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Ed's AV Handbook
Batting practice for the AV Professional
and primer for the novice

Chapter 7  Page 1

An Acoustical Strategy
for the small room

Truck

Mr. Hardesty Lightened the Load

        The late Richard Hardesty was a respected audio engineer, consultant, writer, audio reviewer, and I read he was a respectable pianist.  He was also a pioneer of high-end audiophile retailing in Southern California. 

        Mr. Hardesty acknowledged that in his early retail days, he drove to his customers' homes with a heavy load of audio test gear to deal with room acoustics.
As time passed, the amount of heavy gear decreased as succeeding visits became a near acoustical replay of the previous visits.  Although test gear must have certainly looked cool in the customer's eyes, much of it became unnecessary.  It had become apparent that many of their rooms were just another enclosed box with predictable issues and routine acoustical solutions.

        The goal of this chapter is to skip the heavy-gear phase.  And solve acoustical issues with a practical mix of advice from Mr. Hardesty, the Master Handbook of Acoustics, Auralex Acoustics, long-forgotten sources, plus many of my colleagues. 

The Acoustical Small Room ..... it's your room.

        As outlined in Handbook Chapter 2, wavelength, amplitude, and frequency are used to describe sound waves.  Sound wavelengths range from 56.5 feet to about 3/4 of an inch.  Any room with dimensions comparable to acoustical wavelengths is acoustically a small room.  Therefore, rooms with a dimension of 56.5 feet, or less, is an acoustically small room.

Small Rooms Distort

        Acoustical small rooms can erupt into resonating cavities of agitated-low-frequency energy.  The small-room is also an acoustic reflecting mirror of high-frequency sound that competes with the direct sound from the speaker system.  It may even have a noisy neighbor. The acoustical-small room is your room with a predisposition to generate acoustical distortion.

Acoustical Distortion

        Audio distortion is the corruption of reproduced sound.  Acoustical-distortion is an audio distortion created by the room.  Three types of sound perception are affected by acoustical-distortion: timber, imaging, and spatial impression.  Each influenced by the acoustical-elements of room modes, specular reflections, comb filtering, absorption, diffusion, and noise.  However, the management of room modes, specular reflections, and noise is the principle-concern in the acoustical-small room.

Room Modes

        Each boundary of the acoustical-small room causes low-frequency sound waves to resonate, much as waves in the ocean. They are acoustical-described as standing waves or room modes.  The primary axial room modes of opposite boundaries (length, width, height) are the most significant.  The wavelengths of the three primary axial-modes are equal to dividing half the speed of sound by its room dimension -- length, width, height.

Handbook Note: Speed of sound = 1130 ft /second

        Their resonating energy can produce an uneven distribution of low-frequency sound that causes deviations from flat frequency response.  That is a principle definition of distortion.

Axial Modes

Specular Reflections

        Specular reflections are room boundary reflections of a short-acoustical wavelength that compete with the direct sound from a speaker.  Their physics is identical to a ray of light reflecting off a mirror.  Formally defined as the angle of incidence equals the angle of reflection, specular reflections distort imaging, and spatial impression.

Specular Reflections

Noise

        Noise is a random unwanted competing sound.  Noise generated within a room or adjoining areas distorts imaging, timber, spatial impression, and compresses the dynamic range of sound.  Noise is the most corruptive acoustical distortion.

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Ed's AV Handbook   
Copyright 2007 Txu1-598-288 Revised 2021


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