Asia Noise News

Acoustics Glossary

Get a better understanding of acoustics with our glossary of terms. Let Geonoise Instruments help you solve your noise problems today!

Arranged by:

Adetia Alfadenata

Acoustic Engineer

Geonoise Indonesia

Asia Noise News


Sound measurement is one of the measurements which is considered to be important in a lot of different industries. For example, automotive, manufacture, HSE, research and so on. One of the aspect that is important for all measurements are its calibration. Calibration is a process of documenting and adjusting the reading of measurement instruments with a traceable reference. 

The frequency range of acoustic measurement in air is wide, from infrasound to ultrasound. From tenth hertz to 200 kHz. It is measured in a wide range of dynamic range too, from 20 micropascal to 20 kilopascal. Therefore, to be able to conduct these measurements in a wide range of frequency and dynamic, different kinds of microphone is used.

Most measurement microphones and reference microphones is condenser microphone. This type of microphone is widely chosen because of its flat frequency response and a good mechanical stability. The standard used for measurement microphone is IEC61094-4 which is called working standard microphones, abbreviated WS. WS microphones are categorized into 3 types based on its diameter which are 23.77 mm, 12.7 mm and 6.35 mm. These three microphones are called WS1, WS2 and WS3 respectively.

Another standard is used for Laboratory Standard Microphone, abbreviated LS, which is IEC61094-1. Similarly to WS, LS can be categorized by its diameter, which are LS1 with 23.77 mm and LS2 with 12.7 mm diameter. LS microphone is designed so that it can be fitted into calibration coupler and is normally used by national metrology institute as a national reference in a country. Both of the standards mentioned above specify dimension, sensitivity, frequency response, acoustic impedance, dynamic range, ambient influence and stability.

Condenser microphone is a reciprocal transducer. This microphone can work as a microphone by converting acoustic signal into electric, as well as working as a sound source by converting electrical input into acoustic output. This is why condenser microphones can be calibrated by a calibration method called reciprocity.

Before we discuss further about the calibration method, it is useful to discuss about sound field and the type of microphone used to measure in such fields. There are three types of sound field in general. In a cavity which dimension is smaller than a quarter of the measured wavelength, the soiund field is called pressure-field. This field happens in a calibration coupler for microphone calibration, telephone and hearing aids, for example. Sound field in an anechoic chamber or outdoor where sound can propagate without obstacles is called free-field. While sound field in a reflective room is called diffuse-field. 

All types of microphones can influence the sound field which is being measured, including condenser microphone. Microphones that are used in cavities should have a stiff diaphragm, or in another word has a high acoustic impedance. For free-field condition, microphones that are chosen ideally has a diameter less than 5-7% from the wavelength of the sound being measured. In practice, this rarely happens, so that the influence has to be taken into account in the measurement results. Similar situation happens in diffuse-field, although the influence is relatively smaller.

Note that the influence in the free-field and diffuse-field depends only on the dimension of the microphone’s body. Because of this reason, the influence only have to be measured once for the same microphone model. After the influence is defined, it can be applied to all the same microphone of the same model. 

Let’s go back to reciprocity calibration. This method was invented in the 1940s. This method has been developed and standardised which makes the method one of the most widely used calibration techniques to determine microphone’s response in pressure-field and free-field. The calibration method is based on transfer function of two microphones which are coupled as microphone and sound source.

The two microphones are coupled in a well-defined acoustic environment. The transfer function which is the ratio between output voltage of the sensor and input current of the source is measured. This ratio is called electrical transfer impedance (Ze). Furthermore, by knowing the acoustic transfer impedance (Za), the product of the sensitivity of the two microphones can be defined by this equation.

Where M1 and M2 is the sensitivity of microphone 1 and microphone 2, Ze/Za is the ratio between electric and acoustic transfer impedance.

By using three microphones (1,2,3) and defining three impedance ratio equations (A,B,C) for three possible combinations (1-2, 1-3, 2-3), the sensitivity of three microphones can be calculated by solving these three equations.

Some national metrology institutes are doing reciprocity calibration for laboratory standard microphones. The frequency ranges from 20Hz to 10kHz for LS1 and 20Hz to 20kHz for LS2. Some of the institutes has experience in calibrating lower or higher frequency range.

Building Accoustics

Researchers Develop ‘Acoustic Metamaterial’

Boston University researchers, Xin Zhang, a professor at the College of Engineering, and Reza Ghaffarivardavagh, a Ph.D. student in the Department of Mechanical Engineering, released a paper in Physical Review B demonstrating it’s possible to silence noise using an open, ring-like structure, created to mathematically perfect specifications, for cutting out sounds while maintaining airflow.

They calculated the dimensions and specifications that the metamaterial would need to have in order to interfere with the transmitted sound waves, preventing sound—but not air—from being radiated through the open structure. The basic premise is that the metamaterial needs to be shaped in such a way that it sends incoming sounds back to where they came from, they say.

As a test case, they decided to create a structure that could silence sound from a loudspeaker. Based on their calculations, they modeled the physical dimensions that would most effectively silence noises. Bringing those models to life, they used 3-D printing to materialize an open, noise-canceling structure made of plastic.

Trying it out in the lab, the researchers sealed the loudspeaker into one end of a PVC pipe. On the other end, the tailor-made acoustic metamaterial was fastened into the opening. With the hit of the play button, the experimental loudspeaker set-up came oh-so-quietly to life in the lab. Standing in the room, based on your sense of hearing alone, you’d never know that the loudspeaker was blasting an irritatingly high-pitched note. If, however, you peered into the PVC pipe, you would see the loudspeaker’s subwoofers thrumming away.

The metamaterial, ringing around the internal perimeter of the pipe’s mouth, worked like a mute button incarnate until the moment when Ghaffarivardavagh reached down and pulled it free. The lab suddenly echoed with the screeching of the loudspeaker’s tune.

How acoustic metamaterial works – Geonoise Instruments
How acoustic metamaterial works – Geonoise Instruments

Now that their prototype has proved so effective, the researchers have some big ideas about how their acoustic-silencing metamaterial could go to work making the real world quieter.

Closer to home—or the office—fans and HVAC systems could benefit from acoustic metamaterials that render them silent yet still enable hot or cold air to be circulated unencumbered throughout a building.

Ghaffarivardavagh and Zhang also point to the unsightliness of the sound barriers used today to reduce noise pollution from traffic and see room for an aesthetic upgrade. “Our structure is super lightweight, open, and beautiful. Each piece could be used as a tile or brick to scale up and build a sound-canceling, permeable wall,” they say.

The shape of acoustic-silencing metamaterials, based on their method, is also completely customizable, Ghaffarivardavagh says. The outer part doesn’t need to be a round ring shape in order to function.

“We can design the outer shape as a cube or hexagon, anything really,” he says. “When we want to create a wall, we will go to a hexagonal shape” that can fit together like an open-air honeycomb structure.

Such walls could help contain many types of noises. Even those from the intense vibrations of an MRI machine, Zhang says.

According to Stephan Anderson, a professor of radiology at BU School of Medicine and a coauthor of the study, the acoustic metamaterial could potentially be scaled “to fit inside the central bore of an MRI machine,” shielding patients from the sound during the imaging process.

Zhang says the possibilities are endless, since the noise mitigation method can be customized to suit nearly any environment: “The idea is that we can now mathematically design an object that can block the sounds of anything”.


Asia Noise News Building Accoustics

Exploring Jakartan Public Transportation Through The Sound

Jakarta, the capital city of Indonesia, is home to 10 millions of Indonesia population. Recently the Indonesian government is being sued by a group of activists and environmentalists due to the unhealthy air quality in Jakarta. The plaintiff hopes that through the lawsuit, the Indonesian government can improve existing policies to address the air pollution issues.

On 18 Jul, according to the Switzerland-based pollution mapping service AirVisual, the Air Quality Index (AQI) of Jakarta is 153, categorized as unhealthy and may cause increased aggravation of the heart and lungs. The recommendation upon this condition is to wear a pollution mask and use air purifiers inside the room. The AQI Measures five criteria air pollutants (particulate matter, sulphur, dioxide, carbon monoxide, nitrogen dioxide, and ozone), and converts the measured pollutant concentration in a community’s air to a number on a scale of 0 to 500.

Air Quality Index – AirVisual

Jakarta is one of the largest urban agglomerations in the world. The uncontrolled increase in urban population is proportional to the number of the vehicle in Jakarta. According to Badan Pusat Statistik (Statistics Indonesia), the growth of motorized vehicle in Jakarta is 5,35% every year, on the other hand, this growth will increase the number of pollution in Jakarta. This statement is supported by the acting head of Jakarta Environment Agency, Andono Warih, the fuel residue of motorized vehicles was the main contributor to severe air pollution as 80 per cent of vehicles powered by diesel fuel operated from Jakarta Greater Area (Jabodetabek) to the capital.

Jakartan can contribute directly to overcome air pollution issues. Public transportation is an environmentally friendly mode of getting around. Because public transportation carries many passengers on a single-vehicle, thus it can reduce the number of vehicles as well as reducing the number of emissions from transportation in a dense urban area. Further, public transportation can help Jakarta to reduce the smog, to meet air quality standards and to decrease the health risk of unhealthy air quality. 

The urban transportation system in Indonesia consists of buses, trams, light rail, metro, rapid transit and ferries. Particularly in Jakarta, urban rail-based transportation, such as Commuter Line Train, Light Rail Transit (LRT) and Mass Rapid Transit (MRT), provides mobility and access to the urban area.

MRT Jakarta Phase 1 – MRT Jakarta

The first phase of MRT Jakarta (MRT-J) has been operating since March 2019. In daily operation, the train runs from Lebak Bulus Grab Station to the Bundaran HI Station. There are 13 stations along the railway; the underground stations are Bundaran HI, Dukuh Atas BNI, Setiabudi Astra, Bendungan Hilir, Istora Mandiri, and Senayan Station. Meanwhile, the overground stations are ASEAN, Blok M, Blok A, Haji Nawi, Cipete Raya, Fatmawati, and Lebak Bulus Grab Station. The MRT-J only needs 30 minutes to travel along the 16 kilometres railway, starting from Lebak Bulus Grab Station in South Jakarta to the Bundaran HI Station in Central Jakarta.

There are 16 train lines available to take the passengers getting around. Based on the MRT-J website, In weekdays operation, the trains operate at 05.00 WIB to 24.00 WIB with a total of 285 trips. Meanwhile, in weekend operation, the trains run at the same hour with a total of 219 trips.

During the promo operation (1 April – 12 May), the average number of daily passengers reached 82,643, whereas after the full tariff was applied, the average per day was 81,459.

The following pictures will show you the scenes of MRT Jakarta.

So what do you think? Have you tried getting around using MRT Jakarta? If you have never, try immediately and feel the different sensation of Public Transportation in Indonesia.

Further, through this article, I would like to invite you, explore the MRT Jakarta through a different perspective, that may be for a group of people this method is still rarely used, a sound.

Do you realize that sound can tell us about character, place, and time? Sometimes, it informs us in ways visuals can’t, and that is the idea of what we are going to do right now. Later you will hear, a file of recorded sound of MRT-J in its daily operation.

The sound was recorded by the soundwalk method, any excursion whose primary purpose is listening to the environment. It is exposing our ears to every sound around us no matter where we are. We may be at home, walking across a downtown street, or even at the office. Meanwhile, in this case, our environment is inside the line of MRT Jakarta. The goal is to capture any sound sources that exist during the operation of MRT-J, including the activity of the passengers.

The sound was recorded by using a mounted microphone on the iPhone X at a level of 1.2 m above the ground. The following sound is a recorded environment while the MRT-J was travelling from Bundaran HI Station to Setiabudi Astra Station, the duration of recording sound is 4 minutes and 40 seconds. Please use an earphone or any similar devices to listen to the audio for a better experience.

Caution: please set the volume around 50 – 70 % of the maximum volume.

After listening to the sound, can you identify what sound sources are presented in the recording? Here are the sound sources that I have identified:

  1. Engine sound increases speed
  2. Public Address system
  3. Engine sound
  4. Rail friction
  5. Passengers’ activities (cough, sneeze, conversation, footsteps, etc.)
  6. The sound of the door opening
  7. Brake squeak

Now we have identified the sound sources that are presented in the recording. But, do you know how many decibels that I have to endure while travelling using the MRT-J? In this article, manual measurements of noise levels were performed with a sound level meter in the MRT Jakarta with passengers on its usual route. A-weighted sound level measurements were recorded directly from one station to the next during the time between 08:00 and 09:00, using a calibrated microphone on a stand at a level of 1.2 m above the ground. The results of equivalent continuous A-weighted noise levels Leq (LAEq) in the MRT-J with passengers on its usual route from one station to the next is shown in Chart 1. 

Leq is the A-weighted energy means of the noise level averaged over the measurement period. The results from the measurements show that the A-weighted noise level is varied between 77 dB to 82 dB. Further, if we look closely into the Chart, the noise level is fluctuating. It can be caused by a lot of factors, such as: 

  1. The position of MRTJ (When MRT-J inside the tunnel, the noise can be levelled up due to the reflection phenomenon).
  2. Speed (The machine indicates producing a higher noise when in the maximum speed).
  3. Path Crossing.
  4. The Public Address System Volume.

Moreover, the level of continuous noise in Chart 1 represents a quite noisy environment. According to The National Institute on Deafness and Other Communication Disorders, states that Long or repeated exposure to sound at or above 85 dB can cause hearing loss. Thus, according to the measurement results, I suggested you wear ear protection during commuting by MRT-J. The earplug is one of the equipment that we can use to protect our hearing; you only need to spend a few thousand rupiahs for this. Wearing earplugs can help you to reduce the noise by 18 – 34 dB, it depends on the models/brand. For more accurate results, we need to do a complex measurement, such as:

  1. Add measurement point (In this article, the measurement was done only in a measurement point, at the second car of the line).
  2. Add a velocity as a measured parameter.
  3. Add the measurement time; the measurement can be done during the operation hour, non-stop. (05:00 – 24:00 WIB).

Nonetheless, the idea of showing the measurement results is spreading noise awareness. Noise sticks with you around, even common sounds you hear at work or home can contribute to long term hearing loss and other health risks, they are everywhere, but only a few people are aware of it. Noise pollution is a health threat nobody is talking about. Here are some parameters to help you determine acceptable — and dangerous — noise levels:

  • 45 dB: nightly noise ordinance threshold set by many municipalities concerned with industrial noise exposure for residents
  • 65 db+: exposure for prolonged periods can cause physical and mental fatigue
  • 85 dB+: can cause permanent hearing loss if exposed for extended periods
  • 85-120 dB: dangerous over 30 minutes of exposure
  • 120-130 dB: can cause permanent hearing loss for exposure over 30 seconds
  • 130 dB+: not only are these noises painful, but hearing protection should always be used if avoidance is not possible.

Everyone needs to take care of their ears and hearing, as damage to the auditory system could be irreparable. The loss because of the noise exposure is gradual; you might not notice the signs, or you ignore them until they become more apparent. Please do protect your ears. 


Westerkamp, Hildegard (1974). “Soundwalking”. Sound Heritage

Asia Noise News Building Accoustics

Noise in HVAC System; What, Where and How Noise comes from?

One of the things that becomes an unpredictable issue of a building, especially with a centralized air conditioning system, is the noise it creates. Calculating noise from every part of the HVAC system is needed to avoid unwanted noises in adjacent rooms.

What do we need to know to avoid noise from the HVAC system?

Are there simple steps to avoid it?

In this article, we will discuss about the noise that occurs in the HVAC system and how to avoid it.

Every sound that is heard can usually be identified through its frequency range. In relation to frequency range, noise that occurs related to the HVAC system is divided into 3 categories within the frequency range:

  • Low Frequency

Fan Noise, it generally produces sound from 125 Hz to 500 Hz octave frequency bands. Variable Air Volume (VAV) boxes noise is usually from 125 Hz to 500 Hz octave frequency bands.

  • Mid Frequency

Airflow Noise and turbulence-generated noise in a duct range from 31.5 Hz to 1000 Hz.

  • High Frequency

Damper and Diffusers Noises, they usually contribute to the overall noise in the range of 1000 Hz until 4000 Hz octave bands.

All the noise above can be avoided if we know how to design HVAC system acoustically and each of these issues must be addressed:

HVAC System Acoustically – Geonoise Instruments
  • Duct-borne Noise

The sound generated by the fan will travel along with the ductwork both upstream and downstream easily because the velocity of sound is much greater than the velocity of air in ducts.

  • Radiated Equipment Noise

Radiated equipment noise transmits through the wall or floor into the adjacent space or in the case of rooftop equipment to the environment. It is generated by vibration of the fan casing and motor.

  • Duct Break-in Noise

Noise inside ceiling plenums or from air conditioning equipment, plant room, etc, can break into the duct and then be carried into rooms or spaces downstream. So, where possible, avoid ducts passing through noisy areas as this can significantly increase noise through the air conditioning system, avoid lightweight ducts as well, replace them with heavier ducting such as sheet steel.

  • Duct Break-out Noise

Along with the ductwork, however, transmits through the wall of the duct, thus impacting the adjacent space. Generally, it happens from noise passing through the duct, aerodynamics noise from obstructions fitting in the duct, and turbulent airflow causing duct walls to vibrate and rumble radiating low-frequency airborne noise.

  • Terminal Noise

The final links in the distribution chain are the terminal air devices. These are Grilles, Diffusers, Registers, and Vent Cover that go over the duct opening in the room. Streaming air noise from diffusers and from transitions can cause additional noise in the receiving room. So that for this issue, we need to concern choosing the proper specs of supply and return air devices. We need to try to find out the NC (Noise Criteria) rating for them from their respective manufacturers.

By knowing those 5 ways of how noise occurs, it makes easier for us to categorize noise that will produce in our HVAC system design and help us in choosing what material, enclosure, duct shape and everything we need to reduce noise.