Room acoustics overview

 see chapter 23 in our text
 (most of this has to do with big halls; most studied)

 * how does sound spread in the open air?
     * 1/r^2 drop off

 * indoors - different story
     * reflected
     * absorbed
     * filtered (different for high/low freq)

 * echos
     * direct
     * reflections  (1st few)
     * reverberant  (many more)
     * different with people in the room...
     * funny effects if funny shapes ( e.g. whisper galleries)

 * precedence effect
     * much of sound arrives omnidirectonal
     * ... but we place source from 1st sound

 * reverberation time
     * defined as time for 60dB decay in f ~ 1000Hz sound
     * can calculate from wall material areas and properties

 * fairly good understanding of what people think 
   sounds "good" for a concert or lecture hall
     * loud enough
     * uniform across room
     * clear
     * some reverb, but localized
     * echo not noticable as such
     * see plot, pg 534

 * needs for speaking vs music are different
     * music wants more reverb
     * voice wants more clarity
     * ... but its hard to design a hall that's variable