Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio

Chapter Seven
An Acoustical Strategy
for the small room 

Page 1

Batting practice for the audio/video pro and a primer for the novice

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         The late Richard Hardesty was a respected audio engineer, consultant, writer, reviewer, and -- I read somewhere -- he could play a mean piano.  He was also an early dealer of true high end audio in Southern California.  
          In his early retail days he drove to customers homes
armed with a heavy load of audio test gear to 'tweak' their audio systems and deal with acoustical issues.  As time passed the amount of heavy gear decreased as most succeeding visits became a near acoustical replay of previous visits.  Although his test gear must have looked cool in his customer's eyes, the heavy gear was seldom needed.  It had become apparent that acoustically most rooms were just another enclosed box with predictable issues and routine solutions.  
          The goal of this chapter is to skip the heavy gear phase altogether and still 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 --- in particular my late friend Steve Mounkes.  

The acoustical small room ...... is your room     
          As outlined in Chapter 2, the compressions and rarefactions of sound waves are described by their wavelength, amplitude, and frequency.  The frequency of audible sound ranges from 20Hz to 20KHz with wavelengths from 56.5 ft to about 3/4 of an inch.  
he acoustical small is any room with dimensions that are comparable to acoustical wavelengths. Therefore, any room with a dimension of 56.5 feet or less qualifies as an acoustical small room (this includes ceiling height).
         This acoustical small room is an environment
that can erupt into a resonating cavity of agitated low frequency energy (300Hz & below).  This room can also act as an acoustical reflecting mirror of higher 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 and it has a predisposition to generate acoustical distortion.  

Acoustical distortion         
          Audio distortion is the corruption of sound.  Acoustical distortion is audio distortion created by the room environment.  Three types of acoustical perception are affected by acoustical distortion: timber, imaging, and spatial impression.  Acoustical distortion is influenced by room modes, specular reflections, comb filtering, absorption, diffusion, and noise.  In the acoustical small room the management of room modes, specular reflections, and noise are the principal concern.

Handbook Note: Speed of sound = 1130ft/sec.  
Room modes
           Each boundary of the acoustical small room causes sound energy to resonate much as waves in the ocean. This resonating sound is defined as standing waves or room modes.  Primary axial room modes of opposite boundaries (length, width, & height) are the most significant.  They can produce an uneven distribution of low frequency sound that causes deviations from flat frequency response.  That is a principle definition of distortion.  The wavelengths of the 3 primary axial modes are equal to dividing half the speed of sound by its room dimension -- length, width, height.

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

          Noise is random and unwanted competing sound.  It is the most corruptive acoustical distortion.
As specular reflections, noise generated from within the room or 
an adjoining area will also distort imaging
and the spatial impression of sound fields.  But noise also erases musical harmonic detail, muddys timbre, and reduces dynamic range.  Noise distorts audio.

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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2017