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Noise: Quantification and Perception

Noise: Quantification and Perception. Architectural Acoustics II February 11, 2008. Symphony Hall, Boston. Symphony Hall, Boston. http://www.nytimes.com/2007/06/03/arts/music/03kram.html. http://www.allposters.com/-sp/Symphony-Hall-Boston-MA-Posters_i1119076_.htm. Symphony Hall, Boston.

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Noise: Quantification and Perception

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  1. Noise: Quantification and Perception Architectural Acoustics II February 11, 2008

  2. Symphony Hall, Boston

  3. Symphony Hall, Boston http://www.nytimes.com/2007/06/03/arts/music/03kram.html http://www.allposters.com/-sp/Symphony-Hall-Boston-MA-Posters_i1119076_.htm

  4. Symphony Hall, Boston http://upload.wikimedia.org/wikipedia/commons/thumb/5/57/Symphony_hall_boston.jpg/800px-Symphony_hall_boston.jpg

  5. Symphony Hall, Boston From Beranek, Concert and Opera Halls: How They Sound

  6. Outline • Measuring noise • Sound-level meters • Noise metrics • Speech intelligibility metrics using noise levels • Basic noise control concepts • Intensity measurements

  7. Sound Level Meters • Time constants • Given a sound raised instantaneously to an SPL of L, the meter should display (L – 2) dB within one time constant. • Why 2 dB? If SPL L has energy E, the meter registers (1 – 1/e)·E in one time constant. • e = 2.718, 10log10(1-1/e) = -2 • Time Response • Slow: Time constant = 1 sec • Fast: Time constant = 125 ms • Impact: Time constant = 35 ms rising, 1.5 sec falling Image from www.bk.dk, B&K 2260 Investigator

  8. Sound Level Meters • Frequency Response • Linear, A-weighted,C-weighted • Full bandwidth, 1/1-octave, 1/3-octave • Classes (ANSI S1.4-1983) • 0 (Laboratory): ±0.2 dB, 22.4 – 11200 Hz • 1 (Precision): ±0.5 dB, 22.4 – 11200 Hz • 2 (General Purp.): ±0.5 dB, 63.0 – 2000 Hz ±1.0 dB, 22.4 – 11200 Hz • Orientation • For free-field measurements, point the meter at the noise source (normal incidence) • For diffuse-field measurements, the meter orientation is not too important (random incidence) Image from www.bk.dk, B&K 2260 Investigator

  9. A-Weighting Review

  10. C-Weighting

  11. From dB(A) to NC/RC • dB(A) is typically insufficient to describe interior noise conditions (no spectral information) • NC (Noise Criterion) and RC (Room Criterion) metrics were developed to better describe interior noise, specifically that generated by mechanical systems • These metrics better approximate the human response to various noise spectra and provide us with more detailed analysis information From Paul Henderson

  12. Noise Criterion (NC) • Single number rating based on octave band levels • 63 Hz to 8,000 Hz frequency range • Compare measured spectra with NC curves (tangent basis) • 5 point resolution (NC-15 to NC-65) From Paul Henderson and MJR Fig. 8.2

  13. Calculating the NC Rating • The NC Rating is the lowest NC curve that lies entirely above all measured data points • In this example, the noise is NC-40, and it is limited by the 500 Hz octave band From Paul Henderson

  14. Limitations of the NC Rating • Provides no limits to low frequency noise below the 63 Hz octave band • Permits excessive high frequency noise above 2,000 Hz • Provides no information on spectrum balance or sound quality From Paul Henderson

  15. Room Criterion • Introduced in 1981, approved by ASHRAE in 1995 • Two-parameter rating based on octave band levels • 16 Hz to 4,000 Hz octave band range • First parameter is the SIL(3) (arithmetic average of noise levels in the 500, 1k, and 2k Hz octave bands) • Second parameter is a sound quality rating (Hissy, Neutral , Rumbly, Tonal, Vibration) From Paul Henderson

  16. Room Criterion • Each line has a -5 dB per octave slope • The RC-X line crosses X dB at 1000 Hz MJR, Figure 8.3, p. 165

  17. Finding the RC Limit Curve • Draw an RC line ( ) with slope -5 dB/oct that intersects the 1000 Hz band at the SIL(3) • The limit curve (- - -) is 5 dB above the RC line at and below 500 Hz and 3 dB above the RC line at and above 1000 Hz RC-36 From Paul Henderson

  18. Determine the RC Quality Rating • (R) for rumbly if data exceeds limit curve at or below 500 Hz • (H) for hissy if data exceeds limit curve at or above 1000 Hz • (N) for neutral if spectrum is below limit curve • (T) for tone if audible (any one band is at least 5 dB above both of its neighboring bands) • (V) for noise induced vibrations in light-weight structures (above 75 dB at 16 or 31 Hz, 80 dB at 63 Hz) From Paul Henderson

  19. Other Noise Metrics • Balanced Noise Criterion (NCB) • Proposed by Beranek in 1989 • Extend lower in frequency than original NC curves • More stringent at high frequencies than original NC curves • Similar quality ratings (e.g. rumbly and hissy) to RC rating system http://ceae.colorado.edu/~muehleis/classes/aren4020/handouts/lecture24/nc_rc.pdf

  20. Other Noise Metrics • Room Criterion Mark II • Proposed by Blazier in 1997 • More stringent at low frequencies than the original RC curves • Uses a Quality Assessment Index (deviations from RC curve in low, mid, and high frequencies) to qualify the numeric rating http://ceae.colorado.edu/~muehleis/classes/aren4020/handouts/lecture24/nc_rc.pdf

  21. Blazier and RC Mark II • Three factors influence the subjective response to HVAC-related background noise • The loudness of the noise relative to the noise created by “normal” activities in the space • The potential for “task interference” e.g. the reduction of speech intelligibility • The “quality” of the noise, e.g. a neutral-sounding noise spectrum will be judged mainly by its loudness but a hissy or rumbly noise spectrum is inherently more irritating regardless of loudness Blazier, W., "RC Mark II: A refined procedure for rating the noise of heating, ventilating, and air-conditioning (HVAC) systems in buildings," Noise Control Eng. J. Vol. 45, no. 6, pp. 243-150. Nov-Dec 1997.

  22. Blazier and RC Mark II • RC Mark II Rating takes the form RC xx(yy) • xx is the value of the RC reference curve corresponding to the arithmetic average of the levels in the 500, 1k, and 2k Hz octave bands • yy is a qualitative descriptor • N = neutral • LF = low-frequency dominant (rumble) • LFA = substantial sound-induced vibration • LFB = moderate sound-induced vibration • MF = mid-frequency dominant (roar) • HF = high-frequency dominant (hiss) Blazier, W., "RC Mark II: A refined procedure for rating the noise of heating, ventilating, and air-conditioning (HVAC) systems in buildings," Noise Control Eng. J. Vol. 45, no. 6, pp. 243-150. Nov-Dec 1997.

  23. Recommended Background Noise Levels MJR Table 8.1, pg. 168

  24. Recommended Background Noise Levels MJR Table 8.1, pg. 168

  25. Various Levels • LEQ (Equivalent (Continuous) Sound Level) • Given a time-variant sound-pressure level measured over time T, the LEQis the constant SPL which contains an equal amount of energy over time T • LDN (Day Night Equivalent Sound Level) • A 24-hour LEQ calculated with a 10 dB penalty for levels measured between 10:00 PM and 7:00 AM • LD = daytime LEQ, LN = nighttime LEQ • Ln (Exceedance Level) • SPL equaled or exceeded n% of the time during a measurement period. L10 is often used to represent the maximum level and L90 is often used to represent the ambient level

  26. Various Levels • TNI (Traffic Noise Index) • TNI = 4·(L10 – L90) + L90 – 30 (dBA) • NPL or LNP (Noise Pollution Level) • NPL = LEQ + σk • σ = standard deviation of the time varying level • k = 2.56 (found from studies of subjective response to time-varying noise levels) • Uses A-weighted LEQ

  27. Various Levels • SEL (Sound Exposure Level) • Li = level for a given one-second period • N = number of seconds in the measurement period • SENEL (Single Event Noise Exposure Level) • SEL of a single sound event calculated over a period in which the level is within 10 dB of the maximum level. Often used to quantify noise for individual aircraft fly-overs • CNEL (Community Noise Equivalent Level) • CNEL = SENEL +10log10(ND + 3NE + 10NN) – 49.4 (dB) • ND = number of daytime flights (7 AM to 7 PM) • NE = number of evening flights (7 PM to 10 PM) • NN = number of nighttime flights (10 PM to 7 AM)

  28. CNEL Corrections http://www.sfu.ca/sonic-studio/handbook/Community_Noise_Equivalent.html

  29. A Few More… Long Figure 4.22, p. 143

  30. Average intensity (I) if total power (W) is radiated uniformly over a spherical surface. Noise Source Directivity • Q (directivity) of a source is • For a source against a wall (for example) Total power (W) is radiated uniformly over a hemispherical surface.

  31. Noise Source Location MJR, p. 174

  32. OSHA and Noise Exposure • OSHA is the Occupational Safety and Health Administration • They provide guidelines (legal limits) for workplace noise exposure or noise dose • Noise Dose • where Ci is the total daily exposure time to a specific noise level (e.g. 90 dBA) and Ti is the maximum permissible exposure time for that level • D > 1is illegal

  33. OSHA and Noise Exposure Noise dose is measured with a noise dosimeter. http://www.nonoise.org/hearing/hcp/25.gif MJR Table 8.2, pg. 169

  34. Speech Intelligibility • Statistical Measures: Human Listeners • Modified Rhyme Test: Listeners are given lists of 6 rhyming or similar-sounding words (e.g. went sent bent dent tent rent OR cane case cape cake came cave) and are asked to choose which has been spoken • Diagnostic Rhyme Test: Listeners are given pairs of rhyming words and are asked to choose which has been spoken • Machine Measures • Percentage Articulation Loss of Consonants (%ALCons) • Calculated using RT, speaker-to-listener distance, room volume, and speaker directivity • Speech Transmission Index (STI) • Changes in the modulation of speech intensity are measured for listener positions • Articulation Index • Speech Interference Level

  35. Articulation Index • Combines the effects of source level, background noise, and hearing sensitivity • Given the source level and the background-noise level (in octave bands), calculate the signal-to-noise ratio in each band: SNR = LSource – LNoise (dB) • If SNR > 30, SNR = 30 • If SNR < 0, SNR = 0 • Then…

  36. Articulation Index • Use this table of weighting factors to calculateAI = ΣSNR · weighting factor • AI ≥ 0.7 is desired, < 0.5 is unacceptable

  37. Speech Interference Level • SIL (or PSIL) evaluates the impact of background noise on speech communication • SIL(3) is the arithmetic average of the SPL in the 500, 1,000, and 2,000 Hz octave bands • SIL(4) is the arithmetic average of the SPL in the 500, 1,000, 2,000 and 4,000 Hz octave bands From Paul Henderson

  38. SIL and Distance MJR Figure 8.1, pg. 162

  39. Speech Interference Level Foreman, Sound Analysis and Noise Control, Fig. 7.4

  40. MTF and STI • Modulation transfer function (MTF) • Start with the idea that speech is well represented by modulated bands of noise • Speech is interfered with by reverberation and background noise which effectively modify the modulation Long, Fig. 4.28, p. 151

  41. MTF and STI • The effect of background noise is independent of the modulation frequency, while the effect of reverberation is not • Skipping a few details, the modulation reduction factor is

  42. MTF and STI • m(fm) is calculated for • fm from 0.63 to 12.5 Hz in 14 1/3-octave steps • 7 octave bands of noise, from 125 Hz to 8 kHz • The result is a graph like this with 98 (7 x 14) values Long, Fig. 4.28, p. 151

  43. MTF and STI • Now find the apparent signal-to-noise ratio for all 98 values of m • And the average LSNapp weighted by octave band • Finally Long, Fig. 4.28, p. 151

  44. STI Comparisons Long, Fig. 4.29

  45. STI Comparisons Long, Figs. 4.30

  46. STI Comparisons Long, Figs. 4.29

  47. RASTI = RApid STI Long, Figs. 4.33

  48. Basic Noise Control • Address the source • Enclose it • Modify it to reduce noise production • Address the path • Add a barrier between the source and receiver • Add absorption • Address the receiver • Distribute ear plugs or other hearing protection and enforce their use

  49. Noise Barrier Performance http://www.ashraeregion7.org/tc26/pastprograms/Outdoor_Noise/barriers.pdf

  50. Noise Barrier Performance • Barrier attenuation: SPL reduction provided by the barrier under free-field conditions (no ground absorption considered) • From MJR • ∆L= 10·log10(20N + 3) where • N = 2δ/λ (called the Fresnel Number) • δ = length of shortest path from S to R over the barrier minus the length of the direct path from S to R • λ = wavelength • From every other noise control reference

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