Help: SIBR or Room Modes or Anything?

[veeceem]

So, I just bought Anthem STR preamp with has ARC (RC) built-in. By measuring using its microphone, I found out that the 200-400hz area dip alot (~8dB)
I've been trying to measure after difference placement of speakers/seat or removing some acoustic panels.
Below is 3 different setup that I measure, both seem to slightly differ.
No.1: my 1st measurement ever, seat 30cm from wall, speakers ~50cm from the wall behind them.
No.2: seat 92cm from wall, speakers ~25cm from the wall behind them.
No.3: Seat ~50 cm from wall, speakers ~50cm from wall behind them. The acoustic panels at back of seat are removed.
ROOM is 4m2x6m6, speakers placed on the long wall

1/ Of those 3, which is considered better?
2/ What could be the cause of this dip?
3/ Solution to improve?
4/ Can the Room Correction recommended by ARC cure this dip? Or need other treatments?

Thanks

 

[Theriverlethe]

So, I just bought Anthem STR preamp with has ARC (RC) built-in. By measuring using its microphone, I found out that the 200-400hz area dip alot (~8dB)
I've been trying to measure after difference placement of speakers/seat or removing some acoustic panels.
Below is 3 different setup that I measure, both seem to slightly differ.
No.1: my 1st measurement ever, seat 30cm from wall, speakers ~50cm from the wall behind them.
No.2: seat 92cm from wall, speakers ~25cm from the wall behind them.
No.3: Seat ~50 cm from wall, speakers ~50cm from wall behind them. The acoustic panels at back of seat are removed.
ROOM is 4m2x6m6, speakers placed on the long wall

1/ Of those 3, which is considered better?
2/ What could be the cause of this dip?
3/ Solution to improve?
4/ Can the Room Correction recommended by ARC cure this dip? Or need other treatments?

Thanks
View attachment 39487

No. 2 is clearly the best, but they are all mostly within a range that ARC can correct. No. 2 would be easiest on your speakers and amplifiers with room correction applied.

 

[DonH56]

SBIR = speaker-boundary interference response, see e.g. http://www.acousticfrontiers.com/what-is-speaker-boundary-interference/

At 200 Hz it is more likely to be that than a room mode. SBIR causes cancellations when the reflected wave interacts out-of-phase with the direct wave. "Perfect" cancellation occurs at f = c/(2d) where c is the speed of sound (roughly 1127'/s or about 343 m/s) and d is the difference in path length. At 200 Hz, d is about 0.86 m or 86 cm. That is probably a reflection from the wall beside the speakers reaching the listening position though floor (or ceiling) bounce is also possible. If it is wall bounce you could add panels on the walls (absorbers or diffusers, though the latter may be more expensive and larger at 200 Hz) at the first reflection points, but as @Theriverlethe noted a ~5 dB dip should be well within ARC's ability to correct.

HTH - Don

 

[veeceem]

@DonH56 @Theriverlethe
I wonder if ARC's correction of that ~200hz range will make it sounds nearly the same as a "proper" room treatments, such as diffuser/absorber? If that's possible, I think I should just move back to No.2 and leave the rest to ARC
Thanks

 

[Theriverlethe]

SBIR = speaker-boundary interference response, see e.g. http://www.acousticfrontiers.com/what-is-speaker-boundary-interference/

At 200 Hz it is more likely to be that than a room mode. SBIR causes cancellations when the reflected wave interacts out-of-phase with the direct wave. "Perfect" cancellation occurs at f = c/(2d) where c is the speed of sound (roughly 1127'/s or about 343 m/s) and d is the difference in path length. At 200 Hz, d is about 0.86 m or 86 cm. That is probably a reflection from the wall beside the speakers reaching the listening position though floor (or ceiling) bounce is also possible. If it is wall bounce you could add panels on the walls (absorbers or diffusers, though the latter may be more expensive and larger at 200 Hz) at the first reflection points, but as @Theriverlethe noted a ~5 dB dip should be well within ARC's ability to correct.

HTH - Don

Since it gets worse when he moves closer to the rear wall without treatment, I’d think it’s more likely a bounce off of that.

 

[DonH56]

@DonH56 @Theriverlethe
I wonder if ARC's correction of that ~200hz range will make it sounds nearly the same as a "proper" room treatments, such as diffuser/absorber? If that's possible, I think I should just move back to No.2 and leave the rest to ARC
Thanks

It will correct the frequency response but cannot do anything about the root cause -- the actual reflection itself. Whether that matters is up to you; reflections make a room sound more "spacious" but superimpose the room on the recorded response and can change the image (sound stage). Since it is free I would do as you said: move to #2, run ARC, and see how it sounds.

 

[veeceem]

No.2 with a little adjustment. 6 points measurement. Consider a little improvement over the previous no.2?