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db, dbspl, spl???

Discussion in 'Recording' started by Dave Nyberg, Aug 29, 2003.

  1. Dave Nyberg

    Dave Nyberg Guest

    I have been the witness of a rather overheated topic on a forum about caraudio. The discussion goes about decibells. I think that every 3db doubles the sound pressure (law of nature). But what is the exact difference between the different types of db measures? Can anyone shine a light on it to solve this discussion.

    If this is the wrong forum for this topic please excuse me :)
     
  2. sheet

    sheet Well-Known Member

    We can help.

    First, the dB is relational. You can't take one measurement in dB's and apply it to another. For instance, you sound pressure level question.

    A 3dB increase (50%) in Sound Pressure Level (dBSPL) requires doubling the power, or 100% increase. This increase is funtion of LOG, so to double the dBSPL of a loudspeaker would require almost 20 times the power.

    Now in IASCA, where my brother's old car placed in the top three nationally (mid-level power category), they use the same type of system's (like Ivie's, Audio Control's, etc) that we do.

    There are different "weights" of dB's, dBA, dBB and dBC.

    A-weighting uses a wide bandpass filter centered at 2.5 kHz, with ~20 dB attenuation at 100 Hz, and ~10 dB attenuation at 20 kHz. It rolls off the low end. It compensates with an inverse 30 dB-SPL equal-loudness curve of Fletcher-Munson. A-weighting helps pad the poor performance of gear. Anything with this A-weight minus 10dB is the actual noise level.
    C-weighting uses flatter, but limited bandwidth, with -3 dB corners of 31.5 Hz and 8 kHz. C is the most realistic.
     
  3. Dave Nyberg

    Dave Nyberg Guest

    Thnx for clearing this up. I'm no expert so the weighted db's don't say me much but what i really want to know is what's the difference between sound pressure and volume (loudness). The discussion goes mainly about soundpressure versus volume. Are they the same?
     
  4. sheet

    sheet Well-Known Member

    Well, it is just what I said.

    If a speaker is putting out 110dB SPL, there must be a weight to it. So you won't (shouldn't) find a spec on some speaker without an A, B or C behind it, which tells what they used. A is typically what most people use, even though it rolls off the bottom end. So that spec would look like 110dBA SPL.

    Loudness is measured and weighted, then represented in dB's, which is the sound pressure level.
     
  5. bgavin

    bgavin Guest

    Speaker SPL specs are also subject to a whole lot of marketing (read: bull sh*t). They often take an SPL measurement at 1 KHz, because the driver shows the best output here. It makes the driver "more competitive" in the market.

    A 1 KHz response is utterly meaningless for bass drivers or subwoofers. The story in the bottom end (my area of knowledge) is much different. The weighting type is entirely appropriate, as the human ear is non-linear in its response to low frequency stimulation.

    Don't expect to get anywhere in your car audio forum. Speaker discussions are dominated by those who insist the world is flat, and the sun orbits the earth. If you hit 'em with measurements and facts, they will strike back with faith-based arguments such as "I trust my ears." This is like arguing with a drunk: pointless.
     
  6. Dave Nyberg

    Dave Nyberg Guest

    Lol, i noticed :D Thnx guys!
     

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