Does anyone understand Audio Decibels ?

Discussion in 'The Projects Forum' started by Enfield, Jan 10, 2009.

  1. Enfield

    Thread Starter New Member

    Jan 10, 2009
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    Hi
    I have been working on a couple of projects at home to reduce the noise generated by two machines.

    I have borrowed a decibel meter and obtained some readings but now having obtained the readings I am struggling to interprate the results.

    The first machine runs at around 73.20 dB, I then reduced this to 63.02 dB i.e. a reduction of 10.18dB

    The second machine runs at around 70.87 dB, I have reduced this to around 66.60 dB i.e. a reduction of 4.27 dB

    What I would like to do is convert these figures into a % reduction i.e. determine that I cut the sound down by %50 or whatever

    I can certainly hear that I have made big difference but would love to have a calculation to back this up and understand exactly how much by.

    I have looked on the web an understand that in acoust terms decibels are not on a linea , that is a 5dB reduction is not half that of a 10dB reduction.

    I am good with the mechanical stuff but when it comes the electronic (and related stuff) I leave it you clever guys :D

    Thanks for your help

    Enfield in NZ
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Because you are talking about the power of the sound the formula is:

    Power in dB=10*log(power not in dB)

    to find (power not in dB)

    (power not in dB)=10^[(power in dB)/10]
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Since we are talking watts I believe 3DB is ½ the power, 6 DB is ¼ the power, and so on. Every 3 DB is half, this is a logaritmic scale.

    The reason I asked is with voltage or current it is 6DB. The DB in sound is a watt measurement, I believe.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    3dB is twice the power.
    -3dB is half the power.
    The change in sound level is barely noticeable.

    10dB is 10 times the power and sounds twice as loud.
    -10dB is 1/10th the power and sounds half as loud.
    -20dB sounds 1/4 as loud but is 1/100th the power.

    Your hearing's sensitivity to sound level is logarithmic so it has a huge range from hearing a pin dropped in the next room to hearing the space shuttle takeoff nearby.
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    You are right Bill. With power -3dB means divide by 2. With voltage or current -3dB means divide by sqrt2.
     
  6. duffy

    Active Member

    Dec 29, 2008
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    It's a loaded question, but here's the equation for db of sound:

    1x10^-^1^2x 10^{}(db/10) = watts

    so 73.2 db is .0000209 watts and 63.02 db is .000002 watts, which corresponds to the 1/10th power Audioguru mentioned, and to the chart on this page:

    http://en.wikipedia.org/wiki/Sound_power_level

    But, of course, it won't sound 1/10th as loud. That's why it's a loaded question.
     
  7. Externet

    AAC Fanatic!

    Nov 29, 2005
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  8. Enfield

    Thread Starter New Member

    Jan 10, 2009
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    Thanks for the very quick replies, it seems like I can not express the data as a percentage. The decibel is a very strange unit of measurement indeed. I know the modifications I have done work (I can hear the difference) I was just hoping be able to put a figure to the noise drop that would equate to what you hear i.e. it sounds half as loud as before.

    Thanks

    Again

    Enfield
     
  9. mik3

    Senior Member

    Feb 4, 2008
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    Once you convert the dB value to a normal number then you can express it as a percentage with respect to another value. For example, if the power was 200 and it reduced to 150, the percentage reduce is:

    [(200-150)/200]*100
     
  10. floomdoggle

    Senior Member

    Sep 1, 2008
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    Hey Enfield,
    Don't forget that decibels are a pressure measurement.Not power. Even though they mat be interrelated.
    Dan
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    Decibels are not used only for pressure measurements but for other quantities as well.
     
  12. duffy

    Active Member

    Dec 29, 2008
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  13. floomdoggle

    Senior Member

    Sep 1, 2008
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    Hey Mik,
    Of course you are correct. However audio decibels, and bels, are a measure of sound pressure, not wattage.
    The electronics of decibels, to me are unknown. I was trying to answer the OP's question as stated. As always, I'm just the handyman.
    Dan
     
    Last edited: Jan 11, 2009
  14. floomdoggle

    Senior Member

    Sep 1, 2008
    217
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    Hey Enfield,
    What are the machines you are trying to muff? Might be helpful. Vibration, in one form or another is the enemy of quiet, smooth running machines. There are some cases where the noise is acceptable, even necessary for ultimate operation. There maybe other options available, if you state the machines.
    Dam
     
  15. steveb

    Senior Member

    Jul 3, 2008
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    Isn't it more reasonable for you to take the time to understand Decibels, rather than expecting the rest of the world to stop using them?
     
  16. Externet

    AAC Fanatic!

    Nov 29, 2005
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    The deciBel is now an unnecessary complication that sixty years ago simplified some calculations. Measurements and levels can instead be expressed in proper simple units. As per a previous post in this thread, you cannot always translate dB to %.:(

    Miguel
     
  17. steveb

    Senior Member

    Jul 3, 2008
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    The deciBel is not a complication, it is a simplification. Like many simplifications, there is a learning curve needed to understand them and use them fluently.

    Further, dB can always be expressed in %. However, which is easier; to say that the power is 100 dB higher, or 1,000,000,000,000 % higher. Decibels are always a ratio. Sometimes the ratio is established to a reference value (i.e. dBm is referenced to 1 mW).

    It is true that measurements and levels can be expressed in simple units, but that has always been true, and yet the dB is alive and well.

    What is the point of trying to abolish the dB. Its use is already firmly entrenched in minds, books, manuals, reports, test equipment, software etc. Do you want people coming out of school to be unable to communicate with other engineers, technicians and scientists; or unable to read and understand books; or interpret graphs from test equipment or software programs?
     
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