Sound reduction

SOUND REDUCTION IN AIR-HANDLING SYSTEMS 9.8.1 Basic Concepts... 

At very low frequencies the movement of the panel will be controlled by the stiffness, as inertia is a dynamic force and cannot come into effect until the panel has measurable velocity. Stiffness controls the performance of the panel at low frequencies until resonance occurs. As the driving frequency increases, the resonance zone is passed and we enter the mass-controlled area. The increase in the sound-reduction index with frequency is approximately linear at this point, and can be represented by Figure 42.8. 

ATTENUATION - The sound reduction process in which sound energy is absorbed or diminished in intensity as the result of energy conversion from sound to motion or heat. 

S. Diez-Gutierrez, et al.. Technical note. Impact sound reduction of crosslinked and non-crosslinked polyethylene foams in suspended concrete floor structures. Journal of Building and Acoustics 10 (3) (2003) 261-271. 

There are two kinds of hearing protection devices (see Figure 28-2). Muffs fit over the ears to keep sound from entering the ears. Plugs of various types fit into the ear canal. In either case there must be a good seal between a device and the head or ear canal. Plain cotton plugs are not acceptable for hearing protection, since they have little sound reduction. 

Coefficients of internal friction of relevance to acoustic damping are given in Table 3.1-275 [1.299, 302]. As lead is used in sound attentuation applications, acoustic transmission data of selected single-skin and double-skin partitions with and without lead are given in Table 3.1-276 [1.299]. The sound reduction versus frequency is given in [1.299,304]. 

SRI sound reduction index RW weighted sound reduction STC sound transmission classification... 

Hearing protectors should be chosen to reduce the noise level at the wearer s ear to below the recommended limit for unprotected exposure (e.g. to 85 dBA which is a practicably achievable figure in almost all industrial situations). This cannot be done from simple A-weighted measurements of the noise level, because sound reduction will depend upon its frequency spectrum. Octave band analysis will provide the necessary information to be matched against the overall sound attenuation of different hearing protectors which is claimed by the manufacturers in their test data. 

The sound reduction, attenuation or insertion loss is defined as the difference in sound pressure level or sound power level before and after the enclosure (or any other form of noise control) is installed. The performance of the enclosure will be largely dependent on the sound reduction index (SRI) of the outer wall , assuming approximately 50% of the internal surface is covered with mineral wool or other absorption materials °. Typical values of sound reduction index for materials used for enclosures are shown in Table 20.3. Absorption coefficients are shown in Table 20.4. 

Table 20.3. Sound reduction indices for materials commonly used for acoustic enclosures...

Sound reduction or insertion loss increases with length. 

Similarly for cylindrical silencers, the overall performance improves with length and the addition of a central pod. Performance would be limited by the sound reduction achievable by the silencer casing and other flanking paths. Typically no more than 40-50dB at the middle frequencies could be expected without special precautions. 

Noise reduction ratio The number of decibels of sound reduction actually achieved by a particular hearing protection device. 

Sound reduction index (transmission loss) A logarithmic criterion for the sound insulation of a partition-... 

A mean sound reduction index is often indicated which (for sound incidence from all directions) is obtained as the arithmetical mean in the practical range of 100 to 3200 Hz adopted in architectural acoustics. As a rule, it corresponds to insulation at a frequency of 500 Hz. According to DIN 52210, Sheet 4, the adjusted value R should preferably by used as a single-figure rating to indicate the sound reduction. For "single-leaf" partitions, approximately Rw = Rm - 2. 

Walls which, for whatever reason, do not present a fully closed surface often have sound reduction indexes of less than 30dB (see VDI2571). 

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