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Up In The Air: Factors Contributing To How Sound Behaves In The Great Outdoors

Looking at various environmental factors which can affect sound propagation that simply don't exist in the controlled indoor environment.

By Andy Coules January 3, 2018

The most obvious difference between indoor and outdoor sound is the lack of reflecting surfaces. In my previous article, Unique Signatures, we explored the acoustic behavior of sound in enclosed spaces, so now our attention turns to acoustic behavior in the free field, i.e., outdoors.

It means we don’t have to deal with reverberation, standing waves, room modes or any of the other issues that make mixing sound indoors so challenging. However, there are various environmental factors which can have a impact on the sound propagation that simply don’t exist in the controlled indoor environment. To understand the impact of these environmental factors, first we need to take a look at how sound propagation works.

That which we perceive to be sound is actually acoustical energy traveling, as waves, through a physical medium, be it gas, liquid or solid, so when looking at live sound we’re most interested in sound propagation through air (Figure 1). We don’t immediately think of air as a physical medium, because we can’t see it, but without it there would be no sound.

The transmission of acoustical energy is achieved by adjacent air molecules vibrating backwards and forwards against each other, while the molecules do move they don’t travel with the sound, they merely bang against each other in waves of compression and rarefaction much like the suspended balls in Newton’s cradle.

The speed at which sound travels through a medium depends primarily on it’s composition, sound actually travels slowest in gases, faster through liquids and faster still through solids. The fastest possible medium for sound transmission is a stiff material such as diamond through which sound can travel about 35 times faster than in air.

The temperature of the medium can also affect the speed of sound transmission as molecules at higher temperatures have more energy and thus vibrate more vigorously enabling the sound waves to travel faster.

The speed of sound waves in air is approximately 1,132 feet per second (or about 772 miles per hour) which represents the speed at 74 degrees Fahrenheit. If the temperature drops to 32 degrees F, the speed drops to 1,086 feet/second, and if it raises to 95 degrees F, the speed is 1,155 feet/second.

Changes in air pressure as a result of changes in altitude will also affect the temperature of the air so this too can have a bearing on the speed of sound transmission – all of the figures quoted above represent the speed of sound at sea level. Therefore, if we are reliant on the air molecules to transmit sound waves, then it stands to reason that anything that has a direct effect on those molecules will also affect sound propagation.

Key Issues

The three most common environmental factors that can impact sound propagation are wind, temperature and humidity.

Wind is the natural currents which move air molecules around so it’s bound to have an influence on sound. In a direct sense, wind can be a source of noise on exposed microphones simply by moving air molecules against the diaphragm in the same way that sound waves do.

Many microphones have built-in wind screens (i.e., within the capsule), but we typically augment them with external wind shields when using microphones outdoors. These take the form of foam sleeves which slip over the business end of the mic, the pores of which provide a longer path to the capsule, which helps reduce the energy of any unwanted noise.

This will, of course, also impact the wanted sound by reducing the overall level and rolling off the top end a bit but it’s usually a price we’re willing to pay. I always try to only use wind shields when absolutely necessary (i.e., when the stage area isn’t adequately shielded from the elements).

Wind shields should not be confused with pop shields, which are more commonly seen in the studio environment. Pop shields comprise a mesh of fabric stretched across a circular frame which is positioned between a vocalist and the microphone with the specific aim of reducing the “P” and “B” plosive sounds without affecting the overall level or frequency content of the sound.

Wind can also have an effect on sound once it issues forth from loudspeakers, the most common result being refraction. If sound encounters a crosswind it can be pushed sideways making it appear to come from a different location, whereas if sound travels into or against the wind, the resulting resistance or assistance can prevent the sound traveling far or carry it further.

However wind does not travel at the same speed at all altitudes; it typically travels slower closer to the ground where there are more obstacles and faster higher up, which creates wind gradients (Figure 2).

This causes sound traveling in the same direction as the wind to bend downwards while sound traveling into the wind will bend upwards, the result being an attenuation of the sound levels for anyone standing upwind while the levels downwind will be higher.

Thankfully, under normal still conditions these effects are minimal, as the average wind speeds will be relatively slow compared to the speed of sound; however, shifting wind can easily mess with the stereo image. Something that has a greater effect on outdoor sound is varying temperatures.


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About Andy

Andy Coules
Andy Coules

Sound Engineer, Tour Manager, Audio Educator
   
Andy Coules is a sound engineer and audio educator who has toured the world with a diverse array of acts in a wide range of genres.
http://andycoules.co.uk

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