What is Acoustic Fingerprint?

Acoustic Fingerprint is an advanced noise event detection system, available within the optimus green sound level meters from Cirrus Research.

The system allows templates to be created from a number of rules which can be combined to create anything from very simple level based triggers to a sophisticated set up which can use tonal noise, rate of change and level based rules.

The optimus green sound level meters can hold up to 5 independent templates, any or all of which can be active at any time, and up to 20 independent rules. A template contains rules and can be made up from a single, simple rule or a combination of different types of rules.

This allows the optimus green sound level meters to be used to detect noise events and then take certain actions which are described below.

This advanced detection methodology enables the optimus green sound level meters to be used in a wide range of applications where the recording of noise levels along with audio recordings is required.

When used with the CK:680 outdoor measurement kit, the optimus sound level meter can communicate remotely over a 3G/GPRS network and provide both data download and live noise information to the NoiseTools PC. GPS location data is also provided allowing noise measurements to be stamped with the location and time/date.

Detection Rules

Three different detection rules are available (depending upon instrument type) and these can be used either on their own or combined to develop a sophisticated detection template.

Each of these rules is covered briefly below. For a more detailed explanation and examples of these templates, please refer to Technical Noise 28 – Audio Recording with the optimus sound level meters.

The threshold (or level) rule is the simplest of the three available. It uses a simple noise level in dB as the starting point, for example “Trigger when the LAeq is above 75dB for more than 5 seconds”.

This can be used, for example, to trigger a recording when a train passes or when a noise is detected within a frequency band.

Example: Trigger if the LAeq is above 75dB for more than 5 seconds
Example: Trigger is the LAS goes below 30dB

The Rate of Change rule is very different to the Threshold rule. It is used to trigger recordings when the level changes at a certain speed and removes the link between level and triggering.

The rate of change is the “speed” at which the sound level changes, and it is usually expressed in dB per second (dB/s). It shows how fast the sound is changing.

Example: Trigger if there is an increase in the LAeq of more than 10 dB in 5 sec (10dB/5sec)
Example: Trigger if the LAeq has increased by at least 10dB since 5 seconds ago

The Tonal Noise rule allows recordings to be triggered when a tone is detected according to either the ISO 1996-2:2007 Simplified Method or the Cirrus improved method

This feature is available on the CR:171C & CR:172C instruments.

Example: Trigger if a tone is detected in any band
Example: Trigger is a tone at 3,15kHz is detected

Actions & Alerts

When a template within the system has been triggered, there are a number of actions that can take place, depending upon the instrument and any accessories to which is it connected.

Each time a template is triggered the noise profile, or time history data, is marked and this can be seen when the data is downloaded to the NoiseTools software.

This is enabled as default and provides a range of very useful information which can be used for further analysis and reporting.

In a standard optimus green sound level meters, the most basic action is to start an audio recording when a template is triggered.

When the criteria for the event have been met, the recording will start and continue until those conditions are no longer met or the maximum duration has been exceeded. Pre and post triggers are available along with maximum durations.

Audio recordings can be made using either standard quality, which provides 16bit/16kHz WAV files, or studio quality, which provides 32bit/96kHz WAV files.

When the optimus sound level meter is used with the CK:680 outdoor kit, the instrument can send an SMS or Text message, via the optimus cloud system, to a range of different numbers when a template is triggered.

When the optimus sound level meter is used with the CK:680 outdoor kit, the instrument can send an email , via the optimus cloud system, to a range of different addresses.

When the optimus sound level meter is used with the CK:680 outdoor kit, the instrument can send an Tweet , via the optimus cloud system, to a range of different addresses.


To find out more about how Acoustic Fingerprint can help with your noise measurements, call us on 0845 230 2434 or click here and send us a message.