Frequency, Amplitude, Distance

[bjmsam]

Moderator: Please move this thread to the Room Acoustics and General Speaker Discussions section if that is more appropriate.

The traces below represent measurements taken with a calibrated UMIK-1 mic positioned on-axis at distances of 0.5m, 1.0m, 1.5m, 2.0m, and 4.25m from a speaker against the front wall of a room 8.75m long. Does frequency (and amplitude, in the case of 40Hz) shift due to room response (e.g.- 200Hz) but not shift due to speaker response (e.g.- 1200Hz)?

Here is the spectrogram for a 4.25m trace:

Here is the corresponding SPL & Phase:

Here it is again with Audyssey XT32 Flat engaged:

Insight would be appreciated, as a better understanding of what is going on will help inform a strategy for mitigation.

 

[bjmsam]

Does frequency (and amplitude, in the case of 40Hz) shift due to room response (e.g.- 200Hz) but not shift due to speaker response (e.g.- 1200Hz)?

According to Do Room Modes Even Matter by Ethan Winer, frequency changes with distance for boundary-induced comb filtering (proportional to quarter wavelength distance) and does not change with distance for modes (or for speaker response, presumably). Some of my dashed trend lines appear to be wrong (slope), but it appears my room is dominated by the former rather than the latter. How does treatment differ?

 

[dasdoing]

I think it's just the interaction of the modes + increasing direct/reflected ratio.
I don't think this is the best aprouch to treating.
the modes are clearly visibale in the spectrogram. atack the ringing in the bass first. after bass treatment you atack SBIR

 

[bjmsam]

I think it's just the interaction of the modes + increasing direct/reflected ratio.
I don't think this is the best approach to treating.
The modes are clearly visible in the spectrogram. Attack the ringing in the bass first. After bass treatment you attack SBIR.

...and from the Share your in-room measurements? thread:

Yep… boundary interferences, but also in combination with modes in the bass. To mitigate, there are five things I can think of: 1) optimal positioning 2) acoustic treatment 3) choose speakers that radiate much less energy to the sides, back, ceiling and/or floor 4) augment by overlapping affected area with a sub 5) careful EQ if desired but check the response at adjacent areas and elsewhere in the room; don’t expect simple EQ inversion applied to the in-room measurements at the MLP is all that’s needed to fix/correct the “room” — even if intelligently applied.

The article is speaking about listener boundary interference response. You cannot realistically do anything about that except accepting it as a fact of life.

Small Room and Loudspeaker Interaction has a section on Boundary Loading that seems relevant and consistent with what my speakers were engineered to accommodate (pages 5-7) when placed against the front wall (pages 16-20). Additional treatment for the back of my room should help but will be tricky at those frequencies. I will continue researching options and welcoming specific recommendations (e.g.- DIY Tube Bass Trap and frequency tuning?). Thank you!