Every blast releases energy into the surrounding area. Part of it travels through the ground as seismic waves, producing ground vibration. Another part travels through the air as pressure waves, producing air overpressure.
The two phenomena travel differently. Ground vibration moves through rock and arrives first. Air overpressure follows at the speed of sound in air, about 340 m/s—at larger distances, it arrives seconds after the ground motion.
Both are felt long before they damage anything. Air overpressure combines an audible component and a low-frequency component below 20 Hz felt as a concussion, which makes windows and doors rattle. Ground vibration acts directly on foundations. At sufficient levels, both can damage structures, but people perceive them at far lower levels—which is why annoyance occurs long before any risk to property.
Managing these effects starts with measuring them correctly. Reliable measurement data is the basis for assessing compliance, investigating complaints, and protecting noise-sensitive places.
One phenomenon, many names. Air overpressure, airblast, airblast level—different standards, same quantity: peak pressure in dB against the common 20 µPa reference
Air overpressure is dominated by inaudible low-frequency energy, so it must be measured with a flat (linear/Z) response, never dB(A).
Limits differ, measurement logic doesn’t. All major standards require a peak value on a flat response down to ~2 Hz — only the limit values change. That’s why one ISEE-2022 meter like the SV 810 works across jurisdictions.
Calibration is at two stages. Annual traceable calibration is the lab’s job; the operator does the field verification with a Class 1 calibrator (SV 37, 124 dB / 250 Hz) before the shot—a blast has no retake.
Placement and context decide whether the number is valid evidence. The microphone must be exposed to the blast and clear of reflecting surfaces—screening cannot reduce overpressure, and a nearby wall inflates the reading—while distance and weather should be recorded.
Air overpressure is the pressure wave released into the atmosphere by a detonation. As the wave passes, air pressure rises sharply, falls more slowly, then settles back to ambient. Its peak value is the quantity measured, expressed in pascals and usually reported in dB(lin).
The wave has two parts. The part above roughly 20 Hz is audible—this is blast noise, what people hear. The larger part lies below 20 Hz and is felt rather than heard—this phenomenon is concussion. Concussion exerts force on structures and rattles windows, an effect often confused with ground vibration. Air overpressure is the full physical quantity; blast noise is only its audible part.
The same phenomenon appears under several names, and this variation is a common source of confusion. Different standards, from different periods and jurisdictions, use different terms, units, and conventions for the same pressure wave.
BS 6472-2:2008 uses the term air overpressure. Australian guidance and AS 1259.1 say “airblast overpressure” or “airblast level.” Some references split the terms by frequency, while noting that airblast is conventionally applied to the whole wave. This article uses air overpressure throughout.
The level appears in pascals, decibels, or psi. Older North American sources use psi; current standards report dB(lin).
From pressure to decibels: dB = 20 log(P / 2×10⁻⁵ Pa), or in US units, dB = 20 log(psi / 2.9×10⁻⁹), where 2.9×10⁻⁹ psi is simply 20 µPa converted. The logarithm compresses the scale: every +20 dB means ×10 in pressure, and +6 dB means roughly double. Examples:
| dB(lin) | Pa | psi | Reference |
|---|---|---|---|
| 115 | ~11 | 0.0016 | Australian 9-of-10 limit |
| 120 | 20 | 0.0029 | Australian cap; ISEE trigger starting point |
| 128 | ~50 | 0.0073 | Ontario NPC-119 guideline |
| 134 | 100 | 0.0145 | USBM/OSMRE safe maximum |
| 150 | ~6300 | 0.0920 | poorly mounted window can crack |
Air overpressure is dominated by low-frequency energy, so it is measured with a flat response, not the dB(A) used for ordinary noise. Older blasting standards call this flat response linear, dB(lin). Modern meters built to IEC 61672 call the same response “Z-weighting” (“zero”), dB(Z). The two are identical. The Australian guidance confirms this equivalence directly, defining its dB (Linear) Peak as the linear scale (Z), or unfiltered.
The first is the frequency band, set by the high-pass cutoff—the lowest frequency the instrument captures. USBM RI 8485 ties its safe limits to the band of the flat-response system used: 134 dB for 0.1 Hz, 133 dB for 2 Hz, and 129 dB for 6 Hz.
The limits differ because a wider band captures more of the low-frequency energy and reads a higher level for the same blast. (RI 8485 also lists a legacy alternative of 105 dB measured on a C-weighted, slow-response sound-level meter — a different method entirely, kept for ordinary meters that lack a flat low-frequency response; it is not part of the linear-system series.)
The second is the amplitude (dynamic) range — the span of levels the instrument records without overload. The second is the amplitude (dynamic) range — the span of levels the instrument records without overload. This parameter decides whether the reading is trustworthy at the extremes. If the blast exceeds the top of the range, the signal clips and the recorded peak is false — it shows the instrument’s ceiling, not the blast. If the level sits near the bottom of the range, it drowns in the instrument’s own noise floor. The range must therefore cover both the expected blast level and the limit it will be judged against, with a margin.
Its frequency range is 2 to 250 Hz, in line with ISEE 2022 — that is the band. Its weighting is linear (Z) — the flat response. Its dynamic range is 105 to 168 dB peak (3.6 Pa to 5 kPa)—that is the amplitude span. The regulatory limits (115–134 dB) sit in the middle of that span, the internal noise stays below a 95 dB peak, and the 168 dB ceiling leaves headroom for a close, unexpectedly strong blast. If the ceiling is ever exceeded, the meter reports it explicitly as “Overload Time” in the results, so a clipped event cannot pass as a valid peak.
Air overpressure is governed by three different types of standards:
| Country | Standard | Reported as | Lower frequency limit | Limit |
|---|---|---|---|---|
| United States | USBM RI 8485 | dB peak, linear | 0.1, 2 or 6 Hz (limit stated per system) | 134 dB @ 0.1 Hz; 133 dB @ 2 Hz; 129 dB @ 6 Hz |
| United Kingdom | BS 6472-2:2008 | dB(lin) | 2 Hz | no fixed limit; typical UK quarry levels ~120 dB(lin) |
| Australia | AS 1259.1 / AS 2187.2; EM2402 (QLD) | dB(Linear) Peak = linear (Z) scale | 2 Hz (−3 dB) | 115 dB for 9 of 10 blasts; 120 dB at any time |
| Canada (Ontario) | NPC-119 | dB(lin) | — | 128 dB guideline |
These are written for manufacturers and calibration labs, not blasting operators. Three documents matter, and they largely converge:
The convergence is the point: the US and Australian specifications describe essentially the same microphone—2 to 250 Hz, flat, ±1 dB. An operator’s job here is one check: does the meter carry the qualification the local rules ask for? The SV 810 is built to ISEE 2022 and uses linear (Z) weighting, which answers the check for the standards above.
These bind the limit to a defined method: microphone position and height, exposure, and what to record. EM2402 embeds its procedure next to its limit (≥4 m from structures, 1.2–1.5 m height, and full event record). The ISEE Field Practice Guidelines serve the same role for ISEE-type monitoring. A limit is only enforceable against a measurement taken per its procedure.
ISEE defines two sensors separately: a ground vibration sensor (Part II) and an air overpressure microphone (Part III). These minimum response characteristics are what an instrument must meet for its data to compare correctly against the limits.
The sensor must respond across 2 to 250 Hz—the band that carries blast vibration energy. Accuracy is tightest in the main band: ±5% or ±0.5 mm/s (whichever is larger) from 4 to 125 Hz. At the edges (2–4 Hz, 125–250 Hz) the tolerance opens to +5% to −3 dB, because sensors naturally roll off at band limits.
Phase response is controlled: phase shift from 2.5 to 250 Hz must not distort the sum of two overlapping harmonic signals by more than 10%. A blast wave contains many frequencies at once; a phase error would misstate the peak.
Cross-talk between the three perpendicular axes must stay below 5%, so energy on one axis does not leak into the others.
Density should be below 2405 kg/m³ and reported—a sensor heavier than the ground couples poorly and distorts the motion it should follow.
The microphone covers the same 2 to 250 Hz band, with a tighter amplitude tolerance: ±1 dB from 4 to 125 Hz, with defined roll-off at the edges (−3 dB ±1 dB at 2 Hz).
A separate requirement guards against contamination: the microphone’s response to a 50 mm/s mechanical vibration at 30 Hz, from any angle, must be at least 40 dB below its maximum output, or below 106 dB. In short, the microphone must hear the pressure wave and ignore the ground motion arriving at the same point.
Calibration works on two levels:
The unit goes back once a year; the facility does the rest. Each sensor is calibrated before first use, annually after that, and after any repair affecting its response. Reference equipment must be traceable to NIST, NRC, or equivalent. The facility checks what drifts—frequency, amplitude, and phase responses—while stable characteristics (cross-talk, density, and seismic sensitivity) come from the specification sheet. The output is a calibration certificate.
A quick check before and after the measurement confirms the instrument still reads correctly. For the microphone, this is done with a Class 1 acoustic calibrator, such as the SV 37 at 124 dB and 250 Hz: place it on the microphone, compare the reading with the reference, and any deviation flags a problem before the data is taken, not after. This is a check—it confirms the sensor works; the annual calibration certifies its accuracy.
The geophone cannot be field-checked the same way, because a traceable mechanical motion needs a shake table. Between annual calibrations the operator relies on the instrument’s built-in self-check.
A phase error misstates the peak even when every amplitude is correct. A blast wave is many frequency components arriving together, and PPV is the peak of their combined waveform. If the electronics delay some frequencies more than others, the components recombine in the wrong time relationship, and the recorded peak is wrong—too high or too low.
ISEE 2022 controls this: phase shift from 2.5 to 250 Hz must not distort the sum of two overlapping signals by more than 10%, checked against a 30 Hz reference. The requirement applies to the three ground vibration channels, where it also keeps inter-axis timing correct so the vector sum is valid. It is not specified for the microphone, which records a single pressure channel as a peak level.
Weather changes the measured overpressure, so the conditions are part of the measurement. Ground vibration travels through rock and is largely unaffected. Air overpressure travels through the atmosphere—temperature, lapse rate, cloud cover, humidity, and wind all change the level at a given point.
Two conditions matter most. A temperature inversion or heavy low cloud bends the waves back toward the ground and raises levels in the surrounding area—under inversion, the peak can rise by a factor of 5 to 10 for the same blast. A wind blowing from the blast toward a receptor raises the reading there and lowers it upwind.
Two practical consequences follow. First, record the conditions with every event: temperature, gradient, cloud cover, humidity, wind speed, and direction—120 dB under a strong inversion and 120 dB on a calm day do not mean the same thing. Second, when a result looks unexpectedly high, weather is the first thing to check.
Placement rules differ between standards, but all serve two goals: capture what reaches the receptor, and avoid anything that distorts the reading. The two sensors fail differently, so they follow different logic.
A poorly coupled sensor reads PPV too high and frequency too low. The sensor is placed on or in the ground, between the structure and the blast, on the side facing the blast — never on loose fill, slabs, or grass.
The standards agree on the principle and differ on detail:
A wave reflected off a nearby surface reads high. The microphone must face the blast, stay clear of walls, and remain exposed.
A measurement produces a few key numbers. Each describes a different thing.
Ground vibration
Air overpressure
Ground vibration and overpressure are independent — a low PPV can sit beside a high overpressure, and each is judged against its own limit. And a single value means little without context: distance, point location and weather all shape it and all belong in the record.
A microphone measures a pressure wave, so exposure and reflections are everything. Place it exposed to the blast and clear of walls—a reflected wave reads high. Note that screening does not help: BS 6472-2 states that baffles and mounds do not significantly reduce air overpressure because the dominant low-frequency energy passes around obstacles. You cannot shield the microphone into compliance; you position it to represent the receiver. ISEE provides the concrete rule: within 3.05 m of the structure, or less than 10% of the distance to the blast, whichever is smaller.
Distance drives attenuation. For example, the same blast read 117 dB at 434 m but 108 dB at 610 m—identical charge, different distance. Distance also affects timing, because overpressure arrives late. For an overpressure-only measurement, the ISEE rule is simple: set the recording time to at least 2 seconds more than the blast duration so the tail is not cut off.
Set the trigger just above local background: low enough to catch the blast, high enough to avoid false events. ISEE provides a concrete starting point of 20 Pa (120 dB). Then set the threshold results to the local limit—the instrument then flags an exceedance at the moment of the blast, not days later in the office.
A blast is a one-off; there is no retake. Before the shot, run a field check with a Class 1 acoustic calibrator (e.g., the SV 37, 124 dB / 250 Hz). Secure all cables — a loose cable moving in the wind produces microphonics and false triggers. Finally, confirm the meter meets the local system requirements—the SV 810’s 2–250 Hz range satisfies the band requirement.
An authorized SVANTEK consultant will help You with the details such as the required accessories for your noise monitoring task.
