Indoor Acoustics: Noise in Restaurants

Alexandra Gill writing in the Globe and Mail (Canadian newspaper) has an excellent article about noise in restaurants. Here is a taste (sorry for the pun):

So how loud is too loud?

The ideal sound level for normal conversation is 55 to 65 decibels. When the ambient noise rises to about 70 decibels, you have to raise your voice to be heard. At 75 decibels, conversation is difficult. Above 85 decibels, prolonged exposure - more than eight hours - can permanently damage your hearing.

While restaurant noise levels aren't a threat to hearing loss, "they are certainly an issue for communication. Many, if not most, restaurants have noise levels that are too high for comfortable conversation," says Christine Harrison, an occupational audiologist with the Workers Compensation Board of British Columbia.

I recommend reading the entire article. Enjoy.

Noise: Thoughts on quieting a lab

This AP (Associated Press) article about new products on the market for quieting home environments got me thinking about my experiences quieting a few laboratories and what I learned along the way. Here are my thoughts, arranged in no particular order:

• The frequency of the noise matters. High frequency noises can be reduced or eliminated by light-weight materials such as fiberglass, curtains, carpet, and rubber seals (e.g. around doors). Reducing low frequency noise requires MASS; in other words, it takes heavy materials to absorb the large amounts of sound energy contained in low frequency noise, such as from machinery, engines, air handlers, and the like.
• Solid-core doors can make a big difference in the amount of sound coming from the hallway (or leaking out into it, for that matter) and cause little, if any, disruption to the work environment when used to replace existing hollow-core doors. Most doors that come standard in home and office environments are hollow in the middle and therefore don't absorb low frequency sounds very well at all.
  
• If the room has a drop (also called a "false") ceiling, then replacing the standard ceiling tiles with acoustic ones can make a significant difference and is minimally disruptive.
• Wall and door treatments, such as barium-loaded vinyl hangings and acoustic panels, offer the benefit of being able to be installed to an existing room without requiring tearing down the walls, but installing them may require significant effort and it will almost certainly be quite disruptive to the existing workspace (think about shelves and such that are on the walls).
• If you have hollow walls (e.g. gypsum board on stud), then blowing insulation into the wall cavity can reduce middle and high frequency noises without being too disruptive.
• If full-scale remodeling is an option, consider adding an extra layer of gypsum board or other massive material to the walls. If you go this route, then you should advise the workers to install the boards so that the seams (between boards) of the new layer do not over-lap the seams of the underlying layer. Of course, there may be complications with window and door frames fitting the non-standard thickness of the walls, but a competent carpenter can solve those issues.
  
• Speaking of windows, if the building is located in an area where the external noise (e.g. vehicular traffic or aircraft) is an issue, then it will likely make things much simpler if you pick a room without a window for your lab in order to avoid having to acoustically shield it. Likewise, locating away from internal noise sources, such as elevators and air-handlers, is also a good idea where possible.
• Quieting noises in your lab itself is also a good idea. Computer workstations and other electronic equipment fans are the usual culprits here. A variety of options now exist for quieting computers ranging from low-noise replacement fans to liquid cooling systems that can be added to existing computers.
  
• If the acoustic environment is so poor that you need to go beyond the simpler of the measures listed above, then consider getting professional help (i.e. a consultant who specializes in this area) to evaluate the situation and recommend appropriate steps.
• Choosing a room that isn't a strict rectangular box can be a good idea as it can mean significantly less resonance.

As I said above, my comments are based on my own personal experience on quieting existing rooms. I did not mention options that I am not personally experienced with, such as bass traps. Finally, I have not addressed anechoic chambers or designing sound studios as that is outside of the scope of this post.

Noise Cancellation: Quieting an aircraft

Aviation Week has a short but fascinating article on the design of passive and active noise cancellation for the King Air airplane.  The design seems very sensible for an aircraft application - use passive cancellation (absorption) for the high frequencies and active cancellation for the low frequencies.  

Aside:  For those unfamiliar with active cancellation, this is the same technology that is used in the Bose line of headphones where an out-of-phase signal is induced under the ear cup that cancels out the sounds that leak in from outside, thereby letting you hear the audio being cabled in from the media player/device (e.g. MP3 player, radio, etc.) even in moderately noisy environments.

The design trade-off is governed by the constraints that active noise cancellation does not work very well at high frequencies but does work well at low frequencies, while passive cancellation at the low frequencies requires massive (i.e. heavy) sound absorbing materials, which would weigh the airplane down.  At high frequencies, the materials can be much lighter. So, as you can see, the design trade-off works out very neatly - active at low frequencies and passive at the high frequencies.

One design detail that stood out to me is that they use 24 microphones to provide feedback to the cancellation algorithm so it can continuously adapt to the changing noise environment in the cabin.  Those 24 microphones are coupled to 12 loudspeakers to produce the anti-phase cancellation signal.  The numbers of microphones and loudspeakers tell me that this is a serious system that is designed to reduce noise through-out the cabin, not just in a limited area.  My hat is off to them as this was a seriously challenging design problem.

Human Auditory System: Protecting your hearing

MacWorld has a comprehensive article on protecting your hearing using different aids - ear plugs, muffs, noise-canceling headphones, etc.

Noise: Learning problems associated with noise

The issue of noise in everyday life seems to be getting more and more attention from health advocates and researchers.  The latest thing I've come across is this medical news report on schools and noise.  It struck home with me because my wife and I just recently toured an elementary school where the cafeteria was so reverberant that it was uncomfortable and even painful to be anywhere in it.  I didn't have a sound pressure meter with me, but it was easily in the 90+ dB SPL-A range.  My ears are not as young as they used to be, as they say, so I could not tolerate it for long.  I think that we will be hearing more and more about this type of thing (pun intended) and, as far as I am concerned, that is a good thing.

(Source: Medical News Today)

Equipment: Deadly Silence

Advantages and disadvantages of noise canceling headsets on the battlefield.

(Source: StrategyPage.com)

Aero-Acoustics: Making airplanes quieter

Here is an article in the Deccan Herald on redesigning aircraft engines, engine placement, and landing approach procedures to reduce noise levels on the ground by up to 25 dB (a very significant decrease if achieved).  The article is very readable.  Enjoy!