Cohousing-L Mailing List: room acoustics

[Laura Polich (laura.polich]

I'd like to respond to a couple of things in this latest thread on Hearing
in Common Spaces. I am very happy this topic has come up.

I want to point out that some of the conversations in this thread are
actually about reverberation, not noise. Reverberation always degrades a
speech signal, especially as the room gets bigger. Most speech takes place
within rooms so there is always a need to factor in how sound bounces
around the room and "smears" the speech signal. The first thing to think
about with reverberation is: what is the main and most common use of this
space? Hard and smooth surfaces increase reverberation, but they are easier
to clean. For example, in nursing homes, cleanliness always outranks
audibility, which is why you usually find high levels of reverberation in
nursing homes which makes it harder to use hearing aids or hear well in
nursing homes, but the environment's need to be easily cleanable trumps all
other considerations. At Daybreak, we hold our plenaries in our "Great
Room" which is also our dining area. Cleanliness is important so some
amount of reverberation is always going to remain in our Great Room.
Sound-absorbent materials (fabric on furniture, throw pillows, rugs, fabric
curtains, wall hangings made of fabric or porous materials) will
definitely cut down on reverberation and make it easier to hear in a room.
Those same sound-absorbent materials are harder to clean and will make it
harder to keep the room clean. It's always a trade-off.

I would suggest that using phone-app decibel meters is not that useful to
compare two voices or two noises or two rooms. First there are different
decibel scales for different uses. There is no single decibel scale.
Second, and most importantly, decibels increase logarithmically, not
arithmetically. You might think that 60 dB + 60 dB would equal 120 dB, but
that is looking at the scale arithmetically. For one common decibel scale
60 dB + 60 dB (in other words a doubling of the sound pressure) is equal to
63 dB. The other problem is that sound dissipates outward in all
directions. So small changes in where the sound source is located makes a
big difference in how much sound is found at any one point, and if the
sound source moves (e.g.the speaker walks around while talking), the
measurements have to be recalculated. There is a reason the world has
acoustical engineers.

A loop-system does not amplify the sound that is spoken into iits
microphone. It converts it from acoustic energy to magnetic energy which is
broadcast to the room. Telecoils pick up magnetic energy, not acoustic, so
only the magnetic speech signal is picked up and processed through the
telecoil in the hearing aids (or neckloop). That means the listeners with a
telecoil get a cleaner signal, scrubbed of a lot of the noise in the
room.But human ears do not pick up magnetic energy so everyone without a
telecoil is  oblivious to the signal sent through the loop and is not
bothered by it, but also not helped by it. Only people with telecoils
benefit from the noise reduction of the loop system.

I am not against loop systems. They are great for people who have a
telecoil, but they don't help the others in the room. A
soundfield-amplification system helps everybody. And in ways that are not
directly obvious. Less fatigue for everybody when listening, less vocal
fatigue for speakers, less avoidance of going to plenaries due to the
strain of listening.

The ideal would be both systems available at all times, but that expense is
pretty high and you would have to have two sets of microphones which would
be a logistical problem.

Last comment, yes, most audiologists focus on an individual's hearing aids,
not the range of acoustic environments those people will meet, because that
is what they are being asked to do. A better resource about sound in rooms
would probably be Educational Audiologists who work in schools. They are
tasked with thinking about an individual child's hearing aids or CI, but
also how those devices interact with the classroom's acoustics.

Sound is a pretty complicated thing to work with,
Laura
an audiologist on her way to retirement