Acoustic Modelling

Cost effective and operational shipping noise analysis

Shipping brings perhaps the most significant contribution to the total noise budget in the oceans of the world, as daily tens of thousands of vessels navigate across the oceans. Shipping noise is caracterised by low frequencies, with most energy below 200 Hz, which propagates at very long ranges in deep waters. This makes the direct observation of shipping noise impractical, and therefore one must rely on tools based on acoustic propagation modelling.

The report of the Tecnical Subgroup on Underwater Noise mentions the need for modelling in order to get a complete picture of the distribution of sound in the entire marine environment, as monitoring by direct measurements only is not practicable. Acoustic propagation models incorporate in-situ and historical measurements to obtain coherence between observation and modelling outputs.

shippingnoise.com is an informal initiative that aims at the development of a platform that fuses data of different nature to generate dynamic shipping noise maps in quasi real-time. At the scale of years this would allow for establishing indicators such as sound exposition levels (SEL), over space and time of a given area, and to learn how it evolved over time.

Technological development

Compact Digital Streaming and Autonomous Recording Hydrophones

As mentioned in the report of the Tecnical Subgroup on Underwater Noise, widespread monitoring of the trends in ambient noise level is likely to be costly. The suggestion is to perform targeted measurements in areas of known shipping lanes, near any offshore installations that generate noise and low pressure areas for comparison and determination of the trends in shipping noise. This implies to systematically cover large ocean volumes, however maintaining a cost effective measurement. This could be carried out by setting up new autonomous observatories or by using existing installations for the co-location of monitoring devices. According to the TSG, a deployment should ideally be able to stream data to a shore base, stating that such systems are not readily available at this time.

MarSensing Lda. has developed devices, like the digitalHyd SR-1 and digitalHyd DA-1, able to respond to the recomendations of the TSG in a cost effective way. These devices are able to record signals over a large acoustic bandwith (50kHz) using up-to 24-bit for a greater dynamic range with user programmable gains in the pre-amplifier.
The digitalHyd SR-1 was designed as a fully autonomous recorder, hand deployable, with local storage of acquired data.
The digitalHyd DA-1 is a step forward towards meeting such requirement, with focus on real-time streaming of raw data, data reduction through pre-processing, modularity in the analog signal acquisition circuitry and choice of communication interface. These features permit the digitalHyd DA-1 to be a versatile device for integration into large scale cabled observatories, autonomous observatories, buoys or as a standalone autonomous device. With additional features such as the possibility of onboard data storage, data processsing, and data transmission in real-time or differed-time.

Active Research Projects

The RoboNoise project - Aquatic vehicles in noise monitoring activities

The RoboNoise project, linked to noise in deep water and specifically to shipping noise, aims at testing the possibility to employ aquatic vehicles in noise monitoring activities. The objectives of the project are:

• to acquire acoustic data in deep waters near shipping routes.
• to establish contributions of shipping noise to the total noise.
• to test the feasibility of acoustic acquisition with aquatic vehicles in terms of spatial coverage, autonomy of aquatic vehicles, acoustic data significance (when acquired over a established trajectory).
• to establish the impact of towing acoustic recorders on the quality of the acoustic data (e.g. generation of flow-noise and surf-noise).