2007 and accepted for publication August 2007. Discussion of 

this peer-reviewed and approved paper is invited and must be 

submitted to SME Publications Dept. prior to Feb. 29, 2008.

Measurement method for 


determining absorption coefficients 


for underground mines
 P. KOVALCHIK, R. MATETIC, S. PENG, G. COLE 

P. Kovalchik and R. Matetic, member SME, are manager, 

engineering noise control team and cheif, respectively, with the 

Hearing Loss Prevention Branch, National Institute for Occupational 

Safety and Health, Pittsburgh, PA; S. Peng is Charles E. Lawall 

chairman of the Mining Engineering Department, West Virginia 

University, Morgantown, WV; G. Cole, member SME, is computer 

engineer with the Pittsburgh Research Laboratory, National 

Institute for Occupational Safety and Health, Pittsburgh, PA.Paper 

number TP-07-001. Original manuscript submitted online January 

Abstract 

Previous studies conducted by the National Institute for 

Occupational Safety and Health (NIOSH) have shown that 

approximately 90 percent of coal miners and 49 percent 

of metal/nonmetal miners had a hearing impairment by 

age 50 (Franks, 1997). Mine workers are exposed to additional noise levels underground due to the reflection of 

machine-generated noise that would otherwise dissipate in 

an aboveground setting. Determining the sound-absorption coefficients is essential in modeling the sound field in 

an underground mining environment. Sound absorption 

is a measure of the amount of acoustic energy that strikes 

a surface and is absorbed rather than being reflected. The 

acoustic environment that mining machines operate in is a 

critical factor affecting the sound pressure level exposure 

for mining machine operators. Impedance tube testing is 

an inappropriate method for determining underground 

absorption coefficients due to the brittle composition of 

materials such as coal and slate. Classic absorption coefficient estimation using T60 measurements will not work 

well in an underground environment because the theory 

assumes a finite room, a diffuse field and a relatively uniform absorption. None of these assumptions are true in 

an open-ended mine entry. This paper presents a method 

using a ray-tracing technique to determine absorption coefficients for underground mines.Absorption coefficients are 

determined and presented for octave bands from 63 Hz to 

8 kHz. The absorption coefficients are essential for determining and predicting potential overexposure to machine 

operators in different mine environments.1 

1

The findings and conclusions in this report have not been 

formally disseminated by the National Institute for Occupational Safety and Health and should not be construed 

to represent any agency determination or policy. 

Introduction 

Underground miners are exposed 

to more noise than aboveground 

miners. This is because mine ribs and 

roof reflect sound that would normally be lost to the atmosphere in an 

aboveground operation. The amount 

of sound reflected is a function of 

the mine’s geometry and the type of 

material being mined. The acoustic 

environment that mining machines 

operate in is a critical factor affecting 

sound pressure levels. Underground 

mines consist of enclosed areas that 

create diffuse sound environments. 

The geometry and composition of the 

surfaces determine the number of sound rays reflected or 

absorbed, which influences the overall sound level. This 

property is known as the Sabine absorption coefficient 

(A), which is used to describe the degree of reflectivity of 

the walls, floor and roof. However, not all of the sound is 

reflected, some is absorbed by the surfaces of the walls, 

floor and roof. Determining the sound absorption coefficient is essential in describing the sound field in an underground mining environment. 

Classic absorption estimations are usually made using 

an impedance tube or by conducting T60 measurements. 

Impedance tube testing is an inappropriate method for 

determining underground absorption coefficients due 

to the brittle composition of materials such as coal and 

slate. Classic absorption coefficient 

estimation using T60 measurements 

do not work well in an underground 

environment because the theory assumes a finite room, a diffuse field 

and a relatively uniform absorption. 

None of these assumptions are true 

in an open-ended mine entry.Therefore, a new method for determining 

absorption coefficients is needed for 

underground mines. 

Concept for determining

the absorption coefficient 

The concept assumes that the 

mine entry is an “infinite” duct, i.e., 

very little of the acoustic power traveling down the duct is 

ever reflected back into the source area.To determine the 

absorption coefficient, the measurements are matched to 

a ray-tracing-based model (or image-source model) of 

the acoustic field radiated from the source in an “infinite” 

duct. 

This modeling approach utilizes a ray-tracing-based 

technique that can be used to predict sound fields in 

mines of various shapes and sizes. A computer program 

models the noise source as a point or series of points 

emitting rays of sound energy in a random or deterministic fashion. Each ray is reflected and scattered by surfaces, 

barriers and objects as it travels until it reaches a receiver. 

A contour map generated from an array of receiving 
 

 

FIGURE 1	 

Modeling plan using ray-tracing techniques. 

points predicts the sound field. 

The mine entry is modeled as a finite-length entry 

with 100 percent absorption (A= 1.0) at both ends to 

make it appear infinite (see Fig. 1). This model is used to 

calculate the sound-pressure level (SPL) at the measurement positions with varying absorption coefficients at the 

roof and ribs of an underground mine. 

Because the entry is fairly wide, approximately 4.5 m 

(15 ft) across, this method should work well, even at low 

frequencies. 

Underground measurement procedure 

The calibrated noise source (fan) is positioned near 

the end of a crosscut or the edge of the mine entry, as 

shown in Fig. 2. Figure 3 shows a picture of the calibrated 

noise source used for conducting the underground mine 

tests. 

FIGURE 2 

Measurement scheme used in determining the absorption coefficients in an 

underground mine environment. 

The noise source (fan) is placed at the center of the 

crosscut with equal distance horizontally and vertically 

between the roof, floor and ribs.The exact position of the 

noise source is recorded and documented for analysis 

procedures to determine the absorption coefficients. The 

octave-band sound power levels for the calibrated noise 

source are shown in Table 1. 

Additionally, the dimensions of the crosscut are measured and recorded. The crosscut is divided into equal 

areas or subsections and the center of each subsection 

represents a measurement location, as 

shown in Fig. 2. Several cross-sectional 

measurement planes are marked in the 

entry at increasing distances from the 

noise source and divided into six subsections. Within each subsection, the 

sound pressure level generated by the 

calibrated noise source is measured 

and recorded. A Brüel and Kjaer 2260 

Investigator (Fig. 4) is used to collect 

the octave band data. The Brüel and 

Kjaer 2260 Investigator is a handheld 

real-time octave and one-third octave band device with 

frequency analysis, statistics and logging capabilities. 

Utilizing the measurement scheme from Fig. 2 for 

underground testing, the calibrated noise source (fan) is 

placed just inside the left open end of the crosscut.A total 

of 48 octave-band measurements are performed using the 

Brüel and Kjaer 2260 Investigator for characterization of 

the acoustical mining environment. Two measurements 

are conducted at each of the 24 monitoring points within 

the crosscut. At each monitoring point, a measurement 

(bottom point) is located 0.7 m (2.3 ft) from the floor and 

the second measurement is performed 1.4 m (4.6 ft) (top 

point) from the floor. Figure 5 shows the measurement 

locations (1 through 24) and the position of the measurement planes with respect to the noise source. 

Raynoise program 

Raynoise is the core noise modeling software (LMS 

International, 2003). It uses a ray-tracing-based technique 

to calculate the noise characteristics of a given source/ 

room configuration. A key aspect of the package is that 

the noise sources and material properties are defined 

in terms of a full octave rather than a one-third-octave 

format. It has the capability of importing model information into the AutoCAD DXF format. This package also 

has command file capabilities that allow complete model 

and test definitions to be written and processed. This is 

convenient for processing a variety of 

tests in bulk and for varying parameters for a single test. It also has command-line operation capabilities that

allow it to be used as a “black box” by 

other software. 

A limitation of this package is 

that it can only make calculations 

for a specific frequency. To generate 

overall results, a model must be run at 

each octave band frequency and the 

resultant SPLs for each band are then 

combined outside of the Raynoise 

software. Another limitation is the 

breakdown of the model in the near 

field. When measurement points are 

placed too close to the noise source, 

the calculations grow increasingly inaccurate. Experience, suggests measuring points be separated by at least 

0.5 m (1.6 ft), preferably a full meter. 

Procedure for determining

absorption coefficient 

The field data are downloaded to 
 

the PC in Excel format, one file for each 

measurement point.These files are processed 

by a custom utility program to merge their 

results into one file (text format, comma delimited). If the data are in one-third octave 

format it is converted to full octave format 

with the overall value based on the logarithmic sum of the one-third octave values. 

FIGURE 3 

Calibrated noise source for underground

testing.

FIGURE 4 

Brüel and Kjaer 2260 

Investigator. 

The test conditions documented from 

the underground testing in Fig. 5 are used 

to create an equivalent underground model 

utilizing the Raynoise package. The specific 

information used for input into the Raynoise package consists of: 

• 	 the X, Y and Z coordinates of the calibrated noise source; 

• 	 the height above the floor of the bottom 

and top measurement points, as shown in 

Fig. 2; 

•	 the total length of the crosscut being 

examined; 

•	 the crosscut height and width at each 

measurement plane location; and 

• 	 the sound power output of the calibrated 

noise source in full-octave band format, 

as displayed in Table 1. 

Table 1 

Octave-band sound power levels for calibrated noise. 

 Octave band, Sound power, 

Hz dB 

63 79.8
 

125 83.9
 

250 84.7
 

500 84.9
 

1,000 87.5
 

2,000 87.7
 

4,000 85.7
 

8,000 83.6
 

Using the test information described 

above, an equivalent Raynoise model is constructed.The 

geometric information for the equivalent model is created through the development of an AutoCAD drawing 

of the crosscut and imported into the Raynoise package. 

The measurement points within the model are then defined using the underground measurement data to define 

the top and bottom measurement points for input into 

the Raynoise model. The calibrated noise source is then 

placed in the position as performed for the underground 

measurements and the sound power output is set. The 

equivalent Raynoise model is shown in Fig. 6. 

A full octave band test of the model is then processed 

using the Raynoise program.The calculated linear sound 

pressure level (SPL) results are output to a file (textbased, comma delimited) for comparison to the measured 

data. Both files (overall measured and calculated) are 

then imported into Excel. Charts are generated plotting 

measured vs. calculated SPL at each measurement point 

for each octave band SPL and the overall SPL. 

Comparing the results at each octave band, the modeled absorption coefficients are adjusted to bring the 

calculated SPL results closer to the measured and the 

model is processed again. These steps are repeated until 

the average of the error between measured and calculated has been minimized. The result is a set of octave 

band absorption coefficients for the mine entry. 

Results 

The predicted sound pressure levels were most accurate at the middle to higher frequency levels, 500 Hz 

through 4 KHz.Table 2 lists the average errors across the 

measurement points for all of the octave bands. For the 1 

through 8 KHz frequencies, these errors were essentially 

uniform across all of the measurement points. Predicted 

values closely tracked the measured values with correlation coefficients of 0.96 or better.

Table 2 

Average error between measured and calculated 


values for each octave band.
 

 Average error between

Octave band, measured and calculated, 

Hz dB 

63 2.0
 

125 1.8
 

250 2.1
 

500 1.1
 

1,000 0.6
 

2,000 0.7
 

4,000 1.0
 

8,000 1.6
 
 

■

FIGURE 5 

Measurement locations for determining acoustical properties underground. 

FIGURE 6 

Equivalent Raynoise model of field test. The diamonds represent planar measurement points.

At the lower frequencies, 63 through 500 Hz, error 

levels increased with distance from the noise source. 

The correlation between measured and predicted values 

begins to decrease dramatically from the 250 Hz band 

(correlation coefficient of 0.87) down to the 63 Hz band 

(correlation coefficient of 0.42).The poor performance at 

these frequencies may be due to the noise source output 

decaying into the background noise levels. This would 

indicate a need for a calibrated noise source with stronger 

output levels at these frequencies. The poor performance 

could also be due to additional noise from other sources 

not accounted for at the far end of the mine entry. In other 

words, the background noise may not be uniform along 

the entire entry. 

Once the data is filtered using an A-weighting algorithm, the larger error differences in the lower frequency bands become negligible due to this type of filtering. 

Therefore, the differences in measured and calculated 

sound pressure levels of the model provide a good fit, 

and the final absorption coefficients determined from the 

model are shown in Table 3. 

Table 3 

Final absorption coefficients determined from model 

runs. 

Octave-band, 

Hz Absorption coefficient 

63 0.03
 

125 0.04
 

250 0.20
 

500 0.14
 

1,000 0.15
 

2,000 0.19
 

4,000 0.28
 

8,000 0.45
 

It should be noted that, due to the background noise 

levels being so close to the noise source output levels at 

the frequencies of 63, 125 and 250 Hz frequencies, the 

confidence in the absorption coefficients for these frequencies is not very high, especially at the 250 Hz frequency.While the value noted above 

provides the best fit to the available 

data, it does not seem to follow the 

general trend. 

Conclusions 

Determining the sound absorption coefficients is essential 

in describing the sound field in an 

underground mining environment 

through computer-based modeling. 

Once the absorption coefficients are 

known, the sound level exposure 

for the operator can be predicted 

based on the acoustic characteristics 

of the environment. This technique 

provides a viable method for determining the octave band sound absorption coefficients in underground 

mines. The absorption coefficients 

will be essential for predicting possible overexposure of machine operators in different mine environments 

via computer-based modeling. 

The poor performance at the lower frequencies could be improved by 

accounting for the background noise 

levels from other sources at the far 

end of the mine entry. In addition, 

using a calibrated noise source more 

powerful in the lower frequencies 

could also help alleviate background 

noise interference. 

However, once the data is filtered 

using an A-weighting algorithm, 

these differences are diminished. 

Therefore, the differences in measured and calculated sound pressure 

levels of the model provide a good fit, and this provides a 

viable method for determining the octave-band sound absorption coefficients, not only in underground coal mines 

but in all underground mines. 

References 

Franks, J.R., 1997, “Analysis of audiograms for a large cohort of 

noise-exposed miners,” Internal Report, National Institute for Occupational Safety and Health, Cincinnati, OH, pp. 1-7. 

LMS International, 2003, Raynoise Revision 3.0, Building Acoustics and Industrial Noise Simulation, User Manual.