From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Study on the methods for measuring and classifying tire noise.
| Identifier | ExFiles\Box 158\1\ scan0125 | |
| Date | 22th January 1934 guessed | |
| -2- The first difficulty in the study of tire noise is that of suitable methods of measurement. Audiometers or accoustimeters of various types are available, and they have been used* for the measurement of tire noise, or of combined car and tire noise. While it is possible to analyze complex sound mixtures, and to plot a sound curve showing the intensity of sound at different frequencies, it is difficult to interpret such a curve in terms of practical car experience, or to determine which frequencies are troublesome, and which are harmless. Tire noise at constant speed may be largely a fundamental note with a single frequency, or it may be a very complex mixture of tones and noise with a wide range in frequency. While total noise within any desired range of frequencies may be measured and expressed in decibels, it is doubtful whether this is of real value, since the character of the noise is fully as important as its total volume. The ear is more sensitive to vibrations of certain frequencies than to those of other frequencies, and unless this is considered in audiometer measurements, the results are likely to be in error. Because of these and other objections which can be raised against the audiometer methods of determining tire noise, we have preferred to use direct tire-to-tire comparisons on cars under actual operating conditions, or on test wheels sufficiently close to operating conditions to give reliable results. After a little experience, it is possible to make comparative tire-noise tests on cars on a range of road surfaces with quite reproducible results, if direct comparisons are made within a short interval. However, it is difficult to remember sound intensities and characteristics, so whenever it is necessary to test a new type of tire, it is advisable to make direct comparisons with a previous standard. Still more accurate comparisons can be made by running the tires alternately on a large test wheel. In this way two or more tires may be readily compared and the observers can listen repeatedly to the noise from each until very definite conclusions can be drawn as to the characteristics of the noise. Phonograph records may also be made under these conditions to serve as permanent records of the sound. The howling or squealing type of noise produced when cars are driven rapidly around turns can be studied most satisfactorily by special types of road tests which will be described later. CLASSIFICATION OF NOISE: A moving tire, under normal conditions, can produce noise in a number of ways, each of which may yield a different type of noise. First, there are sounds of definite frequency which may be described as notes. This type of sound, is characterized by well defined pitch, which rises as the speed is increased. It originates from impacts at very short but regular intervals of similar portions of the tread design against the road. A similar noise of much lower intensity can be caused by wind interference with the regularly spaced design units. If the impacts occur at irregular intervals, the sound is best described as noise, since a mixture of frequencies is involved. Examples illustrating these and other points will be described later in this paper. Second, there are other noises of irregular and usually low frequency, which apparently are not due to impacts, but to a trapping of air in certain portions of * W.F. Billingsley Instruments 5 273 (1932). Anon.Auto Topics 106 475 (July 2,1932) Joseph Geschelin Automotive Industries 67 676, 713-717 (1932) " " " 69 640, (1933) | ||