By Kevin Cox, CSI, CDT
The Ancient Greeks and Romans are credited for inventing modern-day architectural acoustics. However, much of this occurred by mistake. Greeks and Romans were trying to solve line-of-sight issues with deeply banked seating arenas in a semi-circle configuration with broad front trumpeting performance areas. This left little to impede the line of sight from any seat in the auditorium or arena. In doing so, they created a good acoustical performance system. Later, the Romans would build large, slanted roofs above the sides of acting areas. Much by mistake again, this architectural addition resulted in reflecting sound to the rear of the auditorium and elevated acoustical performance.
The first reference to architectural acoustics is made by Vitruvius in 1st century BC in his book “De Architectura” (On Architecture). In these writings, Vitruvius describes “echeia” or sounding vases. These vases were placed on the sides of open-air stages to create a chamber that produced reverberation giving sounds a richer quality.
The Middle Ages had only an elementary knowledge of acoustics. Halls and churches are characterized by their overwhelming excessive reverberation and poor speech intelligibility. It is not until late 1900 the father of modern-day acoustics comes on the scene⸺Wallace Clement Sabine (1868-1919). Sabine used pipe organs as sound sources and seat cushions as sound absorbers in a Harvard University lab in 1898 to examine and measure the properties of sound. Sabine went on to assist in the design of the Boston Symphony Hall in 1900, considered one of the most acoustically proficient concert halls of its day. In 1888, Sabine discovered a reverberation time formula that does not appear in print until three years after his death in a collection of papers on acoustics. Today, the formula—known as Reverberation Time 60 dB—is used by acousticians all over the world in its original form. Through experimentation, Sabine was able to determine a definitive relationship between quality of acoustics, sides of the chamber, and amount of absorption, with the most important factor being reverberation time.
Sound can be described as a disturbance or turbulence passing through a physical medium in the form of longitudinal waves from a source to a receiver causing a sensation of hearing. This medium does not have to be the atmosphere but can be a solid, fluid, or gas. The speed of sound through these different media differs due to their molecular composition. The more solid a medium, the more rapidly sound is passed through it. This disturbance or turbulence is in the form of a wave that has alternating high and low pressures and moves in a longitudinal direction of propagation. This pressure creates peaks and valleys, and the distance between two pressure peaks or valleys is a wavelength. A period is the time it takes for one complete oscillation, measured in seconds, and represented with the letter ‘T.’ Frequency is the number of oscillations per second and is measured in Hertz or Hz. Amplitude is the distance between a crest (the highest point), and a valley (the lowest point), and it is measured in decibels (dB). The greater the amplitude, the louder a sound; the lower the amplitude, the quieter or less dB a sound will produce. The fewer Hz or oscillations a sound has, the deeper and lower pitched it is. The greater the number of oscillations, the higher the pitch. These variables make up all the sounds that are audible and inaudible to the human ear.
Sound wave behaviour
Once a sound is introduced into a space, it is interrupted by objects, people, and boundaries of the space itself. Materials are varying degrees of two types: those that allow sound waves to pass through, and those that do not. When encountering barriers, sound waves are likely to behave in the following ways: absorption, transmission, refraction, reflection, diffusion, and diffraction.
• Absorption occurs when a sound wave hits an obstacle and some sound energy is lost through its transfer to the molecules of the barrier; this energy is said to be absorbed. Thickness, porosity, frequency, and amplitude affect amount of absorption from a sound incident striking a surface.
• Although some sound energy is lost molecularly in the composition of the obstruction, some will make it through and be audible on the other side. This is transmission, or a sound being able to transfer through an obstruction. When specifiers and designers are looking at sound privacy of one space to another, they are trying to control transmission.
• Refraction is a variation of transmission. The difference is the soundwave bends as it passes through the obstruction.
• The sound not absorbed or transferred through an obstruction often bounces back into a space. This phenomenon is called sound reflection, and is what specifiers and designers try to control in large, untreated echoic spaces.
• A sound incident ray will often mirror off a flat hard surface, but additional rays can splay out from the incident ray. This is a process known as diffusion, the scattering of reflected sound rays.
• Diffraction is also splaying of rays, but due to indirect impact. It is a product of sound moving around an obstruction and rays splintering, or diffracting, due to this physical relationship.
• Sound produced in a space will bounce off reflective surfaces, gradually losing energy. When these reflections are mixed with each other, this is known as reverberation. Too much reverberation has a negative impact on speech intelligibility, and too little reduces the rich warm sounds like those from live music in an orchestra or concert hall setting.