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What you need to know about Room Acoustics

In the Southeast Asia region especially, acoustic properties of residential buildings are often neglected by designers, developers, contractors, and even home buyers. Noises from both internal and external environments affects occupants’ daily lives, causing nuisance which can strongly deteriorate one’s living quality as a long-term effect. In this article, we will investigate building/room acoustics, and the actions that can be undertaken to improve the acoustical environment inside a building.

Room acoustics

In general, the acoustics of rooms can be divided into two groups: low frequency and high frequency. Sound in rooms can be highly affected by the reflective properties of the surfaces in the room. This is because multiple reflections may occur if the room surfaces are highly reflective, which then leads to a reverberant field in addition to the direct field from the source especially at higher frequency range. Therefore, at any point in the room, the overall sound pressure level is influenced by the energy contained in both the direct and reverberant fields (Crocker, 2007).

Sound transmissions in buildings

Sound can be transmitted within a building by transmitting through air in the spaces bounded by walls or roofs/ceilings, known as airborne transmission. Another way would be through structural transmission through the structural assemblies of the building, or impacts.

Airborne sound originates from a source that radiates sound waves into the air, which would then impinge on the building surfaces. A good example of airborne sound will be speech, or music from a television or loudspeaker. On the other hand, impact sound is being generated when an object strikes the surface of a building. The commonly heard impact sounds that we can hear in buildings are footsteps, furniture-dragging sounds, cleaning, and other equipment that is used directly on the floor surfaces. To overcome these noises, good sound isolation should be considered for all the possible paths for sound and the junctions between walls and floors, not just at the direct path through common wall or floor.

Sound insulation – airborne and impact

It is imperative to consider the control of airborne and impact sound transmission through the building elements like walls, ceilings, or floors, as stated above. In this case, sound insulation methods will be crucial. Different methods can be implemented for airborne, impact and flanking sounds (Crocker, 2007).

For airborne sound, insulation can be applied at any building element. This is because when sound hits on a surface, a very small fraction of the incident energy will be radiated from the other side. The sound transmission loss (TL), which is the ratio of the incident sound energy relative to the transmitted sound energy is typically measured. TL can be expressed in decibels (dB), and it is sometimes known as sound reduction index (R) in European and ISO standards. The elements to be used in buildings for sound insulation are measured in accordance with standards, where the commonly seen method would be the two-room method. A test specimen would be mounted between a reverberant source room, and a receiver room such that the only significant path for sound to transmit through is the specimen, and other possible transmission paths would be suppressed. As such, it will be useful to determine the TL of the building elements/materials so that one can estimate the airborne sound insulation performance inside the building space.

As for impact sound which typically radiates from a floor into rooms below or horizontally, insulation can be done via floor coverings or floor slabs. This is because the applications of these items can reduce the impact sound pressure levels that travels into the receiver room. The typical methods of insulation are adding soft floor coverings on concrete slab, increasing the thickness of concrete floors, or implementing floating floors.

Single number ratings

To know the acoustic information of an insulation element, the standard method would be to refer to the single number ratings of that element. These ratings would be assigned to building materials based on their sound transmission spectra by the means of reference curves or weighted summation procedures.

The most used single-number rating for airborne sound insulation is the Sound Transmission Class (STC), which is in accordance with the American Society for Testing and Materials (ASTM) E413. There is another equivalent number called the Weighted Sound Reduction Index (Rw), which is based on the International Organization for Standardization (ISO) standard ISO 717.

The figure above shows an example of STC contour fitted to a concrete slab’s data. The differences between data points below the contour line and the value of contour are called the “deficiencies”. According to ASTM E413, the sum of deficiency should not be greater than 32 dB, and each individual deficiency should not exceed 8 dB (also known as the 8-dB rule). The reference contour for ASTM covers the frequency range from 125 Hz to 4000 Hz. The Rw contour from the ISO 717 has the same shape, except that it covers a broader frequency range of 100 Hz to 3150 Hz. Also, there is no 8-dB rule in ISO 717. Comparing both standards, the numbers from both ratings are usually close. However, the weighted summation method developed in ISO 717 accounts for the higher importance of low frequencies in traffic noise and modern music systems. As such, this method allows corrections/spectrum adaptation terms to be produced that can be used in conjunction with the Rw rating.

As for impact sound insulation, the sound pressure levels are often collected using a standard tapping machine and normalised, which will then be used with a reference curve to calculate its rating, typically the Impact Insulation Class (IIC), or the weighted index Ln,w. In fact, these ratings are commonly used in building codes. Again, the rating curves are identical in each standard, but there are some differences among them still. For instance, the ASTM IIC method does not allow any unfavourable deviation to exceed 8 dB. An increasing IIC rating would indicate that the impact sound insulation improves. Conversely, the Ln,w rating would decrease as the impact sound insulation gets better. We can take the relationship between both ratings as follow (assuming that the 8-dB rule is not invoked):

However, there is debate regarding the usefulness of ISO tapping machine data obtained on different types of floors. Therefore, the latest version of ISO 717-2 proposed the use of C1, a spectrum adaptation term to consider low-frequency noise that is normally generated below a lightweight joist floor.  is the unweighted sum of energy in the one-third octave bands (50 or 100 Hz – 2500 Hz) minus 15 dB. According to the standard, this rating is expected to have a better correlation with the subjective evaluation of noise coming below floors, especially for low frequency ones.

The single rating numbers mentioned above are all useful when it comes to determining the level of acoustic insulation a material can provide. With the explanation above about room acoustics and the insulation measures that can be implemented, it will give a better idea on how one should tackle and handle the room acoustics in a building.

References

Crocker, M. J. (2007). Chapter 103: Room Acoustics. In C. H. Hansen, & M. J. Crocker (Ed.), Handbook of Noise and Vibration Control (pp. 1240-1246). Adelaide, South Australia, Australia: John Wiley & Sons, Inc. doi:ISBN 978-0-471-39599-7

Crocker, M. J. (2007). Chapter 105: Sound Insulation—Airborne and Impact. In A. C. Warnock, & M. J. Crocker (Ed.), Handbook of Noise and Vibration Control (pp. 1257-1266). Ottawa, Ontario, Canada: John Wiley & Sons, Inc. doi:ISBN 978-0-471-39599-7

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Acoustic of Small Studio

Studio Acoustic

Small studios are now widely used in the recording industry due to their high feasibility and them being economically friendly, which allows those working in the recording/music industry to be able to work remotely without needing to travel to big studios that much. With a good implementation of acoustic treatments, music recorded in small studios can still be high in sound quality, sometimes even suitable for commercial release.

So, what makes a recording studio good?

In today’s article, we will look into the acoustics of small recording studios, where music is performed as recorded (Everest & Pohlmann, 2015).

Ambient conditions

A quiet environment is a must for a studio to be useful, which is sometimes quite hard to achieve. First, noisy sites should definitely be avoided as many noise and vibration problems will not arise by just choosing a site in a quiet location for your studio. Avoid places near loud areas like train tracks, busy road intersections, or even an airport. The ultimate idea is to reduce the external noise spectrum, then keep the background noise within the criteria goal by implementing sound insulations in the building. However, the construction costs of effective insulation elements like floating floors or special acoustically treated walls/windows/doors may cost greatly. Hence, the best way, that is more cost-effective, will be to choose a quiet site in the first place, rather than isolating a studio located at a noisy place.

The HVAC system, which includes heating, ventilating and air-conditioning systems should be designed such that the acoustics meet the required noise criteria goals. The noise and vibration coming from motors, fans ducts diffusers etc. should be brought to the minimum so that low ambient noise levels can be achieved.

Noise

Similar to any other quiet rooms, a small studio needs to comply with the acoustical isolation rules and standards. It is important to construct the building elements with high transmission loss and decoupled from external noise and vibration sources to ensure that the ambient noise levels are low enough for good recording quality. Not only that, but these constructions will also act as an isolation that prevents loud noise (music) levels in the studio from affecting the neighbouring spaces.

Studio acoustical characteristics

Inside a studio, the types of sound present, and may be picked up by microphones, are the direct and indirect sounds. Direct sound is basically the sound coming from the source (before it hits a surface). Indirect sound follows right after the direct, caused by various non-free field effects characteristic of an enclosed area. In short, everything that is not direct sound is considered as indirect or reflected sound.

It is known that the sound pressure level in an enclosed space will vary according to the distance from a source, while also being affected by the absorbency of the room or space. If all the surfaces in a room are fully reflective, it means that the room is fully reverberant (like a reverberation chamber), therefore the sound pressure level would be the same (as of the sound from the source) everywhere in the room as no sound energy is absorbed. It can also be assumed that there is relatively no direct sound since most of the sounds are reflected, hence indirect. Another component that causes indirect sound comes from the resonances in a room, which is also the result of reflected sound.

Indirect sound also depends on the materials used for room construction (e.g., doors, walls, windows, floors, ceiling etc). These elements can also experience the excitation by the vibration of sound from the source, hence able to decay at their own rate when the excitation is removed.

Reverberation Time

The composite effect of all the indirect sound types is reverberation. Many would say that reverberation time is an indicator of a room’s acoustical quality, but in reality, measuring reverberation time does not directly reveal the nature of the reverberation individual components, giving a small weakness of reverberation time being the indicator. Therefore, reverberation time is often not the only indicator of acoustical conditions.

Reverberation time is, by definition, the measure of decay rate, and is usually known as T60. For example, a T60 of 1 second represents that a decay of 60 dB takes 1 second to finish. Some may say that it is inaccurate to apply the reverberation time concept to small rooms, as a genuine reverberant field may not exist in small spaces. However, it is still practical to utilize the Sabine equation (for reverberation) in small-room design to make estimations on the absorption requirements at different frequencies, provided that limitations of the process are taken into account during the estimation.

It is not good to have it being too long or too short. This is because for a room with reverberation time that is too long, speech syllables and music phrases will be masked hence causing a worsening speech intelligibility and music quality. Conversely, if the reverberation time is too short, speech and music will lose character therefore suffer in quality, whereby music will typically suffer even more. Despite that, there is no specific optimal value for reverberation time that can be applied for any rooms, because too many factors are also involved besides reverberation. Things like the types of sound sources (female/male voice, speed of speech, types of language etc) will all affect the room’s acoustic outcome. However, for practical reasons, there are approximations available for acousticians to refer to, where certain amount of compromise has been implemented to make it usable in many types of recording applications.

Diffusion
A high diffusion room give a feeling of spaciousness due to the spatial multiplicity of room reflections, and it is also a good solution to control resonances effects. To create a significant diffusing effect, the implementation of splaying walls and geometrical protuberances works well. Another way will be to distribute absorbing materials in the room, which also increases the absorbing efficiency of the room apart from diffusion. Typically, modular diffraction grating diffusing elements (e.g. 2- x 4-ft units) can provide diffusion and broadband absorption, and can be easily installed in small studios. Still, there will not be much diffusion in a studio room, in practice.


Examples of acoustic treatment
So, what are the acoustic treatment elements that you can use to improve your studio? These items below can be considered (Studio, 2021):
1. Bass Traps
This is one of the most important tools to have in a studio. Bass traps are normally used to absorb low frequencies, also known as bass frequencies, but in fact they are actually broadband absorbers. This means that they are also good at absorbing mid to high frequencies too.

2. Acoustic Panels
Acoustic panels work similarly like bass traps, but rather ineffective at absorbing the bass frequencies. One thing good about acoustic panels as compared to bass traps is that since they are much thinner, they offer more surface area with less material. Therefore, acoustic panels are capable of providing larger wall coverage with less cost as compared to bass traps.

3. Diffusers
Diffusers may not be as effective as compared to bass traps and acoustic panels if used in small studios. So, this really depends on users, whether they find diffusers useful for their application.
Now, where should the acoustic treatment products be placed at?
There are three key areas of the room to be defined in this case:
– Trihedral corners
– Dihedral corners
– Walls
The priority for coverage goes from trihedral corners, dihedral corners to the walls. This is because acoustic treatments should ideally be placed at areas which have the greatest impact. At trihedral corners, for example, three sets of parallel walls converge, hence if there is absorption material located here, it catches the room modes from all three dimensions, giving three times the initial effectiveness. Same concept goes for dihedral corners and walls, but with two dimensions and one dimension respectively.

 

References
Everest, F. A., & Pohlmann, K. C. (2015). Acoustics of Small Recording Studios. In F. A. Everest, & K. C. Pohlmann, Master Handbook of Acoustics (6th Edition ed.). McGraw-Hill Education – Access Engineering. doi:ISBN: 9780071841047
Studio, E.-H. R. (2021). CHAPTER 3: The Ultimate Guide to Acoustic Treatment for Home Studios. Retrieved from E-Home Recording Studio: https://ehomerecordingstudio.com/acoustic-treatment-101/

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Building Accoustics Building Acoustics Environment Home Industrial Noise and Vibration Product News Noise-th Uncategorized Vibration

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.

Select your sound absorber from https://www.blast-block.com/

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Asia Noise News

Ultrasound Selectively Damages Cancer Cells When Tuned to Correct Frequencies

Doctors have used focused ultrasound to destroy tumors in the body without invasive surgery for some time. However, the therapeutic ultrasound used in clinics today indiscriminately damages cancer and healthy cells alike.

Most forms of ultrasound-based therapies either use high-intensity beams to heat and destroy cells or special contrast agents that are injected prior to ultrasound, which can shatter nearby cells. Heat can harm healthy cells as well as cancer cells, and contrast agents only work for a minority of tumors.

Researchers at the California Institute of Technology and City of Hope Beckman Research Institute have developed a low-intensity ultrasound approach that exploits the unique physical and structural properties of tumor cells to target them and provide a more selective, safer option. By scaling down the intensity and carefully tuning the frequency to match the target cells, the group was able to break apart several types of cancer cells without harming healthy blood cells.Their findings, reported in Applied Physics Letters, from AIP Publishing, are a new step in the emerging field called oncotripsy, the singling out and killing of cancer cells based on their physical properties.

Targeted pulsed ultrasound takes advantage of the unique mechanical properties of cancer cells to destroy them while sparing healthy cells.

“This project shows that ultrasound can be used to target cancer cells based on their mechanical properties,” said David Mittelstein, lead author on the paper. “This is an exciting proof of concept for a new kind of cancer therapy that doesn’t require the cancer to have unique molecular markers or to be located separately from healthy cells to be targeted.”

A solid mechanics lab at Caltech first developed the theory of oncotripsy, based on the idea that cells are vulnerable to ultrasound at specific frequencies — like how a trained singer can shatter a wine glass by singing a specific note.

The Caltech team found at certain frequencies, low-intensity ultrasound caused the cellular skeleton of cancer cells to break down, while nearby healthy cells were unscathed.

“Just by tuning the frequency of stimulation, we saw a dramatic difference in how cancer and healthy cells responded,” Mittelstein said. “There are many questions left to investigate about the precise mechanism, but our findings are very encouraging.”The researchers hope their work will inspire others to explore oncotripsy as a treatment that could one day be used alongside chemotherapy, immunotherapy, radiation and surgery. They plan to gain a better understanding of what specifically occurs in a cell impacted by this form of ultrasound.

Written by:

Phawin Phanudom (Gun)
Acoustical Engineer

Geonoise (Thailand) Co., Ltd.
Tel: +6621214399
Mobile: +66891089797
Web: https://www.geonoise.com
Email: phawin@geonoise.asia

Credit: Publishing AIP

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