We know that sound behaves quite differently under water, moving faster and further, but how well can we predict what sound will do under water based on the information that we have? Dr Craig Radford from Leigh Marine Laboratory decided it was time to find out.

A new model for sound in shallow water

Dr Radford and his team thought that sound might be providing a cue for animals to find and settle on reef habitats.

They had demonstrated that animals such as crabs can hear the sounds of the reef and navigate towards them and that they preferred reef sounds to beaches or estuaries. However, when they published their results, they were challenged.

Until now, the models that scientists used were quite simplistic and predicted that the noise of a reef wouldn’t be able to be heard more than 1 kilometre offshore, which would mean that animals such as crab larvae floating in the currents up to 5 kilometres out from the shoreline wouldn’t be able to hear the reef.

Did Dr Radford’s team have to rethink their ideas around crab larvae navigation, or were the sound models wrong for this environment?

Collecting data

Dr Radford realised that they needed to learn more about how sound travels in shallow water to defend his prediction that sound was providing the settlement cue for crab larvae. How far out from the coast was it possible to hear a reef?

He designed an experiment using hydrophones – underwater microphones designed to detect and record sound – and the location he used was the Hen and Chicken Islands north of Auckland.

He started with a reference hydrophone placed close to the reef. This stayed in the same place throughout the experiment. Reef sounds normally fluctuate up and down during the day, and he needed to be able to control for this, so the reference hydrophone recorded constantly while other measurements were taken to compare against the background noise coming from the reef.

Refining the model

Dr Radford then travelled away from the reef and measured the sound levels at different distances – 500m, 1km, 2km, 3km, 4km and 5km.

The existing model predicted that fish wouldn’t be able to detect any reef noise more than 1 kilometre out, but when he compared his results with the reference hydrophone, he could clearly detect reef noise well beyond the 1 kilometre limit that had been previously predicted.

Amazingly, within the 1 kilometre distance, there was almost no loss of sound energy at all – it sounded as if you were still right next to the reef! Dr Radford has referred to this as the ‘reef effect’ that extends the range at which you can hear the reef noise away from the source. This is related to the size of the reef (particularly length) and the angle at which the receiver (hydrophone or fish) is to the reef.

Dr Radford is rewriting what we know about the movement of sound waves in shallow water.

This work has been critical in proving that small animals such as crab larvae could navigate to a reef habitat using sound. There is still work to be done to figure out just how well the crab larvae can hear, what frequencies they hear best and just how far out a reef can be heard, but the story is coming together.

The team at the Leigh Marine Laboratory are starting to piece together an understanding of how important underwater sound might be to the creatures that live there.

Nature of science

An important part of science is peer review, where scientists have the opportunity to critique the work of other scientists. In this study, Dr Craig Radford developed an explanation about how sound might travel under water. When this was challenged by other scientists, he designed new research that has helped him successfully defend his explanation.

    Published 10 May 2011