Acoustics FAQ: Answers to some common questions about sound and construction
The Internet is an amazing thing. (Did you know it was “invented” by an acoustical consultant? Don’t scoff. Just read this article, then make up your own mind.) I particularly like the Q&A sections of most software, device or OEM websites. If I have a problem with something that someone else made and visit the Support section of their website, it is somehow comforting that there is another soul out there that ran into the same brick wall I did, and thought a little further than I did to steer a path around it.
So, in homage to Support sites on the information highway, I will introduce the first Cinema Acoustics Support Q&A Forum. Truthfully, these are questions and answers we have received from experienced designers of cinema facilities that don’t get addressed very often in forums such as this.
What do you say to a client who asks for “soundproof” walls?
This question is not limited to cinema designers, but is posited by homeowners, apartment dwellers, and dentists located in strip malls adjacent to boxing-fitness clubs. I imagine pest-management professionals (bugproof), sunscreen makers (waterproof), and All-In-One AV remote-control designers (idiot-proof) also face these questions.
The true answer is, nothing is “[fill in the blank]-proof,” except for probably death and taxes—and of course the perfect vacuum of outer space, for if you have no air in which sound energy is to be propagated, you will have no sound.
For something that is mostly subjective from the consumer’s perspective, as acoustics is, the level of control depends greatly on the hearing mechanism and perception of the concerned listener. But for all practical purposes, “soundproof” in cinema acoustics usually translates to: “Are the patrons in an auditorium going to be disturbed by the movie sound in the adjacent auditoriums, and if so, what do we do to address the problem?” The answer to that question, were it historically answered, could fill a museum (which I would like to be curator of one day!) called “The Museum of Soundproof.”
But we live in a practical world and in that verifiable realm, “soundproof” means that you have controlled the transmission of sound enough that listeners of “normal” or “reasonable” sensibilities will not be disturbed by the level or frequency of sound intruding into their space. When determining how much noise control is necessary to achieve that goal, one must also consider the frequency content and level of background noise (ambient noise or activity noise) that already occurs in the listening space. Any cinema acoustical designer can tell you truthfully that hundreds of millions of dollars in construction costs have been saved by properly considering these factors in correct proportion.
When does structure-borne sound transfer become a problem?
Structure-borne sound transfer occurs when sound or vibration energizes the structural components of a building and then in turn energizes the architectural elements connected to that structure that demise an adjacent space. However, in cinemas, movie sound will not usually energize an exposed structural element such as a column, girder or beam and carry along that beam into an adjacent space in which that element extends. Because of the concept of wavelength associated with the frequencies involved, movie sound will enter and energize the demising element (drywall or concrete slab), enter the connected beam or column, then transfer into the connected demising element in the adjacent space. By the time all that transmission occurs, we usually have to first address airborne sound transmission, which is why you always hear your acoustical consultant going on about mass in demising construction.
What is the difference between STC and NC?
STC stands for Sound Transmission Class, which is an acoustical laboratory-determined rating of the ability of a specific construction element, such as a wall, ceiling or floor, to impede the transmission of sound through the element, and is independent of how much sound transmits around the element. An important distinction in the use of STC in cinema acoustics is that the ratings only describe low frequencies down to 125 Hertz. A wall can have a very high STC rating but not be usable in acoustically separating auditoriums because it does poorly at lower frequencies, down even to the limit of human hearing near 20 Hertz.
NC stands for Noise Criteria and is a method of rating the level and frequency content of the ambient (or background) noise levels in a given space. When we speak of the “NC rating” of a cinema auditorium, NC-25 to 30 is usually expressed as the proper goal for ambient noise levels in the space. A higher rating, like NC-40 or NC-50, will be used to describe the level of HVAC or plumbing noise that was measured in an auditorium in which a bit of noise-control work is necessary. The important distinction for cinemas associated with NC ratings is that they only address continuous noise, such as from HVAC, lighting, electrical or plumbing systems and equipment. NC ratings are not applied to sound intrusion from adjacent spaces, which is typically a short-term (transient) occurring noise.
There are other interesting questions about cinema acoustics that are forwarded our way on a regular basis, including the most recent trend involving…“That concludes this Help Session. For continued assistance, please deposit 50¢. Good Day!”
Seriously, though, we hope to be able to continue this type of forum. Please feel free to e-mail questions to firstname.lastname@example.org.
Brian Kubicki is a principal with acoustical consulting firm Acoustical Design Group, Inc. of Mission, Kansas.