Noise attenuation

Noise abatement Noise attenuation Noise insulatois... 

Attention to noise may be required beeause antisurge valves usually have large pressure drops. Appropriate valve style, noise attenuation trim, and in-line deviees, as well as attention to line veloeities and pipe sehedules, ean eliminate or minimize the noise. 

Applications such as the noise attenuation panels that line engine inlets and exhausts require a special form of high temperature adhesive known as a reticulating adhesive. These bond honeycomb sandwich assemblies have inner facesheets with numerous small holes (Fig. 13). The holes and underlying honeycomb core... 

Fig. 13. Engine inlet noise attenuation panel. Reproduced by permission of the Boeing Company.

The engine room itself must additionally provide sufficient noise attenuation as shall be deemed necessary, and there must also be adequate fuel storage, reception and handling facilities. 

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is very low at higher frequencies. One reason for this is that the Bode plot cannot really help because the open-loop gain is very small at these frequencies. The other reason is, even a tiny stray parasitic capacitance (e.g., across the power switch and inductor) presents such a low impedance to noise frequencies (whatever their origin) that almost all the noise present at the input migrates to the output unimpeded. In other words, the power stage attenuation (which we had earlier declared to be Vo/Rin) is also nonexistent for noise (and maybe even ripple) frequencies. The only noise attenuation comes from the LC filter (hopefully). 

Winterization, tropicalization, and/or noise attenuation details, when required. 

The inside surface of the ducts must be smooth, except if specific equipments are required (noise attenuator, damper, etc.). 

An obvious way to avoid signal noise attenuation for slowly relaxing carbons is to add a delay time to the pulse interval. This relaxation delay should be at least in the order of 3 7]. However, accumulation of pulse interferograms becomes time consuming by this method. 

FIGURE 2.11 Common mode noise attenuation by shielded isolation transformer. 

Noise attenuation is achieved by combining several steps. Part of noise energy is ... 

Personnel protection devices must be used to protect the ear in an industrial plant. These are earplugs made of rubber or plastic that fit snugly in the ear canal without discomfort and effectively protect the ear. They are also available as a foam cylinder that can be compressed and twisted to be inserted into the ear canal. There are helmets available that have noise attenuating electronic components and communication features. In selecting these helmets, safety engineers must exercise caution and must take steps to ensure that the devices are properly selected and used by the workers without distraction and annoyance. 

For EMI shielding applications, numerous processes are used, some require conductive fillers. These applications include parts molded with conductive filler and conductive paints. Conductive fillers used in commercial applications include aluminum, silver, nickel, and copper flakes and powders, stainless steel fibers, and fibers and flakes coated by nickel and silver. Thermoplastic compounds can provide up to 65-70 dB of electromagnetic noise attenuation but obtaining values over 45 dB is difficult. Static dissipative compounds (ESD) are mostly produced with carbon black which accounts for approximately 90% of the market but many other fillers are also used. 

Figure 3.12. Eliminating noise with analogue filters. Spectra were recorded over a spectral width of 5 kHz with no sample in the probe. The analogue filter window was set to (a) 6 kHz, (b) 3 kHz and (c) 1 kHz and the associated noise attenuation is apparent, although the cut-off point is ill defined.

Howell, G. P. 1983. Effects of nonlinear propagation on long-range noise attenuation. AIAA Paper No. 83-0700. 

Active HeoriM ProtecHoe Devices—are noise-attenuating helmets that provide active noise cancellation, communications features, and attenuation. They reduce noise by introducing destructive cancellation that applies opposite-phase sound waves at the ear. Some are designed for hearing protection and others for one- or two-way communication. 

Figure 23-4. Effect of enclosure leaks onbarrier sound transmission loss (TL). Enter the chart with thebarrier transmission loss for a fully enclosed noise source. Move vertically to intersect with the curve representing the percentage of the barrier area presented by cracks or openings. From the intersection, move left to read the noise attenuation actually achieved.

The absorption coefficient values at low frequencies from 50 to 200 Hz show erratic response for all the granule sizes. One possible reason is that the superpositions of incident and reflected waves at very low frequencies create a region of maximum particle velocity. This usually occurs at V wavelength and it is common practice among acoustics engineers who design noise attenuation panels. 

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