Rat Shack SPL correction?

Discussion in 'Archived Threads 2001-2004' started by Tony Richardson, Jan 22, 2001.

  1. Tony Richardson

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    I just bought the Rat Shack SPL meter and calobrated all my speakers except the sub.
    WOW what a difference??
    Any way, I have heard there is a spread sheet or something that has the corrections for the meter for low frequencies.
    Does anybody know where I can get this?
     
  2. RichardH

    RichardH Supporting Actor

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    Tony,
    I've just sent you the spreadsheet.
    edit:
    that just got returned undeliverable. Email me and I'll send it to you. The following numbers are slightly off of the spreadsheet....
    Edit: The following is the work of Michael Sims, as listed below. I am simply pasting it from another post. Sorry if anyone got the wrong idea....
    ------------------------------
    The Radio Shack meter is a wonderful unit. About a two and a half
    years ago, I did a calibration curve for the RS meter using lab
    equipment, with Eric Busch from DLC Design adding the low bass down
    to 10 Hz. This was published in PSACS Sound Bytes in two issues.
    These are the corrections that should be added to the meter readout
    in order to achieve the correct SPL. These corrections are only
    valid for the meter set to C weighting, using 1/3 octave pink noise
    (easily available from various CDs), with the mic pointed at the
    speaker.
    Both my analog meters and my digital meter measured the same in
    October, 1996.These are corrections, they are to be added to the
    meter readout for the correct response in dB SPL.
    10Hz +20.5
    12.5Hz +16.5
    16Hz +11.5
    20Hz +7.5
    25Hz +5
    31.5Hz +3
    40Hz +2.5
    50Hz +1.5
    63Hz +1.5
    80Hz +1.5
    100Hz +2
    125Hz +0.5
    160Hz -0.5
    200Hz -0.5
    250Hz +0.5
    315Hz -0.5
    400Hz 0
    500Hz -0.5
    630Hz 0
    800Hz 0
    1KHz 0
    1.25Khz 0
    1.6KHz -0.5
    2Khz -1.5
    2.5Khz -1.5
    3.15Khz -1.5
    4KHz -2
    5KHz -2
    6.3KHz -2
    8KHz -2
    10Khz -1
    12.5KHz +0.5
    16KHz 0
    20KHz +1
    To measure the in-room response of your speakers with the SLM, find
    a quiet and undistracted time, obtain a 1/3 octave pink noise CD,
    mic stand, and worksheet. With the speakers in their normal
    positions and using a mic stand for the SLM (I've had good results
    just holding the meter too, pink noise is forgiving), place its
    microphone where your ear would be at your favorite position or
    "sweet spot". Set the meter to "C" and "slow". Play the 1 Khz, 1/3
    octave pink band and set the level on the amp or preamp, and the
    meter range, so the meter reads 80 dB at 0 dB on the meter. Higher
    levels might cause driver compression in the frequency extremes,
    rolling off the response. Now go back to the first ISO center 1/3
    octave band on your CD (25 Hz on mine) and record the response:
    3.5, or -1.0 , or -5.0, or whatever it is. Now step through the
    pink noise bands, recording the meter level each time. This is the
    raw data.
    Keeping the meter in the same measuring position, using an accurate
    CD, having a quiet room, repeating the measurements for accuracy,
    understanding what you are measuring, fresh batteries, not talking
    while measuring, having your meter calibrated for overall level
    (relative level comparison is unaffected), and doing only one
    speaker at a time if possible to avoid comb filtering (variations
    of plus or minus 2 dB are possible) are all important factors.
    Sometimes if I have to measure two speakers at a time, I move the
    meter around in a figure eight pattern, slowly, and try to obtain
    an average reading, as the microphone moves in and out of the
    combing peaks and troughs. Best accuracy would be obtained from
    taking several measurements at different listening positions, if
    one has the time and patience.
    Now take the raw data and make the corrections on the work sheet,
    entering the new values in the appropriate column. Time to either
    moan or marvel, since this is the actual in-room 1/3 octave pink
    noise frequency response of your speaker/system at that listening
    position. Plotting the results on graph paper in different colors
    for left, right, center, etc. makes it look cool.
    Yes, this way is tedious, but it is very inexpensive. And very
    accurate. For the third octave pink noise, I use Carver's Amazing
    Bytes CD, GRP Z-9907; other CD's with 1/3 octave pink noise ISO
    centers are:
    My Disc, Sheffield 10045-2-T
    Sheffield/Coustic Test and Demonstration Disc, Sheffield 10040-2
    Autosound 2000 CD #103, $18, 800-795-1830
    IASCA Setup and Test CD
    HI-FI News and Record Review "CD-II", $30 Japan Audio Society Audio
    Test CD-1, YDDS-2
    --- these last two available from DB Systems, 603-899-6415.
    For the others: http://www.audioxpress.com/ http://www.mcmelectronics.com/main.html http://www.parts-express.com/
    There are other CD's that have warble tones on them, but I am not
    comfortable using them. I also extensively use track 19, disc 2,
    from Delos "Surround Spectacular", DE 3179.
    This has a slow sine sweep from 160Hz to 20Hz with the voice of
    David Ranada announcing the frequencies as they
    descend. A fantastic help when used with the Radio Shack SLM, you
    can easily tell where the room peaks and dips are.
    http://www.delosmus.com/
    You have permission to copy and distribute this information freely,
    as long as no commercial gain is involved.
    Radio Shack is your friend.
    Here is how I did my calibration:
    The $34.99 Radio Shack 33-2050 analog sound level meter has been
    around for over 25 years. Its predecessor, the 33-1028, was
    reviewed favorably in Stereo Review, (Julian Hirsch, "Equipment
    Test Reports", Stereo Review, August 1972). It has a much
    different curve than the ones I tested.
    To verify the accuracy of the newer version, I compared it to an
    Audio Control 3050 RTA, the same one Tom Nousaine used for years in
    his test reports for Car Stereo Review until he bought MLSSA. The
    overall SPL accuracy of all three of my Radio Shack sound level
    meters -- 2 analog and 1 digital 33- 2055-- were within 1 dB of 75
    dBC SPL compared to the Audio Control. I then checked the
    frequency response, comparing it while set to C weighting and slow,
    with pink noise, 1/3 octave band by 1/3 octave band, to the Audio
    Control RTA in the SPL mode. Using the same official PSACS
    calibrated PSB loudspeaker and a pink noise CD, I made a
    calibration curve that can be subtracted from the results obtained
    by the Radio Shack in your living room to obtain accurate,
    repeatable measurements for about $60, including pink noise CD.
    Make sure your meter is set to C weighting. The digital meter and
    my second analog meter (6 years newer than the test unit in 1996)
    were the same as the test SLM. Response below 25 Hz done by Eric
    Busch with sine waves and B&K equipment at Dave Clark's DLC Design
    in Michigan.
    Michael Sims
    Prairie State Audio Construction Society
    --------------------------
     
  3. JohnnyG

    JohnnyG Screenwriter

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    Is there anyway to utilize the 'output' connector on the meter in conjunction with a pink noise sweep to capture a frequency response graph on a computer?
    I've never tried hooking this connector up to anything, so I have no idea what the output is. I'm sure I'm not the first person to think of this though, so there must be something I'm missing.
     
  4. Christian Sonntag

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    Richard,
    It certainly sounds like you have done a lot of work to calibrate the RS meter at many frequencies. And I don't doubt your findings one bit.
    However, is it possible that not every RS unit will require the same corrections? I work in a pharmaceutical manufacturing environment, and we utilize transducers to send realtime data regarding facility temperature, humidity, and room pressures. We calibrate each of these transducers twice yearly, and each one ends up with a calibration curve that is unique compared to the other transducers of the same type. This was true even when they were new, so aging is not necessarily the culprit here.
    Is it possible that other units of the same model would require their own correction factors?
    Regards,
    Chris
    ------------------
    Christian A. Sonntag
    "The world owes you nothing. Take responsibility for your own actions!"
     
  5. TerryC

    TerryC Stunt Coordinator

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    Richard,
    I thought those readings were an average taken by a group of members from the Prairie State Audio Society a long while back? Are you part of that group?
    Christian,
    You are correct about it not being 100% accurate, the only way to do that is buy a calibrated mic or calibrate ones own rat meter against a calibrated mic. Unless one is going for absolute perfection I am sure these are good enough for the average user and will get them in the ball park.
    Johnny,
    Take a look here where I did just that: http://www.hometheaterforum.com/uub/...ML/002593.html
    Terry
     
  6. RichardH

    RichardH Supporting Actor

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    Hey all,
    No, I'm not part of that group that did the readings. I just copied it off of another post and pasted. Sorry if I misled anyone.
    As to each meter being slightly different, yes that is the case, but it seems to be that each meter is no more than a dB or so off at any frequency. That is, they're all off by approximately the same amount at specific frequencies.
    RichardH
     
  7. BruceD

    BruceD Screenwriter

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    Regarding test tones.
    I may be wrong, but I thought the reason for using warble test tones vs. pure test tones was to eliminate the possibility for wave reinforcement in small rooms.
    This means warble tones would tend to give you a more accurate reading of the actual frequency under test.
    Am I completely off my rocker on this one?
    By the way I used the radio shack SPL meter's RCA output port connected to the mini-phone plug line in on my notebook PC and a software program called ETF 5.0 to do in-room frequency analysis on my speakers. It was about $199 for the sw I think, and they have a great example demo on their home page. You can even try it out for free (just can't save or print any collected results).
    www.etfacoustic.com
    I then went that extra step and ordered a calibrated microphone (comes with a calibration file on floppy) and a battery operated pre-amp kit from this website:
    http://orca.st.usm.edu/~jmneal/preamp1.htm
    The software program allows you to load the mic calibration file to automatically compensate the input.
    I'm using this combination to optimize my mains to sub crossover, sub and speaker locations and room treatments. Needless to say I'm learning alot.
    Just to clarify, I have no personal or business connections with either of these sites, I'm just a happy user.
    BruceD
     
  8. Patrick Sun

    Patrick Sun Moderator
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    Just wanted to warn folks about measuring above 10KHz with the RS SPL meter: Don't rely on the readings. The mic simply doesn't do a good job picking up high treble response from 10KHz to 20KHz. So if you do some measuring of full range speakers, keep this in mind. Unless it audibly sounds super harsh, your tweeter should behave if the crossover is properly designed in your speaker system.
    ------------------
    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     
  9. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    Im new to this forum. I have to tell is the best I have found on the Net.
    Regarding the "accuracy" of the measurments that the RS SPL meter can do beyond 10khz, is there a way to know, exactly, the limitations?
    I have seen the corrections table, and beyond 10khz there is no to much change in dB like in low frequencies, why?
    Thanks
     
  10. Julian Data

    Julian Data Second Unit

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    It's the mic cartridge that the limitation. There's a site that tells you how to modify the meter to get it pretty flat.
    I'll look for it.
    ------------------
     
  11. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    hey
    thanks!
     
  12. Julian Data

    Julian Data Second Unit

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  13. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    Thanks Julian, the mod is beyond my capacities, but will find someone to help me!
    Now (for everybody), about the measurement using the RS SPL I have a lot of doubts. I know that it will not be reliable in HF (beyond 10,000hz)until some mic modifications, but I want to use it right now to make some tests.
    Im using it with SpectraLab software but I cant obtain a flat reading in all the spectrum. LF appear to have a lot more power than HF, so the "line of response" using a frequency sweep from 20hz to 20,000hz (english is not my first language, so please excuse me if sometimes I dont know how to express myself) lose power as it aproach from the LF to the HF
    Is this normal? is there a way to obtain a flat response (to the limits of my speakers of course) like in the magazines?
    Im using the "c" weighting on the spl meter and spectralab is using a 44100 sampling rate with an fft size of 16384, smooth window is hanning.
    Also, if there is a place on the net where I can learn to test HIFI equipment (using a program like spectralab or the other that some people mention around here) I will thank very much the link!
     
  14. Julian Data

    Julian Data Second Unit

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    Hi Manuel,
    Most of those results in the mags are resulted from either NF (Nearfield) or close miking of the driver or from 1m distance using 2.83V.
    I recall hanning is for use with THD testing?
    As for testing equipment, I think it would be best to visit Parts Express or AudioXpress for literature.
    ------------------
     
  15. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    Julian,
    Thanks for the info of the mags methodology, what I dont understand is the use of the voltage (2.83)?? how?? where?? with what?? ;-)
    And about the hanning... what should I use instead?
    My results become a lot better (at least visually on the graph) if I set the spectralab to "a" weighting, is this ok?
    ------------------
     
  16. Julian Data

    Julian Data Second Unit

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    2.83V = 1 Watt
    Use "uniform" if you just want to do frequency response.
    ------------------
     
  17. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    Im so a newbie in all this...
    Do you know where (on the net) can I learn more about the correct use of SpectraLab software (or other analysis software)
    Thanks and best wishes!
    ------------------
     
  18. Julian Data

    Julian Data Second Unit

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    Hi Manuel,
    The Spectra home page might be of help or the help file.
    jd
    ------------------
     

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