RI 8508 Airblast Instrumentation and Measurement Techniques for Surface Mine Blasting

Jul 10, 2000

V. Stachura, D. Siskind & A. Engler

RI 8508 Airblast Instrumentation and Measurement Techniques for Surface Mine Blasting 
 
    
 AIRBLAST INSTRUMENTATION AND MEASUREMENT
TECHNIQUES FOR SURFACE MINE BLASTING
 
 Virgil J. Stachura, David E. Siskind, Geophysicists
Alvin J. Engler, Electrical Engineer.
 
 Twin Cities Research Center, Bureau of Mines, Twin Cities, Minn.
 
 ABSTRACT
 
 The Bureau of Mines has investigated techniques and instrumentation that measure 
accurately the airblast overpressures from surface mine blasting. The results include 
equivalencies between broadband research instrumentation and commercially available 
impulse precision sound level meters measuring: root-mean-square, peak, fast, slow, 
impulse, A and C weighting, C-weighted sound exposure level (CSEL), and linear (flat) 
response. These values were obtained from field measurements and broadband FM tape 
recordings of production blasts at area and contour coal mines, limestones quarries, and 
iron mines. Frequency response was determined for 14 commercial systems.
 
 INTRODUCTION
 
 The surface mining industry has seen extensive regulation of blast effects, which has 
caused a need for uniform instrumentation and measurement techniques. Airblast is 
particularly hard to regulate because it varies widely in generation, propagation, and 
effects on humans and structures. Abnormal levels of airblast sometimes occur far from a 
surface mine, and so they can involve a much larger area than is usually associated with 
groundborne vibrations. The weather conditions can cause anamalous airblast propagation 
through focusing caused by temperature inversions and intensification from wind (7).3 The 
level and character of an airblast are also strongly affected by the degree of explosive 
confinement afforded by the burden, stemming, and geologic conditions.
 
 The general airblast can be characterized as an impulsive noise primarily in the infrasonic 
range. Most of the energy in an airblast is inaudible, because its frequency content is 
below the range of human hearing (20 Hz to 20 kHz). Airblast level can be expressed in 
decibels, with the following equation for sound pressure level (SPL):
 
 SPL = 20 log P/Po, where Po is the reference pressure 20 x 10(exp -6) N/m²
 
 or 2.9 x l0(exp -9) psi, and P is the overpressure in N/m² or psi.
 
 The reference pressure has been experimentally determined to be the threshhold of 
hearing for young listeners, at 1,000 hz. This corresponds to 0 dB. Many people can hear 
levels as much as 10 to 20 decibels lower in amplitude. Initial discomfort and pain 
thresholds for steady-state sounds are 110 and 140 dB, respectively (23).
 
 Copyright © 2000 International Society of Explosives Engineers
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 Some of the sources of airblast can be seen in figure 1. The white plume just left of center 
is the source of stemming release pulses. To the right of center, a hole has "cratered," 
transmitting more energy into the atmosphere than into breaking rock. These gas releases, 
combined with the stress wave energy transmitted from the rock and the heaving motion 
around the collar, contribute to the higher frequency portion of airblast. A general swelling 
of the shot area, including the free face, produces the "piston effect," a low frequency 
component of airblast. The high-frequency component is generally above 5 to 6 Hz, while 
the low-frequency portion is in the 0.5-to-2-Hz region (28). The phenomenon of airblast 
generation has been studied extensively by Wiss (39) and by Snell and Oltmans (33). 
 
 Airblast can be separated into two types, which are identified by their frequency content. 
Type I airblast has considerable more energy above 6 Hz than the Type II airblast. Both 
 
 Copyright © 2000 International Society of Explosives Engineers
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 types are dominated by low-frequency energy (below 2 Hz), but the former has a 
secondary band of frequencies (over 6 Hz), which is less than 15 dB below the 
low-frequency energy level. The Type I airblast is more troublesome because of its energy 
in the resonant frequency range of structures (28). Efforts to document the environmental 
effects of this acoustic energy require highly specialized instrumentation, which takes into 
account the frequencies and amplitudes generated by the source.
 
 In this report, sound pressure levels are expressed in decibels and overpressures in pounds 
per square inch (psi). Other units used in acoustics are millibars and Newtons per square 
meter (N/m²), also known as Pascals (Pa). Sonic boom levels are often expressed in units 
of pounds per square foot (psf). A conversion chart is shown in figure 2. The two 
overpressure scales are slightly offset and not symmetrical.
 
 1 psi = 6897 N/m² or Pa
1 psi = 144 psf 
l psi = 69 mb 
 
 ACKNOWLEDGMENTS
 
 The authors acknowledge the generous cooperation of the coal companies, iron mines, and 
quarries that assisted us in obtaining the data presented in this report. Special thanks are 
expressed to A. B. Andrews of E. I. du Pont de Nemours & Co. for helpful suggestions, 
and to George W. Kamperman of Kamperman Associates, Inc., for technical assistance. 
Additional thanks go to Vibra-Tech Associates, Inc., VME-Nitro Consult, Inc., and Dallas 
Instruments, Inc., for supplying instruments for frequency response testing.
 
 PREVIOUS INVESTIGATIONS
 
 The Bureau has studied the problems of airblast and instrumentation, starting as early as 
1939 (13, 22, 37-38). Most of this work involved unconfined or poorly confined blasts that 
were dominated by acoustic energy in the audible range (20 Hz to 20 kHz) and that could 
be measured by standard commercial sound measuring systems. In 1973, Siskind and 
Summers (32) surveyed airblast noise from conventional quarry blasting, using instruments 
with a variety of frequency responses. It was evident that much low-frequency energy 
(less than 2 Hz) existed but that the instruments produced distortion and ringing" from 
insufficient microphone low-frequency response. A sound system that could respond at 
0.l-Hz low-frequency was then built for subsequent studies. Airblasts could be accurately 
captured to analyze structure response, damage, and annoyance potential (28-29, 35). In 
an interim report (32), Siskind and Summers recommended that instruments have a 
frequency response of 5 Hz or lower. This recognizes that houses have natural frequencies 
in the range that will respond to infrasonic vibrations, and that such vibrations are the most 
serious airblast problem in surface mining. Prior to this, many measurements, made with 
20-Hz systems, could not be correlated to complaints or damage.
 
 The main transient noise sources that cause annoyance and damage are sonic booms, 
surface blasts, artillery, explosive testing, nuclear blast simulation, accidental explosions, 
 
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 and partially confined blasts (mining, quarrying, ditching, construction, and excavation). 
To analyze these sources, a variety of sound descriptors have been developed or adopted 
from methods that characterize steady-state noise (table 1). Some are quite complex, in an 
attempt to be all inclusive. Others involve unproven simplifying assumptions so they can 
be applied to transients in general, and blasting in particular.
 
 Kryter (18) examined sonic boom effects on structures based on peak overpressure levels, 
and also determined severity equivalencies between peak overpressures and "perceived 
noise levels" (Lpn, also labeled PNdb) from subsonic jets. Lpn levels are rms values 
calculated from the "noy" values of highest noise level in each octave (1/3 octave) band. 
The noy was derived from judgment tests of perceived loudness conducted in a laboratory. 
Noy values cannot be directly measured, so Schomer (27) described their involved 
calculations. A later study by Kryter (19) involved a more complex "effective perceived 
noise level" (Lepn) based on the largest Lpn calculated from band measurements every 
0.5 sec, including a tone correction for turbine whine. Schomer (27) summarized Kryter's 
studies and also proposed "composite noise ratings" (CNR), a 24-hour integration with a 
10-dB nighttime penalty. Schomer stated that the fear of property damage is related to 
complaints, both of which are different from psychological annoyance. The distinction is 
significant for the mining industry.
 
 A 1978 review by Schomer (26) gathered the results of a variety of sonic boom studies, 
including those of Kryter. With Lpn the most widely analyzed descriptor, Young (40) 
studied annoyance effects from unconfined impulsive sources (for example, artillery) and 
utilized the concept of "sound exposure" levels, (Ls), which are weighted rms values 
integrated over the duration of the event and normalized to 1 sec. Unfortunately, labeling 
among studies using LS has not been uniform. The C-weighted sound exposure level has 

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