A— MATHEMATICAL AND PHYSICAL SCIENCES 39 



and direct reading, and can be put into the hands of an untrained observer. 

 Furthermore, as a dial instrument, it is well adapted to the enforcement 

 of regulations or specifications. 



The Analysis of Noise. 



The wave-form of the microphonic current of a sound can be readily 

 displayed by means of a cathode-ray oscillograph, but the analysis of 

 such wave-forms is usually troublesome, and the composition of a sound 

 or noise is determined more conveniently by other means. The methods 

 of spectrum analysis in optics are not convenient in the case of sound, 

 by reason of the much longer wave lengths of sound and their wider 

 range, the visible spectrum covering a range of only about one octave, 

 whereas the range of sound to which the ear responds extends over about 

 ten octaves. A diffraction grating of the order of size of a garden fence 

 would in fact be required to deal directly with a normal acoustic spectrum. 



For the analysis of sound, the earlier experimenters used acoustical 

 resonators which were tuned to respond to the different components 

 present in the complex sound. In present-day methods, the oscillatory 

 pressure variations which constitute the sound are first converted, by 

 means of a microphone, into corresponding electrical oscillations ; and 

 the problem is reduced to the analysis of a complex electrical oscillation, 

 a process which may be effected by tuning or filtering circuits or by the 

 use of a search or heterodyne tone. Such filters may be either of the 

 high-pass or low-pass type, or alternatively may be arranged to pass bands 

 of frequencies extending over, say, an octave or a fraction of an octave. 

 This latter method is particularly valuable in the case of noises which 

 are not of a steady nature or composition, for example, traffic noise. 



Steady sounds or noises readily lend themselves to analysis and yield 

 typical line spectra, but the matter is less simple with transients, that is, 

 acoustical impulses which start suddenly and die away rapidly. Examples 

 of transients are speech (and in particular certain consonants like p and 

 b), whip cracks, rifle shots, blows, footsteps, hand-clapping, percussion 

 instruments, e.g. the piano, drum, and most musical instruments. All 

 such sounds prove to be largely of the band-spectrum type with super- 

 posed line spectra and nearly all contain very high frequencies. 



Sound analysis may be of particular service in studying machine noises 

 as a means of tracking the source of major components. Synthesis of 

 sounds is sometimes resorted to for particular purposes. Much progress 

 has been made of late in the commercial development of instruments for 

 acoustic analysis and synthesis. Here is an analyser which will alniost 

 instantaneously reveal the composition of a noise, while here also is a 

 synthesiser, kindly lent me by Messrs. Comptons the famous organ 

 builders, in which pure harmonics of controllable intensity can be so 

 blended as to simulate, for example, church bells with a fidelity which 

 is very arresting. 



Noise-level Measurements. 

 In connection with problems of noise abatement, the National Physical 

 Laboratory has made measurements and analyses of many noises of very 



