S = signal (signal unit)
Sref = signal reference (signal unit)
n = number of sources
Ls = signal level from each single source (dB)
The signal units depends on the nature of the signal - W for power, Pa for pressure and so on.
Note! - adding sound pressure levels.
For sound power it is common to use 10-12 W as the reference sound power. Total sound power from two identical fans each generating 1 W in noise power can be calculated as
Lt = 10 log (2 (1 W) / (1 10-12 W))
= 123 dB
Sound power and sound power level are often used to specify the noise or sound emitted from technical equipment like fans, pumps or other machines. The "sound" measured with microphones or sensors (meters) are sound pressure.
Adding equal signal sources can be expressed graphically
Note! Adding two identical sources (doubling the signal) will increase the total signal level with 3 dB (10 log(2)).
|Number of |
|Increase in |
Sound Power Level
The total signal level from sources with different strengths can be calculated as
Lt = 10 log ((S1 + S2 ... + Sn) / Sref) (2)
The total noise power from two fans - one with sound power 1 W and the other with sound power 0.5 W - can be calculated as
Lt = 10 log (((1 W) + (0.5 W)) / (1 10-12 W))
= 122 dB
Adding two signal sources with different levels can be expressed graphically in decibels as
Download and print Adding Sources with different Signal Levels.
|Signal Level Difference |
to Add to the
Highest Signal Level
The sound power from one of the fans in the example above can be calculated as
Ls1 = 10 log((1 W) / (1 10-12 W))
= 120 dB
The sound power from the other fan can be calculated as
Ls2 = 10 log((0.5 W) / (1 10-12 W))
= 117 dB
The difference in decibel is
Ls1 - Ls2
= (120 dB) - (117 dB)
= 3 dB
From the table or diagram above a difference of 3 dB requires that 2 dB must be added to the highest sound pressure source as
Lt = (120 dB) + (2 dB)
= 122 dB
Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.
Measurement and instrumentation strategies.
Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.
Logarithmic unit used to describe ratios of signal levels - like power or intensity - to a reference level.
Sound pressure filters that compensates for the hearing sensed by the human ear.
The rules of logarithms - log10 and loge for numbers ranging 1 to 1000.
Sound attenuation vs. frequency in rotating heat exchangers.
Estimate noise generated by air flow in ducts.
Sound power noise generated by blade dampers in ventilation systems.
The transmission of outdoor sound through and around barriers - the Fresnel Number.
Introduction to decibel, sound power, intensity and pressure.
Sound power from sources like fans, jet engines, cars, humans and more.
Sound Pressure is the force of sound on a surface perpendicular to the propagation of sound.
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