How do you use the Rat Shack SPL meter?

Discussion in 'Speakers' started by EdNichols, Jul 17, 2003.

  1. EdNichols

    EdNichols Second Unit

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    I just bought the meter and the instructions say to point the microphone at the source. But my surround speakers are to the side so I don't think I will get a true reading from them. Should I just point the microphone stright up so the sound will be received equally from all the speakers? Also, I know the meter has a certain amount of error in the lower HZ range and I have seen a table to allow for the error when using a the AVIA disk. Does it also apply when using the test tones? To get the sub. to equal the rest of the speakers I have to lower the output quite a bit so should I be allowing for some increase in DB? Any suggestions?
     
  2. DanaA

    DanaA Screenwriter

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    This is the recommended method as per the Home Theater Basics section of the forum.

     
  3. EdNichols

    EdNichols Second Unit

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    Thanks for the info.
    Now for my second question, is there an allowance for error on lower hz when using test tones as there is when using a test disc?
     
  4. Bob McElfresh

    Bob McElfresh Producer

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    Ok - there IS a conversion chart for the Radio Shack SPL meter to compensate for it's in-accuracies to what a standards lab would call XX db.

    But you are NOT trying to measure the exact SPL. You are using the meter to compare the SPL from 6 speakers. While the meter is in-accurate from a standards point of view - it's got the same problem for all the speakers. So it works fine for COMPARISON.

    (and I can't find a post with the conversion table right now

    [​IMG]
     
  5. EdNichols

    EdNichols Second Unit

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    Thanks, I have seen that chart. I will try to hunt it down. But I was thinking it only applied to the lower HZ since the meter had a larger error rate in that area. I will find the chart and see!!!
     
  6. Jeff Gatie

    Jeff Gatie Lead Actor

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    Someone asked about the correction factors in Basics and how they applied to "pink noise". The answer to this also applies here:

    Pink noise is a sum/average of all frequencies. Most test tones are made of pink noise. The Rat Shack meter correction tables list corrections for specific frequencies (i.e the meter may be -5dB at 25Hz and +2dB at 18 Hz, etc.). Since pink noise is composed of all frequencies - which correction factor are you going to use? The answer - none of them. Vince Maskeeper did a test on this effect and found the correction values really mean nothing when adjusting via pink noise. Just adjust to the proper level on the SPL meter and forget about the correction values.
     
  7. EdNichols

    EdNichols Second Unit

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    Thanks Jeff!! That makes it a lot easier.
     
  8. Edward J M

    Edward J M Cinematographer

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    Unlike the surround speaker calibration tone, the subwoofer rumble tone is obviously comprised of a bunch of bass frequencies, largely grouped between 30-45 Hz.

    Since the RS meter is C-Weighted, one only needs to look at the C-Weighted frequency response curve to realize that any C-Weighted sound pressure meter will read about 2-3 dB below the actual SPL for any frequencies grouped in the 30-45 Hz region.

    A good rule of thumb is to assume any C-Weighted SPL meter will read about 2-3 dB low on the typical subwoofer calibration rumble tone.

    The RS meter Correction Factors correct for both the C-Weighted Scale and for inherent inaccuracies of the meter. These inherent inaccuracies start to become significant below 25 Hz.

    Here is a chart of the C-Weighted Correction Factors vs. the RS Correction Factors. You can see they are about the same down to 25 Hz, and then they start to diverge significantly.

    Regardless, at a minimum the C-Weighted Correction Factors must be applied to even the highest quality SPL meter if it is C-Weighted. The only time you would not use any correction factors for the bass region would be if your equipment is designed to provide a true unweighted response and is rated flat down to at least 10 Hz.

    Frequency / C-Weighted CF / RS CF
    10.0 / 14.3 / 20
    12.5 / 11.3 / 16.5
    16.0 / 8.4 / 11.5
    20.0 / 6.2 / 7.5
    25.0 / 4.4 / 5.0
    31.5 / 3.0 / 3
    40.0 / 2.0 / 2.5
    50.0 / 1.3 / 1.5
    63.0 / 0.8 / 1.5
    80.0 / 0.5 / 1.5
    100.0 / 0.3 / 2
     
  9. EdNichols

    EdNichols Second Unit

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    Edward,

    If I understand your chart correctly, the rumble tones of the sub. will be between 30-45 hz and the meter will show them to be approx 2-3 db below actual?
     
  10. PaulT

    PaulT Supporting Actor

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  11. Edward J M

    Edward J M Cinematographer

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  12. Arthur Vino

    Arthur Vino Stunt Coordinator

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    thread saved in bookmarks..
    [​IMG]
     

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