Live Sound International published an interesting article by Andy Coules in the January 2018 edition concerning sound in the great outdoors. Although the article is mainly focused on large venue installations several points of interest relate to residential and light commercial outdoor sound installs as well. We are going to summarize several of those points here but suggest that you check out the article and get a free subscription to the magazine on their website.
Reflection is everything
The biggest difference between indoor and outdoor sound is the absence of outdoor reflecting surfaces. However that doesn’t mean there are not issues that impact outdoor sound reproduction. Here are some of the articles covered points and topics:
Sound fall off
With typical point source speakers (like all Terra outdoor offerings) sound drops off at a rate of 6dB for every doubling of distance from the source (line arrays have a more complicated fall off pattern). So if you make the speakers loud enough for (relatively) distant listeners they’ll be too loud for those close by. Always figure speaker-to-listener distances when determining speaker placement.
When sound is generated in air it doesn’t create a breeze by forcing air molecules to travel with the sound waves (in spite of misleading “tube demo” you’ll see on television). The molecules pulse into each other in waves of compression and rarefaction.
The speed of sound is different in gases like air (slowest), liquids (faster), and solids (fastest). In fact, the stiffer the material the faster it transmits sound. For example, sound travels through diamond 35 times faster than through air.
Air temperature impact
Temperature impacts the speed and to some degree the direction of sound. Warmer air, faster sound travel and vice versa. Faster sound travel will tilt the sound pattern up; slower travel will tilt it down. For example, warmer air at ground level and cooler air above, as the sun goes down. The sound at ground level will travel faster and therefore tilt up. In the morning the opposite effect can be observed.
Higher humidity will result in high frequencies being attenuated. Low frequency, long wavelengths require more power to generate and are less prone to being reduced by humidity levels. Highs (say above 2k) are much more susceptible to the air molecules frictional distance reduction. High humidity will allow the highs to travel further, expanding their coverage.
Sound can “turn” in the direction the wind is blowing. Typically sound traveling in the same direction the wind is blowing will “tilt” down. Sound traveling against the wind will tend to tilt up. Typically, these effects are minimal but it’s good to be aware that they exist. Strong winds can reduce or enhance sound travel based upon the direction it’s blowing.
Much of this can have some impact on a large residential or commercial installation. The best advice is to install as many speakers as reasonable and put them as close to the listening areas as possible. Of course, there are always compromises involved but being aware of this information should help you make more knowledgeable decisions.