This video is good as it was measured indoors with low background noise and compares the Mavic Air (
MA) to the
Mavic 2 (M2). Since I only have the
MA myself, I used the audio in this video to do some quick comparisons between the two drones while they are in the hovering state.
The noise spectra of these quadcopters is interesting when you run the sound through a spectrum analyzer. You have the motor shaft spectra, which reflects the RPM of the motor. Then you have the 'blade passing frequency' that is often number of blade multiples of the motor shaft frequency, which in the case of a 2 blade propeller used in the DJI drones, is 2X motor shaft frequency. Then you will also see higher harmonics of the blade passing frequency and other broadband frequencies due to aerodynamic inefficiencies with the rotor. Typically noises are generated by blade-vortex generation, blade wake interactions, etc.
From the spectra taken from the above video comparing the drones while hovering, I can deduce the
MA motors spin at a higher frequency than the M2 when hovering steady. The blade passing frequency of the
MA was measured to be 377Hz (motors ~11,000 RPM). The blade passing frequency of the M2 was measured to be 189Hz (motors ~5700 RPM). If someone knows the actual hovering RPM of the
MA or M2 and the above is in an incorrect range (since I couldn't find a source), please let me know but that is what I gathered from the FFT.
Also worth mentioning is that the noise amplitude of the blade passing frequency and all higher harmonics is much higher with the
MA than the M2, suggesting much better aerodynamic efficiency of the M2 compared to the
MA even from the get go before you look at spectral details. Perhaps the rotor design of the
MA is less efficient, such as the fact it lacks any wingtip devices on the rotor and has a simple flat rotor tip. Or perhaps due to rotor wake interactions with the 4 rotors being in such close vicinity to each other in this small quadcopter due to it's small compact size compared with the M2. Surely they can at least improve on the design of
MA's rotor though by adding wingtip devices like they did with the M2 with the raked wingtip to further reduce noise by a bit.
Then you also get to the higher harmonics of the blade passing frequency in multiples. This is another important detail because it shows where the noise spectra is in the frequency range and why it's so audible since human peak hearing perception is generally in the 2kHz to 5kHz range and noise in this range is more sensitive. The
MA with 377Hz (1f) blade passing frequency has measurable and significant higher harmonics spectras of 754Hz (2f), 1.1kHz (3f), 1.5kHz (4f), etc. Much of the dominant higher spectral peaks can be measured between 377Hz-3kHz, which has spectra well represented within peak human hearing range.
The M2 on the other hand with it's 189Hz blade passing frequency (1f) produces major measurable harmonic spectral peaks within 189Hz to 1.5kHz. Much of the major spectral peaks occur below 1.5kHz thus just outside of peak human hearing sensitivity.
In conclusion, the
MA produces higher amplitude of all harmonics when hovering, plus higher noise harmonics occur within peak human hearing range. This is probably why the
MA overall sounds louder than the M2 while hovering. Interesting to note this spectra occurs while hovering. When the drone is flying at speed, the motors will spin faster and some motors spin a different frequencies to each other too, therefore the noise spectra will be completely different than hovering. However, since the
MA produces louder spectra than the M2 at hover, I still expect it to be louder than the M2 under flying conditions, despite the M2 possibly having higher harmonics start to enter the peak hearing range of the human ear when it is flying harder. The M2 just seems like a more aerodynamically efficient beast.