SOUND IS A PRESSURE DISTURBANCE in the form of a longitudinal wave that produces an auditory sensation. Because of this there are always two important parts of the analysis of sound: the source and the receiver. To understand the behavior of sound in buildings, we need to understand the basic physics of sound. In this section we will begin by defining the main characteristics of the sound wave. First, imagine sound as a dominoes effect. Each molecule is excited and triggers the one next to it, but the molecules remain in almost the same position they started. However, the effect has been carried along and you can say something is “traveling.” In architectural acoustics we usually talk about sound traveling through air. This means that a small vibration has created a variation in the air pressure and molecules are beginning to move back and forth, carrying the energy. This energy is spent along the way and thus the level decreases with distance.
The simplest sound is a pure tone. This means that it is composed by only one frequency (tone) and it is represented as a sine wave. The peaks of the sine wave are the points of highest pressure.
SOUND NEEDS A MEDIUM TO PROPAGATE and it travels at different speeds depending on the medium. The more dense the medium, the faster sound will travel. For example, in steel, sound travels at 20,000 ft/s (6,100 m/s), while in air sound travels at approximately 1,125 ft/s (343 m/s). Did you ever wonder why someone would get down and put their ears to the railroad track instead of pointing their ears up toward the train? Well, they would detect the incoming train earlier through the steel than through the air. Sound speed varies depending on the temperature, density, and elasticity of the medium.
SOUND PRESSURE IS THE DIFFERENCE between the pressure of the sound wave and the pressure of the medium (usually air). It is very small compared to the atmospheric pressure and measured in Pascals [Pa].
Amplitude is the difference between the maximum and minimum sound pressures of the wave.