simplifying this! )...
You can evaluate the overall room's response, gaining an indication of the overall 'global' modal response and achieving a general mapping of the room with regards to the modal behavior, and//or you can evaluate a given spot for its behavior relative to the local modal activity.

One local spot may be 'within' a node or anti-node or it may not, as you have discovered. Such information is valuable in siting your listening position such that it is in the most neutral response region. And if one is able to locate such a neutral region that is convenient for listening (as I suspect laying on the floor with your head near the floor may not be the most comfortable or convenient position (just my guess, mind you!) you will be fortunate to accomplish by location, what treatment otherwise attempts to mitigate.

[nixoblivion]

Originally Posted by SAC

One local spot may be 'within' a node or anti-node or it may not, as you have discovered. Such information is valuable in siting your listening position such that it is in the most neutral response region. And if one is able to locate such a neutral region that is convenient for listening (as I suspect laying on the floor with your head near the floor may not be the most comfortable or convenient position (just my guess, mind you!) you will be fortunate to accomplish by location, what treatment otherwise attempts to mitigate.

Hey SAC, how do you think its best to find the Sweet spot + loudspeaker position?

Is it better to choose a good sweet spot (38% springs to mind) then try different loudspeaker positions to find the best and then try a bit with the sweet spot?
Or do you think its better to choose a loudspeaker position, try around with the sweet spot until its perfect and then try to loudspeakers again?

[DanDan]

I suggest that it would be very useful to find where the anomalies lie.
Play a sinewave at around 50Hz. Tune it to the room exactly. The resonance should be very apparent. By walking around, listening or using an SPL meter, find the peaks and nulls. This may point to a better listening position, or a best location for more trapping. The distance between your speakers and the walls and floor can be usefully tweaked also. Check out Wall Bounce Calculator 2D
by Thomas Barefoot, for a virtual look.

DD

[SAC]

Dan is right.

You use measurements to determine the locations of the peaks and nulls, and you empirically verify them.
In other words, you deal with reality rather than assigning primary importance to abstract idealized guidelines.

The abstract rules of thumb are great for abstract general guidelines, but like with mode calculators, they assume a footnote role when you have access to the real space and real tools to determine the real problems.

So, like DD said, measure and determine the actual location o the modal distribution within the room, and verify this empirically - as it should be pretty obvious. Then use the real distribution within the room to determine placement such that symmetry and the mitigation of modal issues, coupled with appropriately placed bass traps and the subsequent establishment of an effective initial signal delay gap (ISD/'RFZ') for the control of the earliest reflections.

And when you are finished with that, then you can work on building a well behaved diffuse early reflection soundfield as appropriate for your intended use.