In aerospace systems, acoustic noise management is a critical concern that directly impacts the user space and/or environment. In this blog, we will explore the main sources of acoustic noise, and the techniques used to reduce it.
Understanding Acoustic Noise
Acoustic noise is the result of mechanical vibrations traveling through a medium—typically air. At sea level, sound travels at approximately 1,087 ft/sec (331 m/sec), though engineers often use 1,130 ft/sec (340 m/sec) for calculations.
The wavelength of sound is given by: λ = c / f,
where c is the speed of sound and f is the frequency in Hz.
Human hearing spans 20 Hz to 10,000 Hz, with peak sensitivity between 1,000 Hz and 4,000 Hz, a range critical for designing systems that minimize perceived noise.
Measurement Methods
dB (Decibel): In acoustics we typically express either sound pressure level (SPL) or sound power level (SWL) in decibels.
SPL reference: p₀ = 20 μPa
SWL reference: W₀ = 10⁻¹² W
dBA: A‑weighting filter models human hearing sensitivity (attenuates low/high frequencies) compared to standard dB
SPL (Sound Pressure Level): Sound Pressure Level (SPL) is reported at 0.91 m (3 ft) in free‑field / hemi‑anechoic conditions unless otherwise noted and includes A‑weighting (dBA) where indicated. Air performance measured per ANSI/AMCA 210 using the same installation category.
Rotron uses a 3-foot standard distance from the fan without ducting or filters.
Where AMCA facilities are not available, equivalent methods ISO 3744/3745 or ISO 9614 are used and clearly identified.
SWL (Sound Power Level): Sound Power Level (SWL, dB re 1 pW) determined per ANSI/AMCA 300, rated per ANSI/AMCA 301 across octave bands (63–8k Hz).
SWL is independent of distance and room; this considers only the source of noise.
Narrowband Analysis: Ideal for tonal and vibration diagnostics.
Octave Band Analysis: Useful for hearing protection and general sound profiling.
AMCA (ANSI/AMCA) Standards—fan‑specific
- ANSI/AMCA 210 – Laboratory Methods of Testing Fans for Air Performance.
Reference this when pairing acoustic data with airflow/pressure ratings so performance and noise come from compatible setups.
- ANSI/AMCA 300 – Reverberant Room Method for Sound Testing of Fans.
Establishes how to measure fan sound (SPL data, derive SWL) in a controlled facility.
- ANSI/AMCA 301 – Methods for Calculating Fan Sound Ratings from Laboratory Test Data.
Defines how to convert measured data to rated SWL/SPL across octave bands and apply installation corrections, ensuring apples‑to‑apples product comparisons.
Weighted Scales
- dBA: Most relevant for human perception and regulatory compliance.
- dBC: Includes more low-frequency content, used at higher volumes.
Reference Noise Values
Sound is measured in decibels (dB), a logarithmic scale.
| Decibel Level |
Description |
| 0 dB |
Threshold of hearing |
| 40 dB |
Quiet office or library |
| 60 dB |
Normal conversation |
| 70 dB |
Vacuum cleaner |
| 85 dB |
Traffic |
| 120 dB |
Threshold of pain |
| 140 dB |
Jet engine |
Rotron Fan Noise Values
The following is an example of noise levels of Rotron Fans.
Maxiax fans for airborne radar, avionics, transmitter cooling and spot cooling with various sizes.
Spartan fans are usually used in severe environmental conditions.
| Noise Level |
Description |
RPM |
| 72 dB(A) |
011355000 MAX50000E28BN,S,2793SF |
8900 |
| 73.5 dB(A) |
Maxiax8 (041224000 MAX80001AN1CL,S,3387JH) |
4850 |
| 88 dB(A) |
011455000 MAX45005,E28XM,S,3115SF |
20000 |
| 86.9 dB(A) |
035977000 MAX57507,AQ2FM,N,3260JH |
11400 |
| 54.4 dB(A) |
020312000 SPTRL,115AC,N,682YF |
3240 |
| 67 dB(A) |
027802000 SPTRL,115AC,N,1395YF |
5470 |
Designing Quieter Fans
There is acoustic noise vs structure-borne noise. We have ways and methods to address both.
Acoustic Noise levels are typically higher on the exhaust side of the fan. This is due to the discharge of high‑velocity, highly turbulent airflow, combined with aerodynamic noise generated by blade tip vortex shedding and trailing‑edge effects.
Acoustic Noise emissions can be mitigated through the use of exhaust ducting, with the degree of attenuation dependent on the duct length, material properties, and geometric design.
| Material Selection |
The choice of materials directly affects how vibration propagates. Rotron selects composites and alloys with high internal damping coefficients to absorb mechanical energy and reduce structure-borne noise. |
| Precision Balancing and High-Grade Bearings |
Imbalances in rotating components are a primary source of tonal noise and vibration. Rotron uses dynamic balancing techniques and high-grade bearings to minimize rotational irregularities. This not only reduces noise but also extends the operational life of the fan. |
| Blade Geometry Tuning |
Blade-passing frequency, the rate at which blades pass a fixed point, is a dominant tonal component in fan noise. By varying the number of blades and adjusting their pitch and curvature, we can shift this frequency away from sensitive ranges or structural resonances. This “frequency detuning” helps avoid amplification through harmonic coupling. |
| Trailing Edge Modifications |
Trailing edges of blades are hotspots for vortex shedding, which generates broadband noise. Rotron modifies these edges, sometimes using serrations or curvature adjustments, to disrupt coherent vortex formation. This technique reduces aerodynamic noise without compromising airflow efficiency. |
| Stationary Vane Variation in Vaneaxial Fans |
In vaneaxial fan configurations, stationary vanes guide airflow and stabilize pressure. By varying the number and orientation of these vanes, we can alter the interaction between rotating and stationary components, minimizing tonal peaks and smoothing the acoustic signature. |
| Airflow Control via Optimized Geometry |
Fan housings, blade profiles, and inlet/outlet geometries are engineered to control airflow velocity and turbulence. Slower, more laminar flow reduces noise generation. |
Using fan laws with speed in relation to sound we find that
ΔL=50log10(N2/N1)
With a 20% increase in speed
ΔL=50log10(1.2)
So, a 20% change in speed results in about 4dB. In perceived noise that is noticeably louder. Applying this logic we have below.
| Speed Change |
Approx. Noise Change |
| +10% speed |
+2 dB |
| +20% speed |
+4 dB |
| +50% speed |
+9 dB |
| 2× speed |
+15 dB |
| −20% speed |
−4 dB |
| −30% speed |
−8 dB |
In testing our Spartan fan (PN 012118000) at 5000 RPM it registered at 62.8dB, when the speed was lowered to 3,000 RPM it registered at 51.4 dB a reduction of approximately 10dB. With a 40% reduction in speed that confirms the approximate noise change per the fan laws.
With this in mind, Rotron created the Spartan 2 fan family. The Spartan 2 is aerodynamically optimized for operating points in the range of 0 to 190 CFM at pressures of 0.7 to 2 in. H2O. As a result, in this operating range the fan exhibits a significant reduction in tonal noise (up to 5 d) and an overall acoustic reduction of 3 dBA. Additionally, the fan is 25% more efficient than the standard Spartan fan.
System Design and Air Mover considerations
Effective noise control requires attention to how fan-generated sound interacts with surrounding components and environments. Engineers can significantly reduce acoustic impact by applying the following system-level strategies:
| Limit Line-of-Sight to Moving Blades |
Direct exposure to rotating fan blades increases the transmission of airborne noise. Using enclosures or ducting to obstruct line-of-sight helps reduce high-frequency sound propagation and tonal noise associated with blade-passing frequencies. |
| Apply Acoustic Damping and Mechanical Isolation |
If the operating frequency of the fan aligns with the natural frequency of the mounting structure, resonance can occur, amplifying both vibration and noise. Engineers should conduct modal analysis and adjust fan speed or mounting design to avoid this overlap. |
| Strategic Fan Placement |
Placing fans farther from sensitive components, such as sensors or user interfaces, reduces perceived noise. |
| Filters to Dampen Sound |
Air filters can smooth turbulent airflow and reduce broadband noise. EMI filters, while primarily used for electromagnetic interference, may also contribute to acoustic damping. |
| Lower-Speed Fan Configurations |
Noise output increases with fan speed. Selecting larger fans that operate at lower RPMs or using multiple fans to distribute airflow can achieve the same cooling performance with reduced acoustic impact. |