User talk:Galeandrews

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Revision as of 09:44, 18 November 2008 by Galeandrews (talk | contribs) (dropped in a stack of dB (SPL) information which might be a technical article some day)
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Definition of the decibel scale

When displaying the waveform in decibels, values range from 0 to -Here is information about decibel levels and perceived volume change. Here’s the quick read info, with supporting documentation below.

   * 3dB = twice the power (note + 3db volume increase requires double the power but  
      does not double the amplitude!) 
   * 6dB = twice the amplitude
   * ~10dB = twice the perceived volume
   * Adding up two 12dB noise sources will get you, on average, 15dB
(which will not sound twice as loud)

My newbie question is what are the y-axis units -- decibels below what reference?

dB is a ratio to the maximum signal, expressed in logarithmic form. Thus, -36 dB means that the amplitude of your sample is 10^(-36/20) = 0.016 of maximum scale (0 dB = 1). You probably discovered already that the scale can be adjusted in Audacity Preferences, Interface tab.

OK, but in this case, a ratio of what to what? For example, in working with WiFi radio power one typically uses dBm -- decibels relative to a milliwatt of power. What is being shown here?

The ratio between the amplitude of the current signal and the maximum possible amplitude. I don't know if i can explain it properly but i'll give it try...

DB is a ratio .sort of a arbritrarily choosen point... .. a ratio comparing the intensity of the sound to a reference level! if the intensity of the is equal to the reference level then that point is called 0db .....

The confusing part sometimes is understanding what is the 0 point... and it really is determined by what your specifically doing and measuring... because it can be different ie: if your recording or if your amplying or if your measure voltage or rf energy or sound intensity... if your looking at the 0 point being nothing or the 0 point be the max of something .. or something in between .. smile... that the bad

for example.... if measuring sound intensity the 0 point is at that point where there no sound... and as sound is heard it will be be measure in terms of +db rating for example a jack hammer running , when measured, will have a intensity of appx +100 db's ... in reference to that ) 0 db where 0db is no sound at all..

or if your sending a signal... well the max signal strength may be considered at 0 db and any thing less than that would be a -xdb ... for example... if you send only half of the maximum signal strenth .. that would equate to appx -3 db......... compared to 0db

now that's only true as a measurement of power .... for voltage and sound pressure amplitude, - 6dB is half as loud (twice as quiet), -12 db is four times quieter -20 db is ten times quieter and -40 dB 100 times quieter

now what does this mean for programs like audacity...

when it come to recording..... 0db is considered the max signal strength before distortion/clipping start occurring ..!! this 0 db is going to be determined by your sound card... what it's circuitry can handle!!

if your amplyfying a signal in your audacity... the 0 db reference is at point where there is no sound signal and everything above that is a indication of signal strength in reference to 0 sound!

now I mentioned that 0db is a arbitrary point.... and it depends on what one ones want it to represent weather it represent a max signal or zero signal... however it can represent something betweend for example in your audacity.... on your wav form... if you notice the 0db ref line is in the middle of the wav form ... it showing the signal strenth above and below the 0 reference line... so in this case the zero is sort of a null point... i am not going to get into any deeper on this smile... but just to say sometimes in music the 0 reference actually is riding at higher point and some you need to bring the music fullyly to a zero refernce there even function in audacity for that.... but that enought of that.. smile..

well here some links to start off with...…?parent=questio…://,,sid44_gci21…

Nice links! The first reference tells that the reference level for sound pressure is a very low value of 0.02 mPa. Indeed it cannot be zero sound, because you can't calculate a ration to 0: Signal/0 = not determined. Thus, in audio, a very low sound that (some) people still can hear is used as a reference.

that right about the sound pressure.. but keep in mind that 0db is arbitrary value and can be giving any value depending on what is to be measured.... for sound pressure that .02 value is assigned based on standards, however if your looking a circutry, a rf device, etc etc etc.... the reference value will be different...!! smile....

What is difference between dB SPL and dB(A) SPL?

The following is from the Acoustics FAQ. A sound level meter that measures the sound pressure level with a "flat" response will indicate the strength of low frequency sound with the same emphasis as higher frequency sounds. Yet our ear perceives low frequency sound to be of less loudness that higher frequency sound. The eardrum- stapes-circular window system behaves like a mechanical transformer with a finite pass band. In EE parlance, the "3 dB" rollover frequencies are approximately 500 Hz on the low end and 8 kHz on the high end. By using an electronic filter of attenuation equal to that apparently offered by the human ear for sound each frequency (the 40-phon response curve), the sound level meter will now report a numerical value proportional to the human perception of the strength of that sound independent of frequency. Section 8.2 shows a table of these weightings.

Unfortunately, human perception of loudness vis-à-vis frequency changes with loudness. When sound is very loud - 100 dB or more, the perception of loudness is more consistent across the audible frequency band. "B" and "C" Weightings reflect this trend. "B" Weighting is now little-used, but C-Weighting has achieved prominence in evaluating annoying community noises such as low frequency sound emitted by artillery fire and outdoor rock concerts. C-Weighting is also tabulated in 8.2.

The first electrical sound meter was reported by George W Pierce in Proceedings of the American Academy of Arts and Sciences, v 43 (1907-8) A couple of decades later the switch from horse-drawn vehicles to automobiles in cities led to large changes in the background noise climate. The advent of "talkies" - film sound - was a big stimulus to sound meter patents of the time, but there was still no standard method of sound measurement. "Noise" (unwanted sound) became a public issue.

The first tentative standard for sound level meters (Z24.3) was published by the American Standards Association in 1936, sponsored by the Acoustical Society of America. The tentative standard shows two frequency weighting curves "A" and "B" which were modeled on the response of the human ear to low and high levels of sound respectively.

With the coming of the Walsh-Healy act in 1969, the A-Weighting of sound was defacto presumed to be the "appropriate" weighting to represent sound level as a single number (rather than as a spectrum). With the advent of US FAA and US EPA interests in the '70's, the dBA metric was also adapted by them. (Along with the dBA metric has come an associated shortfall in precision in accurately representing the capacity of a given sound to produce hearing loss and the capacity to create annoyance.)

[Editor's Note: A single number metric such as dBA is more easily understood by legal and administrative officials, so that promulgation, enforcement and administrative criteria and actions are understandable by more parties, often at the expense of a more precise comprehension and engineering action capability. For instance, enforcement may be on a dBA basis, but noise control design demands the octave-band or even third-octave band spectral data metric.]

The most commonly referenced weighting is "A-Weighting" dB(A), which is similar to that originally defined as Curve "A" in the 1936 standard. "C-Weighting" dB(C), which is used occasionally, has a relatively flat response. ""U-Weighting"" is a recent weighting which is used for measuring audible sound in the presence of ultrasound, and can be combined with A-Weighting to give AU-Weighting. The A-Weighting formula is given in section 8 of this FAQ file.

In addition to frequency weighting, sound pressure level measurement can be time-weighted as the "Fast", "Slow" or "Impulse" response. Measurements of sound pressure level with A-Weighting and fast response are also known as the "sound level".

Many modern sound level meters can measure the average sound energy over a given time. this metric is called the "equivalent continuous sound level" (L sub eq). More recently, it has become customary in some circles to presume that this sound measurement was A-Weighted if no weighting descriptor is listed.