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In the second in a series of blogs, Mike Carrick AMIOA, Head of Acoustics at Siderise Group, looks at sound paths and unravels the complexities of acoustic criteria - steps 3 and 4 to acoustic specification to mitigate against noise.
While it is difficult today to escape sound completely, we experience a wide range of sounds in our day-to-day life and when it is unwanted, we classify it as ‘noise’. If you are considering the specification of an acoustic material, then chances are you have a ‘noise’ problem. In the second in a series of blogs, Mike Carrick AMIOA, Head of Acoustics at Siderise Group, looks at sound paths and unravels the complexities of acoustic criteria - steps 3 and 4 to acoustic specification to mitigate against noise.
Step 3 - Identify the sound path / paths
More often than not, it is not possible to reduce the noise at source or insulate the receiver. When this is the case, the only option is to consider how the sound energy travels from the source to the receiver and by what mechanisms. Is it airborne, impact, structure-born sound transmission, a direct sound path or a flanking sound path (indirect)?
Only once the source, the receiver and the possible sound paths are understood, can a possible solution be considered to mitigate against it and therefore reduce the noise levels transmitted. An example would be a building façade, where the source may be a busy road, airport or railway line. The receiver would be the rooms inside and the path elements would be the façade and its acoustic properties.
In very simple terms, if the receiver was an office, internal desired noise levels may be 45dB(A), but an apartment may be 35dB(A). If you then know what the noise level of the source is, say for example, a road with noise levels of 70dB(A), then you would need the façade to reduce the sound by 25dB for an office, or 35dB for an apartment. Based on an allowance of 5dB difference between laboratory versus site measured values, the façade performance would likely be 30dB Rw for an office, or 40dB Rw for an apartment.
Step 4 - Understand the acoustic criteria
There are many references to the dB, such as ‘Rw’, ‘Dnf,w’, ‘LnT,w’, ‘DnT,w’ ‘Dne,w’ ‘Leq’ ‘Lmax’, to name but a few. It is very important to understand the differences between these as they are not numerically comparable. Stating a dB requirement is not enough. This is like degrees in temperature with 0 degrees Celsius, 32 degrees Fahrenheit, 273 degrees Kelvin and 427.67 degrees Rankine all being the same temperature. So if you say 300 degrees or 45dB it is not enough information. 45dB what?
Commonly used in high rise façade buildings in reference to airborne sound insulation is ‘Rw’, ‘Dnf,w’ and ‘DnT,w’. An Rw value is a laboratory test value for the direct sound transmission of a building component or construction, such as a floor, wall, door, glass etc. A Dnf,w value is also a laboratory test value for the flanking sound transmission of a system, such as a Raised Access Floor, Suspended Ceiling System, Façade System etc. A DnT,w is a final site measured value for ‘room-to-room’ or ‘floor-to-floor’ sound transmission and includes both direct and flanking sound transmission, airborne and structure born.
In my next blog, I’ll look into the design restraints when it comes to acoustic material and how you should always look to treat the lowest performing element.
In the first in a series of blogs, Mike Carrick AMIOA, Head of Acoustics at Siderise Group, examines the source and the receiver, the first two steps to acoustic specification to mitigate against noise.
What is the definition of ‘noise’? It is simply unwanted sound and more often than not it will not only be an irritation but also a health and safety issue. If you are considering the specification of an acoustic material, then chances are you have a ‘noise’ problem. In the first in a series of blogs, Mike Carrick AMIOA, Head of Acoustics at Siderise Group, examines the source and the receiver, the first two steps to acoustic specification to mitigate against noise.
Step 1 - Understand the source
Whatever the source is, it will have a noise level,so the first thing to do is to ascertain that level, either by field instruments using a sound level meter or by manufacturers’ data if the source is a machine or similar. Ideally the data should be presented in Octave Bands or 1/3rd Octave Bands. Otherwise a single numerical value in dB(A), which will refer to either Sound Power (LW) or Sound Pressure (LP).
How does the source emit the sound energy? Is it airborne sound, structure-borne sound or a combination of both? Voices are generally airborne while a hammer hitting an object is generally impact/structure borne. A pump or speaker will generally be both airborne and structure borne, caused by vibrations transmitting through a rigid material. Whatever the source, the percentage attributed to each method will vary.
Step 2 - Understand the receiver
The receiver is quite simply the person or persons who hears the noise and finds it unacceptable.
In some instances, the remedial action would be to reduce the noise at source either by turning the volume down on a radio, or enclose the source with sound insulating materials. For example a pump could be mounted on rubber anti-vibration mats and enclosed with a combination of high mass and sound absorbent linings. Sometimes this is not possible due to the size of source and the practicality, as well as the need to feed the source with air or material etc.
If reducing noise at source is not possible, then it may be possible to insulate the receiver. A good example of this would be an acoustic control room in a noisy factory. This is where the workers are in a relatively small insulated room, or noise haven, inside a larger factory, providing protection from the harsh noise of machinery.
In my next blog, I’ll look at identifying the sound path and consider how sound energy travels from the source to the receiver. I’ll also lift the lid on acoustic terminology and bring some light to this dark art.
Whatever your insulation requirement, whether you are in the construction, marine, transport or energy generation sector, we can provide technical advice and a tailored insulation solution.
Talk to our technical advisory team who will be happy to have a look at your project.