The Physics Behind Capturing Clean Audio Outdoors

Microphone wind noise remains one of the most persistent challenges in outdoor recording. Regardless of how much we invest in premium microphones, a single gust can render our efforts useless within seconds. Understanding the principles behind windshield performance isn't merely academic, it's an essential tool in every sound recordist's arsenal. 

Wind and Audio: Two Distinct Pressure Phenomena 

At the core of windshield design lies a fundamental distinction: wind represents high-velocity local air pressure changes, whilst sound comprises much lower-velocity pressure variations. This difference is key. 

Thin fabrics or open-cell foams can allow sound waves to pass with minimal attenuation, yet still disrupt and decelerate the faster pressure changes created by wind. Foam achieves this through its network of interconnected cells – a labyrinth that breaks up wind wavefronts. However, these same properties affect the audio signal itself. 

At high frequencies, the disrupted wavefronts recombine and cancel, causing noticeable high-frequency loss. At lower frequencies, the waves become muddled or decorrelated. This is typically tolerable for spoken voice, but a compromise nonetheless. Foam, therefore, is never acoustically neutral; it invariably affects both wind and audio. 

Basket Windshields: The Pressure Chamber Effect 

Basket windshields (also known as blimps or zeppelins) employ a thin outer fabric to slow airflow, much like foam. However, their true effectiveness lies in their behaviour as pressure chambers. 

A basket functions as a rigid, largely sealed cavity. To high-velocity wind, it effectively appears solid. When wind strikes the basket, it compresses the internal air like squeezing a sealed container. This creates a pressure increase that occurs simultaneously at all points within the cavity. This is a critical factor when considering how directional microphones operate. 

Directional microphones, or shotgun mics, are pressure gradient devices. This means they measure the pressure difference between front and rear ports via an acoustic delay path. If the pressure changes at both ports are perfectly in phase and matched in level, the microphone detects nothing. This effect is most pronounced at low frequencies, which is precisely where wind noise resides. Consequently, basket windshields excel at controlling the spectral region most compromised by wind. 

Why Combining Foam and Basket Often Fails 

This is where conventional wisdom falters. Foam creates wavefront decorrelation, whilst a basket performs optimally when maintaining correlated pressure changes within its cavity. Combine the two, and you create a system working against itself. Rather than reducing wind noise, the foam disrupts the pressure chamber, leading to degraded performance and increased high-frequency loss.

This explains why having a foam on a mic inside a windshield basket does not improve wind noise reduction. It may actually have a negative audio effect with some microphones. However, this “foam always on” setup can be convenient if constantly moving from indoors to outdoors for minimally affected mics. 

Fur Covers: The Most Effective Solution 

A fur windcover (commonly termed a "deadcat", "fluffy", or "windjammer") covering a basket windshield reduces wind noise through a fundamentally different mechanism: 

This final point is crucial, as wind noise attenuation follows the inverse cube law. This means even a modest increase in distance between the turbulence source and microphone can result in a much larger reduction in wind noise (if using the same design of windshield). The usage of newer, modern materials may also allow smaller diameter windshields to attenuate wind more effectively, such as in the case of the Mini-ALTO. 

Most importantly though, the appropriate design of the outer fur allows the stiffer shell of a windshield to absorb energy whilst generating minimal noise of its own, making it exceptionally effective when it comes to passive wind control. 

The Fallacy of Multiple Layers 

In practice, adding more layers of different types of wind reduction doesn’t improve results. 

The objective isn't about stacking attenuation methods but rather selecting wind noise reduction approaches that complement each other acoustically and provide the appropriate trade-offs for the specific outdoor recording application (i.e. space required, setup speed, weight, durability, etc).  

Conclusion 

Wind noise reduction is very much about understanding the nuances of both physics and acoustics. An effective windshield design aligns to the behaviour of the air pressure, the microphone’s mechanics, and the wind turbulence characteristics. Whether booming in a gale or capturing delicate coastal ambience, understanding these principles enables smarter equipment choices, more effective troubleshooting, and ultimately, cleaner and more reliable audio recordings. 

About the Author: Simon Davies is the Founder, Managing Director and Lead Product Designer at Radius Windshields, based in Devon England. Simon has more than 30 years of experience in designing and manufacturing microphone windshields, shock mounting and other audio accessories. After completing training as a location sound recordist at BBC Wood Norton, Simon began working at Rycote Microphone Windshields in 1997 and held positions there as production engineer, technical director and eventually managing director and owner until selling that business in 2018.