TITLE: Interpreting FFT background noice
QUESTION [0 upvotes]: There is some construction down my street and I think the drills are putting off a high pitch noise so I downloaded a spectrum analyzer. they have an FFT plot here:
The "57-dB-A" I guess is the decibals of the noise at the current time. They have a scale and 57 is within the "normal room noise". Heavy traffic is 80 db-A.

REPLY [0 votes]: The horizontal axis is "frequency". Imagine all sound is made up of a mixture of frequencies - a bit like someone hitting all the keys of a piano at the same time. The spectrum analyzer does some clever mathematics to determine what keys were being hit, and how hard.
The horizontal (X) axis of your plot is frequency (in Hz, with the 'k' meaning 'kHz'). There is quite a lot of low frequency noise - below 500 Hz. Then it gets a lot more quiet, but there is a distinct peak at 4.5 kHz. That is a rather high-pitched sound - and it's sufficiently louder than the other sounds that your ears might be able to pick it up as a separate sound.
There is a "roll off" of the sound starting at 18.5 kHz. This could be the frequency response of your microphone, or attenuation of sound by air.
The vertical axis is calibrated in dB. That is a logarithmic power scale - when the dB increases by 10, there is 10x more power in the sound. I don't know what your "0 dB" point is - that's a bit arbitrary.
The dBa scale is the "acoustic noise" scale. This is a scale that gives different weights to sounds of different frequencies (to get a "equally irritating" sound level you need a different power level at different frequencies).
I think the "57 dBa" measure is intended to indicate the apparent sound level. This requires some absolute calibration of microphone sensitivity - not sure whether that was actually done for your measurement. The plot has a light blue "band" that catches the highest and lowest value of power observed in each frequency band - those fluctuations are natural when you have random noise as input.
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