The agreement sets out a process for what will happen if those preventive measures fail -- with a Music Center lawsuit to collect damages the worst-case scenario.
But Metro and Music Center officials say they're confident that the precautions being taken will allow subways to run silent and run deep (135 feet below street level) in a way that won't impinge upon Disney Hall's acclaimed acoustics.
The danger is not from the typical screeches and loud rumbling noises that passengers waiting on a subway platform are accustomed to hearing. The subway, expected to begin running in 2020, is too deep for those to register in the concert venues.
The threat is vibrations created when metal train wheels pass over metal tracks. Strong vibrations could send energy waves through the ground and into the concert venues, emerging as a low-frequency rumble in the halls. So the key is to stifle vibrations at the source.
Pennington said that Metro's engineering and construction contractors will be asked to find the optimal kind of rubber to use in the noise abatement padding and insulation.
Additionally, he said, the concrete slabs into which tracks are laid will be stiffened as much as possible – another way of making sure vibrations from the tracks won't travel.
The agreement comes after more than a year of talks between Metro and the Music Center that ensued after a subway simulation test at the Colburn School yielded audible low-frequency noise, alarming, among others, Disney Hall’s architect, Frank Gehry, and its acoustician, Yasuhisa Toyota.
Metro and Music Center officials said that ensuing talks culminated in tests aimed at establishing the amount of "ambient noise" that's detectible in Disney Hall when it's empty and quiet – a level that Metro is now committed to keeping intact even with subways running by every few minutes.
Sherman said the cooperation from Metro has been consistently good, with all parties sharing the aim of an inviolate concert and recording experience.
He said he had no estimate on how much those measures might add to the cost.
Sherman said it has been clear that Metro's aim isn't to minimize expenses but to ensure optimal sound in Disney Hall.