Sound Absorption Measurement: Alpha Cabin and Impedance Tube

Abstract

The stage of development of absorbent materials, when necessary to verify their properties in relation to established requirements, plays one of the key challenges in current research. Nowadays, experimentation represents the only reliable way to quantify sound absorption. Thus, the determined sound absorption coefficient is used to compare individual development variants, and also, it is used in a selection of material from the commercial offer. Therefore, the main part of research is devoted to measurements in the impedance tube and in the alpha cabin, because these procedures play one of the most challenging roles in practice. All used experimental methods are based on the theory about transformation of sound energy into other forms of energy in the material. Nevertheless, the physical nature of sound absorption and individual measurement principles are not covered in this chapter, nor are any sound insulation measurements. It deals solely with the sound absorption and determination of the sound absorption coefficient. As a results, this chapter further summarizes basic information on a sound absorption measurement, and mainly, focuses on practical recommendations as well as applicability of results. First and foremost, these individual procedures may represent a considerable international overlap in the field.

1. Introduction

Sound absorption measurement serves as an activity associated with the design, verification and application of suitable materials for solving an acoustic situation of enclosed spaces. These materials have a specific composition, may be applied to large surfaces, and their properties must typically meet many requirements (thermal insulation, mechanical resistance, low dirtiness, compactness, etc.). Absorbent materials are used in building acoustics, the automotive industry and everywhere where humans and noise sources are in a confined space.

This chapter covers the experimental determination of the sound absorption coefficient of industrially produced materials or samples in the stage of development, which are intended for the reduction, or regulation of noise in closed spaces. The chapter also contains a brief description of basic comparison methods and a more detailed description of two laboratory methods, i.e. the measurement of sound absorption in an impedance tube and in an alpha cabin. The goal is to provide a basic description of experimental methods, their comparison, evaluation and determination of accuracy.

In practice:

• the requirements for noise in a closed space are given,

• the properties of absorbent materials are derived from them,

• materials are developed,

• their properties are experimentally verified and compared with the requirements,

• materials are optimized,

• materials are applied,

• verification of the optimized space is carried out, compliance with the space requirements is evaluated.

• The results are put into practice.

2. Basics of measuring sound absorption

Sound absorption is the ability of a material environment to absorb coming sound. It is a process in which sound energy falling on a sample of material is transformed into another form, mainly thermal energy. In the ideal case, the sound energy that encounters the sample (W_INBOUND) is partly reflected (W_REFLECTION), partly transmitted through the sample (W_TRANSMISSION) and partly absorbed into the sample (W_ABSORPTION). Applies to:

W_INBOUND = W_REFLECTION + W_ABSORPTION + W_TRANSMISSION.

When experimentally investigating absorption, certain conditions need to be met so the equation above can be applied. The conditions are:

• It can be assumed that the material sample is fully involved in the energy balance.

• The source of sound energy is controlled.

• The measurement system does not significantly affect the measurement results (by its principle, dimensions and other metrological and non-metrological properties).

There are additional requirements for experimental methods:

• They should enable a statistical approach, respecting the variability of the measurement system as well as the variability within the sample.

• Selected method should satisfy the intended use of the results with its accuracy.

• Selected method should be fast and repeatable.

• The absorption frequency bandwidth should be as wide as possible.

• Method with a perpendicular incident of sound waves on the sample and

• Method with an omnidirectional impact of sound waves on the sample.

To the methods used, it should also be mentioned:

• Determination of sound absorption is always an estimation with a definable precision. A result cannot always be related to a specific application due to variability within samples (each sample taken from the production differs from another).

• A measurement is always indirect, the sound absorption coefficient is always calculated from other measured quantities.

• It is assumed for all measurement methods that the sample is placed near a surface that has zero sound transmission, hence W_TRANSMISSION = 0. Therefore, all incident acoustic energy is absorbed or reflected.

• Most of the methods compare results with and without a sample. Results without a sample assume absorption to be equal to zero.

• Variability caused by the instability of the excitation signal (energy encountering the sample) is averaged. Variability of the measurement system has a defined measurement uncertainty.

3. Physical and metrological basis of sound absorption measurement

A measurement can be considered as the only objective option to determine the sound absorption coefficient α. Other options, such as simulations and modeling in a virtual environment, face problems with an accurate determination of the boundary conditions and with the definition of the internal structure of the material. Validation of materials intended to solve a sound situation in closed spaces requires a determination of the sound absorption factor by experiment, therefore on a real part by objective methods. Requirements are set, for example, by the automotive industry or the building materials industry. Knowledge of the sound absorption coefficient makes it possible to model sound propagation in closed spaces using special software.

Measuring the sound absorption coefficient is a topic mainly for:

1. Independent testing laboratories,

2. Research organizations,

3. Industrial companies involved in the development and production of absorbent materials.

In the next part of the text, 4 measurement methods are described. Two of them are very simple and serve more for a comparison to the reference sample, the other two ones are the most used in practice. A method, which is used mainly by independent testing laboratories and complies the international standards be briefly mentioned.

The following points apply to all described methods:

1. Measurement methods are always indirect. The sound absorption coefficient is always determined by calculation from measurements of other quantities. The evaluation of sound absorption is based on the consequences of the energy conversion, which is a decrease in the sound pressure level (sound intensity) after passing through the sample, a change in the reverberation time after the application of an absorbing material or from the deformation of the reflected wave.

2. A known source(s) is used to excite the acoustic energy (incident acoustic energy).

3. An experiment assumes that the acoustic energy encountering the sample is partially absorbed by the sample and the rest of the energy is reflected. The sample therefore lies/rests on a soundproof surface during measurement. The measurement is always based on the principle that energy from the measuring system is prevented from passing through the sample. The assumption is that the incident energy is reflected from the surface behind the sample and partially absorbed as it passes through the sample. If a relatively large sample is available, the absorbed energy is evaluated using the reverberation time, for smaller samples the resultant wave (sum of incident and reflected wave) is mapped in the near space.

4. The result is a frequency spectrum of the sound absorption factor in the bandwidth allowed by the method.

5. The result is evaluated statistically.

3.1 Approximate and comparative methods of measuring sound absorption

In this chapter are described simple methods of sound absorption measurement. Their description serves rather to complement the technical and historical context of sound absorption measurements. It is based on conditions where it is not possible or preferred to use more advanced methods. These methods:

• Are easily applicable in operating conditions,

• Use a simple calculation model,

• Are relatively fast.

These methods cannot be characterized as laboratory methods for an objective determination of sound absorption. They only serve as quick comparison tests in the optimization of the composition of absorbing layers and can be used directly at the place of application.

3.1.1 Tone burst method

This method is currently used very rarely. However, in the available literary sources, it is still found in various methodological variants [1, 2, 3], it is popular with students who use it in a case when sound absorption is only one of the properties they investigate on materials.

This is a simple method based on the idea that when a sound wave strikes a sample at a certain angle, a reflected wave propagates at the same angle with energy reduced by the absorbed energy. The calculation is based on processing the ratio of incident energy and reflected energy.

The measurement takes place in a space that is as anechoic as possible, ideally using a directional microphone and a directional speaker.

1. Measurement step: a signal with sufficient energy and bandwidth is applied to the speaker and the sound pressure level L_p,d (its frequency spectrum) is measured at the distance A between the speaker and the microphone.

   

2. Measurement step: a testing object of sufficient dimensions is placed on the reflective pad. The distances from the microphone to the point of reflection of the sound waves and the distance of the loudspeaker from the point of reflection of the waves are identically A/2. The identical signal emitted by the speaker is partially absorbed in the absorbing wave, the rest is reflected to the microphone. The measurement takes place according to the diagram below, the important thing is the same angle of incidence/reflection φ. The frequency spectrum of the sound pressure level L_p,r is evaluated at the microphone.

   

3. Step 3: at individual frequencies (1/3 octave), the sound absorption coefficient α(φ;f) is estimated according to the formula:

   $$\alpha \left(\phi ;f\right)=1-{10}^{-\frac{(L_{p,d}\left(\phi ;f\right)-L_{p,r}\left(\phi ;f\right)}{10}}$$E1

Method Notes:

1. The estimated sound absorption coefficient is dependent on the angle φ. Although the method can be included in the group of methods with omnidirectional impact of sound waves, the dependence of the result on the angle of incidence/reflection φ is obvious.

2. For small angles φ, the method is very inaccurate.

3. The Tone Burst Method is very sensitive to background noise.

4. For example, glass can be recommended as a reflective surface, but the assumption that this surface does not transmit sound energy, which ultimately increases the resulting sound absorption, is not always valid.

3.1.2 Sound intensity measurement method

This approximate method requires the use of a measuring system with a sound intensity probe.

In an open field, the reflective surface is covered with a sufficiently large sample of the measured material. A sound source is placed at a sufficient distance from the surface of the sample. At a close distance from the sample (approx. 0.2 distance between the surface of the sample and the speaker), the average sound pressure level L_p and the average sound intensity level L_I reflected from the sample are measured. The sound absorption coefficient is then calculated by the formula:

$$\alpha \left(f\right)=\frac{4}{1+{10}^{\frac{L_p\left(f\right)-L_I\left(f\right)}{10}}}$$E2

The incident energy is proportional to the sound pressure level, the reflected energy is identified as a component of the sound intensity vector. Both quantities are identified by the sound intensity probe.

Advantages of the Sound Intensity Measurement Method may be:

• Low time consumption,

• The measurement does not take place in two steps without a sample/with a sample, but only with an applied sample, which gives the possibility to measure the absorbency of permanent applications.

The disadvantages of the Sound Intensity Measurement Method may be:

• Lower accuracy,

• The impossibility of relating the result only to the absorbent material sample. It is not possible to separate the absorption of the sample and the absorption of the substrate on which it is placed.

3.2 Accurate methods of measuring the sound absorption coefficient

3.2.1 Standard ISO 354

This international standard [4] defines the basic laboratory procedure for determining the sound absorption coefficient in a reverberation space. The procedure can be considered the most accurate procedure leading to the determination of the sound absorption coefficient. The method determines the sound absorption coefficient for diffusing the sound impact and can be used to measure materials with distinct shape structures in the straight and perpendicular direction. It is described in great detail in the standard and is especially suitable for specialized laboratories. The standard places strict requirements on the reverberation space, its dimensions and above all on the dimensions of the sample. The declared frequency range is from 100 Hz to 5000 Hz. The principle of indirect measurement of the sound absorption coefficient is based on Sabin’s formula [5]:

A formula developed by Wallace Clement Sabine that allows designers to plan reverberation time in a room in advance of construction and occupancy. Defined and improved empirically the Sabine Formula [5] is.

$$T\left(60\right)=0.161·\frac{V}{A}\left|s\right|,$$E3

Where:

T(60) = reverberation time or time required (for sound to decay 60 dB after source has stopped) |s|,

V = Volume of room |m³|,

A = the equivalent absorption surface |m²|.

In the test room, the reverberation time is measured with and without the mounted test sample. The reverberation time is the time during which the sound pressure level decreases by 60 dB after the sound source is turned off. This means that the original acoustic energy drops to 1/1000000 of its original size. In the test room, the reason for the decrease is the sound absorption and then the reverberation time is its measure. The equivalent surface is a hypothetical surface of a perfectly absorbing sample that has the same properties as a real sample. The equivalent area is the basis for calculating the sound absorption coefficient.

Advantages of determining the sound absorption coefficient according to ISO 354:

• the possibility of measuring samples with large thicknesses,

• accuracy, repeatability,

• international acknowledgement.

Disadvantages of determining the sound absorption coefficient according to ISO 354:

• Reverberation room volume of at least 150 m³,

• Requirements for the shape of the room,

• Sample area 10 m² to 12 m²,

• Possibly higher price for the testing.

Above all, the requirement for a size of a sample is a problem of using this basic method in the sample development phase, when many possible variants are experimentally verified with subsequent optimization. It is practically impossible for manufacturers of absorbent materials and research organizations that are not directly oriented towards this research to acquire such expensive laboratory facilities.

Specific information on the measurement and calculation procedure is contained in the mentioned standard and it is not the aim of this chapter to discuss them in more detail.

3.2.2 Measurement in an impedance tube

An impedance tube is the most common device used today to estimate sound absorption. In the professional literature, this method is currently mentioned most often. There are more concrete technical versions of the tube, from the own construction of a research workplace to commercially offered versions. As an example, Figure 1 shows the assembly from Brüel & Kjær Impedance Tube Kit 4206 (4206-A), which is described in the following text.

Figure 1.

Impedance tube and absorptive material samples (author’s archive).

The impedance tube principle is based on the creation of a combination of direct and reflected waves in a rigid closed tube with an internal smooth and reflective surface. The skeleton of the tube must be as soundproof as possible. One end of the tube covers a sample that is being measured, on the other end of the tube there is placed a speaker that excites by broadband noise the inner volume of the tube. A plane wave is created between the speaker and the sample, which is a combination of incident and reflected waves. The energy of the reflected wave is reduced by the energy absorbed in the sample. The resulting wave is sampled in the tube and an estimate of the sound absorption coefficient is determined by evaluating the data obtained. The sound wave strikes the sample perpendicularly.

Basic characteristics of impedance tube measurement:

• A dimensionally small sample is measured (100 mm/39 mm for [6]), which is an advantage for the development and optimization of materials, but a disadvantage if the developed materials have significant spatial elements, change on the surface or contain significant non-homogeneities.

• The strike of the sound wave is perpendicular to the sample and the measurement result corresponds to this. The results therefore do not correspond to the behavior of the material in real conditions, where the omnidirectional impact of sound waves prevails. The impedance tube is therefore particularly suitable as a precise comparison platform for the development of absorbing materials.

• The detected sound absorption coefficient represents the minimum ability to absorb sound and it can be assumed that the results will be better with the methods in the diffusion field according to ISO 354 [4] and in the alpha cabin [7, 8].

• The methods are very sensitive to sealing a sample in a tube so that all acoustic energy passes through or reflects off the sample.

• If measurements are made for different sample diameters, it is necessary to unify the results in common frequency bands (principally by averaging).

• If a comprehensive idea of ​​the sound absorption is to be obtained, it is advised to take and measure several samples from the research batch in order to cover all possible non-homogeneities and shape and material changes.

• Despite all the disadvantages, the impedance tube is a suitable and most commonly used platform for estimating the sound absorption coefficient and an important aid for the development of absorbing materials, mainly due to the speed of measurement and sample size.

3.2.2.1 Method using standing Wawa ratio

This method determines the sound absorption coefficient of acoustic materials when the sound is incident perpendicularly. The specific procedure for determining the sound absorption coefficient is described in [9]. The absorbing sample is fixed at one end of the tube. An incident plane sine wave p_i is excited by a speaker at the opposite end of the tube. By superposition p = p_i + p_r of the pressures of the incident wave p_i and the reflected wave p_r from the test sample, a standing wave is created in the tube. The course of the sound pressure level of this standing wave is measured by an adjustable microphone, which is moved along the axis of the tube through the hole in the center of the speaker. The evaluation of sound absorption is based on the difference in sound pressure levels ΔL between the pressure maximum and minimum in the tube.

$$\alpha =\frac{4·{10}^{\raisebox{1ex}{$∆L$}\!\left/ \!\raisebox{-1ex}{$20$}\right.}}{{\left({10}^{\raisebox{1ex}{$∆L$}\!\left/ \!\raisebox{-1ex}{$20$}\right.}+1\right)}^2}$$E4

Moving the microphone and accurately identifying the maximum and minimum sound pressure level reduces the speed of the sound absorption coefficient measurement. Impedance tubes for this evaluation method are more often an individual product of test laboratories, which allows adaptation to the desired frequency band and the way of moving the microphone and evaluating the absorption.

3.2.2.2 Transfer-function method

This test method is similar to the previous method in that it uses the same experimental scheme with a sound source at one end and a sample fixed in an impedance tube at the other end. The procedure is described in detail in [10, 11]. In this test method, plane waves in the tube are excited by a noise source and the sound pressure is measured by microphones located at two fixed points (or by one microphone moved in the tube) and by subsequent calculation of the complex transfer function at a perpendicular incidence of sound waves. The test method is overall much faster than the measurement procedure described in the previous chapter.

The test sample is fixed to one end of a straight, rigid, smooth and sealed impedance tube. Plane waves are excited in the tube by a sound source (noise) and the sound pressure is measured by microphones at two locations near the sample. A complex transfer function is determined from the measured signals, which is used to calculate the sound absorption coefficient. The frequency range of the measurement depends on the dimensions of the tube and the distance between the positions of the microphones. In order to determine the sound absorption coefficient in a wider frequency range, measurements are made on an assembly that contains tubes of two different diameters. Figure 1 shows a measuring set-up that allows determining the sound absorption coefficient for a sample diameter of 100 mm in the frequency range of 50 Hz to 6.4 kHz (for a sample thickness of 440 mm maximum [6]) and for a sample diameter of 29 mm in the frequency range of 100 Hz to 3.2 kHz (for a sample thickness of 200 mm maximum [6]).

Measurements can be done:

1. By a method using two microphones that simultaneously measure the signal in the tube at two clearly defined points,

2. By a method of one microphone, which is moved gradually to two measuring locations during the measurement.

Procedure 1 is quick, accurate and easier to do. It is widespread in practice and much more published.

Procedure 2 requires a specialized excitation signal, has more demanding requirements for processing the measured signals, and is more time-consuming. It better eliminates phase mismatch between microphones and allows optimal selection of microphone locations for each measured frequency. According to [10], this procedure is recommended for evaluating of tuned resonators.

Advantages of measuring in an impedance tube:

• small sample size,

• fast measurement,

• relatively available measuring technology, availability of laboratories,

• strong information and publication background,

• the existence of an international standard.

Disadvantages of measuring in an impedance tube:

• assessment of absorption only for the perpendicular impact of sound waves,

• limited sample thickness.

3.2.3 Measurements in the alpha cabin

The Alpha cabin [7, 8, 12, 13] is an internationally acknowledged measurement platform for determining the sound absorption coefficient at the omnidirectional impact of sound waves. It is therefore close to measurements according to ISO 354, it is based on the requirements of this standard, it respects the methodology as much as possible, but removes the disadvantage of the need for large samples.

The Alpha cabin is a platform that is scaled 1:3.2 to the echo chamber parameters of the Swiss Material Testing and Testing Laboratory (EMPA) in Dübendorf. Figure 2 shows an example of the latest design of the alpha cabin. It is a reverberant space sound-isolated from the outside environment with non-parallel walls.

Figure 2.

Alpha cabin - Technical University of Liberec (author’s archive).

The main technical data of the alpha cabin are:

Internal cabin volume:	6.44 m^3
Frequency measurement range:	400 Hz to 10 kHz (1/3oct.)
Dimensions of a standard sample:	1.0 m × 1.2 m
Surface of absorbent parts:	0.6 m^2 to 2.4 m^2

The formula [7] is used to determine the sound absorption coefficient:

$${\alpha }_S=\frac{\mathrm{0,966}}{S}\left(\frac{1}{T_1}-\frac{1}{T_0}\right)$$E5

Where the measured quantities are:

S = sample area |m²|.

T₁ = reverberation time in the sample booth |s|,

T₀ = reverberation time in the cabin without sample |s|.

The ratios in the diffusion field of the alpha cabin (Figure 3) are practically the same as in the large reverberation chamber, but for three times shorter wavelengths (three times higher frequencies). The Alpha cabin therefore provides comparable results on much smaller sample areas than required by ISO 354. However, the proportional changes in cabin conditions run into one problem. The thickness of the sample is the only geometric quantity that cannot be reduced in a ratio of 1:3, and thus the absorbing surface corresponding to the edges of the sample appears three times larger in proportion to its surface. The problem must be eliminated by edging the side surface of the sample with soundproof material. As standard, it is solved by a metal bounding frame with the dimensions of a standard sample, which is higher than the usual thicknesses of the developed materials. In the case of larger thicknesses, it is recommended to manufacture your own frame and validate it using a reference sample. The importance of sample edging can be shown on the measurement results of the same sample with a thickness of 20 mm with and without a bounding frame in Figure 4. The course shows that the error of determining the sound absorption coefficient increases with increasing frequency (Figure 5).

Figure 3.

Alpha cabin with embedded and framed sample (author’s archive).

Figure 4.

Results of measurements with and without a border frame.

Figure 5.

Frames for delimiting samples (author’s archive).

The Alpha cabin measurement procedure generally requires two measurements.

1. Determination of the reverberation time in a cabin with a frame without a sample T₀,

2. Determination of the reverberation time in the cabin with a framed sample T₁.

The sound absorption coefficient is then calculated according to formula (5).

The Alpha cabin has one more function, which is the evaluation of absorbing objects. These are shaped parts that absorb sound, but the absorbing surface cannot be determined. A typical example is the seat of a passenger car [12], which significantly affects the noise in the closed space of the cabin due to its absorption, but it is not possible to clearly determine the absorbing surface, or to create a sample of standardized dimensions from the seat. In Eq. (5) it is not possible to substitute the absorbing surface S, and thus the measurement result is equal to the equivalent absorbing surface A:

$$A=\mathrm{0,966}·\left(\frac{1}{T_1}-\frac{1}{T_0}\right)\mid {\mathrm{m}}^2\mid$$E6

The equivalent absorptive surface corresponds to the absolute absorptive surface (α_S = 1), which has the same absorptive capacity as the shaped part. Therefore, the larger the equivalent surface area, the more the shaped part is able to absorb more sound energy. An example of the result of measuring the equivalent absorbing surface for a shaped part in the construction of a passenger car is shown in Figure 6.

Figure 6.

Measurement results of the equivalent absorptive surface of a car component.

The equivalent absorptive surface of shaped parts is primarily a comparative parameter when developing or selecting a part for a protected space. However, it can be used in the calculations of the total absorption, because according to Eq. (5):

$$A=S·{\alpha }_S$$E7

Advantages of measuring sound absorption in the alpha cabin:

• Measurement of the sound absorption coefficient for the omnidirectional impact of sound waves,

• Optimal sample size considering the possibilities of developing absorbing materials and the accuracy of the estimate,

• The possibility of determining the equivalent absorption A of shaped parts,

• Test speed and repeatability,

• A recognized platform in the automotive industry,

• Possibility of estimating absorption also for non-homogeneous materials (recycled materials, disordered fibrous materials, loose substrate, etc.),

• Possibility of estimating the absorption of decorative panels and artworks designed to solve the acoustics of closed spaces.

Disadvantages of measuring sound absorption in the alpha cabin:

• relatively large size of the sample compared to the impedance tube,

• limited sample thickness due to the need for lateral sealing,

• large and expensive measuring equipment.

4. Conclusion

Measurement is still the most accurate and fastest procedure for determining the sound absorption coefficient. The practical use of absorbing materials in practice requires an objective determination of absorption for the purpose of optimizing the acoustic properties of enclosed spaces.

The current development of absorbent materials is predominantly still using fibers or porous raw materials with an emphasis on other important properties, such as ecology, usability of waste and recyclable resources, esthetics, non-flammability, etc. For products designed in this way (mats, panels, absorbent elements) the main principle of absorption is the conversion of sound energy into heat by friction of the internal structure of the absorbing element. In general, the elements then have optimal efficiency starting from the frequency that is determined by following equation:

$$f=\frac{86000}{H}\mid \mathrm{Hz}\mid$$E8

Where:

H = the thickness of the absorbing element |mm|,

f = frequency |Hz|.

The optimal thickness equals to a quarter of the wavelength of a perpendicular incident wave, so it can be considered the minimum value at which the material is able to use its full potential to absorb sound. With omnidirectional impact, it can be assumed that the optimal bandwidth will shift to lower frequencies. Figure 7 shows an example of the measurement result in the Alpha cabin of a sample of absorbent material with a reference thickness of 22 mm. A frequency of 3.9 kHz (4 kHz 1/3 octave) corresponds to a thickness of 22 mm. Due to the omnidirectional impact of sound waves, the maximum absorption value is maintained even at lower frequencies (2 kHz).

Figure 7.

Sound absorption of a sample with a thickness of 22 mm - measured in the alpha cabin.

There is an inverse proportional relationship between the optimal frequency and the thickness of the material. When considering declared frequency ranges of individual measurement methods, Figure 8 provides a comprehensive overview of the methods and their practical use for materials testing.

Figure 8.

Measurement methods in relation to the optimization of the thickness of the absorbent material.

It is clear from Figure 8 that the lowest declared measurement frequency achievable in the impedance tube is at frequency of 50 Hz. This would correspond to the optimal absorption of materials with a thickness of approx. 1700 mm, which is technically impossible. An impedance tube of such dimensions is not used in practice, but a sufficiently wide frequency band is available for an objective assessment of commonly used materials. The Alpha cabin starts at a frequency of 400 Hz, which corresponds to roughly 200 mm of material thickness when optimally used. This thickness is usable for the Alpha cabin. The ISO 354 standard declares a minimum frequency of 100 Hz, which corresponds to 860 mm of optimal thickness, which is also acceptable given the dimensions of the space and the area of ​​the sample. It should be emphasized that the measurement according to the ISO 354 standard and in the Alpha cabin is based on the omnidirectional impact of sound waves, the impedance tube is based on only a perpendicular impact.

From the Figure 8, an uncovered bandwidth of sound absorption measurements up to 100 Hz can be seen in the case of omnidirectional impact of sound waves. It should be emphasized that physical and technical obstacles to the use of independent methods are encountered here. The optimal thickness of the materials is greater than 860 mm and ends at 4.3 m for 20 Hz, which is the lowest frequency of the audible band. However, this range of thicknesses of absorbing materials is difficult to use in the real world for practical reasons. The exception is specialized anechoic chambers with high volumes. Here, the effectiveness of absorbing materials is assessed by measuring the reverberation time directly during implementation.

If the commercially usual area of absorbent materials (up to a maximum thickness of 200 mm) were to be evaluated, it can be seen from Figure 9 that the optimal platform is the alpha cabin.

Figure 9.

Measurement methods in relation to thickness optimization of conventional absorbent material | author’s archive |.

Recommendations for the design and experimental verification of the properties of absorbent materials

If the absorbing material is to fulfill the expectations, its structure must be properly designed. This is a matter of material development respecting other requirements (legislative requirements, esthetic requirements, applicability in specific conditions, other specific customer requirements). An objective assessment of sound absorption can only be achieved by measuring on an existing sample. Below is the basic procedure for determining the sound absorption coefficient by experiment.

1. To perform the measurement, a sample must be taken that clearly states:

   • what stage of development it is in, whether it is a measurement for validation or for comparison,

   • how should it be applied (directly on the wall, with an air gap, in combination with other materials)?

2. It is necessary to establish what are the requirements for the accuracy of the sound absorption coefficient measurement result. The accuracy of the result is influenced by the combination of measurement speed, used HW and SW, adherence to sample size, variability of measurement conditions and adherence to methodology. If it will be a comparative measurement that is carried out using the same method, a lower accuracy of the result can be accepted. If it is a question of describing the final form of the absorbing material, it is advisable to recommend methods with a higher quality. Standard ISO 354 can be considered the method with the highest accuracy.Lower accuracy methods:

   • Tone Burst Method

   • Sound Intensity Measurement Method

   Higher accuracy methods:

   • ISO 354 standard

   • Measurement in an impedance tube

   • Measurements in the alpha cabin

3. The sound absorption coefficient is determined by the selected method. It is advisable to apply a statistical approach, perform repeated measurements, determine the result and its uncertainty.

4. It is evaluated whether the measurement result met the expected goals.

5. Discussion

This chapter summarizes experimental methods for determining the sound absorption coefficient α. The chapter addresses the user (researcher, customer, project solvers) who are tasked with designing (developing) absorbing material and need to verify it during the development stage or after application. Available approximate and exact methods, their starting points, limitations, advantages and disadvantages and usability in practice are described. The solver can thus choose a suitable method for the individual stages of the project solution or correctly formulate requirements for external laboratories. The chapter further helps to understand the measurement results in relation to the application to a specific space and guides the project solver to be aware of possible limitations and problems.

Recommendation:

1. The most accurate measurement of the sound absorption coefficient is according to the ISO 354 standard, but it requires a large sample area and expensive measuring equipment. It is not suitable for the phase of development and optimization of the properties of absorbent materials.

2. Objective results for practical application are provided by the alpha cabin, which optimally combines sample size and measurement accuracy. However, the cost of measuring equipment is relatively high.

3. For the phase of development and optimization of properties, an impedance tube is ideal, which requires a minimum sample size, but the results are only suitable for comparing individual variants.

4. Approximate methods are suitable when sound absorption is not an essential property or it is necessary to estimate the properties of an already applied material.

5. For material design and evaluation of sound absorption, it is necessary to have at least basic knowledge in the field of acoustics.

6. Sound absorption is dependent on sound frequency and optimal absorption can be achieved for material thicknesses according to Eq. (8).

7. Sound absorption and sound insulation are different properties of a material.