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Sound in games - porous absorbers

, 12 Oct 2012
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How to calculate damping in a room with a porous absorber in front of the wall.

Introduction

This article focuses on how to calculate the absorption coefficient by placing porous absorbers in front of a wall or other hard surfaces. This means that if you want to construct an absorber of mineral wool of your own, in you home studio, garage or any other a room, you could use the calculated values and use them in calculating the reverberation time (which you can read more about here:

This is quite a narrow article, but it's an important aspect of many rooms, and how they will behave. It is also necessary to have knowledge of these these properties when you want to calculate more advanced features in a room, and use it for sound reproduction.

Background  

There are two different main paths you could follow when you want to calculate the absorption and the complex wavenumber in a porous absorber. They both use the dimensionless parameter E and that is defined by the equation below:

One should take note that the p0 is usually set to the weight of air, as most porous absorbers have roughly 98 % air in them. The Flow resistivity is often given in datasheets as MKS rayl m-1, and this is one of the most important parameters in the equation, so it's important to know how to find it.

The two researchers Delany and Bazley published a formula for approximating based on the averages of many normal porous absorbers. The formula they published gave one for the wavenumber and one for the characteristic impedance, and these two are given below:

These formulas can be used in mid and high frequency without any problems, but they fail at the low frequency. Luckily Mechel developed a different formula, using measurement for the mid and high frequencies and a theoretical model for the low frequencies. The formulation for the low frequency part are as follows:

Where the  is the adiabatic constant for air, roughly equal to 1.4, and the  is the porosity of the material, usually in the area around 0.98, meaning the absorber contains roughly 98 % air in its volume.  The normal, or formulas valid for the mid and high frequencies are given below:

Mechel also sets some boundaries for where the transmission between the two different formulas interact, these are given in the program and not discussed here. The implementation of that would give out a class containing these two parameters are given below:

Module Porous
    ''' <summary>
    ''' Calculates the Material coefficients based on FlowResistivity and Frequency
    ''' </summary>
    ''' <param name="Frequency">The frequency the calculation is valid for</param>
    ''' <param name="FlowResistivity">Flow resistivity of the porous material</param>
    ''' <param name="rho"></param>
    ''' <param name="c"></param>
    ''' <returns>most of the prous materials consists of roughly 98% air,
    ''' so its assumed that this is the caracteristic properties
    ''' of most of these kind of absorbers</returns>
    ''' <remarks>returns the complex wave number ia j*k'.
    ''' This is not useable directly in some formulas</remarks>
    Public Function DelanyBazely(ByVal Frequency As Double, ByVal FlowResistivity _
           As Double, Optional ByVal rho As Double = 1.21, Optional ByVal c As Double = 340) _
           As Acoustics.SimpleMaterialPropertiesHolder
        Dim result As New Acoustics.SimpleMaterialPropertiesHolder
        Dim E As Double = rho * Frequency / FlowResistivity
        result.CharacteristicImpedance = rho * c * (1 + 0.0571 * Math.Pow(E, -0.754) - New Complex(0, 0.087 * Math.Pow(E, -0.732)))
        result.PropagationCoeficcient = New Complex(0, 2 * Math.PI * Frequency / c)
        result.PropagationCoeficcient *= (1 + 0.0978 * Math.Pow(E, -0.7) - New Complex(0, 0.189 * Math.Pow(E, -0.595)))
        Return result
    End Function
    ''' <summary>
    ''' Calculates the Material coefficients based on FlowResistivity,
    ''' Adiabat constant, Porousoty of the material and the Frequency
    ''' </summary>
    ''' <param name="Frequency">The frequency for the calculation</param>
    ''' <param name="FlowResistivity">Flow resistivity of the porous material</param>
    ''' <param name="Porousity">A percentige of air in the material,
    ''' set to 0.98 by default, mening it has 2% of fibers in a volume</param>
    ''' <param name="Adiabat">This is the compressability of air</param>
    ''' <param name="rho">The weight of air</param>
    ''' <param name="c">The speed of sound in air</param>
    ''' <returns>most of the prous materials consists of roughly 98% air,
    ''' so its assumed that this is the caracteristic properties of most of these kind of absorbers</returns>
    ''' <remarks>returns the complex wave number ia j*k'.
    ''' This is not useable directly in some formulas</remarks>
    Public Function Mechel(ByVal Frequency As Double, ByVal FlowResistivity As Double, _
           Optional ByVal Porousity As Double = 0.98, Optional ByVal Adiabat As Double = 1.4, _
           Optional ByVal rho As Double = 1.21, Optional ByVal c As Double = 340) _
           As Acoustics.SimpleMaterialPropertiesHolder
        Dim E As Double = rho * Frequency / FlowResistivity
        Dim lessPart As New Acoustics.SimpleMaterialPropertiesHolder
        lessPart.PropagationCoeficcient = New Complex(0, 2 * Math.PI * Frequency / c)
        lessPart.PropagationCoeficcient *= Complex.Sqrt(New Complex(1, -Adiabat / (2 * Math.PI * E)))
        lessPart.CharacteristicImpedance = New Complex(0, -rho * c ^ 2 / (2 * Math.PI * Frequency * Adiabat * Porousity))
        lessPart.CharacteristicImpedance *= lessPart.PropagationCoeficcient
        Dim NormalPart As New Acoustics.SimpleMaterialPropertiesHolder
        NormalPart.PropagationCoeficcient = New Complex(0.2082 * E ^ (-0.6193), 1 + 0.1087 * E ^ (-0.6731))
        NormalPart.PropagationCoeficcient *= New Complex(2 * Math.PI * Frequency / c, 0)
        NormalPart.CharacteristicImpedance = New Complex(1 + 0.06082 * E ^ (-0.717), -0.1323 * E ^ (-0.6601))
        NormalPart.CharacteristicImpedance *= rho * c
        Dim FinalResult As New Acoustics.SimpleMaterialPropertiesHolder
        FinalResult = NormalPart
        If E > 0.04 Then
            Return FinalResult
        End If
        Dim Elimit As New ElimitHolder
        If E < Elimit.GammaReal Then
            FinalResult.PropagationCoeficcient = _
              New Complex(lessPart.PropagationCoeficcient.Real, _
              FinalResult.PropagationCoeficcient.Imaginary)
        End If
        If E < Elimit.GammaImg Then
            FinalResult.PropagationCoeficcient = _
              New Complex(FinalResult.PropagationCoeficcient.Real, _
              lessPart.PropagationCoeficcient.Imaginary)
        End If
        If E < Elimit.Zreal Then
            FinalResult.CharacteristicImpedance = _
              New Complex(lessPart.CharacteristicImpedance.Real, _
              FinalResult.CharacteristicImpedance.Imaginary)
        End If
        If E < Elimit.Zimg Then
            FinalResult.CharacteristicImpedance = _
              New Complex(FinalResult.CharacteristicImpedance.Real, _
              lessPart.CharacteristicImpedance.Imaginary)
        End If
        Return FinalResult
    End Function  

    Private Class ElimitHolder
        Public GammaReal As Double = 0.04
        Public GammaImg As Double = 0.008
        Public Zreal As Double = 0.006
        Public Zimg As Double = 0.02
    End Class
End Module

Mechel has given other more complicated formulas leather, that includes the effect of among other things curtains, were you don't know the porosity and that would need to be calculated in order to arrive at the correct formulas for propagation constants and characteristic impedance.

There are also alternatives to the two methods that I have just shown, namely the Rayleigh model, which uses a purely theoretical approach and is similar to Mechels low frequency formula, by assuming that a porous absorber could be modeled as a matrix of tubes or a perforated plate.

Mechel also have a different formula that separates different types of mineral wool. He separates Glass fibers,and mineral fibre (basalt or rock wool), and gives two different expressions for it.  It is however given in the book "Acoustic absorbers and diffusers", and Mechel gives a full account of different ways of calculation porous absorbers in "Formulas of acoustic" (his account is on roughly 50 pages, and that is just intended to be a formula reference book!)

There are also two more models, one by Attenborough and  one by Allard and Johnson, the model of Allard and Johnson do however use some of the results from Attenborough in their formulas. These are relative complex models that take more parameters into account when calculating the properties of the medium. I may revisit this article in the future and expand it to include their model as well.

How to do calculations  

This is actually just as difficult as calculating the coefficients of the porous materials used in the equation. We need to start off with some theory first.

A layer in front of an infinitely reflecting surface would always have a surface impedance that could be expressed the following way:

Take note that we are here talking about the wave number and not the propagation coefficient (the difference is Propagation coefficient = jk. So if we have the propagation coefficient we need to multiply it by -j in order to get the wave number, which, confusingly enough could be complex in itself.)

Once we have the surface impedance of this simple material we would then proceed by calculating the reflections from our material relative to the air in front of the material:

This represents the reflection, but we are interested in the losses so we must take 1 - the reflected wave to find out how much of the energy is depleted. And in acoustics the reflection factor is meant to be the amplitude of the reflected wave, and we want the changes in intensity, which is proportional to the pressure squared. This is called the absorption factor and is calculated as shown below:

The formulation calculates the normal incident of a sound wave, meaning that it does not take into account a diffuse waves that are encountered in rooms.

Using the code

The code that calculates the absorptions coefficient are as follows:

''' <summary>
''' Returns the absorbtion coefficient of a porous absorber in front of a hard wall
''' </summary>
''' <param name="Sigma">Flow resistance in the porous material</param>
''' <param name="ThicknessOfPorousAbsorber">Thickness of the porous material</param>
''' <param name="DelanyBazely">If set to true you
''' use Delaney-Bazley, and if false it uses Mechels approximation</param>
''' <returns>Normal incident absorbtion coefficient</returns>
''' <remarks></remarks>
Public Function CalcualtePorousAbsorption(ByVal Sigma As Double, _
       ByVal ThicknessOfPorousAbsorber As Double, ByVal DelanyBazely As Boolean) As OctaveBand
    'Store the calcualted result
    Dim Result As New OctaveBand
    'Speed of sound in air
    Dim c As Double = 340
    'Mass of air in kg/m^3
    Dim rho As Double = 1.21
    'Caracteristic impedance of air
    Dim z0 As Double
    z0 = c * rho
    For i As Integer = 0 To Result.Count - 1
        'Center frequency of Octave band 
        Dim f As Double = Result.Okt13(i)
        Dim MaterialProperties As New SimpleMaterialPropertiesHolder
        If DelanyBazely Then
            MaterialProperties = Acoustics.Materials.DelanyBazely(f, Sigma)
        Else
            MaterialProperties = Acoustics.Materials.MechelGrundmann(f, Sigma)
        End If
        'Calculate the surface impedance
        Dim z As New Complex(0, -1)
        z *= MaterialProperties.CharacteristicImpedance / _
             Complex.Tan(MaterialProperties.PropagationCoeficcient * z * ThicknessOfPorousAbsorber)
        'Calculate the reflection coefficient relative to the main propagation material
        Dim ReflectionCoefficient As New Complex
        ReflectionCoefficient = (z - z0) / (z + z0)
        'Calculate the absorbtioncoefficient and store it
        Result.Item(i) = 1 - (Complex.Abs(ReflectionCoefficient)) ^ 2
    Next
    Return Result
End Function

And that completes the application.

History  

  • 12.10.2012 - Initial release.

If you take a look at the two different types of calculation the absorption from a layer of porous material backed by a hard wall, you don't see much difference, except at the low frequency range. Normally Delany-Bazley are considered valid from 250 Hz and above, so it's normally always advisable to use Mechels formula instead.

References  

The books I list here offers a good explanation of what I describe, and are good further references if you want to know more. They are listed in the order of how much I have used them in the article.

  • "Building acoustics", First edition (2008), Tor Erik Vigran, CRC Press
  • "Acoustic Absorbers and diffuseres - Theory, design, and application", Second edition (2009), Trevor J. Cox and Peter D'Antonio, Taylor & Francis.
  • "Formulas of Acoustics", 2nd Edition, F.P. Mechel, Springer
  • "Fundementals of Acoustics", Fourth edition, Kinsler, Frey, Coppens and Sanders, John Wiley & Sons, Inc.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Kenneth Haugland
Engineer
Norway Norway
I hope that you like the stuff I have created and if you do wish to say thank you then a donation is always appreciated.
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