English version machine-translated from the Vietnamese original — under review.
Some basic concepts of Silencers and Acoustic Louvers:
Noise-reduction performance of a Silencer: Denoted IL, short for INSERTION LOSS. The unit of measurement is dB 2. Noise-reduction performance of an acoustic louver: Denoted TL, short for TRANSMISSION LOSS. The unit of measurement is dB 3. Similarities and differences between IL and TL: Transmission Loss (TL) and Insertion Loss (IL) are both concepts related to the reduction of sound or signal level, but they have important differences: Similarities: • Unit of measurement: Both TL and IL are measured in decibels (dB). • Purpose: Both measure the degree of noise attenuation through a silencer or an acoustic louver. Differences: Transmission Loss (TL): • Definition: TL is the degree of attenuation of noise as it passes through an acoustic louver. • Application: Used to calculate the noise-reduction effectiveness of an acoustic louver in application (Noise level after the acoustic louver) Insertion Loss (IL): • Definition: IL is the degree of attenuation of noise as it passes through a silencer. • Application: Used to calculate the noise-reduction effectiveness of an acoustic louver in application (Noise level after the silencer)
Why there are two concepts, TL and IL, even though they are almost the same Although TL and IL both measure the degree of noise attenuation, they are used in different contexts. TL focuses on measuring the degree of noise attenuation through an acoustic louver, while IL focuses on measuring the degree of noise attenuation after a silencer. Having these two concepts makes it possible to identify more clearly the cause and location of noise attenuation in different systems: TL expresses the noise-reduction capability on the other side of the acoustic louver (usually the outside, the terminating side...), while IL expresses the noise-reduction capability after a silencer is added in series to the system (inside, enclosed...)
PRESSURE DROP, also called RESISTANCE, in English "Static Pressure Drop", is the reduction in static pressure as air or a fluid moves through a silencer or louver. It indicates the degree of obstruction that the silencer or louver imposes on the airflow passing through it.
Velocity or Face velocity is translated from the English term "Silencer Face Velocity", the speed at which air moves across the internal face of the silencer. It is usually measured in feet per minute (fpm) or metres per second (m/s). This velocity is calculated by dividing the airflow (ACFM) by the remaining cross-sectional area (Free Area) of the silencer or louver (ft² or m²)
Distinguishing dB and dB(A): The units dB (decibel) and dB(A) (A-weighted decibel) are both used to measure noise level, but they have some important differences:
dB (decibel): This is the unit for measuring sound level or sound pressure. It is a logarithmic scale, meaning that each 10 dB increase corresponds to a 10-fold increase in sound level. dB is used to measure sound without any weighting, i.e. without adjusting for the way the human ear perceives sound. dB is calculated by means of a logarithmic calculation.
dB(A) (A-weighted decibel): This is the unit for measuring weighted sound level, adjusted to the way the human ear perceives sound. The human ear is more sensitive to sound frequencies in the range from 500 Hz to 6,000 Hz, and dB(A) reflects this by applying an A-weighting filter. This makes dB(A) a more accurate measure of the noise level that people perceive. dB(A) is the reading of a sound level meter, or the noise level that the human ear hears.
For example, a sound with a noise level of 70 dB may be perceived as 65 dB(A) if it lies outside the frequency range to which the human ear is most sensitive.
Alex Dao, NSCA
