I just finished a course on sound privacy and speech masking in architecture from Cambridge Sound Management.  This is a topic that many of our clients have great interest in whether out of a general sense of propriety or the requirements of their workplace.

This course explained how privacy is measured and quantified and gave a overview of good practices.

It was interesting to learn that based on research surveys about 46% of people in the workplace identified themselves as “collaborators” while the remaining 54% said they worked on individual tasks, hopefully in solitude.  Failing to provide that solitude propelled acoustic complaints to the top of the gripe heap, even above thermal comfort.  I have to recommend here, the book Quiet by Susan Cain which discusses introversion and extroversion in our society, and why we all seem to be jumping on the collaboration bandwagon.  An excellent read with great insights into why we tend to do things the way we do..

But back to acoustics. When you add in workplace problems resulting from legal privacy requirements, such as in healthcare or in the HR department, you can see how inattention to this topic could lead to big problems.

The three basic tools we have in controlling the acoustic environment can be boiled down to absorption, blocking and covering.  Absorption keeps ambient sound from reflecting back again and again to the listener by dissipating the sound energy, usually by vibrating tiny fibers on a fuzzy surface.  NRC, or noise reduction coefficient, is a number that rates a material in terms of what percentage of the sound reaching it is absorbed and what is reflected back.  A good ceiling tile, for example can have an NRC in the 90’s which tells us less than 10% of the sound hitting it bounces back to us.

Blocking is simply putting a solid barrier between us and the sound source, and is measured in terms of STC or sound transmission class.  Of course frequency of sound is a factor here too—low frequency rumbles carry more energy and can often be heard even when the higher portion of the sound spectrum is blocked out.  Think of the dull sound of voices in the next room, or a tractor working outside.

So a lead-lined room with walls one foot thick would have excellent STC rating, you wouldn’t hear the loud party outside,  but pretty poor NRC, as your screaming to get out would make quite a ruckus reflecting back to you. A poor work environment indeed.

Cover is provided by background noise that doesn’t eliminate the offending sound as much as it makes the sound unintelligible and non-distracting.  That too is measured and the rating is called NC or noise criterion.

As the trend in workplace environments moves more toward open space with lower partitions, if any, lots of glazing and fewer of the old acoustic tile ceilings it becomes harder to wield all three these tools effectively.

The key to introducing background masking sound is in the signal to noise ratio, a term loosely thrown around these days, but especially appropriate in this case.  The trickling fountain (noise) doesn’t have to wholly drown out your conversation (signal), it just has to reach a certain percentage of that volume to make it impossible for a nosy third party to pick out the meaning.  People converse at about 60 to 65 decibels (dB). If the masking sound comes within 15 dB of that level, you have attained “speech privacy,” assuming your eavesdropper is some feet away.  Sound masking systems typically come in at a uniform 48 dB.  By comparison, a quiet library is about 30 dB, a vacuum cleaner is about 70 dB, a jet engine 100 feet away is about 140 dB.  Remember decibels are on a logarithmic scale so a small increase can be quite noticeable (and a jet engine is not just three times louder than a library).

It is important to realize that a sound masking system does not lower sound levels in a space; quite the opposite.  It makes unwanted sounds unintelligible and therefore ignore-able for someone concentrating on something else.  It is the equivalent of dazzle camouflage.  Low visibility camouflage attempts to make the figure blend in with the background.  Dazzle camouflage breaks up the figure so it is not clear which parts are background and which parts are not.