Noise
From WikIRI
Noise
Sound is nothing more than rapid fluctuations of atmospheric air pressure
Sound Level
- is expressed in decibels, dB
- depends on level & frequency
- Acoustic Frequency is measured in Hz (cycles/second)
- Humans can hear acoustic frequencies from 20 Hz to 20 kHz, if the sound level is high enough
Octave Bands Center Frequency Frequency Range 1 63 Hz 44 Hz to 88 Hz 2 125 Hz 88 Hz to 177 Hz 3 250 Hz 177 Hz to 355 Hz 4 500 Hz 355 Hz to 710 Hz 5 1000 Hz 710 Hz to 1,420 Hz 6 2000 Hz 1,420 Hz to 2,840 Hz 7 4000 Hz 2,840 Hz to 5,680 Hz 8 8000 Hz 5,680 Hz to 11,360 Hz
Loudness
- varies with individuals
- Threshold of hearing is about:
- 0 dB at 2,000 Hz
- 25 dB at 100 Hz
- 70 dB at 20 Hz
- Threshold of pain is about 120 dB
Sound Pressure Level Lp = 20 log10(p/pref)
- Lp is sound pressure level (dB ref. 20 microPa)
- p is the acoustic pressure (Pa)
- pref is the reference sound pressure (20 microPascals, 2x10-5 N/m2)
- Typical values range from 20 to 100 dB
- Lp values vary with frequency
- Noise sources are typically not rated by sound pressure level because Lp varies with distance and direction from the source
Sound Power Level Lw = 10 log10(Ws/Wref)
- Lw is sound power level (dB ref. 1 pW)
- Ws is the source sound power (watts)
- Wref is the reference sound power (1 picoWatt, x10-12 watt)
- Typical values range from 40 to 120 dB
- Lw values vary with frequency
- Noise sources are rated by sound power level because Lw does not vary with location of the listener
The "A" weighted scale is used in the industry to compensate for the fact that the ear is not equally sensitive at all frequencies. When comparing two separate noise sources, even thought the SPL values may be the same, one may appear to be noisier if the sound power is centered around a frequency in which the ear is more sensitive. It is because of this that sound level meters us the "A" weighting scale.
SPL = LwA - 20log r - 0.6 + C
- SPL = "A" weighted Sound Pressure Level (dBA re 20 Pa) represents the A-weighted sound pressure level (including all octave bands) at a point
- LwA = "A" weighted Sound Power Level (dBA re 1 pW) represents the overall A-weighted sound power level (including all octave bands) of a source
- r = Radius from noise source, feet
- C = Correction term for temperature and pressure
Noise
- defined as unwanted sound
- characterized by 3 factors:
- -1. the level or loudness of the sound
- -2. its acoustic frequency content
- -3. the time-varying nature of the sound
Sound level:
The current European standard claims that all generators = 2kW may not exceed LWA95.
LWA95 corresponds with 70 dB(A) at a distance of 7m.
We can say that dB(A) on 7m = LWA - 25. dB(A) on 4m = LWA – 20 dB(A) on 0m = LWA – 0 = LWA
The sound level depends on the type of engine (gasoline or diesel), the number of revolutions and the cooling system. In general we can say that: ° A gasoline engine makes less noise than a diesel engine ° 1500 rpm is more quiet than 3000 rpm ° Water-cooled is more quiet than air-cooled Nivel sonoro: La norma europea actual reclama que todos los grupos electrógenos = 2kW no pueden exceder Lwa95.
LWA95 corresponde con 70 dB(A) a una distancia de 7m.
Podemos decir que dB(A) en 7m = LWA - 25. dB(A) en 4m = LWA - 20 dB(A) en 0m = LWA - 0 = LWA
Machining Apolo and Armoni machines
Noise Comparable to a Vacuum Cleaner
