We talked about the effects of wind on outdoor live sound in a previous TTOTD (see the TTOTD from 5/8/03). At that time we promised to address other issues such as Temperature and Humidity, which we will do in this article. Much of the following information can also be found in the Yamaha Sound Reinforcement Book available on our webstore.
The speed of sound is affected by temperature and humidity. Because it is less dense, sound passes through hot air faster than it passes through cold air. For this reason, temperature gradients cause refraction effects, which are for the most part, nothing to worry about. For instance, morning is a time when the ground is still cool from the night before but the upper air is already warming due to the sun. Under these conditions, sound can bounce between the gradient and the ground, forming regions of higher and lower sound intensity. Also, as sound propagates through air, the air absorbs energy from the sound wave, attenuating (weakening) it. The effect is significant only at frequencies above 2 kHz, and increases with frequency. This is the reason why when we hear thunder in the distance it is only a low rumble. The high frequency “crack” has been attenuated more rapidly than the low frequency portion of the noise. The attenuation of sound in air is affected by the relative humidity. Dry air absorbs far more acoustical energy than does moist air. This is because moist air is less dense than dry air (water vapor weighs less than air). As with our previous article concerning the effects of wind on live sound, temperature and humidity have very little affect on sound for most outdoor live shows.