What's going on here?

[89-300ce]

I just ran this test last night and have no experience with reading a RTA.

The hump betwen 2k - 8k im guessing is horn hump. I can fix that, but what is going on on the bottom?

I've tried moving the speakers about, changing the porting, and opening doors with little effect.

Would this be the room, the cabinets, or the crossover, or am I running the tests wrong?

The room is 22 x 14 x 8 with acoustic ceiling, wood paneling on studs with fiberglass insulation, and a short commercial style rug.


Jorg

ps. For this test the box is tuned for 35Hz.

[Hoerninger]

Some detailed information about your the speakers would be helpful:
homebrew, chassis, network in detail ...
position of mikrofon ...
___________
Peter

[89-300ce]

Homebrew.

JBL K-145, Altec 511b with BMS 4552nd, and homemade Altec N1201 crossover. The box is 4.8 cu.ft MDF tuned to 35hz, 26" wide x 22" high x 15.5" deep ID's. I think the box is adequately braced, but there is minimal fiberglass stuffing, 18x18x2 suspended verticaly just behind the driver.

Microphone is on a boom tilted down (off of a tripod that is too high) to get it at ear height, and pointing at the centre between the speakers.


Jorg

[Hoerninger]

Wondering whether the network is a good choice, there seems to be hardly an improvement except for high passing.
The room seems to be dominating, others may judge the bass tuning.


___________
Peter

[JBL 4645]

Was the microphone pointing downwards, instead of upwards at a 45° angle?

[89-300ce]

I should mention that for this test I was running R3 at 0 ohms (full bypass), and R1 at -2db.

I was planing to incorporate a fixed lpad before the comression driver

http://www.audioheritage.org/vbullet...03&postcount=2

to further attenuate the horn and deleting the R5 resistor entirely.

Jorg

[89-300ce]

Was the microphone pointing downwards, instead of upwards at a 45° angle?

Yes, down, but not quite 45 deg.

Jorg

[Andyoz]

Looks like room mode behaviour to me below about 150Hz.

The large "suck-out" around 80Hz may be due to boundary interference effects, i.e. phase cancellation between the direct sound and the reflected sound off the rear wall or floor. Is the distance from the woofer to the floor about the same as the distance from the rear wall. Try moving the speakers up against the rear wall and the centre frequency of the dip should move a bit.

[89-300ce]

Looks like room mode behaviour to me below about 150Hz.

The large "suck-out" around 80Hz may be due to boundary interference effects, i.e. phase cancellation between the direct sound and the reflected sound off the rear wall or floor. Is the distance from the woofer to the floor about the same as the distance from the rear wall. Try moving the speakers up against the rear wall and the centre frequency of the dip should move a bit.

Yes, the centre of the woofer cone is about 18" off the ground and about 18" from the wall. I have tried moving them out a bit (6") but didn't see a big change. I can't move them back much. I will try again tonight and look closer for any change.

Jorg

[Andyoz]

At a guess, I think the suck out around 170-180 Hz correlates better with the 18" speaker to wall and floor spacing.

The suck-out at 70-80Hz would be consistent with the speakers being about 4 feet from the side wall, with the right speaker being further away from the side wall when compared to the left speaker.

As with anything, this could be completely wrong.

[89-300ce]

At a guess, I think the suck out around 170-180 Hz correlates better with the 18" speaker to wall and floor spacing.

The suck-out at 70-80Hz would be consistent with the speakers being about 4 feet from the side wall, with the right speaker being further away from the side wall when compared to the left speaker.

As with anything, this could be completely wrong.

That's a pretty good guess. The right speaker is also about 4 foot from the side wall, but the large opening to the fireplace might make it seem like the wall is further back.

So, if I can confirm this to be true where does that leave me? The Tact fixes all of this, but it's just borrowed from a friend and my guess is he will want it back . Should I be looking at diffusers to break up those reflections? The wall paneling actualy flexes when pushed on.

Jorg

[89-300ce]

Was the microphone pointing downwards, instead of upwards at a 45° angle?

I guess I'm a little slow. What is the correct orientation of the microphone?

Jorg

[Andyoz]

Not much that can be done unfortunately. Diffusion won't work at these frequencies. Also, trying to correct for it electronically causes more problems than it solves.

I guess this shows why soffit mounting is so good for the low-frequency response. These boundary interference effects no longer apply.

Post some plots with the spekers moved say 2 feet closer to the side wall. Tracking the effects won't be as simple as it sounds as the speakers will interact with the room modes in a different manner with every relocation.

[89-300ce]

Post some plots with the spekers moved say 2 feet closer to the side wall. Tracking the effects won't be as simple as it sounds as the speakers will interact with the room modes in a different manner with every relocation.

Nuts! I'm stuck between a wall and a wall unit.

It took me a while to figure out that as I move away from the side walls I'm getting closer to the wall unit, which just amplifies the 60Hz problem again. If I try to move away from the back wall the hump at 120Hz starts to climb.

I'm guessing the dip at 800-1000 is the woofer coil and cap. I'm going to try removing some capacitance to see if I'm on the right track.

I'm starting to get a feel for it, through trial and error. Thanks for the heads up on the walls.

Jorg

[Robh3606]

Those are your raw room curves?? They look pretty good. Have you tried the correction?? Do you know what the smoothing is and what it uses to measure MLS or Sine?? This is a system curve for my current DIY set-up. With smoothing of course. One is Sine the other MLS. The measurements are reasonably close.