Why the "SPL" from an Acoustic Camera Is NOT the Same as from an IEC 61672 Sound Level Meter: A Technical Guide

Introduction: The Common Misconception About Acoustic Camera SPL

If you've ever used an acoustic camera like those from HERTZINNO, you've noticed that the software interface displays SPL (Sound Pressure Level) values alongside colorful sound maps. It's tempting to treat these numbers as you would the reading from a precision sound level meter. But technically speaking, this would be a mistake.

The International Standard IEC 61672 (and its equivalent Chinese standard GB/T 3785) defines the performance requirements for sound level meters with extreme precision—particularly regarding frequency weightings and frequency response tolerances. When we examine these requirements against the physics of acoustic camera operation, the fundamental incompatibility becomes clear.

First, a clarification: Sound level meters measure equivalent continuous sound level (Leq)—the average level over time—as well as instantaneous SPL with specific time weightings (Fast/Slow). Acoustic cameras typically display instantaneous or short-term RMS SPL corresponding to each video frame. However, whether it's Leq or SPL, if it comes from an acoustic camera, it cannot be considered a certified measurement traceable to IEC 61672.

Let's break down exactly why the SPL displayed on an acoustic camera is a reference value—useful for comparison, but not equivalent to a certified sound level meter measurement.

Part 1: What IEC 61672 Requires from a Sound Level Meter

1.1 Frequency Weightings: A-Weighting, C-Weighting, and Z-Weighting

IEC 61672 requires sound level meters to implement specific frequency weighting networks that simulate how the human ear responds to different frequencies :

When you see "LAeq" or "LAspl" on a compliant sound level meter, it means: The sound energy has been filtered through an A-weighting network with mathematically defined precision, and the result falls within strict tolerance limits at every frequency.

Important limitation: These weighting networks are designed only for the audible range (typically 20 Hz – 20 kHz). For frequencies above 20 kHz (ultrasonic), IEC 61672 defines no weighting characteristics, so traditional sound level meters themselves cannot effectively measure ultrasound.

1.2 Frequency Range and Tolerances: Class 1 vs Class 2

This is where the difference becomes stark. IEC 61672 divides instruments into Class 1 (precision) and Class 2 (general purpose), with specific requirements :

Class 1 Sound Level Meter:

• Frequency range: 10 Hz to 20 kHz

• Tolerance at 1 kHz reference: ±0.7 dB (expanded to ±1.1 dB including measurement uncertainty)

Class 2 Sound Level Meter:

• Frequency range: 20 Hz to 8 kHz (minimum)

• Tolerance at 1 kHz reference: ±1.0 dB (expanded to ±1.4 dB)

Note: Neither Class 1 nor Class 2 sound level meters cover frequencies above 20 kHz. Attempting to measure ultrasonic signals with an IEC 61672 compliant meter is outside the standard's scope.

These tolerances are verified in an acoustic laboratory using calibrated reference sound sources, with the meter positioned to minimize reflections—typically using an extension cable to distance the microphone from the body .

Part 2: Why Acoustic Cameras Cannot Meet These Requirements

2.1 Physical Structure Destroys the Free Field

A HERTZINNO acoustic camera consists of:

• An array of dozens to over a hundred MEMS microphones

• A rigid mounting plate holding them in precise geometric arrangement

• A central optical camera for visual overlay

• Processing electronics and housing

The problem: This structure is acoustically "large" and highly reflective. Sound waves approaching the array diffract and reflect off the surface before reaching the microphones. The very geometry that enables spatial localization (beamforming) inevitably distorts the frequency response at each individual microphone.

Contrast with a sound level meter:

• Slender, conical shape

• Microphone at the end of an extension cable

• Minimal reflective surface area

• Designed to be acoustically "invisible"

An acoustic camera cannot achieve the free-field response required by IEC 61672 because the device itself modifies the sound field it's trying to measure.

2.2 Beamforming Algorithms vs. Single-Point Measurement

A sound level meter measures sound pressure at a single point in space, applying filters that exactly match standardized weighting curves. It outputs instantaneous SPL or time-averaged Leq for that point.

An acoustic camera measures sound pressure at multiple points (each microphone), then applies beamforming algorithms to calculate:

• Phase differences between microphones

• Time delays of arrival

• Spatial distribution of sound sources

These algorithms optimize for source localization, not for preserving absolute amplitude accuracy. The process of beamforming involves spatial filtering that changes the signal characteristics. The SPL value displayed is derived from a focal point in the reconstructed sound field—introducing uncertainties unacceptable under IEC 61672.

2.3 Calibration Complexity

Sound level meter calibration:

• Single microphone

• Calibrated with an acoustic calibrator at one or more frequencies

• Traceable to national standards

• Simple and verifiable

Acoustic camera calibration:

• Every microphone must be phase-matched and amplitude-matched

• Calibration must account for acoustic scattering of the array structure

• Spatial calibration is as critical as amplitude calibration

• No single-point calibration can guarantee IEC 61672 compliance across all frequencies

Part 3: The Ultrasonic Range — Where IEC 61672 Is Completely Inapplicable

3.1 Acoustic Cameras Excel at Ultrasound Detection

Many industrial applications produce acoustic energy concentrated in the ultrasonic range (typically 20 kHz to 100 kHz):

• Gas leak detection: Pressurized gas escaping through small orifices generates ultrasonic noise 

• Partial discharge detection: Corona, floating, and surface discharges emit ultrasonic frequencies 

• Bearing fault diagnosis: Early-stage bearing failures produce distinctive ultrasonic signatures 

This is where HERTZINNO acoustic cameras excel—using wideband MEMS microphone arrays and tailored beamforming algorithms to visualize and locate ultrasonic sources. The frequency range of modern acoustic imagers typically extends from 1 kHz to 100 kHz or beyond .

3.2 Why Weighting Curves Don't Apply Above 20 kHz

IEC 61672 and ultrasound are fundamentally unrelated because:

• Frequency range not covered: The standard's upper limit is 20 kHz (Class 1) or 8 kHz (Class 2)

• Weightings not defined: A, C, and Z curves are undefined above 20 kHz

• No legal metrology basis: No mandatory standards based on IEC 61672 exist for ultrasonic exposure

Therefore, when a HERTZINNO acoustic camera displays SPL values in ultrasonic mode, these numbers are completely outside the scope of IEC 61672. They help users compare signal strengths across different areas in the ultrasonic image—not provide traceable sound pressure measurements.

Part 4: What the SPL on a HERTZINNO Acoustic Camera Actually Means

4.1 Reference Values for Comparative Analysis

The SPL displayed on a HERTZINNO acoustic camera serves one primary purpose: comparative analysis. When you see a red "hot spot" at 78 dB SPL and a blue area at 65 dB SPL, you know the red area is significantly louder. This allows you to:

• Prioritize which noise source to address first

• Compare different frequency bands to characterize the sound

• Track changes over time at the same location

• Identify developing faults in rotating equipment 

As noted in the FLIR Si2 user manual: "When comparing the SPL produced by different bearings, the bearings that are in bad condition will show a significantly higher SPL reading than bearings in good condition" .

But you cannot:

• Use this value in a legal environmental noise report

• Compare it directly to regulatory limits (e.g., 85 dB occupational exposure limits)

• Claim it meets any national or international metrology standard

4.2 SPL vs. Leq: Understanding the Time Dimension

Sound level meters output:

• Instantaneous SPL (very short time constant)

• Leq (energy average over a specified period)

Acoustic cameras typically update at video frame rate, displaying SPL corresponding to instantaneous or short-term RMS for each imaging frame. These values are subject to the same physical and algorithmic influences as the spatial image, and therefore lack legal metrological significance.

Class compliant measurement systems (those meeting IEC 61672) are required when "you need to prove in court that your levels were not in violation of a noise ordinance" . Acoustic cameras are not designed for this purpose.

Part 5: Practical Applications Across Industries

5.1 Power Utilities: Partial Discharge Detection

When using a HERTZINNO acoustic camera to locate partial discharge on high-voltage equipment, the absolute SPL value matters less than:

• The spatial location of the sound source

• PRPD patterns identifying discharge type

• Changes in sound level as you approach the source

Even without legally traceable decibel numbers, you can confidently repair based on what you see. Modern acoustic imagers support PRPD analysis for discharge type classification .

5.2 Petrochemical Plants: Gas Leak Localization

A leaking valve showing 65 dB SPL on the acoustic camera versus 58 dB background noise—that 7 dB difference flags the leak for repair. The absolute value might differ from a precision sound level meter by ±2 dB, but you've found the leak .

Recent industry applications demonstrate detection efficiency improvements of over 60% compared to traditional soap bubble methods .

5.3 NVH Engineering: Bearing Fault Diagnosis

Comparing noise from different motor components: the acoustic camera shows which bearing produces the whining sound. Relative levels guide diagnosis, supported by additional metrics like Crest Factor and Kurtosis that indicate bearing condition .

Conclusion: Two Tools, Two Standards, One Goal

IEC 61672 exists to ensure sound level measurements in the audible range are consistent, repeatable, and legally defensible. Acoustic cameras solve a different problem: finding sound sources quickly and intuitively—audible or ultrasonic.

At HERTZINNO, we design acoustic cameras to excel at localization—with clear visual feedback, intuitive software, and robust hardware for industrial environments. The SPL values we display help you compare and prioritize, but we never claim they replace certified sound level meters.

If you need to prove compliance with noise regulations, use an IEC 61672 sound level meter. If you need to find partial discharge, gas leaks, or bearing faults before failure, use a HERTZINNO acoustic camera.

Two tools. Two standards. One goal: a safer, more efficient industrial world.