ARCHITECTURAL ACOUSTICS 103 



On the other hand, if the walls are hard, little energy is absorbed 

 at each impact and very many reflections will occur before the 

 sound dies out. This produces slow decay or reverberation, 

 which is the most common defect in audience halls. 



The effect of reverberation on speech is readily understood. 

 If the sound of one word is prolonged so as to blend with that 

 of a subsequent one no word will be heard distinctly. In 

 music reverberation produces an effect similar to that of the 

 loud pedal in a piano. Some prolongation and blending of 

 notes in music is desirable ; but the mixing of words in speech 

 is never an advantage. If a hall is to be used for both a middle 

 course must be steered. The reverberation must be made 

 somewhat too long for speech, somewhat too short for music, 

 yet fairly satisfactory for both. 



We have seen that a succession of reflections produces 

 reverberation ; in some circumstances a single reflection pro- 

 duces an echo, and echoes are the next most common defects in 

 audience halls. The conditions necessary for the production 

 of an echo are that the wall from which reflection takes place 

 should be at such a distance from the hearer that the im- 

 pressions formed by the direct and reflected sounds are quite 

 distinct. The minimum interval that must elapse between 

 the two effects for this to be the case is about rJ^ of a second, 

 though, of course, it varies for different observers. As the 

 velocity of sound at 15° C. is 1,120 ft. per second this gives the 



minimum distance of the reflecting wall as = 37 ft. The 



farther off the wall the more distinct is the echo. 



A curved wall, as will be shown later, exercises a focussing 

 action and produces regions of undue intensification and of 

 comparative silence ; it is almost inevitably a menace to good 

 acoustics. 



Besides these main defects it is also possible for the acoustics 

 to be affected by resonance. Where a wall is elastic, but not 

 too rigid, it may happen that the sound waves impinging on it 

 have the natural period of the wall itself, in which case the 

 wall will be set in considerable vibration, just as a heavy bell 

 is set swinging by the small but suitably timed impulses of the 

 ringer. Reinforcement will occur for sounds of this particular 

 period, but not for others. 



The effect on a band playing will be to accentuate some 

 notes at the expense of others, and so to produce distortion. 

 Similarly, it may happen that clear speaking is impossible for 

 a voice of a certain modulation. 



Resonance may occur not only by the vibrations of the 

 walls, but also by the vibration of the column of air in the room 



