RTA - Frequencies below 15.75 Hz - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 4 Old 06-18-15, 01:30 PM Thread Starter
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mojave
 
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RTA - Frequencies below 15.75 Hz

The Spectrum and RTA graphs in the RTA Window don't show any data below ~15.75 Hz. Is it possible for this to be extended lower with a few more octaves?

Here is why I am asking:
  • I'm able to route movies directly through REW using my MOTU 1248 AVB mixing panel.
  • I used an 80 hz crossover with a 24/db per octave slope for all channels (to match the THX filter and speaker sum).
  • I routed the subwoofer channel to REW's input digitally using the MOTU mixer.
  • I used REW with ASIO input to receive the subwoofer signal.
  • I set levels in REW per Blue Sky's http://abluesky.com/support/blue-sky...on-test-files/ test tones and methodology to make it easy to reproduce. The REW SPL Meter was set to C weighting and slow.
  • SPL by Frequency - I used the REW RTA in RTA 1/48 octave mode. With an FFT length of 131072, each FFT bin is only .18 Hz wide. I also used flat top windowing to provide accuracy to .01 dB. Averaging was set to None to accurately track the peaks.
  • SPL by Time - I used the REW SPL logger with the SPL Meter set to Z weighting and fast. From the manual, it looks like this updates every 125ms.

I want to see the SPL at various frequencies and where the peak SPL occurs. I also want to know the peak SPL requirements for playback at Reference Level. For a test, I used a clip from War of the Worlds. I simultaneously ran the SPL logger and the RTA to get SPL by Frequency and by Time. The SPL Meter provided peak SPL. Below are the graphs. For accuracy, I would like to record as low as possible and not be limited to ~15 Hz.





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post #2 of 4 Old 06-18-15, 04:53 PM
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Re: RTA - Frequencies below 15.75 Hz

The spectrum will show data down to the FFT bin width or 2 Hz, whichever is higher.

The RTA is limited in part because at the lowest frequencies the RTA bins become smaller than FFT bins (for example, at 15 Hz 1/48th octave spans 0.2 Hz). A pseudo RTA mode is possible by scaling FFT bins according to relative width when they drop below the RTA bin width, but it gets awkward for shorter FFTs so I picked a reasonable lower bound that works for all FFT lengths. Displaying lower frequencies in RTA mode is one of the things on my TODO list but it's not near the top.
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post #3 of 4 Old 06-19-15, 12:12 PM
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Re: RTA - Frequencies below 15.75 Hz

Now this is something I had never really thought about, sound below 15hz. Very interesting.

Good Listening

Jack

"For those who believe no proof is needed for those who don't believe no proof is possible"
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post #4 of 4 Old 06-20-16, 01:37 PM Thread Starter
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mojave
 
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Re: RTA - Frequencies below 15.75 Hz

Thanks for adding this in V5.15. Works great!
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