104 SCIENCE PROGRESS 



as a whole. This is not as likely to occur in large as in small 

 rooms — it is very marked in a small empty room such as a 

 bathroom — but it does occur in buildings of a cruciform shape, 

 as many churches are. 



Sound waves are also subject to interference and to diffrac- 

 tion, and no treatment of the problem is satisfactory which 

 ignores these. Interference occurs when the condensations of 

 one set of waves fall on the rarefactions of a similar set and 

 operates in the sense of producing a region or regions of silence, 

 there being corresponding regions of intensification elsewhere. 

 Diffraction consists in the bending of waves round obstacles 

 and will generally assist in even distribution. 



It is popularly supposed, and there are probably few ideas 

 which are more prevalent and yet have so little foundation in 

 experiment, that faulty acoustics may be cured by stretching 

 wires across a room. How the idea arises it is difficult to 

 imagine, for the small emission of sound by an isolated wire 

 is an indication of its small capacity for absorption, and the 

 effect on the sound in a room would be similar in degree to that 

 of a fishing line on water waves. Indeed Sabine cites a case 

 where the stretching of five miles of wire in a room made no 

 perceptible difference to the reverberation. 



A second idea is that sounding-boards, usually consisting 

 of plane horizontal boards mounted above the speaker, have a 

 beneficial effect. If they do so in certain cases it can only 

 be regarded as fortuitous, for it is impossible to prescribe a 

 general remedy for a defect due to so great a variety of condi- 

 tions. 



It has further been suggested that use might be made of the 

 ventilating system to improve the acoustics of buildings. The 

 idea has its origin in two observations, viz. that sound travels 

 more readily with the wind than against it, and that in certain 

 cases currents of hot air in the centre of a building have acted 

 as a sound-screen, apparently carrying the sound away. 



This matter has been investigated and discussed in detail 

 by Sabine (2) and Watson (3), and their conclusion is that little if 

 any improvement can be effected by a special system of venti- 

 lation. The case of the wind is not an analogous one, and in 

 ordinary conditions of ventilation the difference of temperature 

 between incoming and outgoing air is insufficient to permit 

 the utilisation of reflection and refraction effects. Acoustic 

 disturbances may, however, be caused by the presence of a 

 hot stove in the centre of a hall, and the least injurious effect 

 is probably produced by the commonest mode of ventilation 

 in which the room as a whole acts as a kind of chimney. 



It is assumed too that perfect acoustics are to be expected 

 when a new auditorium is modelled on an old one whose 



