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Noise Barriers

Noise barriers are designed to resist the sound waves in the propagation path from source to receiver. In general, the closer the barrier is to the source the more effective it becomes. For simple plane barriers the height and length are the most important factors determining the degree of screening achieved and simple design rules have been developed to determine the reduction in overall noise levels.  These are based on the path difference between the direct path from source to receiver through the barrier and the shortest path passing over the top of the barrier. The greater this path difference the greater the screening. The shadow zone of the barrier is the region where the receiver cannot see the source and here the greatest reductions in noise levels are recorded. Some sound will always be diffracted over the top and around the edges of the barrier into the shadow zone so it is not possible to eliminate all noise from the source. However, typical barriers of a few metres high can achieve a worthwhile noise reduction of the order of 10 dB(A). This corresponds to halving the subjective loudness of the sound.

 

Figure (a)

Figure (b)

For more complex barriers simple methods are not appropriate and numerical methods such as the Boundary Element Method (BEM) have been used to produce accurate solutions.

Many different types of barrier have been installed using a wide variety of materials including wood, steel, aluminium, concrete and acrylic sheeting. Some of these designs have absorptive facings on the traffic side which reduce reflected sound. Barriers over 8 m in height have been used for some applications and novel capped barriers and angled barriers have been tested.

Barriers that may offer improved performance over simple plane barriers can be grouped under the following broad headings.

The above fig (a) shows the Main pathway of the sound propagation from the source to the barrier’s edge for sound walls with or without source-side absorption. Fig (b) shows Absorption material construction.

If smaller vehicles passing by the barrier, the reflection off the vehicle it does not play much of a role. Multiple reflections can only occur if noise barriers are built along both sides of the highway or train tracks.

In the case of large noise emitters, the implementation of source-side absorbent noise barriers can prevent the so-called zigzag effect

  1. Absorptive barriers—that is, barriers incorporating elements on the traffic face that absorb a significant proportion of incident sound and hence reduce reflected sound which could contribute to overall noise levels in the vicinity.
  2. Angled barriers—that is, barriers that are tilted away or have contoured surfaces angled to disperse the noise, the aim being to prevent significant sound reflections into the area where screening is required.

 

ABSORPTIVE BARRIERS

Where a plane vertical barrier is erected on one side of the road then sound reflections to the opposite side take place as illustrated in fig 1(a). In addition, reflections between vehicles and the barrier may lead to loss of screening performance as shown in fig (b). Where plane vertical barriers exist on both sides of the road, as shown in fig(c), they are normally parallel to each other and, in this situation, sound is reflected back and forth between the barriers again leading to a loss in performance. Absorbing panels located on the sides of the barriers facing the traffic can reduce this reflected contribution by absorbing the sound energy from the incident wave.

ANGLED BARRIERS

An alternative to using sound absorptive barriers is to angle the barrier or parts of the barrier away from the road such that the reflected wave from the traffic face of the barrier is deflected upwards, so reducing the contribution to noise at receptor positions relatively close to the ground. The performance of such barriers has been measured at full scale at TRL’s unique Noise Barrier Test Facility (NBTF).  The noise source used consisted of an 800 W speaker that can be positioned in front of the test barrier on a specially laid strip of hot rolled asphalt, thereby representing the traffic source on motorways and all-purpose dual carriageway roads. Microphones can be positioned to measure the noise level in the shadow zone of the test barrier at any point on a wide flat grassland area free of reflecting objects. To measure the acoustic performance of the barrier, recorded noise in a broad frequency range is broadcast and noise levels are measured at standard locations behind the barrier. Corrections can be made for variations in speaker output and wind speed and direction. In this way the screening performance of the barriers for a typical traffic noise source can be evaluated.

The above fig shows angled noise barrier.

Source : Various books and research journal

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Noise caused by construction works at night in Netizen

A video she had filmed of the construction site shows the drone of the machinery could be heard reverberating along with the backup beeper of a truck at the site. This can go on up to 3 am as reported by the netizen.

Singapore — A member of the public has taken to Facebook after she realized that construction works were being carried out near her apartment even at night, causing a lot of noise in the area. 

She put up a post on the Facebook group “Complaint Singapore” to seek advice from other netizens who might have encountered a similar situation before.

In her post, a member of the public also included a video she had filmed of the construction site. In the video, the drone of the machinery could be heard reverberating along with the backup beeper of a truck at the site.

According to the caption of the post, this was not the first time that such an incident had occurred. The member of the public also mentions that the construction works had gone on until 3 AM on a previous occasion. As such, she asked other netizens for help on who to contact on the issue, since the sounds generated by the construction works can be quite loud and disruptive.

Other netizens shared their views on the matter and offered suggestions in the comments section. 

A few netizens chalked the nighttime construction down to urgency, saying that there might be an emergency that needs fixing quickly and promptly.

Some other netizens thought that carrying out construction work at night, would impede the flow of traffic less since there are fewer commuters during the night.

A few other netizens suggested that the poster bring the issue up to the relevant authorities such as the National Environment Agency (NEA), the Land Transport Authority (LTA), or the Singapore Police Force (SPF).

After contacting NEA, the poster replied that they were helpful in stopping the works at an earlier hour.

According to NEA, construction sites need to observe the noise level and exercise construction noise control with effect from 1 Oct 2007. 

Credits: www.theindependent.sg

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The power of trees can reduce noise.

The way most workers need to complete tasks have significantly changed the way companies use their spaces. Quiet spaces are needed for deep, focused work. Technology enabled meeting rooms and collaboration spaces are used for productive meetings. Ideally, an office is designed in such a way that it enables team members to do their best work.

Unfortunately, it can be difficult to make sure a design includes all these aspects. As a result, designers and architects still often have to leave space for cubicles and open office spaces, a big contributing factor to general noise levels.

Did you know? Planting trees in your home or office not only helps to cool the internal temperature, increase the oxygen in the air give a feeling of freshness, and help relax only. But plants can also HELP ABSORB NOISE!

One creative way to both combat office noise and bring biophilic elements to a design is to incorporate plants and greenery into a space. Studies have shown that both plants and living green walls are an effective way to absorb sound and noise pollution.

Beyond their sound absorbing qualities, plants and biophilic elements can help to improve a worker’s overall well-being. Access to natural elements like greenery, natural light, and organic textures have been found to both improve employee productivity and reduce absenteeism. Plants have been found to be a mood booster and a stress reliever for team members, which can in turn, help to improve an employer’s bottom line.

Do Plants Help to Absorb Sound?

There is quite a bit of research on the subject, but the short answer is yes. The flexible and porous nature of indoor house plants acts as natural sound reducers. There are three ways that house plants can reduce the sound in your home or office: deflection, absorption, and refraction.

Most people do not understand the sound absorption benefits of houseplants. However, they really do help with absorption sound.

How Plants Reduce Indoor Noise Levels?

As mentioned above, plants reduce noise levels through three different methods: deflection, absorption, and refraction.

  • Deflection – Sound waves tend to bounce around off hard surfaces. That is where all that added noise comes from. Walls are rigid and will amplify sound, while plants are flexible and help to deaden the sound by breaking up the sound waves into other forms of energy.
  • Absorption – Plants are great at absorbing sound because of the leaves, branches, and wood. Wood is a great sound absorber. Have you ever walked through a forest and been amazed at the silence? That is because the trees are absorption all the ambient noise.
  • Refraction – Refraction is taking away the echoes of the sound bouncing off the hard surfaces. Plants will help to refract this noise and eliminate the echoes which are responsible for much of the added noise in your home or office.

The indoor plants that work best at absorbing sound such as:

  • Ferns: have a lot of surface space to help reduce sound. Their wide leaves spread out and cover quite a bit of area.
  • Baby’s Tears: Baby’s Tears are a dense plant that looks almost like moss. The plant has a way of draping itself over the pot and makes a great sound reducer when elevated off the ground.
  • The Peace Lily: The Peace Lily can absorb some of the sounds with their leaves and do a great job of bouncing the sound to the other plants and is a great sound absorbing plant you can put in your home. Their true noise absorbing properties are in their thick, broad leaves.
  • The Rubber Plant: The beauty of this plant is just how big it can get. Rubber plants cover a large surface area which only serves to enhance their sound absorbing properties.
  • Fiddle Leaf Fig: The fiddle leaf fig is another plant with broad, thick leaves. They can grow tall, and the cupped shape to the leaves make for an effective sound absorber.

Reference :

พลังจากต้นไม้ ลดมลพิษทางเสียง

https://bettersoundproofing.com/best-sound-absorbing-indoor-plants/

The Top Sound Absorbing Plants For The Workplace

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SCOPE OF ARCHITECTURAL ACOUSTIC CONSULTANT’S WORK

What should an architectural acoustic consulting firm do? This question is very commonly asked when an acoustician is asked to submit a work proposal for a project. In this article, we will describe the scope of work of an acoustic consultant with reference to the type of mixed-use high-end building project. Because in this type of project an architectural acoustic consultant is required to be able to describe all the scope of work in one project with high complexity.

Details of the scope of work of acoustic consultants in mixed-use high-end building projects are as follows:

1. Criteria Formulation
At the beginning of the project, the acoustic consultant must recommend design criteria/targets for various rooms and areas within the building such as retail, apartment units both for bedrooms and living rooms, and commercial areas such as meeting rooms, multifunction rooms, spas, fitness, restaurants. , club lounges, etc. These criteria are determined based on studies and summaries of the applicable standards in the country, international standards, client recommendations, and the building operator concerned.

2. Schematic
With so many rooms that fall into the scope of work of an acoustic consultant with this type of project, it is highly recommended that an acoustician provide schematic designs for several important rooms for the attention of other consultants in the early stages of the project. Examples are MEP rooms, building structure connections, placement of HVAC equipment above the ceiling, and draft wall partition configurations.

3. Noise Review from the Environment Around the Building
The acoustic consultant must review potential sources of noise from aircraft, train stations, transportation on highways, outdoor MEP equipment, and all things around the building that have the potential to interfere with audial comfort to the interior of the building to ensure the targeted acoustic criteria are achieved. At this stage the acoustician must be able to convey the results of modeling and simulations for several points around the building in the form of drawings that can be understood by clients and other consultants. At this stage, a building fa konfigurasiade configuration can be recommended that takes into account the noise from the area around the building.

4. Noise HVAC (duct-borne)
Discussion and review of noise from all HVAC be it from air handling unit (AHU), axial and centrifugal fans, fan coil unit (FCU), etc. The ducting system will be analyzed to determine the noise level in the critical room from the nearest diffuser ducting system outlet. From this analysis, the need for silencers, lagging or duct linings will be recommended in order to achieve the acoustic criteria that have been determined. The analysis will be carried out on all HVAC systems without exception, with the greatest attention being on residential areas, spas, hotels, etc.

5. Sound Propagation in Building Structures (Structure-Borne)
All matters relating to the propagation or vibration of sound via the building structure, whether it is due to human footsteps on the top floor or vibrations from the installation of MEP machines above the ceiling or floor. The acoustic consultant must be able to evaluate according to the natural frequency of the building structure and provide recommendations on floor slab elements to meet operator and client standards applied.

6. Machine Vibration Control
The acoustic consultant should conduct an in-depth discussion on the vibration isolator for the installed machines. This is done by taking into account the deflection of the floor slab and its relationship to the static and dynamic loads of the machine (eg chiller, pump, cooling tower, AHU, etc.). In addition, ensuring the insulator is efficient to withstand vibrations to the building structure.

7. Room Insulation
Discussion on the isolation of certain rooms by providing technical calculations both with the “indoor room” and “floating floor” methods so that sound and vibration do not propagate to all elements of the building, especially the room around the isolated area.

8. Acoustic Interior
Reviewing and calculating room acoustic parameters on interior design elements of commercial spaces such as ballrooms, meeting rooms, and other areas where the clarity of speech or music is crucial.

9. Detailed Drawing
The acoustic consultant must provide or recommend specifications for building skin elements such as faades, walls and floor slabs in CAD format on a cut or plan basis. This will make it easier for relevant consultants to apply these specifications in their construction drawings.

10. Noise Isolation Due to Impact
Collisions in the fitness area, whether it’s due to aerobic activity or lifting weights, are a special concern for acoustic consultants. In addition to different forms of acoustic treatment, the time span of these activities must also be included in detailed technical calculations, and of course measurable.

11. Review of Related Consultant Drawings
After all acoustic treatments have been adapted to construction drawings by the relevant consultant, the acoustician must review all these drawings to ensure that all treatments have been described correctly, before entering the tender phase.

12. Coordination with Selected Contractors
The acoustic consultant must allocate time to coordinate the design and answer questions from the selected contractor and sign all forms related to material approval if it is in accordance with the acoustic intentions.

13. Final assessment
Before handing over the project to the next party, the acoustic consultant must conduct a final assessment of the building elements designed by the consultant. Next, compare the measured value to the design target and pre-determined criteria.

by Ramadhan Akmal Putra 

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Accelerometer mounting

One of the challenges in measuring vibration using accelerometer is how to mount the accelerometer on the surface of the object that is being measured. Choosing the proper mounting can affect both to the measurement results and practicality when we are conducting the measurement.

 

Accelerometer mounting affects the measurement results because it can shift the resonance frequency of the accelerometer. Accelerometers have a significant amplification factor at its resonance frequency. This implies that in conducting measurements using accelerometer, it is important to choose mounting techniques that does not shift the resonance frequency into our frequency of interest.

 

Generally, there are four ways to mount accelerometer which are:

  1. Stud mounting: this technique is done by bolting the accelerometer into the object. This option is often considered as the mounting technique that produces the best measurement result compared to other options. Stud mounting has a high resonance frequency that in most cases a lot higher than our frequency of interest. To increase the performance of stud mounting, coupling fluid such as oil, petroleum jelly or beeswax can be used.

The downside of this technique is that not all object has a possible location to be bolted at the surface. If this is the case, then we will need to modify the surface and might leave a hole on the object.

  1. Adhesive: there are few adhesives that are commonly used to mount accelerometers such as epoxy (usually chosen for permanent mounting), wax, glue, and double-sided tape. Use of adhesive has lower resonance frequency compared to stud mounting, but in majority of cases still high enough that it does not affect the measurement at the frequency of interest. Of course, this depends on the type of adhesive that is being used as well.

Usage of adhesive however, especially for temporary mounting, has its own problem which is it can leave stain on the surface of the object that we are measuring, as well as on the accelerometer itself.

Another option of mounting related with adhesive is to use adhesive mounting pad, which is a pad that can be mounted on the surface that we want to measure using adhesive, and then we can mount the accelerometer on the pad. This will allow us to move one accelerometer to few locations more easily. From practicality perspective, adhesive mounting pad has an advantage if we want to repeat the measurement. Also, by using adhesive mounting pad, we avoid direct contact of adhesive to the accelerometer so that it will not need cleaning.

  1. Magnet: For metal surfaces, one of the options that is easy and does not leave stain is by using magnetic mounting base on the accelerometer so that we can attach the accelerometer to metal. This is the reason magnetic base is one of the best options especially for short-term and temporary measurement on metal.

However, this mounting technique produces lower resonant frequency compared to the other two options that we have discussed above. If the frequency that we want to measure is high enough, say above 1 kHz, this mounting technique might influence the measurement results.

  1. Handheld: In some of the cases, the three options above are not possible to be chosen, and it leaves us with the last option which is holding the accelerometer by hand. In this kind of cases, a probe tip can be used so that we can put pressure by hand on the surface that we are measuring easier.

We will have to pay more attention to the frequency range that we are measuring if this mounting technique is used. Because this option will reduce our frequency range significantly, generally only in the range of 10 – 100 Hz. 

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Industrial Noise Control Measure

In industrial places that are normally full of machineries or mechanical systems, noise is definitely inevitable, and in fact, very loud. This can sometimes be harmful to the workers hence causing occupational health and safety hazard. Therefore, in this article, we will look into noise control measures that can be used to overcome industrial noise in workplace.

Noise sources

Let’s begin with a recap on how noise is being produced:

Sound in general, is produced by vibration, or sometimes due to aerodynamic systems. Vibration-induced noises can be caused by multiple reasons, for example:

  • Mechanical shocks and friction between machinery parts like hammering, rotating gears, bearings, cutting tools etc.
  • Moving parts that are off-balanced
  • Vibration of large and heavy structures

As for aerodynamic noises, they are caused by air or fluid flows through pipes, fans, or pressure drops in air distribution systems as well. Typical examples of aerodynamic noise sources are:

  • Steam released through exhaust valves
  • Fans
  • Combustion motors
  • Aircraft jets
  • Turbulent fluid flow through pipes

Steps to control noise in workplace

To properly control the noise in the workplace, these steps should be carried out:

  1. Identify the sound sources (i.e., vibrating sources or aerodynamic flow)
  2. Identify the noise path from source to worker
  3. Determine the sound level of each source
  4. Determine the relative contribution to the excessive noise of each source and proceed to rank the sources accordingly. The dominant source should always be prioritised and controlled first in order to obtain significant noise attenuation.
  5. Understand the acceptable exposure limits as written in the health and safety legislation and find out the necessary sound reduction.
  6. Find out solutions while taking the degree of sound attenuation, operation, productivity restrains and cost into consideration.

To reduce exposure to noise

In general, noise exposure can be reduced by the elimination of noise source if possible, otherwise substitution of source with a quieter one or the application of engineering modifications works too.

The most effective way to minimise the exposure of noise is to engineer it out at the very beginning: the design stage. It is suggested to always choose equipment features that can reduce noise level to an acceptable level. For new installations, again select a quiet equipment, and make sure to have a procurement policy that opts for using quiet equipment, and finally eliminate any design flaws that may lead to noise amplification.

Engineering modifications refer to changes that can affect the source, or the sound path. This is usually the preferred solution for noise control in already-established workplaces (those without noise protection measures during design stage). This is because engineering modifications are known to be more cost effective, especially to control the noise at the source than along the path.

Administrative controls and the use of personal protective equipment (PPE) are also effective as measures of noise control applicable on workers themselves. A combination of both may be taken into consideration when the noise exposure would not justify the implementation of engineering solutions that are more expensive. However, it is important to always note that administrative control and PPE may not be as effective as implementing engineering noise control during the starting stage or modifications of sound path. Therefore, they should be categorised as the last resort.

Engineering solutions to reduce noise

Different solutions can be applied for vibration-induced noise and aerodynamic-noise.

For vibration-induced noise, the key point is to reduce the amount of vibration at the source. The typical solutions include modification of the energy source such as lowering the rotating speed of fans, or reducing the impact force of hitting tools etc. Adding damping materials onto vibrating surfaces due to mechanical forces can help to reduce vibrational effects too, especially for thin structures. To prevent unwanted damage due to friction or impact, the damping material may be sandwiched between the surface of equipment and another material that is resistant to abrasion. This treatment is called the constraint layer treatment.

Other methods to reduce vibration-induced noise include minimising gaps in machine guards and cover them with acoustic-absorbent material, replacing metal parts with plastic parts whenever possible, and replacing motors with quieter ones.

On the other hand, to treat aerodynamic-induced noise, specialists recommended to implement engineering practices that are capable of reducing noise associated with unstable fluid flow, for example minimising fluid velocity, increasing pipe diameter or minimising turbulence by utilising large and low speed fans with curved blades.

Besides those mentioned above, there are also passive noise control measures that can be used. These include using enclosures and isolations by storing noisy equipment in enclosed spaces/rooms that have special acoustic features like isolation, louvres or sealings. Installations of acoustic barriers (sound-absorbing panels) in workplaces, or silencers inside ducts and exhausts works well in attenuating unwanted noise too.

General measures to keep in mind

Finally, here are some general methods that one can take to ensure that workplace noise is under controlled.

Regular maintenance should always be performed, where the focus should be on identifying and replacing any worn-off or loose parts, lubricating any moving parts, and make sure that the rotating equipment does not get off balance to avoid vibration-induced noise.

Noisy processes should be taken note about and be substituted with quieter ones. Sound reverberation in the room should be reduced. Reverberation is when sound produced in an enclosure hits reflective surfaces and reflects back into the room in addition to the original noise paths. In some cases, reverberated sounds may dominate the original sound. A good method to help in such conditions will be to add padding onto the reflective surfaces with sound absorbing materials so that noise level can be reduced. Another way will be to arrange the equipment in the room so that they are not too close to too many reflective structures.

Conclusion

In conclusion, always take measures to identify the sound sources in the industrial workplace and find out suitable ways to solve the noise issues to achieve noise limits in accordance with exposure limits set in the health and safety legislation published by the local authorities. It is utmost important to obey the noise exposure limits to ensure the hearing health of workers in the workplace.

Reference

https://www.ccohs.ca/oshanswers/phys_agents/noise_control.html

https://www.who.int/occupational_health/publications/noise10.pdf

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Singapore to Ban Older Motorcycles

noisy motorbikes banned in singapore quiet future
noisy motorbikes banned in singapore quiet future

As motor vehicle emissions regulations tighten around the world, it should come as no surprise that Singapore is set to restrict motorcycles registered before July 1, 2003, and effectively ban them after June 30, 2028. The island city-state is under 800 km2 in area, yet is home to 5.7 million residents (denser than Hong Kong), and is known for its strict laws, cleanliness, and low crime rate.

The restrictions were introduced in 2018, and Singapore’s National Environment Agency (NEA) released new information and guidelines on the emissions regulations and standards in April of this year. The laws come into effect on April 26, 2023, after which time, qualifying motorcycles must meet tighter emissions standards of “4.5% Carbon Monoxide (CO) by Volume; and 7,800 ppm Hydrocarbons (HC) (for 2-stroke engine) or 2,000 ppm HC (for 4-stroke engine)”, as per the NEA documentation. After June 30, 2028, these motorcycles will either be banned, or the owner can apply to be given a temporary permit with usage stipulations, until such time as they are eligible for Classic Vehicle status, which in Singapore is 35 years or older.

The NEA estimates that most of the eligible motorcycles will be able to pass the required emissions standards with proper, regular maintenance. Owners can have their vehicles tested at an approved inspection station to ensure they pass the standards.

noisy motorbikes banned in singapore quiet future

The new laws will apply equally to locally owned, as well as foreign motorcycles entering Singapore. Foreign motorcycles must also meet local noise regulations, and must not emit any smoke or visible vapor. Enforcement is planned via random emissions testing blitzes at land entry checkpoints.

For owners of older motorcycles, the NEA is offering an early de-registration incentive of up to $3,500, to encourage owners to get these bikes off the road. According to the NEA, almost 60% of the 27,000 eligible motorcycles have so far been de-registered via this program.

Sources: motopinas.com, http://nea.gov.sg

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ListenEAR noise dosimeter, noise dosemeter OSHA, ISO9612

Listen Ear Personal Noise Dose Meter Noise & Vibration Registration Instruments and Software, ISO9612, OSHA

LOW COST, PROFESSIONAL NOISE DOSE METER

Compact Light Design Easy Mounting in Hearing Zone Long Life

Rechargeable Battery

• (up to 160 hours/4 Working Weeks)

Time History Data Logging

•10s 10min Data Recording Intervals

•Download to PC via USB Cable

•Real Time Data via Bluetooth to FREE Android App Full Noise Dosimeter Functions:

•Fast & Slow Time Weighting

•A, C, & Z Frequency Weightings

•Equivalent Continuous Noise Dose (LEQ)

•Noise Exposure over 8 Hours (LEX8h)

•Current Dose Projected Dose at 8hrs Temperature & Humidity Readings Complies with IEC61672-1, IEC61252 Visual 2 Tier Alarm Setting Optional External Microphone •AntiKnock, Movement and Orientation Monitoring

•Ability to Pre-Program with Ear Defender SNR Values

ON-SCREEN VIEWS  CURRENT PLUS MIN & MAX SOUNDLEVEL

READ MORE….

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Noisy exhaust eradication a joint effort in Malaysia

noisy motorbike malaysia
noisy motorbike malaysia

Illegally modified motorcycle exhausts, or the eradication thereof, should be a collective effort by all relevant Malaysian agencies, says Transport Minister Datuk Seri Wee Ka Siong. Speaking during the launch of the Diesel Multiple Unit train at Pasir Was, Kelantan, Wee said the import and sale of illegally modified motorcycles and components needed to be addressed, as reported in The Star.

“Some say loud exhausts should be banned but it may not come under the Transport Ministry’s jurisdiction as it could involve the Customs Department and such,” said Wee, responding to social media posts accusing several ministries of allowing the sale of modified motorcycle parts. “Some people questioned how these exhausts entered the market and accused the Domestic Trade and Consumer Affairs Ministry of not stopping the sale of such prohibited items,” he said.

Saying that the sale of modified parts for vehicle use does not fall under the jurisdiction of the Transport Ministry, Wee suggested the relevant ministries collaborate on finding a solution. “We don’t want to trouble the people but there are complaints that the loud sound from these exhausts is causing some to lose sleep,” said Wee.

Wee emphasized the Ministry of Transport is taking an advocacy approach to the issue of noisy motorcycles, with offenders being called up for interviews by JPJ under Article 114 of the Road Transport Act, instead of being issued a summons. Those found guilty of modifying their motorcycle exhaust face a maximum fine of RM2,000 or six months jail, with the seizure of the non-compliant motorcycle an option under Section 64 of the Road Transport Act 1987.

The issue of motorcycle exhaust noise came to the fore around mid-February this year, after police and JPJ took what was felt by some in the riding community to be excessive action. Riders were subject to roadblocks and checks, leading to allegations from the public a minority segment of the vehicle population in Malaysia was being unfairly targeted and persecuted.

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Sound Absorption

What is Absorption?

Absorption refers to the process by which a material, structure, or object takes in energy when waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. The energy transformed into heat is said to have been ‘lost’. (e.g. spring, damper etc.)

 

What is Sound Absorption?

When the sound waves encounter the surface of the material: part of them reflects; part of them penetrate, and the rest are absorbed by the material itself.

Formula for Sound Absorption: –

The ratio of absorbed sound energy (E) to incident sound energy (Eo) is called sound absorption coefficient (α). This ratio is the main indicator used to evaluate the sound-absorbing property of the material. A formula can be used to demonstrate this.

 

α (absorption coefficient) =E (absorbed sound energy)/ Eo (Incident sound energy)

 

In this formula: α is the sound absorption coefficient;

  E is the absorbed sound energy (including the permeating part);

  Eo is the incident sound energy.

 

Generally, the sound absorption coefficient of the materials is between 0 to 1. The larger the numeral is, the better the sound absorbing property. The sound absorption coefficient of suspended absorber may be more than one because its effective sound-absorbing area is larger than its calculated area.

 

Example: If a wall is absorbed 63% of incident energy and 37% of energy is reflected then the absorption coefficient of wall is 0.63.

 

How can we measure Absorption Coefficient?

 

The absorption coefficient and impedance are determined by two different methods according to the type of incident wave field.

 

  1. Kundt’s tube (ISO 10534-2)
  2. Reverberation room (ISO 354)

 

Kundt’s Tube Measurement Method: (ISO 10543-2)

For measurement of small specimen use Kundt’s tube or Impedance tube also called as Standing wave tube.  The result from measurement of absorption factor and acoustic impedance, using the standing wave method, obviously are meaningful only when assuming these to be independent of the size of the specimen, which is normally quite small.  The absorption factor for normal incidence is determined by measuring the measuring the maximum and minimum pressure amplitude in the standing wave set up in the tube by a loudspeaker. 

This basic technique is, an mentioned in the introduction, considered a little outdated in comparison with more modern methods based on transfer was implemented relatively late (1993) in an international standard, ISO 10534-1, after being used for al least 50 years.  Commercial equipment has also been available for many decades.  However, there exists a second part of the mentioned standard, ISO 10534-2, based on using broadband signals and measurement of the pressure transfer function between different positions in the tube.  ISO 10543-2, which implies the specified two microphone method is extended to spherical wave fields.

Normally Placid Impedance tube is used for absorption coefficient and transmission loss measurement. 

(https://www.placidinstruments.com/product/impedance-tube/)

The above fig shows Impedance tube

 

Click here to refer Placid Sound absorption measurement  

Click here to refer Placid Sound transmission loss measurement

 

 

Reverberation Room: (ISO 354)

 

              Reverberation Room method is traditional method, measurement of the absorption factor of larger specimens is performed in a reverberation room.  One then determines the average value over all angles of incidence under diffuse field conditions.  The product data normally supplied by producers of absorbers are determined according to the international standard ISO 354, required for measurement is 10-12 square meters and there are requirements as to shape of the area.  The reason of these requirements is that the absorption factor determined this method always includes an additional amount due to the edge effect, which is a diffraction phenomenon along the edge of the specimen.  This effect makes the specimen acoustically larger the geometric area, which may result in obtaining absorption factors larger than 1.0.  Certainly, this does not imply that the energy absorbed is larger than the incident energy.

 

 

Sound Absorption coefficient of different materials:

The sound absorption of the material is not only related to its other properties, its thickness, and the surface conditions (the air layer and thickness), but also related to the incident angle and frequency of the sound waves. The sound absorption coefficient will change according to high, middle, and low frequencies. In order to reflect the sound-absorbing property of one material comprehensively, six frequencies (125Hz, 250Hz, 500Hz, 1000Hz, 2000Hz, 4000Hz) are set to show the changes of the sound absorption coefficient. If the average ratio of the six frequencies is more than 0.2, the material can be classified as sound-absorbing material.

Application of Sound Absorber:

These materials can be used for sound insulation of walls, floors, and ceilings of concert hall, cinema, auditorium, and broadcasting studio. By using the sound absorbing material properly, the indoor transmittance of sound waves can be enhanced to create better sound effects.

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