Noise & Negotiations: Judging acoustical factors in lease agreements
Welcome back to Film Journal International’s annual look at cinema technology. I have been once again blessed with the opportunity to share some interesting items from the acoustical world through my practice of acoustical consulting.
This should interest numerous parties in the process of designing and constructing cinemas: How should we address acoustical criteria in lease agreements between landlords and the various tenants in their buildings?
Multi-tenant commercial buildings can have a range of tenants, from retail clothing stores and bookstores to restaurants, and of course multi-screen cinemas. Each of these tenants typically enters into a written lease agreement with the landlord identifying the various terms and conditions that the parties agree to in advance of the tenant taking occupancy. Not being an attorney, I can only offer guidance on a particular aspect of these leases, and that, of course, is how the tenants and landlord should agree on suitably controlling sound transmission between tenants.
For many tenants in commercial retail buildings, this is easily addressed with subjective terminology such as “…shall not disturb persons in adjacent spaces…” or “…shall not disturb the operations of adjacent tenants…” In these cases, involved tenants are similar types of spaces. For example, a dress store and a neighboring shoe store will not likely disturb each other because both spaces have similar acoustical criteria. Both spaces sell clothes and probably broadcast background music in their spaces.
Cinemas are very different. They involve many auditoriums and each can be as quiet as a recording studio or as loud as a military airfield. When the auditorium is quiet, such as right before that pivotal screen kiss or right before a frightening image emerges from the darkness, it doesn’t take much sound level in the sports bar on the floor below to be heard in the auditorium. Likewise, when the cinema is busy conveying gargantuan size with rolling low-frequency effects, it takes very little sound energy in this frequency range to cause the Little Princess Boutique on the floor below to have perfume bottles shaking off the shelves.
Subjective terminology will not adequately address these situations. Unfortunately, when leases are being crafted, an acoustical consultant is usually not yet involved.
Many turn to the tried-and-true STC (Sound Transmission Class) ratings for construction separating the cinema tenant space from adjacent tenant spaces on the floor below or adjacent. Remember that STC ratings are laboratory-obtained parameters of the ability of a given construction to limit the transmission of airborne sound through the element.
The single-number STC ratings generally correlate with subjective impressions of sound transmission for speech, radio, television, and similar sources of noise in offices. These noise sources have a specific frequency range that typically includes very little extended low-frequency sound—down where movie sound effects occur in the sub-bass frequencies. The STC rating method is not appropriate for sound sources with considerable low-frequency content.
Using STC ratings in lease agreements involving cinemas is a noise dispute waiting to happen. We can set the required STC rating for walls and floor/ceilings between tenants to STC-90 and if the construction is not properly equipped with mass and structural separation, the construction can do very little in the extended low frequencies. Experience has shown that a much better method of addressing sound transmission between adjacent tenants is to specify a Noise Criteria (NC) rating (see ANSI S12.2) that intruding noise may not exceed. NC ratings extend down into the extended low frequencies, down to 16 Hertz, which is actually below where most of us actually hear sounds anyway.
The appropriate NC rating number selected to not be exceeded should be based on what the typical ambient plus activity noise levels are in the tenant space under consideration. For cinemas, NC-30 is typically set as the not-to-exceed criteria for the lease.
Other parameters such as method of measurements, averaging time, time constants, etc. can be negotiated between the involved parties, and these can be sticking points between the folks involved with the negotiations. But the foundation of NC over STC is solid and verifiable and works to sufficiently protect the involved parties.
Here are a few practical design and construction guidelines that have proven to work for cinemas:
1. The structural slab under the cinemas should at least meet 60 to 65 psf (lbs./square foot) in surface weight. This figure is determined by multiplying the average thickness of the slab by the density of the concrete used. This figure ensures a suitable degree of noise control for most adjacent tenant sounds and controls sound flanking along the slab between adjacent auditoriums.
2. Significant additional sound and impact isolation can be achieved in a floor/ceiling construction by providing an isolated concrete (floating) floor in the cinemas on top of the structural slab. A resiliently suspended multi-layer drywall ceiling can also be suspended below the structural slab in the tenant space below. If given a choice, the resilient ceiling approach is usually preferred because short-circuiting of the isolation system is easier to avoid. Floating floors are very good sound-isolation measures, but short-circuiting avoidance depends on careful monitoring of both design and construction.
3. Regardless of the floor/ceiling sound-isolation solution, be wary of how perimeter walls intersect with the ceiling or floating floor. A perimeter exterior wall with only one layer of drywall that is part of a curtainwall assembly can cause major problems with sound transmission around the floor/ceiling assembly.
Best of luck in your negotiations and, as always, please contact me with questions!
Brian Kubicki is the principal of ADK, L.L.C., and may be reached at firstname.lastname@example.org or (913) 400-3694.