Question about spectrogram of a measured room

20 hertz can come from anything. From the engine of a large car, from a helicopter, from an object falling in the next room, from a slamming door. And even from air movement near the microphone. So no need to worry.

Likely an HVAC fan. I've seen this often in released music tracks that I've demastered. In North America (60 Hz/110v), I noticed that this fundamental frequency is usually ~17 Hz and its harmonics (34, 51, 68 Hz, etc.). The EU will be 14 Hz, etc.

If the HVAC system was on, try temporarily turning it off and measure again to see if it disappears.

This all presupposes that you're in some sort of performance venue, but you haven't described anything about what you're measuring and where that space is, home or commercial, etc.

Other than that, the physics of subharmonic generation comes from very nonlinear parasitic effects, so if that energy is coming from higher frequencies, something very nonlinear is occurring.

Chris

FYI (only)...

Here is an uncalibrated RTA (real-time analyzer) measurement taken in my listening room about 10 years ago, showing local peaks due to in-room laptop and HVAC/refrigerator noise sources.



I'm aware that the 90 Hz and 197 Hz peaks came from the nearfield laptop that I was using. It was August and it's hot in Texas...all the time...so the laptop fan is always ramped up at that time of year. I can't hear the higher amplitude 7 Hz, 12 Hz and 18.5 Hz peaks, since the human hearing system is not sensitive to extreme low frequencies, which is the reason why we typically don't notice HVAC fans. [However, if you happen to be an elephant...the modern industrialized world is a very noisy place indeed--all the time.]

If you had taken those measurements outside prior to a quasi-anechoic polar measurement task for a developmental loudspeaker (the K-402-MEH), even though I live on the edge of a lake and curiously one of the quietest places in the D/FW area due to that fact, you would have seen the following about 8 years ago in January:



The measurement microphone was elevated ~5 ft/1.5 m from the grass-covered ground, and 1/4 and 1/2 wave disturbances can be seen in the RTA spectrum at ~120 and 240 Hz, as well as the higher frequency noise from my neighbor's swimming pool recirculation pump. The ramping-down of the amplitude below ~10 Hz is probably due to the decreasing sensitivity of the measurement microphone itself.

Note the 1/f characteristic amplitude trend. which is also true for music tracks (i.e., the 1/f downward to the right trend). Only in upsweep acoustic measurements do we typically set the amplitude response to be constant amplitude.

Chris