The A-weighted sound pressure level measurement is thought to provide a rating of noise that predicts the injurious effects the noise has on human hearing and has been adopted by OSHA in its noise standards (OTM/Driscoll). In contrast, the Z-weighted measurement is an unweighted scale (introduced as an international standard in 2003), which provides a flat response across the entire frequency spectrum from 10 Hz to 20,000 Hz. The C-weighted scale is used as an alternative to the Z-weighted measurement (on older sound level meters on which Z-weighting is not an option). It can be used to evaluate hearing protection and for characterizing low-frequency sounds capable of inducing vibrations in buildings or other structures. Some references may mention the B-weighted scale, but note that this scale is no longer used.
Most industrial equipment vibrates to some extent. Determining whether or not the vibrating forces are severe enough to cause a problem is accomplished through a comprehensive noise and/or vibration survey. As machines operate, they produce either harmonic forces associated with unbalanced rotating components or impulsive forces attributed to impacts such as punch presses, forging hammers, and shearing actions. Excessive noise can be one result of the vibratory energy produced; however, potential damage to the equipment itself, the building, and/or the product being manufactured is more likely. Quite often, vibration problems are clearly identified by predictive-maintenance programs that exist within most industrial plants.
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Assuming that the root cause or source cannot be effectively modified, the next option for controlling undesirable vibration is to install vibration isolation. Isolators come in the form of metal springs, elastomeric mounts, and resilient pads. These devices serve to decouple the relatively "solid" connection between the source and the recipient of the vibration. As a result, instead of the vibratory forces being transmitted to other machine components or the building, they are readily absorbed and dissipated by the isolators.
Sound-absorption materials are used to reduce the buildup of sound in the reverberant field. The reverberant field exists at all locations where sound waves reflect off relatively hard surfaces, such as walls, ceilings, or inside enclosures, and then combine with the sound waves propagating directly from the noise source. The added effect produces a higher noise level than the level that would have existed in the absence of any reflecting surfaces.
Sound TL materials are used to block or attenuate noise propagating through a structure, such as the walls of an enclosure or room. These materials are typically heavy and dense, with poor sound transmission properties. Common applications include barriers, enclosure panels, windows, doors, and building materials for room construction.
All products sold for noise control should have a TL rating that is determined by ASTM standard. It is important to note that TL rating varies with frequency. TL values generally range from 20 to 60 dB, with the higher number indicating superior attenuation properties. For TL values of common building materials, consult Table 9.12 in The Noise Manual (AIHA, 2003, or latest edition).
Enclosures, both off-the-shelf and custom-design, are available from a number of manufacturers listed in the Noise and Vibration Control Product Manufacturer Guide. It can also be more cost-effective to build enclosures in-house by following the Guidelines for Building Enclosures.
The HRT maintains multipurpose Type I sound level meters and octave band analyzers, which can also be operated as sound intensity analyzers for identifying noise sources and determining engineering controls. In addition, this equipment includes a building acoustics system for measuring noise decay and determining the reverberation characteristics for a given room. Based on the noise decay data, calculations can be performed to estimate potential noise reduction if absorptive materials are applied to room surfaces, such as the walls and ceiling.
As described above, frequency influences sound absorption by materials. Table V-2 shows the absorption coefficient for common building materials at different frequencies. Note that dense materials, such as rough concrete, absorb lower frequencies better than other materials, while high frequencies are better absorbed by less dense materials, such as carpet and fiberglass. Painting concrete creates a smooth surface that greatly increases the percentage of sound that is reflected at all frequencies.
Table V-3 and V-4 show various transmission loss values for common building materials at specific frequencies and material thicknesses. Note that the values in these tables are measured under ideal laboratory conditions as a resource for comparing different materials. In the workplace, the noise exposure experienced by the receiver would not actually be reduced by the reported transmission loss value, because imperfections in enclosures, barriers, or other noise controls made of these materials permit sound to go around the material, leak through cracks or utility paths, or pass through other materials with lower transmission loss values (e.g., a door jamb, window glass) that were also used in construction.
Table V-3 demonstrates how the thickness of two materials (plywood and steel) influences the transmission loss values for the materials, and Table V-4 compares the relative transmission loss values for common building materials.
Enclosures can present difficulties for the production process. Using them can involve many challenges, such as interior heat buildup, limited physical and visual access to the equipment, difficulty getting the product in and out of the enclosure without sacrificing some noise reduction, and maintenance personnel needing to disassemble the enclosure when repairing equipment. It is not unusual for a reassembled enclosure to lose much of its effectiveness due to poor fittings and small gaps or openings in the enclosure.
Note the large partition wall on the right side of the photograph in Figure 40. A barrier should be as tall as possible and be as close to the worker or the noise source (in between the two) as feasible in order to maximize the reduction in noise exposure. Of course, if a receiver is inside a room, reverberations from the ceilings and walls can diminish the effectiveness of a barrier. For this reason, indoor barriers are most effective when workers are in the direct field of sound from the noise source, as opposed to the reverberant field. Even outdoors, it is possible for noise to reflect from nearby buildings and contribute to the noise exposure of the receiver.
The equipment manufacturer, contacted by phone, indicates that one engineering option is to rebuild the drive mechanism and replace the cutters with those of a helical design. According to the manufacturer's technical representative, this will greatly improve the quality of the planed finish and reduce the noise level to about 90 dBA. With the existing administrative controls, everybody's daily exposure level would be reduced to less than 84 dBA. A call to the regional service technician produced a cost figure of $10,000 per planer to retrofit, with no maintenance or production penalty involved.
General guideline 3: Reducing compressed air pressure and volume used can reduce noise levels substantially and can also save on energy costs. It is almost always cost-effective. Other good opportunities for noise reduction are associated with routine maintenance and machine guarding (why not build in noise reduction at the same time?).
Key management is one of the biggest challenges of building an enterprise encryption strategy because the keys to decrypt the cipher text have to be living somewhere in the environment, and attackers often have a pretty good idea of where to look.
This is a recipe for probably the most famous type of bread in Denmark. Rye bread or Rugbrød as it is called in Danish, is a type of bread which is packed with different seeds, grains and cracked rye and is therefore a very healthy alternative to regular white bread.
My Swedish great grandma would go out early in the morning, catch fish, and make ryr bread all before the sun came up to start milking the cows! Hers ryebread was a much smoother, dense, but fluffy recipe without visible cracked grains. I believed she also used molasses. This recipe seems so similar but is very different in texture/color. Hers was a dark brown and felt more like ponds cake in your mouth. Any suggestions or alternatives to make it more like hers?
Hi Kim! I made the bread twice and it came out really good. My boyfriend's mom bought the ikea mix bread and I was looking to do something similar and it indeed was. I couldn't fined cracked rye kernels so I used spelt grains instead and it came out great. Thank you for this lovely recipe!
Thank you for the recipe. This is my first time using sourdough.I live in the UK and did not manage to get cracked rye so just used non-cracked rye instead. I used a sourdough starter and fed for a week and it seem to look how it should and appeared active but the dough did not rise in the bread pan before before baking even though I waited much longer than the recipe suggested.
Thank you for publishing your recipe. I had a bread similar to this one in Chile recently, from a Bavarian bakery and loved it. I can find Rye kernels or flakes here at hone in the US, but not cracked kernels. Which would be better to use? Also is it really 2 tablespoons not teaspoons of Salt?
Two tbls of salt was too much. Bread looked fine, but was much too salty. Going back to two tsps salt. Also, I used rye kernels and chopped them in the blender. From the look of the bread, thatworks fine in place of cracked rye.
Hello, I went to Denmark and absolutely loved your bread. Now THAT is real bread! One question; if I don't have cracked rye and wheat available, can I use other seeds instead, like pumpkin, sesame, chia, etc? Thanks! 2ff7e9595c
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