Noise reduction

1) Introduction

The sound can be air or driving structural conduction. Most sources radiate sound both as otherwise. The following diagram shows that for noise reduction measures can be taken at various levels in the chain: at the source, about the means of propagation, interfaces and/or on the receiver.

2) Noise in buildings

List of sources of noise that can be found in buildings and around these:
Air conduction noise originating from sources outside the building, as for example, road traffic, train, planes and any commercial or industrial activity.
Air conduction noise from interior building sources, as for example: oral communication, televisions and other any activities carried out by the residents.
machinery noise diverse as for example: elevators, plumbing, heaters, systems of ventilation and air conditioning.
drum noise because the objects dropped on the floor, steps, washing machine, etc.

3) The noise in the industry

Noise in industrial environments due to:
• Noisy Processes of manufacture,
• Machines in General,
• Specific mechanical parts,
• Extensive use of compressed air (process, cleanup operations, etc.),
• Operations and noisy handling techniques (hit with hammer, metal-to-metal contact, chirps, swishes, etc.).

 

4) Control of noise at source

Can take various kinds of measures at the level of the sound source:
• replace the source for a quieter.
• use another kind of technology to eliminate or reduce excessive sound levels.
• change handling processes, of work and behavior.
• optimize installation procedures: rebound damping panels for isolation of vibrant elements (with Cork dampers, rubber, etc.).
• maintenance of equipment in good working condition.

 

5) Propagation of noise control

Propagation in air medium:
It is, above all, seek a solution of acoustic correction of locations, through sound absorption and the interposition of obstacles that limit the spread of air noise.

Spread through solid:
try to limit vibration of structures by damping, through modifications in structure and through the creation of discontinuities.

5.1) Control of air seeding

The different solutions involve:

A) Sound absorption

In free field the sound level decreases with the distance to the source, inside an enclosed space this decrease is conditioned by sound absorption characteristics of surrounding.

(B)) Obstacles to the spread

The use of acoustic screens, partitions, total or partial,

Creating closed areas, absorption Chambers, may contain partially noise and increase the level of sound absorption in the workplace.

When the noise of airborne transmission occurs through a duct, use silencers or full absorbers.

 

C) Absorption control

The reflected noise within a room has an impact on the environment and on local acoustic comfort. Depending on the type of room you're in study (gym, restaurant, plant, etc.), various kinds of measures can be taken using the same principle: whenever it adds material absorber, the reverberation time, and so the sound pressure level, decreases, except in the immediate vicinity of sources.

We can recommend several types of treatment depending on the application. Will have to select a material to cover the reflective surfaces or absorsoras, know where this should be placed, etc. Any solution must take into account economic constraints, architectural, functional, Security and other.

Select the reverb time great for a concrete case, Depending on the volume and function of the site, is the first step of an acoustic conditioning plan.

However, the decrease with the distance of the sound pressure level is more important than the reverberation time in an industrial environment.

D) Expansion Chambers

And) Silencers

A silencer or sound Attenuator is a system capable of reducing the acoustic energy that propagates along a duct or pipe. There are fundamentally two types of silencers:

reactive silencers: principle: by varying the cross-section of a tube or creating side arms, a part of the acoustic energy is reflected back to the conduct, in the opposite direction and/or destructive interference generated.

 

dissipative silencers: principle: introducing acoustically absorbent material inside a duct, part of the acoustic energy is dissipated.

F) Obstacles in the path of the noise

In terms of sound rays (optical analogy), the trajectory of noise transmission can be modified by introducing obstacles. You can create shadow areas, Similarly to what happens with the light.

This is the case when using barriers against noise. If these are absorbents, part of the energy will be absorbed, This being an effect to add to the creation of the shadow zone. Of course, the attenuation obtained is limited due to the effect of diffraction, in particular at the top of the barrier, This is especially striking in effect low frequencies.

The use of suspended ceiling absorbers or panels of acoustic screens supported pavement, reduces the noise propagation:
• a plant if placed near the engine room
• in a canteen or cafeteria if placed next to tables.

A screen or acoustic barrier in an enclosed space will only be effective if the ceiling is treated with absorbent material.

G) Influence of frequency

The effectiveness of these barriers depends on the frequency content of the noise. High-frequency sound (short wavelength) It's more directional, easy to reflect and more attenuated with distance (due to the higher absorption in the atmosphere and the effect of any obstacles that arise in source-receiver path). Low frequency sound (large wavelengths) It spreads in all directions, surpasses obstacles more easily (unless these are of large dimensions) and attenuates less with the distance.

5.2) Structural conduction noise isolation – vibration

The vibrations have the origin in your direct excitation of a solid element by a force or a speed. Although some vibrations are relatively small, can produce high levels of noise in a receiver due to structural conduction noise transmission. A tuning fork gives rise to a very weak sound, unless it is against a large surface.

The spread of structural conduction noise, depends on the kind of, especially its physical characteristics (mass, stiffness, cushioning). The different solutions to be applied in this type of problem has to do with:
• the damping of structures and with,
• structural discontinuities.

Structural damping

A little thick wall flexes under the effect of vibration and transmits energy. Within the limits of the wall is in part the reflection of vibratory energy (with the possibility of excitation of resonances of the Panel – own vibration modes) and in part to the solid elements neighbors (beams, pillars, slabs, other walls).
If a Panel is placed to vibrate by a transient excitation, the amplitude of the vibrations decreased more rapidly after cessation of excitement as the internal cushioning material is great.

Discontinuities

In a complex structure (boat or building, for example), the vibration energy is distributed by each element of this structure. The radiated noise level is, therefore, a function of the method of assembling the various elements. It is necessary to make a careful distinction between:
• vibration decoupling between source and receiver through anti-vibráticos support, massive or floating slabs, or resilient floor finishes
• discontinuities in the middle of spreading thanks to the use of flexible materials, resilient and discontinuities, in fact, in the very structure (expansion joints, for example).

For example, the transmission of vibrations through pipes or conduits shall be reduced if they contain sections and flexible joints.

6) Noise control interfaces

This type of measure is directly connected to the coupling between source and receiver. Optimization of interfaces is often considered the main solution to the problems of noise when you cannot take action directly at the sound source. In buildings, for example, You can never control the noise level due to the activities of its inhabitants. On the other hand, You can prevent the annoyance caused by noise if the building is designed in such a way as to limit the transmission of sound.

Control at the Source Interface-means of Propagation

A machine radia audible sound that propagates through the air around the sound source. This machine also produces vibrations that are transmitted to the structure of the building. Can, therefore, reduce the noise of air conduction radiation and conduction noise acoustic surroundings and introducing structural vibration isolators respectively.

Wrapping machines

When it is not possible to act upon the noise at the source (I.E. It's impossible to get a quieter machine), the solution may be to create an engaging acoustics, or canopy, around the machine or just around your most noisy. To be effective, the surrounding acoustics has to be:
• composed of elements with similar sound insulation and, therefore, have mufflers on all exits and air intakes, particularly in vents, as well as insulated doors and Windows compatible too;
• consists of absorbent material in their inner faces (rock wool or glass wool, rubber foam or polyurethane foams) to reduce the sound level inside the envelope,
• isolated, in mechanical terms, in relation to the source
• air-tight, at the level of the outside surface of surrounding (deleting obviously ventilation openings properly treated with silencers as stated above).

Anti-Vibration Decoupling

 

To prevent the propagation of vibrations of the machine for your support structure, These two systems may be decoupled with the help of:
• support anti-rammers
• antivibrático massif
• floating floor on antivibráticos support

The choice of a solution depends on the vibration frequencies and dynamic forces of the structure in question.

The best solution is to isolate the vibrant structure as close to the source as possible to avoid the propagation of vibrations over long distances. For example, an elevator machine is isolated from the structure of the building through the use of elastic supports.

The isolation can be achieved using resilient materials, as elastomers, or a spring system.

Are used in the form of antivibráticos, mantas, etc.

The floating floor is a construction technique often used to isolate slabs, subject often vibrating excitations, the rest of the structure.

Damping of vibrations

The effectiveness of a antivibrático system is directly linked to the relationship between the excitation frequency and resonance of a system. The resonant frequency of the system have to be frankly smaller (3 times, recommended) that the lowest frequency for which the vibration of the machine is significant.

7) Sound isolation between sites

One should not confuse sound insulation between two spaces and sound absorption in a room.

A concrete wall 15 cm thick, It's a very good sound insulation, but your ability absorsora in acoustic terms is zero. On the contrary, a wall consisting of glass wool would have a good acoustic absorption capacity, but it would be a bad insulator.

 

8) Sound radiated from noisy buildings

Make an assessment of the noise emitted from a noisy building is a fairly complex task, due to their large size and directivity. The design of industrial and leisure buildings (for example clubs) should be made taking into account acoustic constraints and according to applicable environmental regulations.

The sound sources must be placed on anti vibration mountings and in atmospheric acoustics, for example.

9) Acoustic receiver protection

As a solution of last resort, When it is not possible to have another type of measures, one can resort to the use of individual protection shell protectors or insertion, in relation to air conduction noise, or anti-vibráticas soles compared to vibrations.

 

Presentation

Then you can view a presentation on this topic.