Is something wrong with my SPL meter?

Discussion in 'Beginners, General Questions' started by Eric Polesovsky, Jul 19, 2003.

  1. Eric Polesovsky

    Eric Polesovsky Auditioning

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    I just picked up a Radio Shack analog SPL meter and I have a few questions (and or problems).
    For example: When I set my meter to "A" weighting, I'll get a reading of roughly 60 db in my theater room. However, when I jump to "c" weighting, the needle will peg to the right. Why is there a large gap between A and C weightings on the meter?

    Also, when I switch to "c" weighting, and set the meter to 70db, I cannot calibrate at this level because the needle is pegged to the right. I can only calibrate at 85db. How can my quiet basement theater be reading over 75db with no test tones playing?

    Could there be a problem with the meter?
    Thanks for any help.
     
  2. Cees Alons

    Cees Alons Moderator
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    Eric,

    What does the booklet (manual) say?

    I'm not familiar with your meter, but most of them have (at least) four settings: "high" an "low" sensitivity, "calibrate" (could that be your "c"?) and "battery". The last one only to check if the battery is still charged enough.

    "Calibrate" does not really measure sound levels, but is used to set the meter nicely given the (remaining) voltage of the battery.

    Only "high" and "low" are for measurements (perhaps "A" and "B" on your meter?)

    But then again, this may not apply to your meter at all.


    Cees
     
  3. Michael Reuben

    Michael Reuben Studio Mogul

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    The difference between the "A" and "C" settings is normal. "C" is the setting used for HT calibration in most situations.

     
  4. JerryCulp

    JerryCulp Stunt Coordinator

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    Wiped out my post...
    Re-reading the thread, I think either your meter is bad or you are getting some low freq noise from another source, washing mashine, furnace, etc...
    Try taking the meter to another place and test and see if the readings change. If not, return it.
     
  5. Bob McElfresh

    Bob McElfresh Producer

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    Short Answer: Use the "C" setting, and get your test-tones to produce 75 db, not 85 db while calibrating.

    There is a difference between the way a machine and a human can perceive loudness. The "A" setting tries to report SPL as a machine/instrument/microphone could. Useful in a sound/recording studio or laboratory.

    The "C" setting takes into account how a human perceives sound. I think the shape of the human-ear is one of the things compensated for and that's why it is called "C-weighting" - the human ear looks roughly like the letter C. (Dont quote me on this one [​IMG] )
     
  6. brentl

    brentl Cinematographer

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    Hey Bob says that "the human ear looks roughly like the letter C. "

    Sorry had to.

    Brent
     
  7. David Ruggiero

    David Ruggiero Stunt Coordinator

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    "A" weighting primarily measures frequencies in the range of 500-10,000hz. "C" weighting measures frequencies from 32-10,000, so essentialy "A" would be leaving out most of your low frequencies, such as those produce by a woofer or subwoofer, and "C" would add them in hence the reason why "C" is louder. It is measuring more sound. You should use "C" to calibrate your home theater. This info can be found on page 11 of your spl meter manual.

    If you are getting over 70 on the meter before the test tones you must have something in your basement producing low frequency noise. If not your meter is defective.
     
  8. Wayne A. Pflughaupt

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    Expanding on what David wrote:

    In every room where the sound is well-balanced - the lows, mids and highs blend well, with nothing apparently exaggerated or lacking - there is a natural, upward tilt from the higher frequencies to the low (i.e., the low frequencies are at the top of the rise).

    Put another way, although in-room response sounds flat, it does not measure flat. The higher frequencies will register lower SPL readings, while the lower frequencies will register higher readings - say, 65dB at 3,000Hz, and 73dB at 50Hz.

    Since the lower frequencies have higher SPL readings, that is why the numbers “go off the scale” when you switch from A to C weighting.

     
  9. Eric Polesovsky

    Eric Polesovsky Auditioning

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    Thanks for all the help.
    When I take the meter out of my basement (my theater room) I get different readings on the C weighting. I don't believe now that there is a problem with the meter.

    There must be a source of low frequency sound in my room that is causing the meter to read higher. (I still don't know what)
    My AC is off, computer is off, and nothing is playing.

    Maybe the hard walls cause some strange reflections or sounds.
    Any ideas why I can't calibrate at 75 db?
    Or should I just go to 85 db and not worry about it? I use Avia for test tones.

    Thanks again.
    Eric
     
  10. Nathan*W

    Nathan*W Screenwriter

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    You may want to pick up the "Video Essentials" DVD, it walks you through a calibration specifically using an analog SPL meter from Radio Shack.
     
  11. Jeff Gatie

    Jeff Gatie Lead Actor

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    Avia'a test tones are 85dB at reference. In order to calibrate to true reference, you should be calibrating Avia's tones to 85dB. VE's and most receiver tones are attenuated to 75dB, hence the confusion.
     
  12. JerryCulp

    JerryCulp Stunt Coordinator

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    Have you tried using the SPL like a divining rod? Walk around the room and see the noise gets higher and track it down that way.
    My next guess would be plumbing. A slight leak (or just a trickle of running water) in the plumbing can cause a rumble in the pipes. Recently we had a flapper go bad in a toilet and it made a low freq noise that resonated through the house. The cold water pipe on the water heater was actually shaking.
     

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