Frequency weighting
An ideal sound level meter initially records all frequencies equally. This evaluation is called "Z" (zero) or flat.
However, the sensitivity of the human ear varies greatly in the frequency range from 20Hz to 20kHz. A sound at 2kHz appears considerably louder than 15kHz. This effect is taken into account in acoustic measurements by attenuating the low and high frequencies. The most common is the DIN "A" curve. Measurements based on such an assessment are denoted by dB(A).
Our ear performs level-dependent
However, our frequency perception is also level-dependent. At higher levels, we perceive lower and higher frequencies better in proportion. The "C" curve is often used here.
The weighting filters are standardized so that the attenuation at 1kHz is exactly 0dB.
You would actually have to use different frequency weightings for each level. This is exactly what is done with the more complex psycho-acoustic measurements , which reflect our hearing ability far better. For historical reasons, however, we use most of the time the A-curve.
The level difference A-C
Simple sound level meters do not have an octave or third octave analysis to specifically evaluate individual frequency ranges. However, with a simple trick, you can also use such a device to determine whether there are significant components at lower frequencies. You measure the sound level with A weighting and also with C weighting. If the difference is greater than 20 dB, this is a clear indication of low frequencies.
What weighting filters does a typical sound level meter have?
The A, C and Z curves are defined in IEC61672-1. The A and C curves are integrated in almost every sound level meter. The Z curve is only found in higher quality devices.
However, in audio technology there are many other evaluation filters
CCIR/ITU-R 468
The A curve is used very frequently. Even simple handheld sound level meters integrate this rating. It is based on the work of Fletcher-Munson and corresponds to the typical hearing ability at 40 phon. However, this curve does not adequately reflect the hearing ability in the presence of noise. The signal-to-noise ratios measured according to A do not adequately reflect the subjective hearing impression. The A curve is particularly unsuitable for noise suppression processes such as pre-emphasis or the well-known Dolby processes (tape recorders).
Therefore, other evaluation characteristics have now become established. The CCIR/ITU-R 468 curve is of particular importance. It drops sharply at high frequencies and emphasizes the area around 5kHz. In this frequency range, the human ear is very sensitive to noise. Assuming that the noise source is white, the A values are around 10dB below the CCIR/ITU-R 468 values with a bandwidth of 20-20kHz.
From a marketing perspective, this may be a reason to use A filters, as the values are generally "better". However, this example also shows that in serious and reliable measurements, the evaluation filters must always be specified, as otherwise the results are not comparable.
This evaluation curve is particularly suitable for measurements on microphones (noise evaluation) and for distortion analyses.
The CCIR/ITU-R 468 curve has its maximum at about 6.3kHz with 12.2dB. At 1kHz it is, as usual, at 0dB.
