声学基本概念
What Is Noise?
Noise is unwanted sound which may be hazardous to health, interfere with speech and verbal communications or is otherwise disturbing, irritating or annoying.
What Is Sound?
http://qwyatt.com/images/tech-1.gifSound is defined as any pressure variation in air, water or other fluid medium which may be detected by the human ear.
What Are The Characteristics Of Sound?
The two most important characteristics which must be known in order to evaluate the sound or noise are it's amplitude and frequency. The amplitude or height of the sound wave from peak to valley determines the loudness or intensity. The wave length determines the frequency, pitch or tone of the sound.
How Are These Characteristics Expressed?
The frequency of sound is expressed in wavelengths per second or cycles per second (CPS). It is more commonly referred to as Hertz. Low frequency noise is 250 Hertz (Hz) and below. High frequency noise is 2000 Hz and above. Mid-frequency noise falls between 250 and 2000 Hz.
The amplitude of sound is expressed in decibels (dB). This is a logarithmic compressed scale dealing in powers of 10 where small increments in dB correspond to large changes in acoustic energy.
What Are Octave Bands?
http://qwyatt.com/images/tech-2.gif Standardized octave bands are groups of frequencies named by the center frequency where the upper limit is always twice the lower limit of the range. Test data for performance of acoustical materials is standardized for easy comparison at the center frequencies. Equipment noise levels and measurement devices (dB meters) also follow the preferred octave bands.
What Is The Difference Between dB And dBA?
http://qwyatt.com/images/tech-3.gif dB sound pressure levels are unweighted. dBA levels are "A" weighted according to the weighting curves shown below to approximate the way the human ear hears. For example, a 100 dB level at 100 Hz will be perceived to have a loudness equal to only 80 dB at 1000 Hz. Other weighting scales (C and B) are also shown. The dBA scale is based on a child's hearing and was originally documented based on actual hearing tests to characterize the human ear's relative response to noise.
Is Hearing Loss Permanent?
Yes! Permanent hearing loss occurs when the tiny hair cells in the cochlea (inner ear) are damaged or destroyed. A healthy cochlea contains approximately 40 thousand hair cells which are necessary to transmit sound vibrations to the brain. Exposure to excessive noise levels will damage the hair cells resulting in permanent, irreversible hearing loss.
The tables at left show the additive effect for adding equal and unequal decibel levels. Unless the two levels differ by 10 or more dB there will always be some increase to the higher level. Frequency levels can also be added together in a similar fashion to get overall dB levels.
For adding several decibel levels
of the same value:No. of
Equal
LevelsAdd the Following
Amount to that Level
to Get the Sum23.0 dB3 4.8 dB 4 6.0 dB 5 7.0 dB 6 7.8 dB 7 8.4 dB 8 9.0 dB 9 9.5 dB 10 10.0 dB N 10 log N dB At the final total, round off to the
nearest whole number.For adding any two decibel levels to an
accuracy of about 1 dB:When Two
Decibel Values
Differ By Add the Following
Amount to the
Higher Value0 or 1 dB 3 dB 2 or 3 dB 2 dB 4 to 9 dB 1 dB 10 dB or more 0 dB When adding several levels, start
with lowest levels first; continue
two at a time until only one final
value remains.
Is A 5 dB Change Significant?
Sound Level
ChangeAcoustic
Energy Loss Relative
Loudness0 dB0 Reference -3 dB50% Perceptible Change -10 dB90% Half as Loud -20 dB99% 1/4 as Loud -30 dB99.9% 1/8 as Loud -40 dB99.99% 1/16 as Loud
Yes! The pressure associated with the loudest known sound is more than one billion times that associated with the faintest sound. Such a large range is unmanageable for measurement purposes. Using a logarithmic scale compresses the range to between 0 and 200 dB. At right, various sound level changes are referenced to relative loudness and acoustic energy loss. A 5 dB change is more than a 50% change in acoustic energy!!!
Is Sound Power The Same As Sound Pressure?
No! While both sound power levels (Lw) and sound pressure levels (Lp) are both expressed in decibels, the referenced standards for each are different. More importantly, the sound power level is the total acoustic energy output of a noise source independent of environment. Sound pressure levels are dependent on environmental factors such as the distance from the source, the presence of reflective surfaces and other characteristics of the room/building/area hosting the source. Actual sound pressure levels will always be higher than sound power levels.
Noise Control Terminology and Definitions
Absorption Coefficient: The absorption coefficient of a material or sound absorbing device is the ratio of the sound absorbed to the sound incident on the material or device.
Acoustical Material: A material used to alter a sound field. The material may be used to absorb, damp or block acoustical energy.
Airborne Noise: A condition when sound waves are being carried by the atmosphere.
Ambient Noise: All the sounds from many sources associated with a given environment.
Anechoic Room: A test chamber which has a lining of absorbent acoustical material to eliminate all sound reflections. It is most often used to determine the sound radiation characteristics of equipment.
Damping: The process of dissipating mechanical vibratory energy into heat. In noise control, a damping material is usually applied to a vibrating surface to reduce the noise radiating from that surface.
Dissipative Silencer: A device inserted into an air duct or opening to reduce noise transmitted through the duct or opening. Noise reduction is accomplished through the use of internal sound absorbing materials.
Flanking Transmission: Noise that reaches an observer by paths around or over an acoustical barrier.
Frequency Spectrum: A graph or plot of the sound pressure level in each band from a set of octave or 1/3 octave bands.
Insertion Loss: The reduction of sound power level attained by inserting a silencer or muffler in an acoustic transmission system (see ASTM E-477).
Loudness: Loudness is the subjective human definition of the intensity of a sound. Human reaction to sound is highly dependent on the sound pressure and frequency.
Mass Law: A rule for estimating the transmission loss of a barrier in its mass controlled region. The rule states that transmission loss increases/decreases 6 dB for each doubling/halving of either frequency or barrier surface density.
Noise: Any undesired sound.
Noise Reduction (NR): The reduction in sound pressure level caused by making some alteration to a sound field.
Noise Reduction Coefficient (NRC): A single number rating which is the average of the sound absorption coefficients in the octave bands centered at 250, 500, 1000 and 2000 Hz expressed to the nearest integral multiple of 0.05 (see ASTM C-423).
Octave Band (O.B.): A range of frequencies where the highest frequency of the band is double the lowest frequency of the band. The band is usually specified by the center frequency, i.e., 31.5, 63, 125, 250, 500 Hz, etc.
Radiation: The process whereby structure-borne vibration is converted into airborne sound.
Reverberation: Reverberation is the echoing of previously generated sound caused by reflection of acoustic waves from the surface of enclosed spaces.
Reverberation Room: A test chamber so designed that the reverberant sound field within the room has an intensity that is approximately the same in all directions and at every point. It is commonly used to measure sound absorption, ASTM C-423 and transmission loss, ASTM E-90.
Sabin: The unit of measure of sound absorption. The number of square feet of sound absorbing material multiplied by the material absorption coefficient.
Sound: Pressure waves that are traveling in the air or other elastic materials.
Sound Absorption: The acoustical process whereby sound energy is dissipated as heat rather than reflected back to the environment.
Sound Level Meter: An instrument used to measure sound pressure level. Sound level meters are commonly either Type 1, precision instruments, or Type 2, general purpose instruments. Both types can have weighting and filter networks to provide dB readings by octave band in the A, B, or C scales.
Sound Power Level (Lw): A measure of the total airborne acoustic power generated by a noise source, expressed on a decibel scale referenced to some standard (usually 10-12 watts).
Sound Pressure Level (Lp): A measure of the air pressure change caused by a sound wave, expressed on a decibel scale referenced to 20礟a.
Sound Transmission Class (STC): A single number rating derived from measured values of transmission loss in accordance with ASTM 413. The rating provides an estimate of the performance of a barrier in certain common noise attenuation applications.
Structure-borne Noise: Mechanical vibration in a structure which can ultimately become audible sound. Until such time as radiation occurs, these vibrations are inaudible and of little concern.
Transmission Loss (TL): The reduction in sound power that is caused by placing a wall or barrier between the source and receiver. Transmission loss is expressed in decibels. Noise Control Product Types
ABSORBERS
http://qwyatt.com/images/tech-4.gifUse: To reduce noise reflection. To dissipate noise energy.
Physical Properties: Porous, fibrous and sometimes covered with protective membranes. Noise enters the absorber and is partly dissipated (absorbed) within the material. Some is transmitted. Some is reflected. Absorber performance is expressed as a decimal value. A perfect absorber is rated at 1.00. The higher the decimal value the more effective the absorber will be.
Effectiveness is expressed as NRC (Noise Reduction Coefficient).
NRC: Percentage of acoustical energy absorbed calculated as an average of laboratory test data at several frequencies.
Noise Reduction Coefficients of MaterialsNRCBrick, unglazed.05 Concrete block .05 1/8" pile Carpet .15 5/16" pile Carpet and foam .35 Concrete floor .00 Plaster, smooth finish .05 Plywood paneling, 1/4" thick .10 Water surface (as in swimming pool) .00 1" thick fiberglass curtain .70 3" thick "SONEX" wedge foam .86 4" thick smooth surface foam .89 4" thick metal panel .95
BARRIERS
http://qwyatt.com/images/tech-5.gif Use: To block transmission of noise.
Physical Properties: Non-porous, high density and usually non-fibrous. Barriers are generally flexible or damped. The noise is blocked, reflected and re-routed in another direction. Barrier materials are tested and rated for their Sound Transmission Loss capability. The number is stated in dB and the higher number signifies the better barrier.
Effectiveness is expressed as STC (Sound Transmission Class).
STC: Single number rating derived from decibel loss data at several frequencies.
Sound Transmission Class of Materials STC 1 lb. density barrier material 26 1 lb. density transparent curtain 26 5/8" Gypsum wallboard 30 3/16" Steel wall 31 2" fiberglass curtain with 1 lb. barrier 29 2" thick metal panel (solid and perforated) 35 4" thick metal panel (solid and perforated) 41 12" thick concrete 53 3/8" plasterboard 26 22 gauge steel 25 Solid core wood door, closed 27 Concrete block wall, unpainted 44 COMPOSITES
Use: To block the transmission of noise and reduce reflections from the barrier.
Physical Properties: Consists usually of a layer of porous material and a layer of dense material. The composite material will have a performance capability as an absorber and as a barrier. Septum barriers are sandwiched between two absorber layers.
Effectiveness is a combination of STC and NRC ratings.
http://qwyatt.com/images/tech-6.gifDAMPING
Use: To reduce noise radiated from vibrating surfaces.
Physical Properties: Viscoelastic. Damping coatings take many forms. There are mastics for spraying, troweling, etc. and there are tapes and sheets with pressure sensitive adhesive. Damping treatments are sometimes combined with absorbers.
Effectiveness is expressed as a "loss factor" which is the damping/stiffness ratio of a material.
http://qwyatt.com/images/tech-7.gifDECOUPLED COMPOSITES
Use: To enhance the performance of the composite material when applied to the inside of an existing barrier. Decoupling creates an air space between the existing barrier and the septum composite barrier boosting transmission loss beyond what could be expected with direct attachment.
http://qwyatt.com/images/tech-8.gifDIFFUSION
Use: To reflect sound waves off convexly curved or uneven surfaces for the purpose of evenly distributing and blending the sound over a broad area. In critical listening environments diffusion can eliminate sharp echoes without eliminating the sound by absorbing it.
http://qwyatt.com/images/tech-9.gifELECTRONIC
Use: To cancel unwanted noise energy through destructive interference by electronically generating a 180?out of phase anti-noise which is equal and opposite in phase and amplitude.
Physical Properties: Equipment includes an input microphone, controller/amplifier, speakers and an error microphone. Works best with noise propagated in a confined/closed loop space such as a pipe or duct. Works best where the noise source is repetitive and not random. Works best in low frequencies up to about 500 Hz.
Effectiveness is expressed as Dynamic Insertion Loss (DIL) for active/electronic mufflers and silencers.
DIL: The noise reduction of sound power level attained by inserting a silencer or muffler in a pipe or duct transmission system under air flow conditions.
Technological Advances: Electronic or active noise and vibration technologies are emerging from research and development to production applications. Recent advances include cancellation headsets, mufflers for automotive and industrial and a variety of consumer appliances.
http://qwyatt.com/images/tech-10.gifFLOW CONTROL Use: To reduce flow/fluid-borne noise transmission traveling through pipes and ducts connected to air/fluid control devices, equipment and systems. Mufflers or silencers use absorptive and reactive designs to allow air passage while attenuating the noise. Fluid-borne flow systems may be air, gases or steam.
Physical Properties: Internal geometry of the flow control device dictates the overall noise reduction that can be achieved and the resultant pressure loss of the system. Absorptive designs can vary the insulation thickness and density in the wall cavity as well as the distance between internal baffles (passage width). Reactive designs can vary the flow control device internal chamber length and volume as well as the number of interconnected chambers and the size and length of choke tubes connecting the chambers.
Effectiveness is expressed as Dynamic Insertion Loss (DIL) for mufflers and silencers.
DIL: The noise reduction of sound power level attained by inserting a silencer or muffler in a pipe or duct transmission system under air flow conditions.
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