Environmental acoustics- noise criteria.pptx

Environmental acoustics- noise criteria.pptx

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  Environmental acoustics Noise Types ofNoise Noise rating curves Noise criterion for various spaces Dr. Priyanka Tabhane Department of Physics Rashtrasant Tukadoji Maharaja Nagpur University, Nagpur
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  Noise • Noise isbasically unwanted sound. • It is measured in dB unit. A Noise can be • Indoor Noise • Outdoor Noise
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  Indoore noise • Indoorenoise from the airborne sound from human activities in adjacent and living space or from mechanical noise travels through the air wall, flors and celling. Indoore source:- 1.Air cooler 2.Air conditioners 3.Television and music system 4.Other offices and Home appliances Outdoor Noise • Outdoor sound propagated is a affected by many factors including the distance evaluation and the line of sight. Outdoor source:- 1.Traffic (road and rails) 2.Generator,Industries, firecrackers and loudspeaker
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  Noise rating curves •Noise that is acceptable for speech interference may be annoying for other activities like reading, listening to music or sleeping. • While two continuous noises with the same frequency spectrum can be treated by comparing their a weighted sound levels, time-varying noises with different spectra require a rating procedure that accounts not only for the intermittent nature of the noise but also for its subjective qualities. • The procedure Generally accepted for rating steady broadband noise containing no significant tones, such as produced by ventilating system and distant road traffic, consist of comparing measured Octane band levels with a family of criteria curves. • Several procedures for rating study noises are in general use different only in the shapes of their criteria curves. • One of the currently accepted method uses the balanced noise criterion NCB curves. • These curves are shown in figure
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  • To determinethe rating of a noise, it's octave and levels are ploated on the same graph as criteria on curve. • The highest curve reached by any octave band level of a noise gives the NCB rating of the noise. • The dashed line in the figure is the spectrum of noise that is rated at NCB = 40. • Determining the sonic quality of the noise spectrum requires two other procedures- 1. calculate the SIL of the noise. If the level of any octave band having centre frequency at or below 1 kHz exceeds the NCB curve Of value SIL+ 3, the noise will appear rumbly (such as the sound made by heavy vehicles or thunder). 2. calculate average level of the octave bands centered at 125, 250 and 500 Hz and identify the NCB curve that intersects that level at 250 Hz. If any band level for the octaves with center frequencies equal to or greater than 1 kHz lies above the NCB curve the noise will sound hissy . • The maximum noise rating recommended for a room depend not only on the intended use of the room but also on such factors as the expectations of the users.
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  Noise Criteria forvarious spaces : An important consideration is that the noise level in various spaces should be low enough so that no damage occurs to the hearing of people who are exposed to the noise. Thus, we design auditoriums, classrooms, and public and private buildings to satisfy the listening desires of people and to control the noise for comfort of the individuals. So the tentative criteria have been established for different spaces based on an analysis of all reliable information on the subject in the literature.
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  1) Criteria forResidential Areas : The management of factories located near residential areas often need to consider the noises made by its operations. Although this problem has always existed the advent of the jet aircraft engine brought the question of neighbourhood noise control into the foreground. Different criterion have been established by several acoustic groups to minimize the noise level in factories.  Response from the neighbourhood. E.g., no annoyance, mild annoyance, strong and mild complaints, threats of legal action, etc.  Assign letters ( A to I) to noise ratings.  Adjust the noise rating upward or downward by assigning certain numbers of the letter. For example a continuous spectrum noise will be represented by the 0 and impulsive noise such from the drop forge will be represented by +1
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  1) Criteria forResidential Areas : The management of factories located near residential areas often need to consider the noises made by its operations. Although this problem has always existed the advent of the jet aircraft engine brought the question of neighbourhood noise control into the foreground. Different criterion have been established by several acoustic groups to minimize the noise level in factories.  Response from the neighbourhood. E.g., no annoyance, mild annoyance, strong and mild complaints, threats of legal action, etc.  Assign letters ( A to I) to noise ratings.  Adjust the noise rating upward or downward by assigning certain numbers of the letter. For example a continuous spectrum noise will be represented by the 0 and impulsive noise such from the drop forge will be represented by +1
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  2) Criteria forOffice Spaces : In office spaces satisfactory speech communication is generally the principal reason for noise control. When a survey was carried out in companies and an educational institution it was found that when the office workers were asked to rate the noise on the scale ranging from very quite to intolerably loud it was found that the workers rated the noise level based on their ease of communication. For example private office, Secretarial office and so forth. The results are shown in the graph below.  Point A, corresponds to normal voice.  Point B, corresponds to a slightly raised voice.