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Room Acoustics: Isolation & Noise Control - page 2

By Patrick Hart

Elements to consider when seeking to maximize the isolation and control the noise from your home theater are:

  • Locating the theater - where, within the house, given the choice, would be the best location for the theater.
  • Internal noise isolation - a dynamic range somewhere greater than 70dB would be a minimum figure.
  • Impact/footfall Noise Isolation - footfalls heard inside a home theater from above means bass can escape out of the HT.
  • Environmental Noise Isolation - A/C has to come into the theater and projector fan noise may require further isolation.

The terminology used describe the degree of isolation achieved outside the theater is quite different than that used for the in-the-theater room acoustics. In parts one through three of The CEDIA Seminars we were focused on damping reflections within the room in an attempt to realize a direct-sound, flat-frequency-response at the listening position. Here, in part four, we seek to achieve as quiet an environment as possible, which adds to the system's dynamic range, while at the same time confining that great sound, to the greatest extent possible, within the home theater. Some terms:

  • Noise reduction (NR) - the measured difference in sound level produced in two enclosed spaces or rooms - a receiving room and a source room - by one or more sound sources in the source room.
  • Transmission loss (TL) - the measured difference (usually in a laboratory) of the sound levels between the source and the receiving room taking into account the area of the common partition and the sound-absorbing characteristic of the receiving room.
  • Noise Isolation Class (NIC) - the single number rating of a partition's isolation value based on field measurement of NR at one-third octave bands.
  • Sound Transmission Class (STC) - the single number rating of a partition's isolation value based on laboratory measurement of TL at one-third octave bands.

Notes:

    • The higher the NIC or STC number, the more overall isolation value a partition has.
    • NIC and STC are not defined below 125Hz. Therefore, understand the limitations of their usefulness for home theater or other applications requiring low-frequency isolation.

One last term: Impact Insulation Class (IIC) - Said to be a measure of foot falls, the new method unofficially known as the "tire drop' is a single number rating of a partition's isolation value based on field measurement of NR at one-third octave bands. This "tire drop" test was developed in conjunction with the Acoustical Society of America and is the best test we now have to measure structure-borne transmission such as with subwoofers.

Note however, that this test can only be made after the entire home theater project is completed. So it is essential that a specialist in home theater isolation design be employed from the project's outset. Each small detail of an isolated room design must be perfectly executed and checked before the next step is attempted. There exists literally dozens of ways for a room's isolation to be "short circuited" by improper mating or termination of a floating part of the room assembly to a fixed part of the assembly. And such a mistake, however minor, can cause gains in a room's Noise Criteria (NC) when there should have been losses.

Locating the Theater - Basements
Home theaters, even in new house construction, are usually already assigned a particular location within the home. If a basement is available, the noise leakage to other rooms will be either airborne noise through the ceiling or possibly duct-borne noise through (or around) the air conditioning ducts.

Considerations for basement home theaters are:

  • The extremely strong (high "Q") bass peaks which are usually set up with rigid cement block construction. Constructing a second semi-floating internal wall will usually alleviate strong bass modes to a great extent.
  • Sound transmission can still find a path through the air conditioning/heating system. Proper attention needs to be paid to the size and routing of the ducts. Also, sound material (covered later) can be added to ducts. And finally, the air conditioning ducts must pass through properly sealed and isolated barrier walls.
  • A floating floor as opposed to cement-with-padding-and-carpet is always more preferable in a high performance home theater. Tactile-feel sound waves, so strong that you can actually experience a realistic simulation of an onscreen event, is the "crème" at the top of a high performance home theater. One need only experience the Cuba Gooding Jr. at-the-guns scene (with two really powerful, flat response subwoofers!) from Pearl Harbor to know what I'm talking about.
  • And no. Sorry… you can't just run down to Home Depot and find some "soft squishy stuff" and hope that will suffice for a floor isolator for c) above. You need an engineered isolator and proper installation to float a floor correctly.

 

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