The second letter in our example is the capital letter “A”.
For indicators, the second letter is commonly "A", "C" or "Z". This is called the "frequency weighting" and indicates that some frequencies within the audio spectrum are, in some cases, given a weighting. i.e. the level of these frequencies is reduced or increased in the measurement.
The reason for these reductions at lower and higher frequencies has to do with our hearing ability. Humans are better at hearing mid-range frequencies than high or low frequencies. The following three sound samples demonstrate this. Each of these samples is recorded with the same level. Yet when you play them, you may perceive that the low (125 Hz) and the high (10 kHz) are softer. Older people may not even be able to hear the 10 kHz signal; with age people lose their ability to hear high frequencies.
Caution: There are many things that can influence your perception of loudness; high-frequency sounds may be more annoying and may thus appear louder, sound reflecting on surfaces (such as your table) may make low frequencies sound louder, size and/or quality of the loudspeakers may compromise low and high frequencies, your system may compress the signal if you play back too loud. Taking all these things into account, to experience the A-Weighting effect, we further recommend that you rather play these files softly through speakers, not headphones!
If we were to play these three samples through one of the channels of an audio mixing desk, because, for example, we wanted to test how the channel responds to each frequency, then we would measure the frequency response of the channel using an audio analyzer without frequency weighting. In other words, all frequencies are treated equally ("Z" = no frequency weighting).
If we were then to play these three samples through a loudspeaker to test how the speaker will be heard by humans, we want to reduce the high and low frequencies measured by the sound level meter so as to represent what humans hear. In this case we apply an "A" weighting to the frequencies. In our example, we would then measure LAFmax.
When sound levels get loud (above 100 dB), humans become more sensitive to both the high and the low frequencies. To represent this in a sound level meter measurement, we apply the "C" frequency weighting. In our example, we would then measure LCFmax.
In summary, the basic rule for frequency weighting is, use "Z" for all audio measurements and acoustic measurements where you do not want to consider the human hearing perception, such as testing the response of a loudspeaker across the entire frequency range. Use "A" for all acoustic measurements under 100 dB. Where sound levels are above 100 dB, use "C".
Consequently, the options for frequency weighting are LZ, LA and LC