The ‘A’ Frequency Weighting Ken Scannell Non-refereed Noise and Sound Services, Spectrum House, 1 Elegans Avenue, St Ives NSW 2075 Previously published as part of an Acoustics Forum in Acoustics Australia, Vol 31 No 1, April 2003, aimed at promoting discussion. Reprinted with permission. Introduction Generally, the low frequency bass Figure 1, this appro.imately follo s tones (i.e. 50 to 250 Hz) sound the inverted Fletcher and Munson The ‘A’ frequency weighting is used slightly quieter than the tones in 40-phon curve 6_ 3 dB7. The 40- extensively in many acoustical the mid-audio frequency range 6i.e. phon curve is based on the noise measurements. Although 1 to 4 k9I7. E.periments ere subJectively reported equal almost exclusively used, it is often carried out by 9arvey Fletcher Z1[ loudness magnitudes at various misunderstood or incorrectly at the Bell Telephone Laboratories frequencies relative to 40 dB at 1 defined even by those who would in Ne Nork, in the early 1E30s to k9I. be expected to have a better determine ho loud tones of knowledge. It is commonly stated different frequencies sounded The symbol for the >A0 frequency in glossaries, even in official subJectively. A series of curves on a eighted sound pressure level, documents or textbooks on graph ere dra n from these measured in decibels is >L PA 0 Z3[ acoustics or noise, as “a scale that e.perimental results. These become although the common abbreviation simulates the response of the flatter in frequency ith higher is dBA or dB6A7. Either of the t o abbreviations could be used but the symbol is preferred as this places the >A0 ith the level and not ith the decibel, hich incorrectly implies there are different types of decibels. 7imit tions of 1A2 .eighting Cue to its simplicity and convenience, the >A0 frequency eighting has become popular and it is an often-used frequency eighting for many different noise sources. It is used for all types of noise assessments from Figure 1. The ‘A2 4eighted fre3uency filter rel tive to occupational noise, building 0 d, at 1 k9B. acoustics, loudness assessments and noise annoyance assessments. human ear” or similar erroneous sound pressure levels and are The World 9ealth ,rganiIation nonsense. kno n as equal loudness contours. 6W9,7 Z4[ has recognised that the From these contours, three curves >A0 frequency eighting is an kno n as A, B, and C frequency overall value hich may simulate Origins of the A- eightings ere developed for use neither the spectral selectivity of .eighting curve in sound level meters. These human hearing nor its non-linear frequency eightings ere specified relation to sound intensity. Quite The human hearing system is not in an American Standard for rong and totally misleading as sensitive to all sounds if they sound level meters in 1E36 Z2[. The statements in glossaries are vary in pitch or frequency. >A0 frequency eighting is sho n in NEW ZEALAND ACOUSTICS Vol. 16, No. 2 19 commonly given for the >A0 normal ear ere desired". first impressions but can vary by up frequency eighting such as “The to 11 dB in the lo frequency 6e.g. >A0 frequency eighting adJusts the The equal loudness contours ere 100 9I7 range. noise level to the subJective re-determined under more response of the Even if the >A0 human ear" or frequency reference is made eighting could to >A- eighted be used as a good decibels0, hich, universal of course do not predictor of e.ist and should loudness it is not be e.pressed as >A0 a good predictor frequency of noise eighted sound annoyance, levels in decibels. particularly for sounds hich Fletcher and differ from those Munson derived hich are the original equal medium level, loudness curves broadband mid- using only eleven audio frequency, observers ho and have constant listened to pure Figure 2. The ‘A2 Fre3uency 4eighting and the E3u l temporal tones through 7oudness Contours from Fletcher and Munson and characteristics. headphones. In their paper Robinson and D dson. It is often stated Fletcher and Munson 61E337 stated stringent conditions in 1E55 using that the >A0 frequency eighting “ait ould be necessary to increase ninety subJects. The re-determined follo s the 40-phon equal loudness the siIe of the group if values more equal loudness contour curves are contour. The confusion comes representative of the average similar to the original curves on from the fact that there are t o sets NOISE CONTROL SERVICES ACOUSTIC PRODUCTS AND DESIGN • Products for industrial and commercial noise control • Acoustic design • Turn-/ey acoustic solutions PO Box 82 126, Highland Park 6 Canon Place, Pakuranga AUCKLAND Phone: +64 9 576 4319 Fax: +64 9 576 3806 E-MailA peterCnoisecontrol.co.nD 20 Vol. 16, No. 2 NEW ZEALAND ACOUSTICS of equal loudness contours R one level can be realised by considering risk of occupational hearing from Fletcher and Munson and the simple case of a >dripping tap0 damage and the A0 frequency another from Aobinson and noise hen trying to sleep. eighted e.posure to noise. It is Cadson 61E567 Z5[. The >A0 ho ever not a good frequency eighting frequency filter is close The >A0 frequency eighting eighting to use hen assessing to the Fletcher and Munson 40- should be used for occupational annoyance from noise hich is phon curve but varies by up to 8 noise assessments 6e.cept peak predominantly lo frequency 6i.e. dB at lo frequencies from the noise assessments7 because there belo about 250 9I7. >more representative0 Aobinson are >kno n0 relationships bet een and Cadson 40-phon curve. This is the statistical risks of hearing a significant difference as it damage and the overall long term References represents close to a 50O change in >A0 frequency eighted noise the perception of subJective e.posure level Z7[. 9. Fletcher and 4.A. Munson , loudness. The t o 40-phon curves, “LoudnessL it0s definition, at the lo frequency end of the The >A0 frequency eighting has, measurement and calculation", J. spectrum are compared to the >A0 unfortunately, never been changed Acoust. Soc. Am. 5, 82-105 61E337. frequency eighting in Figure 2. from the 1E36 American Standard even though it as based on results Americ n St nd rd ASA Many noise sources in the here Fletcher and Munson D24.2. American National environment are lo frequency. indicated that they ere not Standard for Sound Level Meters When assessing these noise sources necessarily representative of the 61E367. the >A0 eighting frequency filter average normal ear. This as later can be regarded as a high-pass filter proved to be the case by Aobinson Austr li n St nd rd, Acoustics ith a cut-off frequency 610 dB and Cadson. 9ence the >A0 – Description and do n point7 at about 250 9I. frequency eighting is not even a 9ence, here a noise source is rough appro.imation 6i.e. about Me surement of dominated by lo frequency, the 50O error7 to the response of the Environment l Noise Part 1L use of the >A0 frequency eighting human ear at 40-phon. General Procedures 61EE77. gives a poor indication of loudness and an abysmal indication of noise ,. Berglund, T. Lindv ll H D. annoyance. Summ ry nd 9. Sch.el 6Eds7 Guidelines for Community Noise World 9ealth Annoyance is multi-dimensional, in Conclusions ,rganisation, Geneva, httpL// . ho.int/peh/noise/ fact, at lo sound pressure levels The >A0 frequency eighting is not guidelines2.html 62002.7 the character of the noise 6e.g. a scale, it cannot be used to temporal structure and frequency >establish a human dose response content7 can become, by far, the relationship0 and it does not 8. 4. Robinson, & R. S dominant factor in the annoyance simulate the response of the 8 dson , “A re-determination of perception. This as clearly sho n human ear. The >A- eighting0 the equal-loudness relations for in research carried out by Scannell should al ays be described in a pure tones", Brit. J. Appl. Phys. 7, Z6[ here subJects compared a lo glossary as the >A0 frequency 166-181 61E567. frequency repetitive impulse noise eighting to distinguish it from a to pink noise for both loudness time eighting. The >A0 frequency G. Sc nnell , “9uman response to and annoyance. 9ere a character eighting must be used for a lo frequency, repetitive impulse correction of up to 15 dB as occupational noise assessments but noise", J. Lo Freq. Noise and found to be required here audible should be utiliIed ith e.treme Mibr. 8647, 122-12E 61E8E7. sounds ere at a very lo sound care hen an indication of pressure level but ere unpleasant loudness or noise annoyance is Austr li nANe. Ze l nd in character. required. St nd rd ‘Occup tion l noise m n gement2 Part 4L Auditory Scannell found that for annoyance, A possible improved description of assessment, AS/N;S 126E.4 any penalty added to the obJective the >A0 frequency eighting isL the 61EE87. o measurement for a source ith >A0 frequency eighting is used as a unpleasant character must be level rudimentary appro.imation to the dependant ith a higher penalty subJective human perception of for lo er sound pressure levels. loudness at lo sound pressure The fact that character is more levels. There is a kno n important than the sound pressure relationship bet een the statistical NEW ZEALAND ACOUSTICS Vol. 16, No. 2 21 .