声学基本概念B

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Noise Control Treatment Strategies
Fundamentals Of Noise Control Problem Solving
Effective acoustical design in the Industrial, OEM, HVAC, Architectural and Environmental markets relates to the simple Source/Path/Receiver model. In most cases the simple model is more complex as there are multiple sources generating the noise, multiple transmission paths and multiple receivers or receiver areas that are targeted for noise control. Furthermore, the source can be airborne and/or structure-borne and the transmission path can be direct and/or indirect (reflected). Each of these areas in the noise control model need to be evaluated to determine where the simplest most cost-effective treatment can be applied while meeting all of the project requirements. Important factors in addition to the overall acoustic performance include cost, safety, accessibility, visual access, ease of installation, useful life, aesthetics and minimizing the disruption of daily operation of the process, system or equipment. The sketches and descriptions on the next page will illustrate in more detail the most basic treatment strategies using engineering controls.</P. class=text1  Please note that prior to implementing noise control treatments, mechanical equipment should be checked for proper installation, balancing and routine maintenance. Poorly maintained equipment will generate higher noise levels.
Proper selection and sizing of equipment or modifications to the operating speeds should also be reviewed prior to instituting engineering controls. Slower operating speeds will generally result in lower noise levels.
In end user applications, administrative controls can reduce employee/receiver noise dose/exposure. This is done by limiting the daily duration of operation for noisy equipment or time shifting employees to bring down the overall time weighted average. Source/Path/Receiver Model
The sketches and descriptions below illustrate in more detail the most basic treatment strategies using engineering controls. "Typical" noise reductions associated with each strategy are listed below.

SOURCE CONTROL	DIRECT PATH CONTROL	INDIRECT PATH CONTROL	RECEIVER CONTROL
6 to 8 dB	10 to 25 dB & up	4 to 6 dB	10 to 25 dB & up

                               
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Industrial Applications
Employees exposed to excessive in-plant industrial noise may be at risk to suffer a variety of physiological and psychological consequences. Other intangible effects have also been hypothesized to be caused by stress associated with noise exposure. Prevention of hearing loss is the focus of hearing conservation programs. It is the only physiological effect that has an undisputed, well documented association to noise exposure in humans. The table below summarizes some of the possible effects that are linked to noise exposure. Total employee exposure includes recreational sources.
Effects Linked To Noise Exposure

Physiological	Psychological	Other
Hearing Loss Hypertension Muscle Reactions Cardiac Disease Ulcers Colitis Heart Palpitations Headaches Nausea	Stress Insomnia Annoyance/Irritation Lack of Concentration Low Morale Learning Disability Mental Fatigue Fear Anxiety	Absenteeism Speech Interference Compromising Safety Sleep Interference Worker Productivity Job Satisfaction Mood Disturbances

Noise Induced Hearing Loss

                               
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The basic mechanism of hearing involves converting sound waves hitting the ear drum to structure-borne vibrations transmitting through bones in the middle ear. From there, vibrations are changed into nerve impulses in the cochlea of the inner ear. The fluid filled cochlea contains 40,000 tiny hair cells like the one shown at right (magnified) that initiate the nerve impulse which is transmitted to the brain. With repeated exposure to excessive noise, these hair cells lose some of their resilience and may even break off resulting in sensorineural or noise induced hearing loss. Hearing loss is permanent because once damaged, the hair cells can never be repaired or replaced.
The Federal Noise Standard For Employee
Exposure As Developed By OSHA
(Occupational Safety & Health Administration)

Permissible Noise Exposures (OSHA)
Duration per Day (Hours)	Sound Level dBA (Slow Response)
8	90
6	92
4	95
3	97
2	100
1-1/2	102
1	105
1/2	110
1/4 or less	115
Note that OSHA Permits a 5dB Increase in Permissible Levels for a Reduction of 2:1 in Exposure Time (Often Referred to as the 5dB Exchange Rate).

Key Points

• 8 hours at 90 dBA equals the permissible exposure level (100% dose)
• 8 hours at 85 dBA equals 50% of the permissible exposure level (PEL)
• The OSHA Standard was formulated to minimize not eliminate the risk of hearing loss
• Dosimeters, not sound level meters, are used to establish employee exposure/ dose
• Feasible engineering controls MUST be implemented when the equivalent dose for 8 hours exceeds 90 dBA or where continuous noise is over 115 dBA

OSHA Derating Instructions For All Hearing Protectors Using NIOSH Method #2

1) Take NRR from package	29 NRR
2) Subtract (7) dB	- 7dB
	22
3) Divide by (2) or 50%	÷ 2
4) OSHA adjusted NRR	11

A comprehensive hearing conservation program guideline was added to the original OSHA standard. The hearing conservation amendment outlines requirements for annual audiometric testing, training and documentation/ record keeping.
OSHA Compliance Strategy Summary

                               
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Caution!
Hearing Protective Devices' (HPD) Noise Reduction Ratings (NRR) May Over Estimate Performance
A recent study by Dennis A. Giardino and George Durkt, Jr. of the Mine Safety and Health Administration published in the American Industrial Hygiene Association Journal compared field performance to the EPA noise reduction rating (NRR). The results from some 1,265 hearing protective device (HPD) evaluations showed that field performance is significantly less than that specified by the NRR, especially for low frequency noise. Results also showed that the NRR is not a good indicator for comparing relative performance of HPD models.
Conducting An Industrial Noise Control Survey
Listed below are several of many possible questions that need to be researched as part of the noise control survey to determine feasible and practical treatments/solutions. Meeting the acoustical requirements alone is not sufficient to solve the problem in most cases. Identifying all the needs of the noise control treatment can best be accomplished by soliciting input from all disciplines that might be affected. Forming a noise control team with representatives from maintenance, operations, quality control, production, safety, engineering, management, etc. will assure the best possible design to meet the needs of all involved. Preliminary input from the parties involved leads to "ownership" and "buy-in" creating a sense of accomplishment for the team.

1)	What is the noise source?
2)	Where is the noise source? Please describe the room or area around the source. Is it inside or outside?
3)	What noise readings do you have from the source and/or the problem area? ______ dBA Enter octave band analysis readings below. 63_____ 125_____ 500_____ 1000_____ 2000_____ 4000_____ 8000_____
4)	Which of the following regulations apply to this project? OSHA______ EPA______ Insurance Policy______ Corporate Standard______
5)	How much reduction is desired?
6)	How many workers are exposed to the noise source and for how long?
7)	If an operator is involved, can he or she be isolated from the machine?
8)	Can the walls and/or ceiling be treated?
9)	How many reflective surfaces are near the source?
10)	Do the workers and/or operators wear any hearing protection? If so, what kind?
11)	Can the machine, equipment or system be enclosed?
12)	How frequent is access to the source necessary?
13)	How critical is visibility of the source? ...to the operator?
14)	What special conditions should be taken into consideration? (i.e. temperature, fire codes, harsh environment, aesthetics, etc.)
15)	Would this be a temporary or permanent installation?
16)	Who would do the installation?

Architectural/Interior Applications
Factors influencing sound propagation indoors include the physical dimensions and geometry of the space as well as the absorptive, reflective or diffuse characteristics of the terminating surfaces (walls, floors and ceilings). Overall sound level intensity and quality within the space are defined by acoustic phenomena such as reverberation, echoes, sound concentrations and room resonance. Depending on the use of the space, the sound quality varies with the reverberation time (RT60) in seconds. This is the time it takes for a sound to decay 60 dB. "Dead spaces" have low reverberation times and are ideal where speech intelligibility is the top priority. Higher reverberation times characterize "live spaces" that are best for performance areas dedicated to music. Reverberation times in-between are best suited for multi-purpose spaces where both speech and music are important. Reverberation times over 3 seconds should be avoided altogether.

                               
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Simplified Room Acoustics: The Sabin Formula
The Sabin Formula is named after Wallace C. Sabine, generally accepted as the Father of Acoustics. The formula allows for quick and easy calculations to estimate the existing reverberation time (RT) and to calculate how much additional treatment, using absorption materials, is required to obtain a lower RT value which is consistent with the intended use of the space.
Any room or indoor space possesses some ability to absorb and dissipate sound waves/energy. Reverberation time calculations using the Sabin Formula vary according to the volume of the space and the units of sound absorption or Sabins in the space. A Sabin is a unit of sound absorption equivalent to one square foot of material with an absorption coefficient of 1.00. For example, a 10,000 ft.2 concrete floor would yield only 150 Sabins of absorption based on the absorption coefficient for concrete at 500 Hz (.015 x 10,000 =150).

                               
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