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Acoustic of Small Studio

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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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