In late 2021, a series of silencers were installed on our backhoe dredge, the Takutai. The dredge will be familiar to locals, as she works the Otago Harbour Channel on weekdays.
While her accommodation block and operator cabin were well insulated for sound, it was a different story outside. Her rear-mounted digger, hydraulic systems' generator and plant room ventilation fans were all significant sources of noise.
Marine Fleet Manager Brandt: "The Takutai was very noisy on deck - measured at 93 and 94dBA at two locations on deck - and the highest level of hearing protection (Class 5) was required to go outside during operation. We worked with silencer manufacturers NCS Acoustics to determine our options for reducing on-board noise. Three solutions - one for each source of noise - were proposed and actioned. Additional mufflers were added to the excavator engine and the generator, and an in-line silencer was installed within the air intake fan."
Takutai Master Alex says the difference is amazing. "It was bloody noisy before. Even with hearing protection, you could not spend a long period on deck when the digger was operating. Now we can have a conversation with other crew. There's no way you could do that before. It's just so much nicer now."
Post-installation testing carried out at the same two locations on deck show the work was exceptionally effective. The two measurements are now 73 and 82dbA, respectively, which is well below the Worksafe 90dBA threshold and means that hearing protection is no longer required on deck.
To put that in perspective, that means the noise for the crew is essentially half what it was - and noticeable quieter for our harbour community too.
How, you ask? The decibel scale is exponential, not linear. The clue is in the name DECIbel - "deci" equals "one 10th". So, for every 10 decibel increase or decrease in sound intensity, the loudness is actually 10 times greater or less. E.g. 90dbA is not just a wee bit louder than 80dbA - it's 10 times louder.
Brandt puts it better: "A 10dB reduction means a 90% reduction in actual sound pressure on the ear, and an apparent volume reduction of 50%. The crew are now working in conditions on deck that are half as noisy. That's a great result."
Port Otago joined forces with Microsoft to develop an accurate automated noise monitoring and classification programme.
Until recently, recordings from the port's four noise monitors were reviewed and classified manually, but the IT team suggested teaching a computer to do the job.
Microsoft was on board from the outset, while the port IT team passed on a year's worth of noise data to help the software "learn" the difference between port activity and say, a passing train or barking dog.
The automated system started in May and is 95% accurate. Occasionally re-training of the software is required to make sure it stays accurate. It is also an opportunity to classify any new types of noise that emerge.
Port Otago will work with acoustic engineers Marshall Day to maximise the software's potential across the operation. The next step is to extend its ability, so it also recognises frequency and can pick up "third octave" data. This would include the problematic Rio Class Ships.