Friday, 2 December 2016

Catch you on the flip side (of sound)

Particle motion - not all animals are pressure sensors

Mammalian ears are pressure sensors, indeed the ears of most animals that spend some time in air are. We sense pressure fluctuations in our environment and our brains translate those to a perception of hearing sound. Our hearing system really is very elegant and interesting (it performs realtime, mechanical Fourier Transforms!), but that isn't really a topic for this blog.

The catch is that to be a good pressure sensor you need to be able to detect pressure. We do this by having lots of compressible air in our middle ear, so that pressure from the outside can make our eardrum vibrate. But everyone who's ever tried to dive knows that having all that compressible air in your head can be a problem!

Many fishes and all invertebrates (as far as I know) do not have any air-filled cavities associated with their ears, so they do not suffer from aural problems when diving. But this lack of air in the ear means that it must be mostly full of water, water that does not like to be compressed. in fact water is over 16,000 times harder to compress than air...

Luckily (for those guys), sound pressure waves travel through fluids by having molecules push each other in an orderly, sequential fashion, see the middle example in the video below. 


video


All the particles oscillate around a fixed point, transmitting the signal/pressure wave by pushing the next particle in line. The two other wave types (left and right in the video) are transverse (shear) waves that only occur in solids, and surface (Rayleigh) waves that occur the surface of solids.

Many of the animals that cannot detect pressure changes very well are adept at picking up certain components of how the particles themselves move in the medium.

This particle component of sound is not understood as well as the pressure component, and with respect to modelling it, we're a little stuck.
Nedelec et al. [2] provides sources for calculating peak particle motion (displacement/velocity/acceleration) from sound pressure, frequency and range from source, but only away from interacting boundaries and under the assumption that the wave is a plane wave i.e. far from the source. So many caveats...
All the following values are peak values; peak particle displacement, peak particle velocity & peak particle acceleration.

Nevertheless I tried to plot the resulting planes in a space given by frequency (1 Hz - 100 kHz) and a large pressure range (0-220 dB re ╬╝Pa). As much as I like 3D plots, to my surprise, what i found when only looking at the effect of frequency on particle motion surprised me more (Figure 2).
Figure 1. Depicting how Particle motion related to frequency and pressure. "Plane wave limit" refers to the lower frequency limit where an assumption of plane waves applies (80 Hz at 10 m depth over sand). The "Far field lower limit" is a simple assumption that over two wavelengths from the source, the field behaves as in far field. Adapted from equations in [2]
Looking at the same data, but in 2D, it becomes clear that something interesting is going on.
In Figure 2 particle acceleration is constant in the near field (left of red line), but becomes dependent on frequency in the far field (right of red line). For the particle velocity the opposite is true; particle velocity is independent from frequency in the far field - not sure this makes intuitive sense, but otherwise it would be no fun!
Note that in this figure i have included the "Plane wave limit" also. This is the limit (here ~80 Hz) at which we can expect sound waves to propagate approximately as plane waves and we can infer particle motion from pressure and frequency [2] (here at 10 metres depth, sandy seabed).
Figure 2. Same as Figure 1, but only looking at the effect of frequency on particle motion.

Fheew.... ok - so why is this important?

First of all most of the animals that we are concerned about rely on coastal (i.e. shallow) or seabed environments in their life-cycle, and protecting them becomes a lot easier if we have an idea about what they experience. For the above example, we have almost no idea about what the particle motion is doing under 80 Hz if we only measure and/or model pressure waves. Close to the seabed (and in it) we start to see propagation as in the video up top, and that makes modelling hard(er). 

The good people from [1] did lots of measurements of particle velocity in a shipbuilding dock showing that for frequencies below 400 Hz the pressure part of the signal no longer correlate "nicely" with the particle velocity.
This is directly taken from [1] and you should really check it out, they explain it very well!

Thanks for reading!
I hope that I have managed to give you a little bit of insight, and hopefully something to think about.

Graphwork from GNU Octave.

References:

Monday, 21 November 2016

Underwater noise course Belfast

November 17-18th in Belfast saw another run of our UW noise course.
We were happy to have participants from Denmark, England, Finland, Ireland, New Zealand, Portugal and USA. 

On day one we were introduced to a new player in the underwater noise field - the tidal turbines. They are a relatively unknown feature in the underwater soundscape, but as nearby Strangford Lough is a bit of a hotspot for these devices. Two specialists from CASE in Belfast came to show us the current state of the art tidal turbines, some of which they are currently testing.

The remaining of day one was focussed on learning about the issue of underwater noise by approaching the issue from several disciplines. We went over the legislation that largely dictates the content of environmental impact assessments. An introduction to marine fauna followed, with an in-depth look at shortcomings in the current legislation's focus on noise limits that do not alway align with the intent in the legal text or the marine biology.
We do this to facilitate understanding between especially biologists and acousticians. 
In line with the recent move in academia towards a better understanding of the role of particle motion in underwater acoustics, we discussed what impact this has now, and will have on future work in the industry.

The second day was the day of modelling. Everyone got working with dSBea to characterise soundscapes in scenarios that we had prepared or in a site of their choice with their own noise sources.

I want to say a big thank you to the participants contributing with their extensive expertise to the conversation, making it a great learning experience for all of us.

I you want to see some dBSea examples yourself, have a look at dBSea's download page and download dBsea Basic and example scenarios to view them for yourself.

Thank you for reading - if you're interested in our courses e-mail me at: rasmus.pedersen(at)irwincarr.com

Thursday, 22 September 2016

Making it move

When pictures just aren't enough

Admitted, this one was mostly for the fun of it, but like most fun it taught me something useful.

I was at a conference recently, and was rather mesmerised by some of the animations of modelled noise sources. But dBSea does not output animations, so I went the long way and got "Autohotkey", a program that lets you automate repetitive tasks, such as running dBSea's modelling algorithms 90 times over and moving the source in small increments between each image export. 
This process is essentially a batch process (i.e. running similar processes repeatedly), and can be very useful if you wish to run many models while systematically changing the input variables.

The below animation is what I came up with, a boat making it's way into Chesapeake Bay to "Bush Park Camping Resort". Note that the boat is travelling 180 km's in nine seconds (for your viewing pleasure), as the full eighteen hour video was a bit over the top for my laptop.
Boat sailing in through the mouth of Chesapeake Bay
It's important to outline that the results do not differ from using the "Moving source" option in dBSea, but for visual representation this is certainly more eyecatching.

Thank you for reading, please feel free to comment or ask questions below.

Resources:
https://autohotkey.com/
http://www.gebco.net/data_and_products/gridded_bathymetry_data/gebco_30_second_grid/

Friday, 9 September 2016

There's a real world out there

Integrating with GIS software

Often the modelling is just the beginning of a project, and inserting your modelling results in a real world context makes them immediately more relevant and easier to interpret.
From dBSea you can export into a range of formats, including .png and ESRI ascii (.asc) raster. The ascii format is very useful because it is geotagged, meaning GIS software will know where to put it, and that all features, such as the raster calculator tool or area calculation, is available to you.

Exporting visible levels to ESRI ascii raster to be used in QGIS

In the below illustration I used the GIS software to assess the extent to which the ferry from Mallorca to Ibiza masks the call of a humpback swimming by. (Masking is the process of making a signal indiscernible by adding noise)
dBSea is not strictly made for assessing masking, so I used the raster calculator in QGIS to find the area where the noise level from the ferry was 6 dB over that of a fictive calling humpback whale heading for Gibraltar. Because levels are exported in ascii format the GIS software is able to alter colours for the sound levels, and I can get creative with the presentation.

Two outputs from dBSea combined to indicate masking effect of underwater noise from ferry, exported from dSBea as ESRI ascii grid raster and imported straight into an "Eckert VI" projection of a world map.
The ability to export directly into GIS software mean that results can be easily and effectively put into context - especially if you're undertaking a large project where noise is just one of many concerns.


Please note that not much in this example is accurate as it is only meant as an example of possibilities.

Resources:

http://www.naturalearthdata.com/downloads/
https://www.qgis.org/en/site/forusers/download.html

Wednesday, 7 September 2016

dBSea 1.3.11

Tweaks for speed and adaptability

dBSea 1.3.11 is out and can be downloaded now.
the Basic version has also got an upgrade, and you don't need a license for that one.

So what has changed?

The nerdy stuff:
  • You can now adjust the dBSeaModes oversampling to suit the detail in your scenario.
  • The soundspeed profile interpolation is more robust regardless of input.
  • More efficient use of multicore when present.
  • Rendering of the results in the graphics window are now even faster.
The big difference:
The new thresholds from NOAA are now incorporated into dBSea, so you can evaluate those 24 hour SELcumulative levels properly. If you're using dBSea for impulsive sounds, we've included those thresholds too.  
Besides the five groups shown here, we also included sirenians (dugongs and manatees) in dBSea
And don't forget to have a look at the new example scenarios on the download page
(If you do not have a full version of dBSea, use dBSea Basic to view them)
Screenshots of some of the examples you can find on dbsea.co.uk/download


Tuesday, 23 August 2016

Less is More?

Is the more accurate model always better?

- Once more a "techy" post -

dBsea can be quite resource-intensive to run, especially if your model demands calculation of many sources and spatially detailed outputs.
Today I'll make a case for always running simple simulations until all parameters are set as wanted, or you are ready to run your model through the night. Even though dBSea is heavily optimised with respect to resource consumption, it is easy to make a model that demands huge amounts of calculations.
A comparison of calculation time versus accuracy of output seems to be the logical way forward.

Figure 1. A comparison of a coarse solve VS a detailed one, 5 minutes VS 3 hours. Top panes are the maximum levels projected to the surface, the middle shows exclusion zones and the bottom shows the 3D version with the calculation grid superimposed.
While the above solves are by no means identical, but sound levels are within circa 10 dB (see the spectrograms in the top panes), and exclusion zones are within a factor of 2. While this might seem like big differences, when you are designing you model, and want a bit off feedback on the changes you've made, a rapid update of the results is very desirable.

Another example is the below example (Figure 2), where two ray trace solves are very close to identical. Scenario "A" took over 30 min to compute, with thousands of outgoing rays from an approximated line source. Scenario "B" was very quick to solve with no calculation of attenuation, and only 20 outgoing rays. Also no frequencies over 1 kHz were included as attenuation means these frequencies do not propagate as far.
Figure 2. Comparison of a high accuracy solve VS a quick solve. dB-levels are only for comparison.
For quick evaluation and feedback during scenario design, simplifying your solves will radically improve this phase, making sure you only need to do the "big" solve once.

Thanks for reading, please don't hesitate to comment.




Friday, 1 July 2016

We get stuck

Today I had to resort to primitive methods for retrieving our Soundtrap logging device.

We had deployed it in Carlingford Lough in Ireland, not as much to measure anything specific as to get some insight into the soundscape of our local patch. The mouth of the Lough is quite busy, with big boats passing regularly, along with the occasional fisher boat and jet ski from the friendly local life guards (RNLI).


The hydrophone was attached to an anchor but with a separate line to the buoy to avoid unnecessary noise from the mooring. It's sitting close to the shipping channel, ready to record any passing ships. I should mention that we might get dolphins or porpoises as well, as this Soundtrap supports super high sampling rate, 576,000 samples per second!



The Soundtrap is deployed, safely hovering a couple of metres above the seabed. Yes, the sticker is upside down, I'll correct that later.

Long story short, our anchor was too good, and we had to send a diver down to retrieve both hydrophone and anchor. This meant waiting for slack water, which occurs when the tide is not moving water in or out of the Lough. Because of 4-5 metres difference in high and low tide, we could not send a diver down during the tidal race!

In the end we got our kit back.
It's always good to be reminded of the forces of nature - and strong anchors!




The sublime view from Cranfield Bay over the Irish Sea and the Cooley Peninsula. 

Thanks for reading!