"50% reduction in sound"

A catalogue tells me proudly that "Everest secondary windows reduce noise from outside by up to 50%."

Just what does that mean?

Next to that information, it also helpfully gives a list of the sound levels of common sounds. For example, a pneumatic drill at five metres measures 100 dB.

So, will the 50% reduction mean that the noise from a pneumatic drill five metres outside your window, would be cut by a huge 50 dB? (Or even by "up to" 50 dB: funny, advertisers never tell you "down to"!)

Well, no, decibels don't work like that. In fact, a reduction of 50% in the power of the sound is always a 3 dB reduction, whether it's from 100 to 97 dB, or from 10 to 7 dB. Not a lot, in fact.

But maybe they aren't referring to power? If they mean a 50% reduction in the air pressure fluctuations that make audible sound, then it would be 6 dB: useful, although not dramatic. Or, could they mean 50% reduction in the subjective loudness? That is usually taken as corresponding to about 10 dB, for an average person. Still not fantastic but very noticeable.

There is yet more doubt: the reduction will depend a lot on what sort of noise it is. The reduction in traffic noise (primarily low pitch) will not be nearly so great as the reduction in squeaky children's voices. Maybe that's where the "up to" comes in? But if so, is it fair to specify the reduction on noise from a tin whistle, when that's not the sort of noise that bothers most people?

With such vagueness, the manufacturer will always be able to justify their figure of 50%, even without the get-out of "up to", so long as their secondary glazing does anything useful at all. So this bit of blurb is impossible to put a real meaning to. Could it be that the publicity people wanted a "scientific" sounding figure for the catalogue, and the engineers gave them one that looks fairly impressive, but that they could hardly fail to meet? I've no idea, but I must admit that if I worked in their technical department, I might just be tempted to do that.

I'm not accusing Everest of being deliberately misleading - in fact I think they are probably being conservative about the performance of their windows. But using percentages to refer to sound power without actually saying so, is a common ploy when the actual reduction in sound is not very impressive. This is often the case with noise reducing headphones, where a moderately useful 10 dB translates into a much more impressive 90% "reduction in sound".

Moral: ignore the percentages, look at the decibels!