Don’t Waste Your Money! Avoid unnecessary expenses when designing for acoustics
Hello again from the cinema acoustical design forum desk! It’s good to be with Film Journal readers once again. Let’s get right to it…
I initially thought my topic would never generate enough material for an entire article, but the more I thought about it, the clearer it became that my real struggle would be staying concise.
Everyone involved with cinema acoustical design is hit with questions about things that are assumed by the questioner to be relevant to affecting the acoustics of their projects, but in reality are misapplied or have no relevance whatsoever to the issues on the table. Why they come up may be because a manufacturers’ sales rep is encouraging applications for their products, or perhaps the cinema designer came across the product or concept in their own research or while working on another project. Regardless of how or why they came up, these are the items most frequently suggested for incorporation into cinema design and construction that are discarded in the flames of irrelevance.
Resilient channels are suggested for use in auditorium wall or ceiling construction at some point on almost every project. These typically light-gauge, metal “Z-shaped” channels are often used in office or residential wall and/or ceiling construction to introduce a measure of structural separation or resilience in the wall or floor-ceiling assembly. They are mounted perpendicular to the studs and the drywall is attached to the channel, with the “leg” of the channel providing the desired resilience. Some employ neoprene as the resilient element. While these devices are very beneficial to their most used applications in offices and residences, the walls in cinema auditoriums that require the highest degree of sound isolation are already composed of double-stud wall construction, which is the most structural separation that can be achieved. The addition of resilient channels to these assemblies is not adding more isolation than can already be realized. So, if an existing design is being reviewed for cost-reduction opportunities, resilient channels are usually the first thing to go. Also, many designs fail to recognize the top sin of using resilient channels: installing them between layers of drywall. If you recall, narrow air gaps in stud-and-drywall construction cause degradation of sound-isolation performance in the lower frequencies due to mass-air-mass resonance.
Sound-absorbing panels are most certainly important to the acoustics of a cinema auditorium. But when clients attempt to address sound transmission—or as more garishly termed, sound bleed—between adjacent auditoriums, the question is often asked whether installing more or thicker sound-absorbing wall panels will improve sound isolation. The simple answer is no. Sound-absorbing wall panels, as well as the lay-in ceilings in the auditoriums, are placed there to improve the sound in the acoustic environment of the auditorium in which they are used. I often describe the situation by noting, “If sound-absorbing panels were all that was needed to control sound transmission between spaces, why not just use drapery to separate adjacent auditoriums?”
Laminated gypsum board is a much-discussed product and it has its benefits to certain projects. For the unfamiliar, laminated gypsum board is similar to laminated glass, except two thin layers of drywall are adhered with a viscoelastic layer. The main benefit to laminated gypsum board acoustically is seen at the coincidence frequencies, which for drywall are in the 2,000 to 4,000 Hertz high-frequency range. At these frequencies, the coincidence dip seen in the transmission loss curve is reduced, improving the performance of the wall or ceiling at these high frequencies. However, as anyone who has experienced sound-transmission problems between adjacent cinema auditoriums can attest, the problem almost always occurs in the extended low frequencies, not in the high frequencies. Even with the coincidence dip of standard drywall, transmission loss values are up in the 55 to 65 dB range, so gaining a few decibels of isolation is not really very relevant.
Sound-retarding doors are often considered for use as auditorium entry or exit doors, particularly when the auditorium exit door may be near a busy roadway or an item of noisy equipment. The reality, though, is that these types of sound-rated doors are primarily designed for application to recording studios or acoustic testing labs. These doors usually possess cam-lift hinges to ensure gravitational force is applied uniformly to the door perimeter seals to ensure that sound leakage around the door where it meets the frame is minimized. This type of hinge is harder to open than a typical door with butt-hinges and may not be appropriate for use in public spaces such as movie theatres and may not meet ADA (Americans with Disabilities Act) opening force standards without incurring the additional costs of automatic door openers. The best door for an auditorium is relatively heavy: one-and-three-quarter-inch-thick solid-core wood or insulated (glass or mineral fiber) hollow-metal doors with adjustable field-applied sound gaskets at the head, jamb and door bottom.
Sloping and shaping sound-absorbing ceilings in cinema auditoriums are often considered as being helpful to the distribution of sound in an auditorium, but that’s a totally unfounded myth. Ceilings in cinema auditoriums are designed to absorb sound, not reflect it. Finish materials in a performance space are shaped to reflect or diffuse incident sound, but in these applications the shaped material is drywall or plaster, which reflects sound instead of absorbing it.
Insulation blankets above lay-in auditorium ceilings are seen in many cinemas. The thought is that the glass fiber or mineral fiber will improve the sound absorption that the ceiling provides, but the ceiling is already designed to absorb sound. A glass fiber lay-in ceiling panel absorbs about 80 to 90% of incident sound and a mineral tile panel absorbs about 55 to 65%. Laying a six-inch-thick blanket of insulation above a lay-in ceiling in an auditorium may add three to five percent to those numbers, but the benefit (not to mention the additional weight the ceiling grid must support) doesn’t meet the additional cost.
I could go on for another hour or two with elements ill-applied to cinema acoustics, but press time is rapidly approaching! Thanks again for reading.
Brian Kubicki of ADK, L.L.C. may be reached at email@example.com.