ITU-T P 32-1988 EVALUATION OF THE EFFICIENCY OF TELEPHONE BOOTHS AND ACOUSTING HOODS《电话间和声罩的效率评估》.pdf

1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 0 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%0(/.%G0G042!.3-)33)/.G0G015!,)4935“3#2)“%23 G0G0,).%3G0G0!.$G0G03%43%6!,5!4)/.G0G0/2) there are no measurement methods recommended for evaluating the transmission quality improvementresulting from the use

2、 of the telephone booth;3) an evaluation of the booth efficiency, based only on the acoustic insulation obtained by traditional methods(acoustic attenuation of the panels of the booth) is not always correlated to the subjective evaluation of thebooth performance;4) subjective measurements either of

3、the intelligibility or of the intelligibility threshold variation give thepossibility of evaluating the efficienty of a booth, but are time-consuming and expensive and also require aqualified and well-trained operator team;5) there are no recommendations giving criteria relating the employement of t

4、he booths to the ambient noiselevel, in order to determine an acceptable quality of transmission,2 Volume V - Rec. P.32methods of measurement as specified below are recommended1):a) evaluating the efficiency of telephone booths and acoustic hoods taking into consideration the intelligibilityindex, o

5、btained from a listener inside the booth with the external ambient noise having a certain acousticspectrum;b) calculating the intelligibility index inside the telephone booth or the acoustic hood by means of theobjective method defined in 3, taking into consideration the acoustic attenuation of the

6、booth and thesidetone of the telephone set used. This objective method allows a rapid evaluation of the boothperformance, sufficiently precise for practical purposes;c) considering the logatom intelligibility as an evaluation criterion related to the booth performance,calculated by means of the arti

7、culation index (AI). The conversion from AI to logatom intelligibility islanguage-dependent and it shall be performed with the appropriate relation;d) evaluating the booth and the acoustic hood at the conditions of utilization, that is, when a user is speakingfrom the inside using a telephone set wi

8、th a determined sidetone and with an external ambient noise havingan average intensity level and a certain acoustic spectrum, both already known.2 Definition and descriptions of parameters of calculationTelephone conversations taking place in conditions of ambient noise are affected by ambient noise

9、 throughthree different paths:1) acoustic noise (Na) at the ear which is not engaged in the telephone call;2) acoustic noise (Nb) at the ear which is engaged in the telephone call, determined by the acoustic leakbetween ear and handset;3) noise picked up by the microphone and directed by sidetone (N

10、s) to the ear which is engaged in theconversation.The acoustic noise flowing through the acoustic leak between ear and handset has a spectrum which changes asa function of the pressure of the handset against the ear. To evaluate the performance of booths, the acousticattenuation (LRNE) of this path

11、can be taken into consideration.The noise Nsis due to sidetone changes according to the telephone set used and it generally has a spectrumwhich is different from that of Nb. In spite of their mutual correlation, the power summation of the respective spectraseems the best estimate of the global noise

12、 (Ng) which affects the ear engaged in the conversation.In addition, the noises at the two ears (Na, Nb) are generally different, both in level and in spectrum;experimental intelligibility measurements 1 2 have demonstrated that this disturbing effect can be evaluated bysubtracting 10 dB from the no

13、ise level (Na) at the free ear.The aforesaid experiment measurements have also shown that the total equivalent noise NTto be used inintelligibility calculations is given by the amplitude sum of noise spectra at the two ears. Consequently, the totalequivalent noise NTis given from the relation:NNNTag

14、=+20 101020102010log dBThe sidetone noise Nsis a function of the mouth-to-ear sidetone loss LMESTand it should be measured at theactual noise level, typically 65 dB SPL, under diffuse field conditions. This is particularly important in the case oftelephone sets with carbon microphones or of electron

15、ic telephone sets with automatic gain control or provided withnoise cancelling microphones.3 Calculation of the booth or acoustic hood efficiencyGiven a particular telephone booth or an acoustic hood, the following procedure shall be followed fordetermining the articulation index in actual operating

16、 conditions._1)Documentation about the specifications in this Recommendation is not yet sufficient to confirm their validity, thus they aresubject to future enhancement and should be regarded as provisional.Volume V - Rec. P.32 3Calculate:a) the noise spectrum Nainside the booth by subtracting the a

17、coustic attenuation of the booth (La) from theexternal noise spectrum (Ne). The attenuation should be measured in third octave bands, with a personinside the booth (or a baffle providing an equivalent acoustic absorption) and in a diffuse field condition;b) the spectrum of the noise Nbby subtracting

18、 the leakage attenuation of the handset (LRNE) from the noisespectrum inside the booth Na;c) the sidetone noise spectrum Ns by subtracting the acoustic sidetone attenuation LRNST2)from the noisespectrum inside the booth Na;d) the spectrum of global noise Ngat the ear pressed against the handset as t

19、he power sum of Nsand Nb;e) the spectrum of total equivalent noise NTas the amplitude sum of noises at both ears, after havingsubtracted 10 dB from the noise spectrum at the ear not engaged;f) the articulation index, AI by Kryters method 3, assuming a listening speech level of 70 dBA, a valuecorresp

20、onding to the limit of the attenuation of the line loss distribution.An example of application of the calculation method is shown in Appendix I.4 Efficiency limits of booths and acoustic hoodsEfficiency of booths or acoustic hoods can be considered satisfactory if an AI equal to 0.6 is guaranteed.Th

21、is value corresponds for most languages to a logatom intelligibility of 80% inside the booth, according to theresults of French and Steinberg 4, in Figure 1/P.32. It can be assumed as the minimum acceptable limit ofperformance, corresponding to the maximum external noise level that the booth can wit

22、hstand in order to guarantee agood quality of telephone transmission inside the booth.Therefore, each booth can simply be classified by specifying a maximum external noise level (MENL), which isthe level that gives AI = 0.6.The MENL that classifies the telephone booth shall be determined by repeatin

23、g the calculation of the AI, as isindicated in 3, with different levels of external noise. By means of the curve representing the values of the AI as afunction of the outside noise level, the MENL corresponding to an AI = 0.6 can be determined. This MENL dependsnot only on the acoustic attenuation o

24、f the booth or acoustic hood, but also on the received speech level which isassumed to have a reference value of 70 dBA, and on the sidetone performances of the telephone set which should bemeasured at a proper sound pressure level, (about 65 dB SPL) and in free field conditions._2)It is important t

25、he room noise sidetone sensitivity LRNSTwhich makes use of a diffuse room noise source within the booth. Itmay also be necessary to include within the booth a manikin to stimulate the presence of a subscriber.4 Volume V - Rec. P.325 Speech privacy of telephone communicationsThe booth can also guaran

26、tee speech privacy of conversation by reducing the vocal signals radiated towardsoutside in order to make them unintelligible. Applying Kryters calculation method of the articulation index of thespeech signals transmitted through the booth to the external ambient at a predetermined noise level, the

27、distance atwhich the logatom intelligibility or AI falls to a pre-determined value (for example, AI = 0.3) can be estimated. Thismethod can be used to determine the curves of equal intelligibility (isophenes) in any direction, increasing distancefrom the booth.Note - The quality improvement of the c

28、onversation for the subscriber at the other end of the telephoneconnection, during a call with a telephone in a booth or acoustic hood has not yet been studied. The evaluation of thisaspect is required in any case to consider a number of other factors such as the natural increase of speech loudness

29、innoisy environments and the effective signal-to-noise ratio of transmitted signals.ANNEX A(to Recommendation P.32)Example of efficiency calculation of a telephone boothThe articulation index (AI) is calculated according to Kryters method.The acoustic attenuation of a telephone booth measured in an

30、echo chamber at each one-third octave band isreported in Table A-1/P.32, column 2. The total noise level outside the booth is 80 dBA and the sound level of thenoise at each centre frequency band is indicated in column 3. The sidetone response characteristics (LRNST) of thetelephone set used inside t

31、he booth is given in column 4.The noise level inside the booth at each centre frequency band (Na) is obtained by subtracting column 2 fromcolumn 3 (column 5). It is supposed that the handset of the telephone instrument used in the booth has the acousticattenuation indicated in Figure A-1/P.32 and re

32、ported in column 6.Volume V - Rec. P.32 5The values of noise (Nb) due to acoustic leakage between ear and handset obtained by subtracting column 6from column 5 are reported in column 7.The values, at each frequency band, of the sidetone noise (Ns) obtained by subtracting column 4 from column 5are re

33、ported in column 8. The global noise at engaged ear (Ng) is reported in column 9 as the power sum of the levelsindicated in columns 8 and 7. The total equivalent noise is obtained by adding the levels of column 9 to the values ofcolumn 5 reduced by 10 dB (column 10). The speech spectrum () is report

34、ed in column 11 and the signal-to-noiseratio corrected by 12 dB (considering the peaks of the speech signal) is indicated, at each one-third octave band, incolumn 12. Kryters coefficients are indicated for each one-third octave band in column 13.The articulation index (AI) is obtained by multiplying

35、 the values of column 12 by those of column 13 andadding the results. By repeating the calculation with other external noise levels, it is possible to draw the diagram of theAI as a function of external noise levels for the considered booth, as shown in Figure A-2/P.32. It can be seen that thisbooth

36、 is designed for withstanding a maximum external noise of about 77 dBA which is the MENL value that classifiesthe booth.6 Volume V - Rec. P.32 Volume V - Rec. P.32 7TABLE A-1/P.32Centralfrequencyone-third octaveband(Hz)Acousticattenuationof the booth,La(dB)Externalnoise, Ne(dB SPL)Acousticsidetoneat

37、tenuation,LRNST(dB)Noise insidethe booth,Na(dB SPL)Acousticattenuationof handset,LRNE(dB)Noise due toacousticleakage, Nb(dB SPL)Sidetonenoise, Ns(dB SPL)Globalnoise atengagedear, Ng(dB SPL)Totalequivalentnoise, NT(dB SPL)Speechspectrum, (dB SPL)Signal + 12 dBNoiseKryterscoefficientProducts(13) x (12

38、)(1) (2) (3) (4) (5)= (3) (2)(6) (7)= (5) (6)(8)= (5) (4)(9) (10) (11) (12)= (11) + 12dB (10)(13) (14)2002503154005006308001000125016002000250031504000101313151414161515151111121277.576.573.574.072.572.072.071.069.568.066.064.062.061.51212119910121299810.5141467.563.560.559.058.558.056.056.054.553.0

39、55.053.050.049.5345678.510.011.513.014.516.017.519.020.564.559.555.553.551.549.546.044.541.538.539.035.531.029.055.551.549.550.049.548.044.044.045.544.047.042.536.035.565.060.156.554.853.651.848.147.347.045.147.643.337.236.468.163.460.058.457.456.153.152.751.950.152.449.244.744.161636465656362616058

40、544947394.911.616.018.619.618.920.920.320.119.913.611.814.36.90.0040.0010.0010.00140.00140.0020.00200.00240.00300.00370.00370.00340.00340.00240.01960.01160.01600.02600.02770.03780.04180.04870.06030.07360.05030.04010.04860.0166TOTAL (dBA) 80.0 66.3 64.7 70.0 AI = 0.52SPL Sound pressure level8 Volume

41、V - Rec. P.32References1 CCITT Contribution COM XII-No. 122, (France), Study period 1981-1984.2 CCITT Document Annex 2, AP VII-No. 115.3 KRYTER, (K.): Methods for the calculation and use of Articulation Index, J.A.S.A. Vol. 34, 1962.4 FRENCH, (N. R.) and STEINBERG (J. C.): Factors governing the inte

42、lligibility of speech sounds, J.A.S.A. Vol. 19,1947.BibliographyCCITT Contribution COM XII-No. 139, (Italy), Study period 1973-1976.CCITT Contribution COM XII-No. 130, (Norway), Study period 1977-1980.CCITT Contribution TD 26, (Sweden), WP Laboratory (Geneva, 17-19 January 1984)KRYTER (K.): The effects of noise on man, Academic Press, pp. 70-77, 1970.