Popular Post El Guapo Posted March 4, 2023 Popular Post Share Posted March 4, 2023 Enjoying Atmos or other formats of immersive music more than a year. Quite pleasant and feel like I can stop upgrading my equipments for a long while, just dig good music. I use ALXO to make FIR filters for multichannel room corrections / bass management. But when listening some very low recording level album like The World of Hans Zimmer I always doubt my system was still incorrect / not properly calibrated because when increasing the volume, the FR just not right and caused music unpleasant. I then keep re-studying documents from Dolby Professional such as Dolby Atmos Music Room - Best Practices for Music Room Configuration, I think I found the culprit -- my incorrect method for speakers calibration. We all know that after done producing the FIR filters, those filters are just based on that SPL when doing log sweep. Like the screenshot below I used -20dBFS signal doing log sweep at 82dBC. Then I sent the -20dBFS pink noise thru convolution for verification. The result was ~70dBC not at the required 82dBC: If I increase the volume to 82dBC... the FR became terrible: My listening environment was not ideal (i.e. have a very sorry room mode / curve). The C-weighted 82dB is not the flat response 82dBC. So I have to change my strategy for speakers calibration.🤔 Here's what I proposed for myself -- change the calibration process to 2-step. Step 1: to make a flat response FIR first: Insert the FIR filters to convolution engine and then use -20dBFS pink noise generated by Dolby Atmos Renderer to adjust the output volume to the required dBC. I know the RTA datum is still uneven but it's just a rough estimate for target SPL 82dBC*. After done SPL calibration for center channel, now it's time to deal with LFE. Based on Dolby documents, the LFE must based on the 1/3 octave RTA datum to adjust the level +10dB (or 89~92dBC if you don't have RTA datum). I used the Step 1's result as reference to adjust the gain for my woofer, from ~70dB increased to ~80dB. Here you see it's not just +10dB to the meter reading directly (82.3 to 92.3 in this case)! The dBC just increased a little. Here's the screenshot from iPhone: Then... Step 2: use the new output volume (cancel the convolution) and do the log sweep again. Produce the FIR filters using Atmos curve: I'm quite satisfied with the step response:😇 Using -20dBFS pink noise generated by Dolby Atmos Renderer to verify the FR: And verify the LFE: Not that bad I think.🤔 Now I have a somewhat correct Atmos target curve at calibrated SPL. All screenshots above I was using iPhone for quick measurement. Here's the screenshot for center channel and LFE from REW: Looks acceptable to me. 😅 After the proposed 2-step calibration process and then back to The World of Hans Zimmer, my subjective opinion is: everything just right, and also apply to all Atmos music from Apple Music. 😃 Hope this would help for our audiophiles who plan to join the immersive world. *Based on Dolby documents the speakers must calibrate at 85dBC, but tolerable to 82dBC for small studio. MFJG and ericcatz 1 1 Link to comment
OCA Posted August 28, 2023 Share Posted August 28, 2023 You may like the Atmos channel sine sweeps for REW linked in this video: Link to comment
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