Metrology for low-frequency sound and vibration

The monitoring of extreme events such as volcanic eruptions, earthquakes, tsunamis or nuclear explosions, rely heavily on the measurement of seismic activity and low frequency sound or infrasound, both in air and in the ocean. Specialised sensor technologies supporting such monitoring are well-established, but their calibration required further development, and currently lacks traceability to international system of units (SI). This project aims to establish both the first primary measurement standards for low frequency sound and vibration, over the frequency range of the applications, and a structure for effective and targeted dissemination of traceability. New calibration capabilities will primarily support the operation of global networks for environmental monitoring and research in areas such as climate change and non-proliferation of nuclear weapons. New measurement services will be launched, based on methods that will be subsequently embodied in international standards, and a series of case studies will introduce key metrology concepts such as traceability and measurement uncertainty into these applications for the first time.

Need addressed by the project

Studies of low frequency sound and infrasound propagation in the atmosphere and in the ocean play an important part of weather prediction and understanding climate change; low frequency sound and vibration phenomena have long been used as indicators of major natural events such as earthquakes, tsunamis, volcanic eruptions etc.; infrasound, low frequency seismic and ocean acoustic measurements are core technologies used for monitoring compliance with the provisional Comprehensive Nuclear-Test-Ban Treaty (CTBT); and not least, low frequency noise nuisance is a significant modern-day problem with less severe, but nevertheless widespread impact.

Despite their widespread use in vital applications for the environmental and society, infrasound and low frequency acoustic and seismic measurements are not fully covered by primary or secondary measurement standards and hence any measurement data lacks traceability, compromising reliability, value and wide acceptance. For example, the global networks monitoring seismic activity and infrasound in the air and in the oceans, which are often found in remote and inaccessible locations, are in need of a suitable calibration infrastructure and improved on-site calibration methods to raise the confidence and reliability of the data they generate. Even the more routine measurement of low frequency noise nuisance is lacking basic measurement traceability for a significant part of the frequency range of interest.


Recognising this critical deficiency, the Consultative Committee for Acoustic Ultrasound and Vibration (CCAUV) formed of the world-leading experts in these measurement technologies, has given this issue high strategic importance. The main need is for novel primary and secondary calibration methods and devices, and for transfer standards suitable for extending traceability into the field. Furthermore, inaccessibility and difficulties associated with taking sensors out-of-service just for calibration, creates a strong need for on-site methods of re-calibration to maintain ongoing traceability.

Furthermore, the environmental conditions at monitoring stations differ significantly from those found in the laboratory. Extremes of temperature and humidity and harsh geographical locations set additional challenges in understanding how sensor performance is impacted by the environme


The overall objective of the project is to extend the frequency ranges for traceable environmental measurements in the field of infrasound, underwater acoustics and seismic vibration to lower frequencies. This includes the development of the required calibration methods, the procedures for validation and dissemination, as well as the on-site transfer to the actual applications at environmental measurement stations.

The specific technical objectives of the project are:

  1. To develop primary calibration methods and devices in the low frequency range for airborne acoustics (40 mHz – 20 Hz), underwater acoustics (0.5 Hz – 100 Hz) and vibration (seismic) sensing systems (10 mHz – 20 Hz), needed for environmental measurements but not yet covered by global calibration capabilities.
  2. To develop laboratory-based secondary calibration methods for airborne acoustics (40 mHz – 20 Hz), underwater acoustics (0.5 Hz – 100 Hz) and vibration (seismic) sensing systems (10 mHz – 20 Hz) as the first step in transferring new primary calibration capability to working standard devices.
  3. To develop facilities and methods for the dissemination of traceability for airborne acoustics, underwater acoustics and vibration (seismic) sensing systems through specific methods for on-site calibrations. Improvements will be tested through a series of case studies with additional evaluation of stability, behaviour, positioning effects, installation conditions and sensitivity to the environment, leading to enhanced knowledge of system performance under operational conditions.
  4. To evaluate the outcome and impact of improvements to current global acoustic, underwater and seismic sensor networks deployment strategies gained by introducing traceable calibration and the application of measurement uncertainty principles, and to propose optimised models and parameters in the applications, leading to increased confidence in measurements.
  5. To engage with stakeholders including; regulators, sensor manufacturers, network providers, users of the traceable data, standardisation committees including ISO/TC 108/WG34, IEC/TC 29, IEC/TC 87/WG 15 and ISO/TC 43/SC 3 and authorities responsible for developing and implementing EC Directives related to the environment, to facilitate the take-up of the project results.

Progress beyond the state of the art and results

This project represents the first consolidated attempt to address the identified need across the three technologies of airborne acoustics, seismology and underwater acoustics.

Primary calibration

For all three technologies, current primary standards are limited at low frequencies by the capability of available facilities or by scientific knowledge required for their implementation. Therefore, new research will be undertaken to overcome these limitations alongside the development of novel alternative approaches that avoid such limitations altogether. In the case of microphones (which measure sound in air) and hydrophones (measuring sound in water), alternative calibration methods will be based on different types of infrasound generators developed within the project, where the sound pressure can be determined from measurable physical parameters. For seismometers (which measure vibration of solids), improved vibration excitation systems to cope with larger/heavier sensors, and novel laser-based techniques measuring the applied acceleration will be developed.

Secondary calibration

Laboratory based secondary calibration methods will be developed for airborne and underwater acoustics, and suitable sensors to act as transfer standards will be identified. Together, these will allow calibration to be transferred to sensors permanently deployed in live monitoring networks.

 RB - Sandia pipe array - hot climate

New calibration and measurement capabilities will be verified by inter-laboratory comparisons and will ultimately form the basis of new calibration services. New calibration methods will be documented through proposals for new international standards, significantly extending the scope of applications supported by the standardisation bodies.

On-site calibration

Laboratory-based calibration capability provides only half of a solution, as field calibration, or so-called on-site calibration of sensors is necessary at periodic intervals to maintain traceability. Current methods are restricted by the lack of calibrated reference devices. Therefore, through access to live monitoring stations operated by the partners in the project, methods for on-site calibration will be devised and evaluated, giving consideration to the use of actively generated test stimuli in air, underwater and in the ground, as well as passive methods using naturally occurring ambient excitation.

For all sensor types the need to understand the impact of the operating environment on performance of sensors will also be addressed. The effect of temperature, humidity and static pressure will therefore be investigated, as well as sensor-specific parameters such as the influence of water depth on hydrophone performance, all leading to the extended characterisation of the various sensor types.

Evaluation of outcome improvements

The technical developments flowing from the laboratory to the field use of the sensor systems are primarily targeted at improving data quality and therefore confidence in the information drawn from it. Improvements in data quality will be evaluated in series of case studies designed to assess the impact of the metrological aspects developed in the project. For example, case studies will be designed to illustrate the impact of traceable calibration, the estimation of measurement uncertainty and the propagation of uncertainty through the whole measurement process. The case studies will be used as input for a good practice guide capturing everything learned and the recommendations that can be made for future operation of global networks. Case studies of the impact of newly gained traceability in environmental noise pollution assessment w


Impact on industrial and other user communities

Operators of the CTBT monitoring system and its monitoring stations are expected to be early adopters of new calibration capability developed in the project, gaining measurement traceability for the first time. Their position will therefore be strengthened through increased confidence in their data and enhanced credibility overall. In this way, the project supports the United Nation’s non-proliferation policy. Similarly, the assessment of ocean noise pollution in response to international treaties (for example under the Oslo-Paris Agreement – OSPAR) and EU Directives such as the Marine Strategy Framework Directive will benefit. Confidence will improve in crucial acoustic measurements used to infer changes in the ocean temperature and polar ice coverage. Other beneficiaries include the maritime transport community, where the environmental effect of ever-increasing ship traffic has been recognised by the International Maritime Organization. Low-frequency noise and infrasound is one of the major environmental barriers to the ‘renewables’ agenda, in particular the expansion of both offshore and land-based wind energy generation. In both cases improved measurement standards for very low frequency sound will provide important confidence in environmental impact assessments, leading to better-informed decision-making for stakeholders on both sides, e.g. in the fiercely contested debates over the environmental impact of wind farms near dwellings or in certain marine habitats. There are other industries that also benefit from an improvement in low frequency noise and vibration measurement capabilities and the inherited reliability and traceability in low-frequency environmental measurements, e.g. mining and oil exploitation applications (including fracking), which rely on environmental measurements in their execution as well as for evidence of compliance with environmental regulations. Environmental and industrial noise control in general will be improved by developments in the verification of sound level meter performance at low frequencies, benefiting the general public and the workforce.

Impact on the metrology and scientific communities

The project will be closely monitored in the global acoustics and vibration measurement community and directly addresses a key strategic goal established by CCAUV for low frequency measurement standards. Uptake of newly developed methods by the National Measurement Institutes is highly likely as soon as publications are available, as are new international measurement comparisons which lead to formal recognition of particular measurement capabilities, in this case for low frequency calibration. The project will also establish the Europe measurement institutes as world leaders and innovators in these technologies. In the wider scientific community, expected benefits include better scientific understanding of the atmosphere and improved accuracy in weather forecasting. Traceable low frequency measurement will also improve the representation of gravity waves in the stratosphere and estimation of wind speed and temperature in the thermosphere, ultimately improving existing models for these upper-atmosphere regions. The benefits also spread to monitoring of climate-related phenomena such as severe weather, thunderstorms and stratospheric warming, providing for improved evaluation of long-term trends.

Impact on relevant standards

The outcomes of the research will feed into current and future international standards for primary and secondary calibration in the fields of sound in air, underwater acoustics and vibration at low frequencies. For airborne sound, standardisation on methods of low frequency calibration has already been proposed in the working group on microphones (IEC TC29/WG5) but has so far lacked the research effort necessary to make progress. This project provides exactly the research required to develop the first draft standard. International standards for sound level meters, the devices used for practical noise measurements will also adopt findings from this project. For underwater acoustics, input will be made to standards on the calibration of hydrophones (IEC TC87) and for monitoring of noise in the ocean (ISO TC43/SC3). The latter strongly supports the implementation of the EU Marine Strategy Framework Directive, where this project provides much-needed research in the low frequency range. For vibration, input will be provided to a series of international standards (for example, TC108 for the ISO 16063), to cover lower frequencies and fields of application. For example, there is a strong need for a new draft document concerning on-site calibration methods, where international interest and a desire to cooperate on this, has already been expressed.

Longer-term economic, social and environmental impacts

Economic impact of the project is achieved through new calibration methods and standard adding recognition and credibility to the performance of the sensors. Introduction of robust uncertainty estimation also adds to the usability of the sensors. Altogether the project developments will facilitate greater uptake and wider use of sensors and monitoring systems, improving sales of such systems which are predominantly developed and produced in Europe. The improved calibration methods will enable manufacturers of monitoring systems and sensors as well as of calibration equipment to more readily demonstrate that their performance meets the application requirements for the instrumentation. In terms of CTBT monitoring, enhancements to the data represents a greater return on the financial investment in establishing and operating the global monitoring system and provides stronger justification for the multi-national investment.

The project will provide a robust metrology infrastructure for low frequency measurements for environmental monitoring. In airborne acoustics and vibration, there is a delicate balance between urban development and increased noise and vibration exposure of the population, for example due to road and high-speed rail developments, which are always heavily contested on environmental grounds, and where low frequency noise and vibration is a significant contributing factor. In all cases an improved ability to measure accurately and with confidence adds scientifically robust factual details to the debate. The project therefore supports the development of strategies and local action plans for monitoring and control thereby contributing to the long-term EU goals for “Living well within the limits of our planet”.

In underwater acoustics, the field of metrology for environmental noise is relatively immature and struggles to keep pace with the rapidly evolving legislative framework. Improved ocean noise measurements will ensure that decisions are informed and underpinned by good acoustic metrology, that the environment is protected without unnecessary barriers to developments, and that existing Directives requiring monitoring can be implemented in a scientifically robust manner, with appropriately calibrated instrumentation.

Many of the environmental impacts noted above have a social impact component. For example, reduction in annoyance caused by environmental noise and vibration, has well-documented impacts on the health and wellbeing of the population, especially in terms of learning ability, sleep disturbance, mental health and hypertension, where there are known associations with heart disease, stroke and dementia. Even at the less severe end of the spectrum, improvements in quality of life are easy to appreciate.

Less obvious is the level of protection offered to society by the monitoring of illegitimate nuclear testing and the consequent international efforts to condemn and prevent further nuclear proliferation. A secondary social benefit is the use of the data for monitoring other forms of natural disaster and for climate change studies, all of which have profound social impact.