# Acoustic Treatment Two 1 Acoustic Treatment Two – Noise and Vibration Control MEBS 6008.

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Acoustic Treatment Two 1 Acoustic Treatment Two – Noise and Vibration Control MEBS 6008

Acoustic Treatment Two 2 Balanced vs unbalanced air flows Exhaust air flow/ supply air flow(EAF/SAF) Sensible Effectiveness 1 0.590% or 0.9 2 0.685% or 0.85 3 0.779% or 0.79 4 0.876% or 0.76 5 0.970% or 0.7 6 1.066% or 0.66 Unbalanced flow increase effectiveness of heat exchanger Heat exchanger transfer less overall heat, why? The following example illustrates the reasons: Balanced Flow Unbalanced Flow 4.7 cub.m. 4.7 cub.m 35.7 deg C 31.9 deg C 28.9 deg C 25.6 deg C 4.7 cub.m. 35.7 deg C 30.0 deg C 33.2 deg C 3.3 cub.m. 25.6 deg C EAF/SAF = 4.7cub.m. /4.7cub.m. = 1 EAF/SAF = 3.3 cub.m./4.7 cub.m. = 0.7

Acoustic Treatment Two 3 Addition of two sound pressure levels – one example to illustrate

Acoustic Treatment Two 4 Sound Power Level The reference value used for calculating sound-power level is 10 -12 watts. Sound-power level (L w ) in dB is calculated using the upper left equation Sound Pressure The reference value used for calculating sound-pressure level is 2 ×10 -5 Pa. Sound-pressure level (L p ) in dB is shown on the lower left equation Sound power is proportional to the square of sound pressure  multiplier 20 is used (not 10). Reference values are the threshold of hearing. Note the unit of the equation

Acoustic Treatment Two 5 Why 42dBA as stated in previous lecture???

Acoustic Treatment Two 6 Noise Control in Ventilation System

Acoustic Treatment Two 7 Noise Control in Ventilation System

Acoustic Treatment Two 8 Noise Control in Ventilation System

Acoustic Treatment Two 9 End reflection loss The change in propagation medium (when sound travels from duct termination into a room)  reflection of sound back up the duct. The effect is greatest at long wavelength (i.e. low frequencies) This leads to a contribution to the control of low frequency noise from the system. Noise Control in Ventilation System

Acoustic Treatment Two 10 The relation between sound pressure level and sound power level in real room may be found by Determination of sound level at a receiver point When a source of sound operates in a room, energy travels from a source to the room boundaries, where some is absorbed and some of it is reflected back into the room. Noise Control in Ventilation System

Acoustic Treatment Two 11 For a normally furnished room with regular proportions and acoustical characteristics between `average’ and `medium-dead’ and room volume < 430 m 3, a point source of source could be found by: - Determination of sound level at a receiver point (continued) Noise Control in Ventilation System

Acoustic Treatment Two 12 FUNDAMENTALS OF VIBRATION A rigidly mounted machine transmits its internal vibratory forces directly to the supporting structure. Vibration isolators is resilient mountings By inserting isolators between the machine and supporting structure, the magnitude of transmitted vibration can be reduced (%). Vibration isolators can also be used to protect sensitive equipment from disturbing vibrations. Vibration energy from mechanical equipment  transmitted to the building structure  radiated as structure-borne noise.

Acoustic Treatment Two 13 Vibration isolation Any residual, out-of-balance force in the rotating parts as a weight located eccentrically. The weight rotates  each part of the machine structure subjected to a cyclic force from inertia of the rotating off-centre height Vertical component of the force is concerned  acting alternately upwards and downwards, at a frequency equal to the shaft rotational frequency. Assumption : machine is rigid for every point (includes mounting feet). Other case of cyclic force example: Combustion loads in reciprocating engines eventually appear as noise energy

Acoustic Treatment Two 14 SINGLE-DEGREE-OF-FREEDOM MODEL The simplest example is the single-degree-of-freedom model. Only motion along the vertical axis is considered Damping is disregarded Valid only when the stiffness of the supporting structure >> the stiffness of the vibration isolator. (mechanical equipment on G/F or basement locations) Natural frequency of the isolator : deflect the spring a little more + suddenly release it  the machine oscillate vertically about its rest position at natural frequency. The natural frequency f n of the system is where k is the stiffness of vibration isolator (force per unit deflection) M is mass of the equipment supported by isolator.

Acoustic Treatment Two 15 This equation simplifies to (try yourself, noting the unit of g) where δ st is the isolator static deflection in mm, k/M = g / δ st. Static deflection = incremental distance the isolator spring compressed under the equipment weight. Isolator static deflection & supporting load  achieve the appropriate system natural frequency

Acoustic Treatment Two 16 The machine settles under its own weight The machine deflects the spring by a certain amount  static deflection of the isolator Static deflection determines the eventual performance of the spring as an isolator when the machine is running. Static deflection depends only upon the static stiffness of the spring, and weight of the machine. Undamped Vibration Use the steel spring as vibration isolator.

Acoustic Treatment Two 17 Damped Vibration Real isolators have a certain degree of internal damping Energy is progressively removed from the system Amplitude of vibration steadily reduces Large amount of damping  movement of the mass back to its rest position after initial deflection will be very sluggish Neither overshoot nor oscillate Critical damping Amount of damping just sufficient for mass to return to its mean position in min. time without overshoot

Acoustic Treatment Two 18 Damping equation Effects of Damping Frequency ratio for maximum transmissibility < Equivalent undamped amount At high forcing frequency, transmissibility varies with amount of damping Figure shown D=20% and D=100%

Acoustic Treatment Two 19 Transmissibility T is inversely proportional to the square of the ratio of the disturbing frequency f d to the system natural frequency f n, or Transmissibility T and Displacement x

Acoustic Treatment Two 20 Vibration isolation begin after f d / f n > 1.4. Vibration transmissibility rapidly decreases. At f d = f n, resonance occurs (the denominator of Equation equals zero) At resonance  theoretically infinite transmission of vibration. In practice, some limit on the transmission at resonance exists due to some inherent damping.

Acoustic Treatment Two 21 A frequency ratio of at least 4.5 is often specified, which corresponds to an isolation efficiency of about 90%, or 10% transmissibility. Higher ratios may be specified, but in practice this is difficult to achieve. Nonlinear characteristics cause typical isolators to depart from the theoretical curve.

Acoustic Treatment Two 22 Equipment mass increased  the resonance frequency decreases  increasing the isolation. In practice, the load-carrying capacity of isolators requires their stiffness or number be increased. The use of stiffer springs leads, however, to smaller vibration amplitudes—less movement of the equipment.

Acoustic Treatment Two 23

Acoustic Treatment Two 24

Acoustic Treatment Two 25 TWO-DEGREE-OF-FREEDOM MODEL When heavy mechanical equipment is installed on a structural floor( especially roof), the stiffness of the supporting structure may NOT be >> the stiffness of the vibration isolator. Significantly “softer” vibration isolators are usually required in this case. Two-degree-of-freedom model for the design of vibration isolation in upper-floor locations.

Acoustic Treatment Two 26 The precise behavior of this system with respect to vibration isolation is difficult to determine. The objective is to minimize the motion of the supporting floor M f in response to the exciting force F. Evaluating the interaction between two system natural frequencies and the frequency of the exciting force  complicated Fraction of vibratory force transmitted across an isolator to the building structure (transmissibility) depends in part the isolator stiffness comparing with that of supporting floor.

Acoustic Treatment Two 27 Ideally, this ratio should be on the order of 10:1 to approach an isolation efficiency of about 90%. Static deflection of the vibration isolator is = incremental deflection of the supporting floor under the added weight of the equipment  50% of the vibratory force Stiffness is inversely proportional to deflection under the applied load, this relationship is shown as a ratio of deflections. To optimize isolation efficiency  static deflection of the loaded isolator>> incremental static deflection of the floor under added equipment weight. Floor deflection  excessive vibration is attributable to upper floor or rooftop mechanical installations.

Acoustic Treatment Two 28 Selection of vibration isolators on the basis of the single-degree-of-freedom model  neglected floor stiffness  inadequate Steps to choose vibration isolators with consideration of floor stiffness Asking structural engineer to estimate the incremental static deflection of the floor due to the added weight of the equipment at the point of loading Choose an isolator that will provide a static deflection of 8 to 10 times that of the estimated incremental floor deflection. Consider also building spans, equipment operating speeds, equipment power, damping and other factorsRemarks The type of equipment, proximity to noise-sensitive areas, and the type of building construction may alter these choices.

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Acoustic Treatment Two 35 Fans and Air-Handling Equipment: Fans with wheel diameters < or = 560 mm and all fans operating at speeds to 300 rpm NOT generate large vibratory forces. For fans operating under 300 rpm, select isolator deflection so that the isolator natural frequency is 40% or less of the fan speed. Fan operating at 275 rpm, an isolator natural frequency of 110 rpm (1.8 Hz) or lower is required (0.4 × 275 = 110 rpm). A 75-mm deflection isolator can provide this isolation.

Acoustic Treatment Two 36 Pumps: Concrete bases (Type C) should be designed for a thickness of one tenth the longest dimension with minimum thickness as follows: For up to 20 kW, 150 mm; For 30 to 55 kW, 200 mm; For 75 kW and higher, 300 mm. Pumps over 55 kW and multistage pumps may exhibit excessive motion at start-up  supplemental restraining devices can be installed if necessary.

Acoustic Treatment Two 37 ISOLATION OF VIBRATION AND NOISE IN PIPING SYSTEMS All piping has mechanical vibration (equipment and flow-induced vibration and Noise)  transmitted by the pipe wall and the water column. Equipment installed on vibration isolators exhibits motion or movement from pressure thrusts during operation. Vibration isolators have even greater movement during start-up and shutdown (equipment goes through the isolators’ resonant frequency). The piping system must be flexible enough to Reduce vibration transmission along the connected piping, Permit equipment movement without reducing the performance of vibration isolators, and Accommodate equipment movement or thermal movement of the piping at connections without imposing undue strain on the connections and equipment.

Acoustic Treatment Two 38 Minimized by sizing pipe so that - the velocity is 1.2 m/s maximum for pipe 50 mm and smaller and - using a pressure drop limitation of 400 Pa per metre of pipe length with a maximum velocity of 3 m/s for larger pipe sizes. Flow noise and vibration can be reintroduced by -turbulence, -sharp pressure drops, and -entrained air. Flow noise in piping

Acoustic Treatment Two 39 Resilient Pipe Hangers and Supports Resilient pipe hangers and supports are necessary to prevent vibration and noise transmission from the piping to the building structure and to provide flexibility in the piping. Suspended Piping. Isolation hangers described in the vibration isolation section should be used for all piping in equipment rooms. The first three hangers from the equipment : the same deflection as the equipment isolators (a max. limitation of 50 mm deflection) Remaining hangers : spring or combination spring and rubber with 20 mm deflection.

Acoustic Treatment Two 40 Floor Supported Piping. Floor supports for piping in equipment rooms and adjacent to isolated equipment: The first two adjacent floor supports should be the restrained spring type, with a blocking feature that prevents load transfer to equipment flanges as the piping is filled or drained. Where pipe is subjected to large thermal movement, a slide plate should be installed on top of the isolator

Acoustic Treatment Two 41 Piping Penetrations Most HVAC systems have many points at which piping must penetrate floors, walls, and ceilings. Risk for a path for airborne noise  destroy the acoustical integrity of the occupied space. Seal the openings in the pipe sleeves by an acoustical barrier such as fibrous material and caulking (between noisy areas, such as equipment rooms, and occupied spaces)

Acoustic Treatment Two 42 Flexible Pipe Connectors (1)They provide piping flexibility to permit isolators to function properly, (2)They protect equipment from strain from misalignment and expansion or contraction of piping, and (3)They attenuate noise and vibration transmission along the piping. The most common type of connector are arched or expansion joint type, a short length connector with one or more large radius arches, of rubber or metal. All flexible connectors require end restraint to counteract the pressure thrust. Overextension will cause failure. Manufacturers’ recommendations on restraint, pressure, and temperature limitations should be strictly adhered to.

Acoustic Treatment Two 43 Noise Control in Practice

Acoustic Treatment Two 44 Noise Control in Practice

Acoustic Treatment Two 45 Noise Control in Practice

Acoustic Treatment Two 46 Noise Control in Practice

Acoustic Treatment Two 47 Noise Control in Practice

Acoustic Treatment Two 48 Noise Control in Practice

Acoustic Treatment Two 49 Noise Control in Practice

Acoustic Treatment Two 50 Noise Control in Practice

Acoustic Treatment Two 51 Noise Control in Practice

Acoustic Treatment Two 52 Noise Control in Practice

Acoustic Treatment Two 53 Noise Control in Practice

Acoustic Treatment Two 54 Noise Control in Practice

Acoustic Treatment Two 55 Exercises would be provided later as this file size is too large. Good Luck in the coming Exam.

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