After several months spent discussing various aspects of electricity for professional audio applications, it’s time to turn in new directions. For those sick of the National Electrical Code, take heart, this month we look at the effects the atmosphere has on live sound production. Just as electricity is integral to audio, so too is air, the fluid medium that we live in, breathe in, and rely on to carry sound waves to the audience. In this article we will investigate some of the influences that air has on the sound that travels through it, and also how atmospheric or weather conditions can influence the experience of “combat audio.” First we discuss a little about air as a fluid, and then move on to practical ramifications for gigging.
Fluids — A Quick Intro
A great place to start the discussion of air is to introduce the concept of a fluid. Fluids are substances that continually deform, or flow, when you apply a shear stress to them. Shear stresses can be thought of sliding forces. Fluids can be different phases of matter, but the most common are liquids and gasses. The components in a fluid have a property called viscosity, which is a measure of a fluid’s resistance to deforming. Another way of thinking about viscosity is that it is representative of the stickiness between molecules. The stickier molecules are, the more they resist deforming with an applied stress. It is the interaction between molecules that transfers the energy of sound as it moves through a fluid.
Air, which is mostly a mixture of nitrogen and oxygen, is a rather unique fluid medium for sound to travel through. As sound travels through air from a source, the collective decrease of intensity of that sound as it travels is called attenuation. Attenuation is made up of multiple components, some obvious, others more subtle. The most basic component of attenuation is what we will call geometrical attenuation.
The Varied Faces of Attenuation
Geometrical attenuation results from the sound wave spreading out in space as it leaves a source. As the energy from the source is spread out over an ever greater area, the sound energy at any given point is diluted. Geometrical attenuation gives rise to the classic inverse square law. Geometrical attenuation is not a lossy process, which is to say that no sound energy is converted into heat energy; it is merely a function of the increasing surface area of an expanding sound wave.
Another type of attenuation is sometimes called effective attenuation. Lumped under effective attenuation are all of the various scattering phenomena, such as reflection, refraction, and diffraction. These behaviors, analogous to the familiar optical behaviors, are also not lossy, but reduce the effective sound energy at a given point by redirecting (i.e., scattering) it in other directions.
After geometrical attenuation and scattering, we are left with attenuation that actually converts some fraction of the sound into heat energy. This attenuation process is called absorption. Classical absorption results from the collected effects of the viscosity introduced above. Here the interaction of molecules, due to the stress of viscous forces, generates some amount of heat from sound passing through. That heat is then distributed throughout the fluid as the molecules bump into each other. This mode of absorption is a comparatively minor effect in air.
For our acoustic fluid air, it turns out that a special class of absorption dominates our observed behavior of sound being converted into heat. This type of absorption is a called molecular relaxation. Relaxation is a function of the time difference between a molecule’s translational action and vibrational action. To think about this physically, imagine a sound wave moving through air that causes the molecules to translate left and right as the sound passes. If there is a delay before the molecule’s vibrations match the translation, the energy spent to create the vibration is no long in phase with the translational energy, and ends up converted to heat (heat is a measure of the average motion of molecules).
The lagging behavior of molecular vibrations results in an interesting effect for air, namely that the diatomic molecules, oxygen (O2) and nitrogen (N2), strongly absorb high frequencies above 4 kHz. This effect is especially pronounced in dry air, where there is little water vapor. In humid air, by contrast, water molecules more easily exhibit vibrational changes, and this reduces the high frequency absorption effect from oxygen and nitrogen.
In summary, we have seen that attenuation results from a mixture of causes. Some of those causes spread sound energy out in space, and some of them directly convert sound energy into heat. We call the heat-producing mechanisms absorption. For the absorption mechanism in air, the most notable effect is the preferred absorption of high frequencies due to the behavior of oxygen and nitrogen.
Practical Ramifications of Attenuation
If attenuation has both lossless and dissipative behaviors, which affects matter in practice at a gig? The answer is “both!” As an example, the sound system sounds dialed in at sound check in an empty room at 4pm with 50% relative humidity. By 10 p.m., with the room now filled with perspiring audience members, suddenly that dialed-in system from sound check is intolerably bright and painful in the high frequencies due to the extra humidity. Another example is the phantom call to the authorities from a location far from the venue, and the hapless sound company trying to figure out how the sound could travel so far. The first circumstance is due to absorption, a lossy process, and the second circumstance results from scattering.
The reality of gigs is that air is an inherently absorptive medium, so even lossless behaviors (like scattering) ultimately drive further losses, because the scattered sound then travels through more absorptive air. It is the responsibility of the system tech and/or the FOH engineer to be aware of the various effects air has on the P.A., and to compensate accordingly.
For absorptive processes, there are numerous curves and equations that can be used to figure out how much absorption is occurring at given levels of humidity and temperature. Some processors even automatically compensate for these effects. In a controlled environment, like tuning an installed sound system, I personally find the curves and/or equations to be useful; the absorption behavior will be reasonably consistent in a climate controlled environment. For an outdoor festival event on a summer day, however, I find that I primarily resort to my ears and “meat computer” to set the relative amount of high frequencies to cover the course of an event. Regardless of the approach, the most important thing for professionals to realize is that air is not a stable, unchanging medium. Attenuation will change throughout the course of events, and it is appropriate to shape the sound system’s behavior to reflect changes in the environment and maintain a consistent sonic signature.
Ramifications from Temperature
In addition to absorption behavior, attenuation effects can have dramatic influence on the way sound travels due to temperature gradients. Since the speed of sound is proportional to temperature, sound travels faster in warm air than in colder air (see sidebar). If you have a region where there are temperature gradients in the air, some dramatic effects can arise.
The first case is known as a temperature inversion, where the air nearest the ground is colder than the air up higher. This sort of circumstance can happen after the sun sets and the ground cools quickly. As sound travels faster in the higher air, this causes the sound to bend (i.e., refract) downwards, much like light refracts in a lens. If you have ever heard a distant campfire on a cold night, there is a good chance a temperature inversion is responsible. There is also a good chance a temperature inversion played a role in that distant noise complaint.
In contrast to a temperature inversion, the other effect seen is called temperature lapse. Here, the air nearest the ground is warmer than the air higher up, and this causes the sound to bend upwards, away from the audience. The effects of temperature lapse can throw off the aiming assumptions of P.A. prediction software, and can influence how far the P.A. will cover effectively. Air absorption effects, coupled with a temperature lapse, can leave the sound company scrambling for some delay speakers when everything seemed fine on paper before the gig.
Conclusion
Most readers of this article will probably have experienced a combination of the attenuation effects above, but perhaps have not seen each of them broken out into discrete entities. The presence of humidity decreasing high frequency absorption is particularly unintuitive to grasp. When split into individual behaviors, the whole of attenuation becomes more understandable. An inescapable conclusion from the behavior of nitrogen and oxygen is that we need to have extra high frequency horsepower available to overcome air absorption. For instance, with modern vertical array sound systems providing three or more compression drivers per enclosure, manufacturers are acknowledging the realities of air in their product lines.
Temperature lapse and inversion are more subtle phenomena. The gradients are smaller, and it is not as easy for sound companies to measure the temperature of the air vertically above the plane of the audience. The effects are quite real, and can be rather dramatic for long-throw applications. An area of future growth for the industry could be including thermal effects in the modeling of array behavior, and tools that facilitate measuring air temperature versus height.
The diligent system tech and/or FOH engineer should endeavor to keep tabs on the environmental changes to the sound system throughout the event. Tools like a humidity sensor, thermometer, and barometer should be considered additions to the toolkit, complementing rangefinders, distros and measurement systems. Voicing the sound system should be considered an ongoing process throughout the gig, sensitive to the changing environmental conditions. One may have to make changes, especially in the high frequencies, to keep the tonal balance developed at sound check consistent late into the evening.
