Flavors of noise

Noise impacts come in two flavors: Noise that is too loud, and noise that sounds awful. There is a degree of subjectivity under the “awful” category. For example; the awful din of your neighbor’s barking dog probably doesn’t meet their threshold of “awful” as readily as it does yours. And of course there is the screaming ‘crotch rocket’ motorcycle “your ear is my hole” conundrum: Somebody likes these sounds…

But there are some noises that are just plain nasty for everybody. The grinding of metal plates on gravel and the sound of shattering glass come to mind. For some people just putting the words “fingernail” and “blackboard” together makes them cringe.

“Too loud” doesn’t have such a subjective continuum. At some point noise (unwanted sound) interferes with what we want to hear. And knowing what the noise is and what the wanted sound is, the “too much noise” threshold is fairly predictable. An example of this is available in any crowded restaurant. In order to overcome the noise of others talking, speakers will raise their speaking voice until competing with the din of the room becomes uncomfortable, at which point they’ll stop talking. The noise level at this point is predictably around 85dBA.

The phenomena of noise interfering with wanted sound it is called “masking.” How loud the masking noise is and how much it overlaps the wanted sound frequency range all figures into how much interference it causes.

95,000+ commercial ships registered with Lloyds ply the seas

95,000+ commercial ships registered with Lloyds ply the seas

Shipping noise is commonly associated with masking in the ocean, as it is one of the more ubiquitous ocean noises. Measured on ocean-basin scales it seems to be mostly a pervasive low frequency noise. In this context it has always been considered a low frequency masker, potentially interfering with the low frequency communication signals of baleen whales.

But a recent paper by Hermannsen et.al examines shipping noise in shallow water and at close ranges to find that ships are also noisy at high frequencies – causing hearing range reduction of >30dB at 125kHz within 500 meters of the ship. This would be a problem for harbor porpoises (as indicated in the paper) as well as dolphins, seals, fish, and diving birds.

And that doesn’t even address the “awful” component of the noise. While I can only surmise from a human hearing perspective, I would imagine that an exceedingly loud, broad-band hissing noise saturating my living space would put me in a bad mood.

Something to ponder this season when picking out that Halloween costume shipped across the sea from China.