____________________________________ 

a Email address: PKovalchik@cdc.gov

b Email address: RMatetic@cdc.gov

A Measurement Method for Determining Absorption Coefficients for 

Underground Mines 

Peter G. 

Kovalchika

National Institute for 

Occupational Safety 

and Health 

626 Cochrans Mill Rd. 

Pittsburgh, PA 15236 

Rudy J. Mateticb

National Institute for 

Occupational Safety 

and Health 

626 Cochrans Mill Rd. 

Pittsburgh, PA 15236

Gregory P. Colec

National Institute for 

Occupational Safety 

and Health 

626 Cochrans Mill Rd. 

Pittsburgh, PA 15236

Adam K. Smithd

National Institute for 

Occupational Safety 

and Health 

626 Cochrans Mill Rd. 

Pittsburgh, PA 15236

 

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.

ABSTRACT 

Previous studies conducted by the National Institute for Occupational Safety and Health 

(NIOSH) have shown that approximately 90% of coal miners and 49% of metal/nonmetal miners had a hearing impairment by age 501

. Mine workers are exposed to 

additional noise levels underground due to the reflection of machine generated noise that 

would otherwise dissipate in an above ground 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 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 some 

of the 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 relatively uniform absorption. None of these 

assumptions are true in an open-ended mine entry. This paper will present 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 will be essential for determining and predicting 

potential overexposure to machine operators in different mine environments. 

1 INTRODUCTION 

Miners are exposed to more noise underground than above ground, because the 

mine ribs and roof reflect sound that would normally be lost to the atmosphere above 

ground. The amount of sound that is reflected is a function of the mine 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 are enclosed 

areas, which results in a diffuse sound environment. The geometry and the composition 

of the surfaces influence the overall sound level by the number of sound rays reflected or 

c Email address: GPCole@cdc.gov 

d Email address: ASmith9@cdc.gov 
absorbed. This property is known as the sabine absorption coefficient, α, 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 

conducting T60 measurements. Impedance tube testing is an inappropriate method in 

determining underground absorption coefficients due to the brittle composition of the 

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 relatively uniform absorption. None of these 

assumptions are true in an open-ended mine entry. Therefore, another method for 

determining absorption coefficients is needed for underground. 

2 CONCEPT FOR DETERMINING 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. In 

order to determine the absorption coefficient, the measurements are matched to a raytracing based model (or image source model) of the acoustic field radiated from the 

source in an “infinite” duct. 

The 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 points predicts the sound field. 

The mine entry is modeled as a finite-length entry with 100% absorption (α =1.0) 

at both ends to make it appear infinite (see figure 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. 

Since the entry is fairly wide (approximately 15ft across) this method should work 

well, even at low frequencies. 
Figure 1: Modeling plan using ray tracing techniques. 

UNDERGROUND MEASUREMENT PROCEDURE 

- Microphone

- Noise source *

Width (w)

Height (h)

*

0 10 20 30 40 50 5

Measurement planes

View across one 

measurement plane 

Source

ft

Crosscut

Figure 2: Measurement scheme used in determining the absorption 

coefficients in an underground mine environment. 

The calibrated noise source (fan) is positioned near the end of a crosscut, or the 

edge of the mine entry as shown in figure 2 above. Figure 3 shows a picture of the 

calibrated noise source used for conducting the underground mine tests. 
Figure 3: Calibrated Noise Source for 

Underground Testing 

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 below. 

Octave-band (Hz) Sound Power (dB)

63 79.8 

125 83.9 

250 84.7 

500 84.9 

1000 87.5 

2000 87.7 

4000 85.7 

8000 83.6 

Table 1: Octave-band Sound Power Levels 

for Calibrated Noise Source 

Additionally, the dimensions of the crosscut are measured and recorded. The 

crosscut is divided into equal areas or sub-sections and the center of each sub-section 

represents a measurement location as shown in figure 2 above. Several cross-sectional 

measurement planes are marked in the entry at increasing distances from the noise source 

and divided into six sub sections. Within each subsection, the sound pressure level 

generated by the calibrated noise source is measured and recorded. A Brüel and Kjaer 

2260 Investigator is used to collect the octave band data and is shown in figure 4 below. 

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. 
Figure 4: Brüel and Kjaer 

2260 Investigator 

Utilizing the measurement scheme from figure 2 for underground testing, the 

calibrated noise source (fan) is placed just inside the left open end of the crosscut. A total 

of forty-eight, 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 twenty-four monitoring points within the 

crosscut. At each monitoring point, a measurement (bottom point) is located 0.7 meters 

from the floor and the second measurement is performed at 1.4 meters (top point) from 

the floor. Figure 5shows the positions of the measurement locations, one thru twentyfour, and the position of the measurement planes with respect to the noise source. 

Figure 5: Measurement Locations for Determining Acoustical Properties Underground 

3 RAYNOISE PROGRAM 

This is the core noise modeling software.2 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 full 

octave, not 1/3 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 SPL’s 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 

1⁄2 meter, preferably a full meter. 

4 PROCEDURE FOR DETERMINING ABSORPTION COEFFICIENT 

The field data is downloaded to the PC in Excel format, one file for each 

measurement point. These files are processed via a custom utility program to merge their 

results into one file (text format, comma delimited). If the data is in 1/3 octave format it 

is converted to full octave format with the overall value based on the logarithmic sum of 

the 1/3 octave values. 

The test conditions documented from the underground testing in figure 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: 1) the X, Y, and Z 

coordinates of the calibrated noise source; 2) the height above the floor of the bottom and 

top measurement points as shown in figure 2; 3) the total length of the crosscut being 

examined; 4) the crosscut height and width at each measurement plane location and 5) the 

sound power output of the calibrated noise source in full-octave band format as displayed 

in table 1. Using the test information described above, an equivalent Raynoise model is 

constructed. The geometric information for the equivalent model is created thru 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 Figure 6. 
Figure 6: Equivalent Raynoise model of field test. The diamonds represent planar 

measurement points. 

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 

(text-based, 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. 

5 RESULTS 

The predicted sound pressure levels were most accurate at the middle to higher 

frequency levels, 500 Hz through 4 KHz. Table 3 lists the average errors across the 

measurement points for all of the octave bands. For the 1 KHz 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 .96 

or better. 

Octave-band (Hz) Average Error Between 

Measured and Calculated (dB) 

63 2.0 

125 1.8 
250 2.1 

500 1.1 

1000 0.6 

2000 0.7 

4000 1.0 

8000 1.6 

Table 3: Average Error Between Measured and 

Calculated Values for Each Octave Band. 

At the lower frequencies, 63 Hz through 500 Hz, error levels increased with 

distance from the noise source. The correlation between measured and predicted begins 

to decrease dramatically from the 250 Hz band (correlation coefficient of .87) down to 

the 63 Hz band (correlation coefficient of .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 utilizing 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 4. 

Octave-band (Hz) Absorption Coefficient

63 .03 

125 .04 

250 .20 

500 .14 

1000 .15 

2000 .19 

4000 .28 

8000 .45 

Table 4: Final Absorption Coefficients 

Determined from Model Runs 

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. 
6 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 provides a viable method for 

determining the octave band sound absorption coefficients in not only in underground 

coal mines, but all underground mines. 

7 REFERENCES 

[1] Franks, J. R.. 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. 

[2] Raynoise Revision 3.0, Building Acoustics and Industrial Noise Simulation, User 

Manual, LMS International