DBA question

[inwoosac]

...lets say ..i had a 30dba fan..
and a 34dba fan...
would the noise be 34dba?..or somehow add a little to the 34dba....??

 

[lifeguard1999]

Short Version:

0. Your answer is 35.45 dB

1. Doubling the intensity of sound means an increase of a little more than three dB.

2. If a sound is not reflected or interrupted, the intensity drops 6 dB ( i.e. 0.25 of its value) every time we double the distance. Thus, if the SL is 30 dB at 2 metres from the source, it will be 24 dB at 4 metres and 18 dB at 8 metres.

Long Version (lifted from britannica.com)

Because of the enormous nonlinearity of the ear in sensing pressure waves, a nonlinear scale is convenient in describing the intensity of sound waves. Such a scale is provided by the sound intensity level, or decibel level.

(dB), unit for expressing the ratio between two amounts of acoustic power or for measuring the relative loudness of sounds. One decibel (0.1 bel) equals 10 times the common logarithm of the power ratio--i.e., doubling the intensity of a sound means an increase of a little more than three dB. In ordinary usage, specification of the intensity of a sound implies a comparison of the intensity of the sound with that of a sound just perceptible to the human ear. For example, a 90-dB, or 9-bel, sound is nine powers of 10 (i.e., 10^9, or 1,000,000,000) times more intense than a barely detectable sound.


decibels......intensity*....type of sound
....130..........10.........artillery fire at close proximity
....120...........1.........amplified rock music; near jet engine
............................(threshold of pain)
....110..........10^-1......loud orchestral music, in audience
....100..........10^-2......electric saw
.....90..........10^-3......bus or truck interior
.....80..........10^-4......automobile interior
.....70..........10^-5......average street noise; loud telephone bell
.....60..........10^-6......normal conversation; business office
.....50..........10^-7......restaurant; private office
.....40..........10^-8......quiet room in home
.....30..........10^-9......quiet lecture hall; bedroom
.....20..........10^-10.....radio, television, or recording studio
.....10..........10^-11.....soundproof room
......0..........10^-12.....absolute silence (threshold of hearing)


*In watts per square metre; ^ indicates exponentiation of the intensity figure.



The rest is NOT lifted from britannica.com

WARNING! MATH AHEAD!

Now the equation used is: SL = 10log(I/Io) where I is the intensity of the sound waves measured in watt/m^2, SL is the sound level expressed in dB, and Io is the threshold of hearing or 10^-12 watt/m^2.

For 30dB, I = 1E-9
For 34dB, I = 2.511886E-9

Add them up (3.5E-9), recalculate SL [ 10*log(3.5E-9/1E-12) ] and you arrive at: 35.45 dB. That assumes that they are in phase, and are not cancelling one another.

A quick check is the rule-of-thumb that 3dB equals a doubling of sound intensity. Say you had two, 30dB fans. Then the sound level is [ 10*log(2.0E-9/1E-12) ] = 33.0103 dB. The difference between the two is 3.0103dB. Hence a doubling occurs every 3 dB.

Class dismissed.

 

[inwoosac]

thx for the long post....much appriciation

 

[damien6]

If you could read that w/o getting headaches or even have a vague inkling of what all that was about (besides looking at them as an odd mixtures of letters, numbers and quotation marks that looks nicely organized in odd sort of way) that means you're either pretty smart or know how to copy and paste pretty good. Regardlessly, even if you copied it, you still thought it was significant enough to save it or know where it was when you needed it and used it in the right (round about way since I'm a dumb fvck and don't know what any of that means) conversational settings, you must be a genius. No, I mean that w/o being sarcastic or anything.........or you work in some field related to aerodynamics which pretty much means the same thing I guess. Of couse you could also be a student who just read up on all this and will probably forget everything about it a moment after the exam or the next party which ever comes first. Damned! I must lost a lot more brain cells in my young'un days than I thought because I feel like mine's only 2% full.


If you ever build your personal cooling system, I want to take a look at it if you would.


Damn, that's a good answer. (I'm still try to decypher what it means though)

 

[lifeguard1999]

Good guesses. I graduated in Aerospace and now work in the field of supercomputers. At the moment, I run a scientific visualization shop though in the past I have been a systems administrator.

The question was just one that I had in my mind as well. He just prompted me to go figure out the answer. I went to britannica because they had answers to my questions in the past. Instead of rewritting their excellent work, I just cut-n-pasted and gave them credit.

Then I started into the math portion to figure out the answer to the question. I realized my post was getting too long, so I divided into the Short Answer + Long Answer section.