Decibels or dB for short are something that you’ve probably come across.
By definition, a decibel (dB) is simply a unit that is used to measure the strength of a sound or the power level of an electrical signal by comparing it with a given level on a logarithmic scale.
One question that I usually get from people in audio production has to do with dBA and dBC, specifically what they’re.
I thought this was an interesting question and I decided to create this article fully explaining this in detail.
With that said,
What is dBA and dBC?
dBA and dBC are filter types that are used to measure dB. The letters A and C are filters with a different sensitivity to frequencies. You’ll find these measurements on most sound meters.
The dBA sound level meter measures mid range frequencies while a dBC sound level meter measures low and high frequencies.
Noise Measurements
A-weighting and C-weighting refer to different scales for loudness perception at various frequencies.
At very low sound pressure levels the human ear is less sensitive to low frequencies and some very high frequencies.
In other words, low frequency tones or sounds like 100 Hz,
Have to be made much louder in order for them to be perceived as equally loud as, say, a 1,000 Hz
tone…. when the tones are presented at a low sound pressure level like for instance 40 dB.
But at higher sound pressure levels….like say 90 dB the ear will perceive the two tones as nearly equal in loudness.
Weighting scales are therefore built into the filters of sound level meters so that measurements taken with those meters can easily mimic this frequency and level dependent on the behavior of the ear.
So let’s break it down further.
The A-weighting scale lets the sound level meter “hear” soft sounds approximately in the same way an average, normal human ear does. This is achieved by making the meter less sensitive to most low
frequency, and some high frequency, energy.
On the other hand, The C weighting scale is
used to measure loud sounds with approximately equal sensitivity at all frequencies, just like the human ear.
A-weighting (dBA)
A weighting basically follows the frequency sensitivity of the human ear at low levels. This is the most commonly used weighting scale, because it also predicts quite well the damage risk of the ear which is of vital importance.
Sound level meters set to the A-weighting scale will filter out much of the low-frequency noise they measure, which is similar to the response of the human ear.
B-weighting (dBB)
B-Weighting basically follows the frequency sensitivity of the human ear at more moderate levels, which is used in the past for predicting performance of loudspeakers and stereos, but not industrial noise. It is no longer used with modern sound level meters.
C-Weighting (dBC)
C Weighting pretty much follows the frequency sensitivity of the human ear at high noise levels. The C-weighting scale is flat, and therefore includes much more of the low-frequency range of sounds compare to the A and B scales.
A & C weighting Applications
Noise reduction
In situations where dBA sound levels exceeds safe or comfortable levels, suggestions for reducing the sound intensity may include limiting the level or volume of the sound, moving farther away from the sound source or using ear plugs to protect the ears.
C-weighting occurs for peak measurements and in noise measurement for the entertainment industry, for example in a live stage event or in running a movie theater business where bass noise transmissions can become a problem.
In sound systems
Professional sound systems sometimes list an A-weighted rating in their printed specifications.
If you find this, it means that an A filter actively hides or filters certain hums or other background sounds.
That manufacturer obviously felt the need to filter some objectionable noises in its sound system.
Most people see this as a positive addition to a system, or while others presume the sound system is not of top quality in the presence of A-weighted filters.
Otherwise, the manufacturer would not feel compelled to filter these unwanted sounds from coming through the system.
It’s all matter of interpretation.
Conclusion
In conclusion…
Sound pressure is denoted in decibels.
Sound pressure decibels are conventionally expressed in relation to average human threshold of human hearing, the smallest sound a human with normal hearing is expected to be able to hear.
With that said, What you should remember in relation to his topic is that A-weighted sound level discriminates against low frequencies, in a very manner similar to the response of the human ear.
However …. The C-weighted sound level does not discriminate against low frequencies and measures uniformly over the frequency range of 30 to 10,000 Hz.
This weighting scale is useful for monitoring sources such as engines, explosions, and machinery. The sound levels measured with these two weightings have units of “dBA” and “dBC”, respectively.
