Help with RatShack Digital SPL Sort of??

Discussion in 'Archived Threads 2001-2004' started by Gregg Loewen, Feb 24, 2001.

  1. Gregg Loewen

    Gregg Loewen Video Standards Instructor, THX Ltd.
    Insider

    Joined:
    Nov 9, 1999
    Messages:
    6,374
    Likes Received:
    32
    Trophy Points:
    6,610
    Location:
    New England
    Real Name:
    Gregg Loewen
    I have been using a RatShack Digital SPL for about 1 year now, and I know that the analog has some descripences when doing low frequency sweeps. Correction tables as well documented to correct for these problems. How about for the digital SPL? I havent really cared too much up to now, but I am wanting to calibrate my 2 subs with my BFD and want every thing accurate.
    Thanks in advance,
    Gregg
    ------------------
    The Sonodome
    The Newest Sonotube
    Gregg's DVDs
    and LDs
    The Family Units
     
  2. Patrick Sun

    Patrick Sun Moderator
    Moderator

    Joined:
    Jun 30, 1999
    Messages:
    38,749
    Likes Received:
    480
    Trophy Points:
    9,110
  3. Marv

    Marv Stunt Coordinator

    Joined:
    Feb 21, 2000
    Messages:
    110
    Likes Received:
    0
    Trophy Points:
    0
  4. Richard Cooper

    Richard Cooper Stunt Coordinator

    Joined:
    Mar 21, 2001
    Messages:
    132
    Likes Received:
    0
    Trophy Points:
    0
    Hi, I borrowed a Sound Level Meter from work today to have a play with, and I'm guessing it is 'A' weighted only as the switchable scale is labeled dB (A) (It is manufactured by 'Castle', model number 'GA 301'). Firstly, I gather the the 'Rat Shack' (cute [​IMG] ) meter has a switchable weighting, does anyone have a correction table for 'A' weightings? I know it probably won;t be accurate for this particular meter, but I guess it's better than nothing.
    Anyway, I set thte meter to 'slow' and started calibrating my Denon AVR 2801 amp with Ruark VITA 100 speakers using internal test tones. What puzzles me is that I've seen posted that many ppl set their SW to HIGHER than the surround/centre. What I found was that with all spkrs at 75 dB and the sub also at 75 dB, the bass was a little overpowering. I dropped it to about 72 Db and it seemed a lot better. Now I know that in the end, it is my ears that should decide, but I don't like being 'against the grain'!
    A couple of ponderings as to why:
    1) The dB meter needs 'calibrating', possibly adjusting levels with a weightings table.
    2) My listening room aids bass reproduction.
    A few details about my listening room:
    11' square (hence v. small speakers.
    Upstairs room with 3 walls brick exterior cavity wall, one single coarse breeze block.
    Floor is 3/4" chipboard laid on joists.
    A large quantity of heavy furniture is present in the room. Actually, it is my bedroom, as I still live at home, and my parents do not believe in HT! I'm kinda new to this level of hi-fi, so any help most appreciated!
    As a little extra, what is the purpose of 'A', 'C' etc. weightings on these meters?
    Richard
     
  5. Sundar Prasad

    Sundar Prasad Stunt Coordinator

    Joined:
    Jul 26, 2000
    Messages:
    54
    Likes Received:
    0
    Trophy Points:
    0
    Richard,
    If memory serves me correctly, the A weighting on SPL meters is set to approximate the response of the human ear (based on research done some time ago and published as the famous Fletcher Munsen curves). This means that a meter set to A weighting rolls off the low frequency end of the spectrum quite quickly and reports SPL levels that are lower than what a C weighted meter would give. The ear-brain system thinks that a 75 dB sound at 4 kHz is quite a bit louder than a 75 dB sound at 40 Hz. If you search on the web, you might be able to come up with some details on the A weighting curve.
    It is no surprise that you feel that bass is pretty heavy after calibrating with A weighting.
    Sundar
     
  6. Richard Greene

    Richard Greene Stunt Coordinator

    Joined:
    Mar 5, 2001
    Messages:
    148
    Likes Received:
    0
    Trophy Points:
    0
    I've had a Rat Shack analog meter since the early 1980's.
    The owner's manual says accuracy is +/- 3dB from 32 to 10,000 Hz. I have no idea how that specification affects the measurements ... but I have compared the Rat Shack meter
    with an expensive sound meter and found it to be accurate
    woithin a dB or two from about 50Hz. to 2,000 Hz. for "C Weighting", so I assume the large deviations from flat are below 50Hz. and above 2,000Hz.
    The "A" Weighting is used to test for potential hearing damage -- this weighting ramps down the bass readings below 1,000Hz so that by 10 Hz. the meter is reading 70dB
    too low! Reason: Loud bass does much less damage to the
    ears than frequencies between 1000 and 5000Hz. ... the
    "A" Weighting weights frequencies by how much damage they are likely to do to our hearing.
    The "C" Weighting adjusts for the fact that most people have difficulty hearing deep bass and high treble frequencies.
    If your system measures flat with a "C" Weighted meter,
    the unweighted SPL will actually have ramped up bass and treble if measured using an unweighted scale.
    (A simple summary: bass up 3db at 31.5Hz.,
    up 6.2dB at 20Hz. and
    up 14.3dB at 10 Hz. ----
    treble up 3dB at 8,000Hz.,
    up 6.2dB at 12,500Hz.
    and up 11.2dB at 20,000Hz.)
    The bass would probably sound good up 6.2db at 20Hz.
    -- bass frequency response that measures flat won't sound flat because the ear has difficulty hearing deep bass at normal volumes. Most people have their subs set a few dB's louder than their satellite speakers (often much too loud
    to create a subjectively flat frequency response -- these people give subs a bad reputation!).
    The treble would probably not sound good up 6.2db at 12,500Hz. -- even treble that measures flat tends to sound too bright.
    Every stereo system needs some kind of custom "house curve"
    (typically some deep bass ramp-up and some amount of treble roll-off) or it is likely to sound deep-bass-shy and too bright at the same time ... even though the frequency response curve measures ruler flat.
    The Rat Shack meter is not perfectly accurate, especially for treble measurements, and the "C" weighting needs to be adjusted to get a real unweighted SPL. But frequency fluctuations in an ordinary home listening room are far larger than the meter's errors ... +/-15dB is a typical listening room and you're very lucky if your stereo/listening room measures better than +/-10dB using a slow sine wave sweep.
     
  7. Richard Cooper

    Richard Cooper Stunt Coordinator

    Joined:
    Mar 21, 2001
    Messages:
    132
    Likes Received:
    0
    Trophy Points:
    0
    "If memory serves me correctly, the A weighting on SPL meters is set to approximate the response of the human ear (based on research done some time ago and published as the famous Fletcher Munsen curves). This means that a meter set to A weighting rolls off the low frequency end of the spectrum quite quickly and reports SPL levels that are lower than what a C weighted meter would give. The ear-brain system thinks that a 75 dB sound at 4 kHz is quite a bit louder than a 75 dB sound at 40 Hz. If you search on the web, you might be able to come up with some details on the A weighting curve.
    It is no surprise that you feel that bass is pretty heavy after calibrating with A weighting."
    Sundar Prasad:
    Hmm, that makes sense, as the meter is used for measuring the sound output of wire skeining machines. I might look into buying a more appropriate one.
    I havn't been able to find any weightings on the net, so I guess I'll have to do it by ear for a while longer!
    Richard Greene: Hmm, if the RS meter is only accurate to 10Khz, it does raise questions as to how good it is for HT use. Don't know any specs for the castle one as it has no instructions with it.
    Thanks for the help though.
     
  8. Jack Gilvey

    Jack Gilvey Producer

    Joined:
    Mar 13, 1999
    Messages:
    4,948
    Likes Received:
    0
    Trophy Points:
    0
     

Share This Page