Acoustics noise levels

Ventilation noise originates primarily from fans and the air turbulence generated inside ducts and around supply air and exhaust air terminal devices. The appearance of the noise is, of course, affected by factors such as the speed of rotation and the power of the fan, and by how the fan is stabilized or in other ways acoustically insulated. The noise level and the frequency characteristics are also largely derermined by the velocity of the air inside ducts and around terminal devices, where factors such as the dimensions and appearance of the ducts and terminal devices may play a decisive role in the appearance of the noise. 

The subject of acoustics involving sound transmission is of prime importance in industrial ventilation. Correct system design will ensure that the designer provides a system that will not give rise to complaints regarding noise levels. 

Mud acoustic attenuation and annulus noise level are being investigated. It is expected that attenuation would be very sensitive to free gas concentration. 

Finally, noise is also becoming of increasing concern, particularly on sites adjacent to urban areas. Acoustic enclosures, supplied by the manufacturer, will normally reduce noise levels to 85 dB, but if further reduction is required, attention must be paid to the material used to construct the buildings and enclosures low noise levels can be achieved only at a cost. 

Percentiles are expressed as the percentage of time (for the stated period) during which the stated noise level was exceeded, i.e. 5 min Lgo of 80 dB(A) means that for the 5-min period of measurement for 90 per cent of the time the noise exceeded 80dB(A). Therefore Lo is the maximum noise level during any period and Lioo is the minimum. Leq (the equivalent continuous noise level) is the level which, if it were constant for the stated period, would have the same amount of acoustic energy as the actual varying noise level. 

Many of the problems encountered with the application of the Acoustic Emission (A.E) technique to corrosion problems are due to the fact that the signal level was of a similar order of magnitude to the background noise level of the instrumentation. 

Moreover, in recent years broad band lasers have appeared which lack any frequency modal structure, at the same time retaining such common properties of lasers as directivity and spatial coherence of the light beam at sufficiently high spectral power density. The advantages of such a laser consist of fairly well defined statistical properties and a low noise level. In particular, the authors of [245] report on a tunable modeless direct current laser with a generation contour width of 12 GHz, and with a spectral power density of 50 /xW/MHz. The constructive interference which produces mode structure in a Fabry-Perot-type resonator is eliminated by phase shift, introduced by an acoustic modulator inserted into the resonator. 

The success of the tap test depends on the skill and experience of the operator, the background noise level, and the type of structure. Some improvement in the tap test can be achieved by using a solenoid-operated hammer and a microphone pickup. The resulting electric signals can be analyzed on the basis of amplitude and frequency. However, the tap test, in its most successful mode, measures only the qualitative characteristics of the joint. It tells whether adhesive is in the joint or not, providing an acoustical path from substrate to substrate or it tells if the adhesive is undercured or filled with air, thereby causing a mechanically damped path for the acoustical signal. The tap test provides no quantitative information and no information about the presence and/or nature of a weak boundary layer. 

To attain efficient, effective, and practical noise control, it-is necessary to understand the individual equipment or process noise sources, their acoustic properties and characteristics, and how they interact to create the overall noise situation. Table 11 presents typical process design equipment providing high noise levels and potential solutions to this problem. 

For Tu < 0.10 %, the transition location is insensitive to Tu. Such low levels of disturbances are typical of acoustic noise that controls... 

BACKGROUND NOISE - Sound other than the wanted signal. In room acoustics, the irreducible noise level measured in the absence of any building occupants. 

---