Document ba0JGm16O923JRNG0GYvk7xq3

338 CHAPTER 25 1959 Guide 75 120 oc < 0 O <E U O O O 6 -rw 75 150 150 300 -200 . _ - ISO 300 600 - --250 ' --200 600 1200 --250 -200 1200 2400 -- $0 . -- 40 70 - 60 - so - 40 - 30 '--15 --*> 2400 4600 4600 9600 -70 -60 -SO ' - 40 50 PHONS130 -- MOO -- 2S0 -200 0 "t- * a* o -3 -- 5 - -S _ "4 oz < _-- 3 0 2 U O so -- -- .5 -- .4 20--* 75 75 ISO . 150 300 300 600 600 1200 1200 2400 2400 4800 4600 9600 30 PHONS-SONES OCTAVE PASS BAND IN CYCLES PER SECOND Dm tckafatod Mol toudnaw h tons* can bo coomitod to loodneu level m pboet by nacnt of & noaogrum at the right. (From Rafavma 3. Used by penmnroej Rg. 2 .... Nomograms Relating L, in Octave Bands to Loudness in Sones Introduction of a continuous spectrum noise properly dis tributed as a function of frequency produces a quieter condi tion even though the background noise is actually higher. Rumbles, rattles, audible single frequency components, beats between components, and intermittency in a noise often make it objectionable even though the noise as measured seems to meet the criterion. Noise Specification . There are at least four possible ways of specifying the noise. The first way is simply to give the octave band spectrum of the noise. This method provides the information that is essential to the design engineer and no substitute can suffice. The other three of these methods of specifying noise are angle-number numerical quantities that indicate something about the in fluence of tiie noise on the average human being. These three methods are useful as summaries of the octave band spectrum, but, taken separately, they are of limited value to the design engineer. The definitions of each of four commonly used ways of specifying noise are as follows: 1. Octave Band Spectrum. The octave band spectrum is com- firised of the readings of sound pressure level in the eight octave requency bands (see the section on Acoustical Terminology). The readings are taken with the sound level meter weighting Sound Control 339 networks set on fiat (C-ecale) and the unite are decibels (dbc). The specification of a complete octave band noise spectrum is necessary for satisfactory acoustic design in all eases. .2. Speech Interference Level SIL. The speech interference level is a single number found by taking the arithmetic average of the sound pressure levels in the three octave frequency bands 600-1200, 1205-2400, and 2400-4800 cps. The unit is the decibel (db). The speech interference level indicates the ease of speech communication in the presence of the noise so rated. 3. Loudness Level Ln. The loudness level is a single number computed from octave band levels according to the procedure following. The unit is the phon. To obtain the loudness level in pbons, obtain the sound pres sure levels in the eight octave bands and read from Fig. 2 the loudness in sones for the respective bands. The sum of the loud nesses will be: Ztf - N. + N, + K, + N, + N, + AT, + N, + N. (10) The loudness in sones of the total noise is given by tchert ' N - AT. + 03 (2N< - N.) (11) Nm = the loudness of the loudest band. 1*N{ = the sum of the loudnesses of all eight bands. To convert from loudness in sones to loudness level in phons, use Equation 9 or the nomogram at the right-hand ode of Fig. The loudness level in phons indicates the subjective loudness but does not indicate the ease of speech communication. 4. Sound Level. The sound level is a single reading taken on the sound level meter with the meter operated as follows:** 4 Sound-Laval Conga Wotghtiag Unit Below 55 db................... 55-85 db.......................... Above 85 db................... A B C (Flat) dba dbb dbc Sound level measured with the A-scale can be correlated most closely with speech interference level but the two are not nu merically the same. The C-se&le is always used with the octave band analyser and is used where otherwise specified. Criteria Noise criteria have been developed from detailed studies and engineering' experience in a number of architectural spaces.* The acceptability of background noise for various types of activities normally performed in rooms and enclo sures can be related to two principal characteristics of the noise: (1) Speech interference level, SIL, and (2) Loudness level. Lit. Definitions of these two quantities are given in the preceding section on Noise Specification. The speech inter ference level is governed by the amount of high-frequency noise (between 600 and 4800 cps) and the loudness level is governed by both the high- and low-frequency noise. For most types of spaces the loudness level in phons is 22 to 25 units higher than the speech interference level in dbc. However, for factories, sports coliseums, restaurants, anH similar spaces where cost of acoustic treatment dictates a maximum amount of compromise, the loudness level may rise as much as 30 units above the speech interference level. Acceptable background noise criteria have been established for these two conditions and are presented in Figs. 3 and 4. Each curve is designated by a number indicating the speech interference level obtained. The numbers in the first mlnmn of Table 1 and Table 2 identify the curVe which defines recom mended noise levels for the various types of spaces listed. With repud to the choice between Figs. 3 and 4, Fig. 3 should be used whenever possible, particularly where there is a possibility that the sound may be fluctuating, such as often occurs with turbulence in a ventilating system. Fig. 4, or numbers in the range between Figs. 3 and 4, may be used when economic factors dictate a compromise and when there is a definite assurance that the noise will not be of a fluctuat ing type. In engineering design, all eight octave hand levels selected from the curves of Fig. 3 or 4 should be used. In construction specifications, reference may be made to The Guide and the selected NC or NCA curve may be referred to by number e.g., NC-35. . Where octave band analyses of the noise cannot be made and a rough approximation of accepted criteria must be ob tained by use of a sound level meter only, the A-scale values which would be read for the noise spectra specified in Column 1 are given in Column 2 for comparison purposes. It should be understood, however, that an A-scale reading gives a measure of the overall noise using a weighted network; it does not specify the spectrum. For example, the noise spectrum shown as NC-25 in Fig. 3 would give an overall A-scale meas- urement of 35 dba, but a measured value of 35 dba does not mean that the actual noise measured has the spectral shape or is as satisfactory as the specified NC-25 spectrum. The lack of octave band analysis can be partly offset by a subjective observation; if the noise sounds unnatural because of excessive amounts of high pitched or low pitched noise or has particularly noticeable amounts of pure tones, the spec trum will probably depart widely from the specified one. In such cases it is advisable to make an octave band analysis of the noise, if possible. KINDS OF NOISE The noise introduced into a room or building by ventilating or air-conditioning equipment may be divided into two gen eral kinds, depending on how it reaches the room: 1. Noise transmitted through the ducts. a. From equipment such as fans, motors, sprays, etc. b. From outside, and transmitted through duct walls into air stream. c. From duct wall vibrations, transmitted into air stream. d. From air currents, including eddying noises. e. Cross talk and cross noises between rooms connected by the same duct system. /. From dampers and air valves located near branch duct take-offs and outlets. g. Noise produced by grilles or diffusers. 2. Noise transmitted (Arouph the building construction. a. From machine mountings as vibration. b. From equipment through room wall surfaces. The following text will present data and discussion of meth ods whereby solutions ol the noise problem can be obtained when the allowable room noise level, and the path through which the noise reaches the room, are known. NOISE GENERATED BY FANS Aerodynamic noise from all types of fans may be broadly divided into a rotational component and a vortex component. The rotational component is associated with the impnlca given to the air each time a blade passes a given point and is hence a series of discrete tones at the fundamental blade pass- ing frequency and harmonics thereof. The vortex component of noise is largely due to the shedding of vortexes from the fan