Room Acoustics: Isolation & Noise Control
Taught by Steve Haas of SH! Acoustics
Our first three courses in the CEDIA Seminars series covered proper system calibration after installation and set-up was complete, followed by two complementary courses on room acoustics. More often than not, however, a great home theater must be capable of co-existing within the framework of the day-to-day goings on in the rest of the house and its occupants. This is where an understanding of how to isolate your home theater so as to control the inevitable noise leakage of dinosaur foot falls and anti-aircraft guns in the rest of your home's living space can be a really good idea.
Steve Haas' company, SH! Acoustics, www.shacoustics.com specializes in room isolation and noise control. In the best case, Steve and CEDIA-certified specialists like him should be called in during a home's initial design so that he can coordinate with the home's architect. Retrofitting of fully isolated home theaters is also possible, though sometimes much more time consuming if, for instance, existing wall structures need significant modification.
Most of the basic principles presented here can also be applied to dual-purpose rooms. It is possible to achieve moderately better overall dynamic range with attention to details such as suppression of A/C noise, plumbing and room- induced rattles. What will not usually be possible with dual-purpose rooms, however, is the attainment of the greater degree of isolation from the rest of the house, especially in the bass frequencies. It is this greater degree of isolation which can add ~20dB of dynamic range to a system. This is a huge expansion of dynamic system capability. And it cannot be achieved by any other (sane) approach.
The goal of room isolation is two-fold:
- Provide an environment free of extraneous noise and vibration intrusion (within the home theater).
- Prevent theater sound from interfering with other home activities or neighbors.
That first statement in bold above should be a goal of all home theater or music listening environments. We will talk about terms and measurements which can be used to describe the absence of extraneous noise in the pages following. The most important point to be made here though is that every 3db of extraneous noise eliminated in the listening environment results, in essence, in a 3dB gain in dynamic range. And properly executed, those 3 dB can add up quickly. Contrast these multiple 3dB gains with the single 3dB gain experienced from a doubling of amplifier power and the efficacy of proper room design becomes much more appealing. Let's define….
Basic Definition - The difference, in dB, between the loudest and the quietest sound in any audio program. For example, Compact Disc audio, with 16-bit encoding, has a theoretical dynamic range of 96dB. In order to be able to actually hear these loudest to softest extremes, however, all components in the playback chain need to have greater capabilities.
For instance, the signal-to-noise ratio of a receiver or pre-pro plus amplifier needs to be greater than 96dB. High quality amplification components most times have no problem meeting this spec, many having S/N ratios over 100dB. So let's proceed to the listening environment. For it is the listening environment which is always the biggest roadblock to extended dynamic range.
A typical audio track, whether music or movie, might have a dynamic range of 80dB (based on the content of the track). A non-isolated home theater with an ambient noise level of 45dB and a peak playback level of 105dB would result in an effective dynamic range of 60dB, or the masking of the lowest 20dB of dynamic audio information on the track. (Note also that 40dB is generally considered the ambient noise level in a quiet neighborhood.)
For reference, let's take a look at a standard, say what THX requires for a home theater to be certified. Their "quietness" spec for a home theater is less then or equal to NC22 (explained below). That's quiet! But the "room quietness" spec on the low end coupled with an 115dB peak requirement for subs and 108dB for satellites (both with no audible distortion) is what gives a THX certified theater its huge 86dB dynamic range (108 - 22 = 86).
Next, for a second standard with which to compare; Harman's Multichannel Listening Lab environment, which I often reference, has an NC of 5!! Thus, when double-blind listening tests are performed at their nominal 75dB level (B-weighted at 3 meters) the Listening Lab has a dynamic range of 70dB. Note that to get down to the NC 5 Noise Criteria figure the door leading into the Listening Lab is approximately one foot thick! And you have to step up eight inches or so onto the fully floating floor!
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