Frequently Asked Questions

Durability is achieved by ensuring that the packed density of a silencer baffle is high enough to ensure the locational stability of the infill. Acoustic performance is achieved by the infill having the correct permeability (flow resistivity) to allow resistive passage of the noise energy into the absorbing core.

The use of fine fibrous materials, such as ceramic fibres, required low density packing in baffles, so acoustic performance could only be achieved at the expense of durability In contrast, the high fibre diameter of GTB basalt fibres allows the packing density necessary to ensure locational stability and still achieves the level of permeability necessary for the best acoustic performance.

High density is essential to maintain the locational stability of the silencer infill when subjected to highly turbulent gas flow. If the silencer infill undergoes repetitive movement, fibres will be broken down and volumetric packing capability will be lost, even if the infill itself remains. Loss of volume will allow the baffles to resonate. This can lead to fractured welds, and, eventually, the possible breaking up of the baffles.

High density is essential to maintain the locational stability of the silencer infill when subjected to highly turbulent gas flow. If the silencer infill undergoes repetitive movement, fibres will be broken down and volumetric packing capability will be lost, even if the infill itself remains. Loss of volume will allow the baffles to resonate.

This can lead to fractured welds and, eventually, the possible breaking up of the baffles.

Three different units are frequently referred to when discussing acoustic performance. These are described briefly as follows:

• INSERTION LOSS (dB) - Refers to the reduction in the measured downstream noise level which occurs when a silencing element is placed within a duct subjected to an upstream noise source. Insertion loss is generally used by a silencer designer to establish the required baffle arrangement and passage length. It can be theoretically modelled using library data obtained from reference baffles.
• ACOUSTIC ABSORPTION COEFFICIENT (a) - Refers to the sound absorption capability of a porous material on a frequency-specific basis. Usually displayed as an acoustic spectrum showing a plot of alpha value (a) vs. frequency (Hz), acoustic absorption coefficients are material-specific. Alpha values assist the suppliers of acoustic materials in assessing the suitability of fibrous infill make-ups for particular applications, provided the predominant frequencies are known or can be reliably predicted.
• FLOW RESISTIVITY (Rayls/m) - Flow resistivity is the single most important factor towards achieving optimum acoustic performance from bulk fibrous materials with favourable sound absorption characteristics. Flow resistivity is typically quoted in MKS Rayls/m and refers to the resistance offered to the passage of airborne pressure waves (sound energy) through the bulk fibrous material (acoustic absorber). The correct flow resistivity will cause the sound pressure waves to "do work" in passing through the bulk fibrous absorber, converting kinetic energy into thermal energy during the process. If the flow resistivity value is too low, the pressure waves will pass through the fibrous material with very little effort, which will barely reduce their kinetic content. If the flow resistivity value is too high, the sound pressure waves will be reflected rather than pass into the fibrous material, again failing to give up their kinetic content. For a gas turbine exhaust silencing application, the flow resistivity value for the core absorber should typically be between 14,000 and 18,000 MKS Rayls/m using a baffle thickness of around 450mm (18").

Thick baffles will absorb lower frequency (longer wavelength) noise more effectively, whilst thin baffles are more effective with higher frequency (shorter wavelength) noise. In GT exhaust silencers the baffles will generally be between 400 and 600mm thick, whilst in intake silencers, where high frequency compressor noise is evident, the baffles will typically be less than half this thickness. Aircraft test cells running very large fan engines may have baffles up to around 1m (40") thick. The drawback with thick baffles is that, unless there is a very long effective path length, they cause re-generated noise due to turbulent flow.

Pre-weighed, pre-sized pillow modules provide a "measured system" approach, offering fast fibre-free installation with no shop-floor waste. Although slightly more expensive, this approach can significantly reduce labour time, especially if there is little previous experience of packing silencers. Bulk materials can be preferable in cases where hands-on supervision is present and the workforce is experienced in packing silencers using bulk materials and capable of achieving uniformity of the packing throughout the silencer.

Not necessarily. However, it is much easier to be confident of the durability of a pillow-based infill system than is the case with a manually packed silencer. This is because the pillow modules are constructed and supplied to pre-determined specifications based upon the operating conditions for the project. In manual packing, although the materials may be the same, operational durability will be influenced by the consistency of the packing process and, in particular, by the diligence which has been applied in correctly locating the facing materials overlaying the core absorber material. Pillows are always recommended in locations where the gas flow is known to be highly turbulent.

There are three frequently encountered causes of poor durability:

1. Insufficient packing density, resulting in a lack of locational stability.
2. Use of inappropriate thermal insulation, containing organic thermally degradable binders.
3. Inadequate protection of fibrous core material, allowing fibre loss in turbulent in gas flow.

Note: Durability can also be affected by damage arising from non-specified operational circumstances.

Provided that the flow resistivity is the same, there should be no significant difference in acoustic performance between pillow-based infill and manually-packed infill.

The "choking" effect of resistive facing materials (acting upon the flow resistivity of the infill as a whole) is frequently underestimated, leading to a shortfall in the acoustic performance. This is most likely to happen when the core absorbing material and the facing material(s) are ordered from different suppliers. The situation can be prevented by ascertaining that the flow resistivity of the facing materials is equal to (or less than) that of the core absorber. A key attribute of pillow modules is that these are supplied with the flow resistivity of the outer envelope "in balance" with that of the core absorber.

A roll-based system comprises a "kit" of the same materials as those used to construct pillow modules of the required specification. The core absorber is supplied together with overlaying felts and/or fabrics in the appropriate width and system quantities, all in roll form. Bulk materials are generally supplied on an individual basis, irrespective of the supply of any other materials.

Zoning refers to selectively packing specific regions within the silencer. It is most effective when applied using pillow modules, as a range of specifications can be used. However, zoning is also possible by using a mixture of pillow modules and bulk materials. Zoning demands a thorough profile of the operating conditions within the silencer (usually provided by CFD data) in order to locate higher and lower specification infill in appropriate "zones" of the silencer. The object of zoning is to provide significant cost savings without compromising reliability. It can be considered as a fine tuning of the "technical/commercial window" (see above).

The generic limiting temperature of basalt fibre is 820°C (1508°F). However, in order to allow an adequate margin of safety in large thermal systems, the maximum operating temperature is normally considered as being 775°C (1427°F). Silica, stainless steel and HT felts and fabrics ensure the retention of fibres at these temperatures. Although maximum velocity capability is influenced by operating temperature, Basalt-based infill systems can be supplied to cope with passage velocities in excess of 100m/s (325ft/sec) at operating temperatures up to 700°C (1290°F).

Not all basalt source rock deposits are suitable for production of fibre for gas turbine exhaust applications. The air-rich exhaust flow will cause oxidation of the ferrous iron content in unsuitable basalt fibre types at temperatures as low as 250-350°C (480-660°F), converting the fibres to red powder. GTB® (GAS TURBINE BASALT) IS A PROVEN MATERIAL IN AIR-RICH GT EXHAUST ENVIRONMENTS AND WILL NOT OXIDISE.

All fibrous materials supplied by MC Resources are unclassified under established regulatory procedures and are therefore considered to be environmentally safe on respect of handling, installation, operation and eventual disposal.

Durability is achieved by ensuring that the packed density of a silencer baffle is high enough to ensure the locational stability of the infill. Acoustic performance is achieved by the infill having the correct permeability (flow resistivity) to allow resistive passage of the noise energy into the absorbing core.

The use of fine fibrous materials, such as ceramic fibres, required low density packing in baffles, so acoustic performance could only be achieved at the expense of durability In contrast, the high fibre diameter of GTB basalt fibres allows the packing density necessary to ensure locational stability and still achieves the level of permeability necessary for the best acoustic performance.

High density is essential to maintain the locational stability of the silencer infill when subjected to highly turbulent gas flow. If the silencer infill undergoes repetitive movement, fibres will be broken down and volumetric packing capability will be lost, even if the infill itself remains. Loss of volume will allow the baffles to resonate. This can lead to fractured welds, and, eventually, the possible breaking up of the baffles.

High density is essential to maintain the locational stability of the silencer infill when subjected to highly turbulent gas flow. If the silencer infill undergoes repetitive movement, fibres will be broken down and volumetric packing capability will be lost, even if the infill itself remains. Loss of volume will allow the baffles to resonate.

This can lead to fractured welds and, eventually, the possible breaking up of the baffles.

Three different units are frequently referred to when discussing acoustic performance. These are described briefly as follows:

• INSERTION LOSS (dB) - Refers to the reduction in the measured downstream noise level which occurs when a silencing element is placed within a duct subjected to an upstream noise source. Insertion loss is generally used by a silencer designer to establish the required baffle arrangement and passage length. It can be theoretically modelled using library data obtained from reference baffles.
• ACOUSTIC ABSORPTION COEFFICIENT (a) - Refers to the sound absorption capability of a porous material on a frequency-specific basis. Usually displayed as an acoustic spectrum showing a plot of alpha value (a) vs. frequency (Hz), acoustic absorption coefficients are material-specific. Alpha values assist the suppliers of acoustic materials in assessing the suitability of fibrous infill make-ups for particular applications, provided the predominant frequencies are known or can be reliably predicted.
• FLOW RESISTIVITY (Rayls/m) - Flow resistivity is the single most important factor towards achieving optimum acoustic performance from bulk fibrous materials with favourable sound absorption characteristics. Flow resistivity is typically quoted in MKS Rayls/m and refers to the resistance offered to the passage of airborne pressure waves (sound energy) through the bulk fibrous material (acoustic absorber). The correct flow resistivity will cause the sound pressure waves to "do work" in passing through the bulk fibrous absorber, converting kinetic energy into thermal energy during the process. If the flow resistivity value is too low, the pressure waves will pass through the fibrous material with very little effort, which will barely reduce their kinetic content. If the flow resistivity value is too high, the sound pressure waves will be reflected rather than pass into the fibrous material, again failing to give up their kinetic content. For a gas turbine exhaust silencing application, the flow resistivity value for the core absorber should typically be between 14,000 and 18,000 MKS Rayls/m using a baffle thickness of around 450mm (18").

Thick baffles will absorb lower frequency (longer wavelength) noise more effectively, whilst thin baffles are more effective with higher frequency (shorter wavelength) noise. In GT exhaust silencers the baffles will generally be between 400 and 600mm thick, whilst in intake silencers, where high frequency compressor noise is evident, the baffles will typically be less than half this thickness. Aircraft test cells running very large fan engines may have baffles up to around 1m (40") thick. The drawback with thick baffles is that, unless there is a very long effective path length, they cause re-generated noise due to turbulent flow.

Pre-weighed, pre-sized pillow modules provide a "measured system" approach, offering fast fibre-free installation with no shop-floor waste. Although slightly more expensive, this approach can significantly reduce labour time, especially if there is little previous experience of packing silencers. Bulk materials can be preferable in cases where hands-on supervision is present and the workforce is experienced in packing silencers using bulk materials and capable of achieving uniformity of the packing throughout the silencer.

Provided that the flow resistivity is the same, there should be no significant difference in acoustic performance between pillow-based infill and manually-packed infill.

The "choking" effect of resistive facing materials (acting upon the flow resistivity of the infill as a whole) is frequently underestimated, leading to a shortfall in the acoustic performance. This is most likely to happen when the core absorbing material and the facing material(s) are ordered from different suppliers. The situation can be prevented by ascertaining that the flow resistivity of the facing materials is equal to (or less than) that of the core absorber. A key attribute of pillow modules is that these are supplied with the flow resistivity of the outer envelope "in balance" with that of the core absorber.

Not all basalt source rock deposits are suitable for production of fibre for gas turbine exhaust applications. The air-rich exhaust flow will cause oxidation of the ferrous iron content in unsuitable basalt fibre types at temperatures as low as 250-350°C (480-660°F), converting the fibres to red powder. GTB® (GAS TURBINE BASALT) IS A PROVEN MATERIAL IN AIR-RICH GT EXHAUST ENVIRONMENTS AND WILL NOT OXIDISE.

All fibrous materials supplied by MC Resources are unclassified under established regulatory procedures and are therefore considered to be environmentally safe on respect of handling, installation, operation and eventual disposal.

Durability is achieved by ensuring that the packed density of a silencer baffle is high enough to ensure the locational stability of the infill. Acoustic performance is achieved by the infill having the correct permeability (flow resistivity) to allow resistive passage of the noise energy into the absorbing core.

The use of fine fibrous materials, such as ceramic fibres, required low density packing in baffles, so acoustic performance could only be achieved at the expense of durability In contrast, the high fibre diameter of GTB basalt fibres allows the packing density necessary to ensure locational stability and still achieves the level of permeability necessary for the best acoustic performance.

High density is essential to maintain the locational stability of the silencer infill when subjected to highly turbulent gas flow. If the silencer infill undergoes repetitive movement, fibres will be broken down and volumetric packing capability will be lost, even if the infill itself remains. Loss of volume will allow the baffles to resonate. This can lead to fractured welds, and, eventually, the possible breaking up of the baffles.

Three different units are frequently referred to when discussing acoustic performance. These are described briefly as follows:

• INSERTION LOSS (dB) - Refers to the reduction in the measured downstream noise level which occurs when a silencing element is placed within a duct subjected to an upstream noise source. Insertion loss is generally used by a silencer designer to establish the required baffle arrangement and passage length. It can be theoretically modelled using library data obtained from reference baffles.
• ACOUSTIC ABSORPTION COEFFICIENT (a) - Refers to the sound absorption capability of a porous material on a frequency-specific basis. Usually displayed as an acoustic spectrum showing a plot of alpha value (a) vs. frequency (Hz), acoustic absorption coefficients are material-specific. Alpha values assist the suppliers of acoustic materials in assessing the suitability of fibrous infill make-ups for particular applications, provided the predominant frequencies are known or can be reliably predicted.
• FLOW RESISTIVITY (Rayls/m) - Flow resistivity is the single most important factor towards achieving optimum acoustic performance from bulk fibrous materials with favourable sound absorption characteristics. Flow resistivity is typically quoted in MKS Rayls/m and refers to the resistance offered to the passage of airborne pressure waves (sound energy) through the bulk fibrous material (acoustic absorber). The correct flow resistivity will cause the sound pressure waves to "do work" in passing through the bulk fibrous absorber, converting kinetic energy into thermal energy during the process. If the flow resistivity value is too low, the pressure waves will pass through the fibrous material with very little effort, which will barely reduce their kinetic content. If the flow resistivity value is too high, the sound pressure waves will be reflected rather than pass into the fibrous material, again failing to give up their kinetic content. For a gas turbine exhaust silencing application, the flow resistivity value for the core absorber should typically be between 14,000 and 18,000 MKS Rayls/m using a baffle thickness of around 450mm (18").

Provided that the flow resistivity is the same, there should be no significant difference in acoustic performance between pillow-based infill and manually-packed infill.

The "choking" effect of resistive facing materials (acting upon the flow resistivity of the infill as a whole) is frequently underestimated, leading to a shortfall in the acoustic performance. This is most likely to happen when the core absorbing material and the facing material(s) are ordered from different suppliers. The situation can be prevented by ascertaining that the flow resistivity of the facing materials is equal to (or less than) that of the core absorber. A key attribute of pillow modules is that these are supplied with the flow resistivity of the outer envelope "in balance" with that of the core absorber.

A roll-based system comprises a "kit" of the same materials as those used to construct pillow modules of the required specification. The core absorber is supplied together with overlaying felts and/or fabrics in the appropriate width and system quantities, all in roll form. Bulk materials are generally supplied on an individual basis, irrespective of the supply of any other materials.

The generic limiting temperature of basalt fibre is 820°C (1508°F). However, in order to allow an adequate margin of safety in large thermal systems, the maximum operating temperature is normally considered as being 775°C (1427°F). Silica, stainless steel and HT felts and fabrics ensure the retention of fibres at these temperatures. Although maximum velocity capability is influenced by operating temperature, Basalt-based infill systems can be supplied to cope with passage velocities in excess of 100m/s (325ft/sec) at operating temperatures up to 700°C (1290°F).

All fibrous materials supplied by MC Resources are unclassified under established regulatory procedures and are therefore considered to be environmentally safe on respect of handling, installation, operation and eventual disposal.

Durability is achieved by ensuring that the packed density of a silencer baffle is high enough to ensure the locational stability of the infill. Acoustic performance is achieved by the infill having the correct permeability (flow resistivity) to allow resistive passage of the noise energy into the absorbing core.

The use of fine fibrous materials, such as ceramic fibres, required low density packing in baffles, so acoustic performance could only be achieved at the expense of durability In contrast, the high fibre diameter of GTB basalt fibres allows the packing density necessary to ensure locational stability and still achieves the level of permeability necessary for the best acoustic performance.

High density is essential to maintain the locational stability of the silencer infill when subjected to highly turbulent gas flow. If the silencer infill undergoes repetitive movement, fibres will be broken down and volumetric packing capability will be lost, even if the infill itself remains. Loss of volume will allow the baffles to resonate. This can lead to fractured welds, and, eventually, the possible breaking up of the baffles.

Three different units are frequently referred to when discussing acoustic performance. These are described briefly as follows:

• INSERTION LOSS (dB) - Refers to the reduction in the measured downstream noise level which occurs when a silencing element is placed within a duct subjected to an upstream noise source. Insertion loss is generally used by a silencer designer to establish the required baffle arrangement and passage length. It can be theoretically modelled using library data obtained from reference baffles.
• ACOUSTIC ABSORPTION COEFFICIENT (a) - Refers to the sound absorption capability of a porous material on a frequency-specific basis. Usually displayed as an acoustic spectrum showing a plot of alpha value (a) vs. frequency (Hz), acoustic absorption coefficients are material-specific. Alpha values assist the suppliers of acoustic materials in assessing the suitability of fibrous infill make-ups for particular applications, provided the predominant frequencies are known or can be reliably predicted.
• FLOW RESISTIVITY (Rayls/m) - Flow resistivity is the single most important factor towards achieving optimum acoustic performance from bulk fibrous materials with favourable sound absorption characteristics. Flow resistivity is typically quoted in MKS Rayls/m and refers to the resistance offered to the passage of airborne pressure waves (sound energy) through the bulk fibrous material (acoustic absorber). The correct flow resistivity will cause the sound pressure waves to "do work" in passing through the bulk fibrous absorber, converting kinetic energy into thermal energy during the process. If the flow resistivity value is too low, the pressure waves will pass through the fibrous material with very little effort, which will barely reduce their kinetic content. If the flow resistivity value is too high, the sound pressure waves will be reflected rather than pass into the fibrous material, again failing to give up their kinetic content. For a gas turbine exhaust silencing application, the flow resistivity value for the core absorber should typically be between 14,000 and 18,000 MKS Rayls/m using a baffle thickness of around 450mm (18").

The generic limiting temperature of basalt fibre is 820°C (1508°F). However, in order to allow an adequate margin of safety in large thermal systems, the maximum operating temperature is normally considered as being 775°C (1427°F). Silica, stainless steel and HT felts and fabrics ensure the retention of fibres at these temperatures. Although maximum velocity capability is influenced by operating temperature, Basalt-based infill systems can be supplied to cope with passage velocities in excess of 100m/s (325ft/sec) at operating temperatures up to 700°C (1290°F).

All fibrous materials supplied by MC Resources are unclassified under established regulatory procedures and are therefore considered to be environmentally safe on respect of handling, installation, operation and eventual disposal.