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Keith Oakley - n/a
18-Sep-07, 06:01 PM
One of the most interesting papers at the noise conference today went a long way to explaining why we sometimes find dbA measurements don’t reflect our subjective feelings about a particular craft’s noise.



DbA is an international standard which measures the overall acoustic power level of a sound but includes a weighting curve that for example discounts a lower frequency noise; because some surveys of folk 50+ years ago found they weren’t so disturbed by low frequency sounds.



The new measure reflects more recent work that says folk find pure tonal noise disturbing, and also sounds where a tonal noise is modulated (turned on and off) by a frequency between 10 and 150hz, particularly around 75hz.



All very techy until you realise it helps to explain why a certain scottish Osprey5 (with a strong single tonal noise from the fan) was judged by many more annoying than a certain big Sevtec (with low frequency but less tonal noise) despite the latter having a slightly higher dbA score. Also why a certain Swedish junior craft with a single cylinder 2 stroke thrust engine has a reasonable dbA score but is judged by many to be stomach clenching. The engine (mostly the exhaust) produces a range of higher frequencies modulated by a deep 65hz frequency.



So do we use something other than dbA? No - it’s a very well accepted international standard and the new ideas are way away from being adopted as a standard. But it would be good to look out for these two types of noise and avoid them if possible to gain more public acceptance, particularly for cruisers.

Paul Fitz - n/a
18-Sep-07, 08:10 PM
This is why we used Noise Criteria curves to define the acceptable sound power limits of fans installed in extraction and HVAC systems. If a fan installation exceeded a specific curve it could often be improved by modifying the installation to improve a specific frequency.



I remember vaguely writing an article in the Mag about NC curves, just too old to remember when http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

hover t - n/a
19-Sep-07, 12:06 AM
Hi Keith



What frequencie was the strong single tonal noise? and what was the static dba noise figure when you tested the Osprey at Ardgartan



The fly past test was 87db at 25mtrs as I remember but its a lot higher when your head is 1.5mtrs from the fan http://hovercraft.org.uk/images/icons/smiley_icons/icon_dead.gif



http://hovercraft.org.uk/images/icons/smiley_icons/icon_idea.gif I think we need some sort of noise cancelling or noise absorbing ducts with soft surfaces to trap some of the nasty noises



The ospreys fan has one of those hi top end screaming noises that will burst your head after a while that's why everyone in my craft wears ear defenders . I will have to take my db meter with me the next time I am out and check the noise level in the craft



Trev

Keith Oakley - n/a
19-Sep-07, 06:45 AM
Trevor

From memory it was about 500hz and it was the fan blade passing frequency apparently coming from the end of the splitter. (left side looking from front if I remember correctly). If craft is 87dba at 25m then using the rule of thumb of a 6db increase each time you halve the distance (eg 12.5m 93,6m 99 etc) then at 1.5m it'll be around 111dba

Keith