radio shack corrections

Discussion in 'Speakers' started by MikeDuke, Sep 28, 2004.

  1. MikeDuke

    MikeDuke Stunt Coordinator

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    This may be a stupid question but if the following are the corections for the radio shack SPL meter
    10hz........+20db
    12.5hz.....+16.5db
    16hz........+11.5db
    20hz........+7.5db
    25hz........+5db
    31.5hz.....+3db
    40hz........+2.5db
    50hz........+1.5db
    63hz........+1.5db
    80hz........+1.5db
    100hz......+2db

    how do you figure out whats in between
    22Hz or 28Hz or 36Hz and so on

    check out my review of the JM Labs SW900 subwoofer
    http://www.hometheaterforum.com/htfo...76#post2400576
     
  2. Eric Ha

    Eric Ha Stunt Coordinator

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    I just split the difference. For example, 28hz would be +4db, splitting the 25hz and 31.5hz values.
     
  3. MikeDuke

    MikeDuke Stunt Coordinator

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    Thanks. Thats what I thought but I wanted more input.
     
  4. Kevin C Brown

    Kevin C Brown Producer

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    Yes, interpolate. [​IMG]
     
  5. Jon W.

    Jon W. Stunt Coordinator

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    Never heard of this but just so I understand, If I read 80 db at 25 Hz on the meter it is actually 85 db?

    Also I have the digital Rat shack meter, (it was all they had in stock) does the same corrections apply?
     
  6. Kevin C Brown

    Kevin C Brown Producer

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    Yes, you add the correction to the value read on the meter.

    To the best of my knowledge, the digital meter and the analog meter are identical, just the display is different.
     
  7. Jon W.

    Jon W. Stunt Coordinator

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    Thx dude! [​IMG]
     
  8. Edward J M

    Edward J M Cinematographer

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    Also keep in mind that these are generic correction factors for the RS meter.

    They are a combination of theoretical correction factors for the C-weighted curve, AND additional correction for inherent inaccuracies in the meter itself.

    Not every RS meter will react in the same fashion, so I would not use these generic CFs for accurate low frequency FR work. They ARE perfectly adequate for correcting the gross errors at the lowest frequencies that are introduced by the c-weighted filter in the meter, and for basic in-room FR sweeps to find/correct large peaks/dips in the response.

    Also, the RS meter tends to get a little screwy near the top end of its operating scale (reportedly 126 dB); anything over about 112-113 dB and the readings can be unreliable.

    The best use for the RS meter continues to be an extremely affordable tool for basic system balancing and calibration.
     
  9. Jon W.

    Jon W. Stunt Coordinator

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    Well Ed, I was about to try and plot my FR. I ordered a BFD yesterday and need to plot my room. I know there are programs that do it but I don't know where and what kind of mic to buy. I really need someone to say download this and buy this, any suggestions?
     
  10. Edward J M

    Edward J M Cinematographer

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    Use the RS meter, the correction factors, a piece of graph paper (or Excel of you are familiar with it), and some sine waves burned to a disc. 1/12 octave resolution should do the trick. You can try snapbug.com for the sines, or Anthony Gomez (HTF member) has a website that hosts 1/12 octave downloads.

    Some people don't recommend sines because they can develop standing waves easier that say a warble tone. You can try a narrow bandwidth warble instead for each test frequency. Seek out Wayne Pflughaupt (he posts here often) for guidance; he's got some neat graphing solutions and can give you more help. [​IMG]
     
  11. BruceD

    BruceD Screenwriter

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    Ed,

    I thought the purpose of using sine wave tones (instead of warble tones) was to excite specific standing waves so they will show up in the frequency response graph. This is the cheapest method of identifying the center frequency for the BFD to compensate for those annoying standing wave peaks (boomy bass).

    Warble tones intentionally don't have a stable center frequency, thus not giving the room a chance to produce much in the way of a standing wave, nor a way to correctly identify the center frequency for the BFD.
     
  12. Jon W.

    Jon W. Stunt Coordinator

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    Ed, That was pretty much my plan. I guess I was looking for the easy way. :b Gonna be a long weekend. Thx for the reply.[​IMG]
     
  13. Edward J M

    Edward J M Cinematographer

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    Hi Bruce:

    Personally I always used sines when I did manual FR sweeps and graphing.

    I don't use that method any more, and instead use the Quick Sweep from TrueRTA. The QuickSweep function is gated, but not in the conventional MLS sense in that it intentionally allows room effects to be included (I think the entire sweep is open for about 1/2 second, but I'd have to check with designer John Murphy to be certain).

    Anyway, I've seen MingL and (I think) Wayne Pflughaupt recommend the narrow bandwidth warbles to avoid the build-up of standing waves. While I don't want to speak for these guys, I just wanted to include their POV and asked the poster to seek out Wayne for further guidance.

    I don't know how long it takes to fully develop standing waves in a room. Is it possible that standing waves don't often fully develop under normal music/movie listening situations? Could the use of a warble to manually plot the FR be more representative of the FR under actual use? I'm just thinking out loud without all the answers and trying to see their POV....what are your thoughts?

    Thanks,

    Ed
     
  14. ChrisBee

    ChrisBee Stunt Coordinator

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    Ed said: "I don't know how long it takes to fully develop standing waves in a room. Is it possible that standing waves don't often fully develop under normal music/movie listening situations?"


    Isn't a room effect "boom" a fully developed standing wave?
    Which is why people use BFDs (etc) to control room effect peaks. :b

    What am I missing here? Apart from the lack manners in questioning the expert's own words? [​IMG]

    Regards
    Chris Bee
     
  15. Brian L

    Brian L Cinematographer

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    Well OK. I have followed a few threads that discuss test tones and measurement, and was left somewhat confused in terms of the recommendation for the best low cost method to measure the room response and then tweak with an EQ.

    I think BruceD is suggesting that the point of the exercise is that you WANT To detect and correct (with proper EQ) room peaks. If your tones are designed to NOT allow those peaks to occcur, then how can they be used for that purpose?

    In my personal experience using a basic RS SPL meter as well as an Infinity RABOS meter (sure wish someone with the right test gear would do some correction factors for that device), you will not get a stable enough reading unless you DO use sin waves. As such, you will be left to interpolate a rapidily bouncing meter.

    Having said all that, I finally got a BFD last weekend. Using pure sin waves, it was quite easy to plot out the low frequency room response, and quite easy to correct it (at one listenning spot).

    Last thing...the link Ed mentioned, in addition to having links to sin waves, has a great spread sheet that allows the user to enter the measured room respnse. It then applies the RS correction factors, and on top of all that, graphs the before/after results. A wonderful tool!

    Here is the full link:

    http://www.snapbug.ws/bfd.htm

    BGL
     
  16. Edward J M

    Edward J M Cinematographer

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    Manners? Don't sweat it - you're one of the nicest guys around. Expert? Hardly - a constant learning process and trying to stay humble is more like it.

    I suppose I answered my own question: The Quick Sweep on TrueRTA definitely shows room gain, peaks, dips, and true nulls (i.e., those dips which do not respond to a PEQ boost). And the Quick Sweep is only about 500 ms (and sound travels around 1100 fps). So I'm not sure why others recommend the warble for plotting FR. Maybe they can chime in or Bruce can shed some light.....
     
  17. BruceD

    BruceD Screenwriter

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    Here is what I do know.

    Using MLS test tones (ETF) to correctly identify standing waves requires a gate time of >200ms, so I would assume this is also true for sine-wave tones.

    Brian's experience of an oscillating SPL meter with warble tones (making it impossible to identify the offending center frequency) is similiar to my experience.

    I found warble tones were very useful for me in identifying the correct "C" weighted SPL settings (C weighting is similiar to what your ears actually hear) on both sides of my crossover bewteen mains and sub @60Hz (Marchand XM9-L with 24dB/octave L-R for both low-pass and high-pass). The 60Hz crossover frequency was chosen because it is an octave above my mains f3 of 32Hz. The crossover has level adjustments for each side and I have level adjustments for each channel on the amps.

    The whole point of the exercise is to capture the actual SPL readings, including those caused by the standing waves (which is why warble tones are not useful). A Parametric EQ doesn't correct the problem, the standing wave, it simply reduces the SPL of the fundamental frequency, thus reducing the intensity of the standing wave, and thus the total SPL energy at that frequency.
     

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