Acoustics alert: If nobody complains, did a tree really fall in the forest?
We are often asked to visit a cinema complex to figure out the cause and best solution to an acoustical problem. It could be that patrons are complaining about “sound bleed” (I always hated that term—so vampirish!) between auditoriums, or they are distracted by noise from the air-conditioning system, or echoes are making it difficult for patrons to understand dialogue in the movie.
At first glance, the common link to all these problems is acoustics. But there is another. Each is alerted by a complaint from a patron.
But what if nobody complains? Is there a problem if nobody notified management that they aren’t happy with the product or service they have purchased?
Everyone who has worked in retail or been a customer of a retail establishment has not been completely pleased with some aspect of the commercial experience. Some will let management know immediately. Others will wait until the problem becomes too objectionable, then they will complain.
Many (and I argue, most) will not say anything and just never return. Anyone who has been in a cinema where the movie starts without sound or there is sound but no picture can attest that the entire audience will not get up and alert a staff person. Clearly, the majority of the audience will not complain—even though there is clearly a problem.
This occurs many times in less obvious acoustical problems. And those who don’t complain to management will often tell others, or use social media to inform others of their less-than-exemplary experience.
So what is an acoustical consultant to do? How do we know there is a problem if nobody complains?
The answer is expressed in one word: “criteria.”
Establishing appropriate acoustical criteria begins every cinema project in design, guides the testing and diagnosis of an existing facility being renovated, and is the standard used when acoustical experts are called to address complaints.
My office library has a binder titled “Facility Acoustical Criteria.” It contains a number of handbooks for designing and constructing cinemas and theatres from traditional, recognized names such as THX®, Dolby®, IMAX® and Omnimax®, among others. Publishing dates go as “far” back as 1985. Newer editions have dates of 1997 and 2003, though the same details and data are usually listed. The only difference in newer versions is an updated and maybe more nuanced text. One particular guide (I won’t implicate the offender!) actually recommended a demising wall between adjacent auditoria composed of a triple wall of concrete with freestanding stud walls on each side. If you have followed my previous articles in these pages, you’ll recall that you are wasting money with a triple wall because of inherent sound flanking, not to mention you might even create a new problem due to mass-air-mass resonance!
Any reasonable design guide for any type of facility is going to address three basic concepts: sound isolation, reverberation, and ambient noise levels. Setting goals for each of these concepts establishes a level of consistency that guides design, construction and maintenance of cinema exhibition worldwide.
Some of the basics:
Sound isolation describes the degree of reduction of sound transmission that occurs between adjacent auditoria. Sound Transmission Class (STC) ratings are single-number descriptors obtained from tests conducted in an acoustical laboratory of the ability of a wall, floor/ceiling, door, window, etc. to impede the transmission of sound. The higher the rating number, the better the sound isolation—usually.
However, care should be applied in cinema evaluation because STC ratings are not sensitive to low-frequency sound, where movies generate lots of energy. You can actually have a wall that has a very high STC rating and is awful at controlling low-frequency sound.
So in cinema work we look for construction that possesses good STC ratings, with particular attention paid to extended low-frequency sound isolation performance. Since STC is measured in a lab, performance verification is accomplished with other ratings systems. Field Sound Transmission Class (FSTC) can be used as one option, but Noise Isolation Class (NIC) ratings are typically used in cinemas because room volumes and common wall areas are fairly uniform.
Reverberation describes the decay of sound in a space. Sound decays after it leaves a source at a predictable rate due to distance travelled and due to absorption by surfaces the sound waves come into contact with. We rate reverberation by measuring the time an interrupted sound decays in a room, standardize it to a given amount of decay, and call it Reverberation Time, or T (sometimes T60). In most cinemas, the design goal is to limit the T to as low a figure as can be practically achieved (a much different “Low T” than the one you hear on radio and TV ads these days!). Less than 1.0 second (mid-frequency average) is generally accepted as the optimum goal. Echo control is a sub-chapter of the reverberation question, and is generally well-controlled when all the cinema’s finish surfaces are treated with efficient sound-absorbing materials.
Ambient noise levels in any enclosed space are defined as the noise levels measured in an unoccupied space (no activity noise) with no machinery operating other than that associated with building services (mechanical, electrical, plumbing, lighting, etc.).
In cinemas, ambient noise is composed of contributions from HVAC systems, lighting, plumbing, and electrical distribution systems. Ambient noise should be controlled to acceptable levels so that noise from these components does not interfere with enjoyment of the movie experience. Noise Criteria (NC) curves are groupings of sound-pressure level versus frequency, and are used to approximate noise levels subjectively perceived to be equal in annoyance in the audible frequency spectrum from 16 Hertz at the low end to 8,000 Hertz at the high end. A single-number rating system is used for ease in identifying each grouping of sound-pressure levels. Generally, the lower the NC ratings number, the quieter the ambient noise environment.
NC ratings generally applicable to cinemas are: NC-25 in auditoria, NC-40 in projection areas, and NC-35 in adjacent corridors and lobbies.
Work with these criteria and numbers and you can be reasonably satisfied that even the patrons who won’t voice complaints will leave the cinema happy with the experience!
I, as always, greatly respect your time, and welcome your feedback!
Brian Kubicki, P.E., heads the acoustical consulting firm Acoustical Design Kubicki and may be reached at firstname.lastname@example.org or 913-400-3694.