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Old 8th August 2020
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Help needed, Problems at 70Hz

Dear Gearslutz community,

I have been working on the acoustic optimisation of my room for some time now and in this way i have encountered a problem for which i currently know no solution. Maybe one of you knows what to do? Here are the facts:

The room
-Room dimensions LxWxH: 425x320x260cm
-the two long sides consist of approx. 20cm presumably brickwork
-the two short sides are exterior or load-bearing walls and consist of approx. 40cm of brickwork. The outside wall has additionally approx. 30cm heat insulation on the outside.
-the ceiling is plasterboard, probably with wooden beams and compacted gravel.
the floor consists of wooden planks, probably screed underneath and has a cellar (ground floor).

The target
-Mainly production of music, recording of various instruments via MIDI or DI, -reference listening of mixes/masters and recording of vocals
-Optimized frequency response, reverberation time from approx. 200ms to approx. 200Hz (roughly based on EBU Tech. 3276)
-Getting a grip on initial reflections in the listening position
-However, the room should not sound completely "dead" and be fun to make music in!

The idea
-sort of "Dead End" in the listening position
-sort of "Live End" at the other end of the room for vocal or acoustic guitar recordings

The implementation to date
-Measured the room modes diagonally across the room
-I tested the speakers at different places in the room and roughly determined the best position.
-then I tried to get the bass range under control with porous absorbers. To do this I stacked Sonorock packages in the corners and made measurements with REW in different setups. Except for the longitudinal mode at 41Hz I got most of it under control.
-There was a peak at about 130Hz. The Room Simulator of REW shows me that the octave of the floor-ceiling mode (0-0-2) is approximately there. At 65Hz (0-0-1) it shows more of a dip. So I put an absorber (16cm Sonorock) above the listening position.


The Problem
the ceiling absorber reduced the peak at 130Hz by about 5dB, but now I have a huge dip of 24dB compared to the measurement without ceiling absorber at about 70Hz.

My Guesses
-During my first measurements, from the floor (loudspeaker) to the diagonally opposite corner of the room (microphone), a large dip at 67Hz is already visible. This dip was well compensated by the superchunks in the corners, so it must have something to do with the ceiling absorber.
-In my opinion it can be ruled out that the valley at 70Hz is caused by SBIR, because the speakers would have to stand about 122cm away from the wall for a phase cancellation of 70Hz (Wavelength ~490cm).

-Since I can't hang anything from the ceiling (plasterboard), I put the ceiling absorber on battens at a height of about 2m. Is it possible that the height of the ceiling absorber plays an important role?
-If I play 70Hz as a sinus tone and roll on my office chair towards the back wall, the sound under the absorber is much quieter. The Spectrogram shows that the decay of 70Hz increases again in volume. Could it be that the difference in ceiling absorption between the front and back of the room leads to the valley at 70Hz? Should I therefore also equip the rear part of the ceiling with absorbers?

I have attached drawings, pictures and REW measurements.

I would be very happy if you could help me with my questions.
Thanks in advance and best regards,

schlorian
Attached Thumbnails
Help needed, Problems at 70Hz-img_1467.jpg   Help needed, Problems at 70Hz-bildschirmfoto-2020-08-08-um-19.43.39.jpg  
Attached Files