In-Room Acoustics

For a professional installation (which this is definitely not), a quality acoustical design would typically involve hiring an acoustician who would ideally create and run detailed simulations of the room to model the results and arrive at the optimal design. Since I wasn't prepared to hire someone or run these acoustical simulations, I opted for the next best approach: reviewing technical information, recommendations from several experts, and insights from well-regarded audio websites. This information was then used to design the room's acoustical elements.

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In this section, I'll summarize the primary considerations gathered from the research, as mentioned earlier, and then discuss the design decisions for the theater room. Some tradeoffs were made, which impacted room acoustics. At some point, measurements are needed to assess the results of these tradeoffs and identify the required tweaks, but to date, I have not been motivated to gather this data.

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The main objectives for the room's audio performance goals are summarized as follows:

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1. Clear, intelligible dialog

2. Precise sound localization

3. Seamless sound movement around the room

4. Reproduce accurate and even tonal balance with full dynamics

5. The audio experience is consistent for primary seating positions

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A small room is not an ideal environment for audio reproduction. The room itself creates some distortions that "muddle" the audio. Designing the acoustics within the room aims to fix this. A small room with highly reflective walls (e.g., typical drywall) suffers from several "distortions" to the audio that should be addressed. Some of these are:

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• General Reverberation consists of sound bouncing around multiple closely spaced reflections on the room's boundaries.

• Lateral reflections are typically sidewall reflections that create perceived phantom sources.

• Early reflections reach the listener shortly after the originating sound (typically <15ms), which confuses the brain; many lateral reflections are also early reflections.

• Standing waves, also called room modes, create a very uneven frequency response that varies with location within the room. This is most pronounced in the lower frequencies.

 

Breaking the problem (and the room) into two based on sound frequency ranges is helpful when addressing these issues.   In the lower frequencies, the dominating issue is room modes. In the upper-frequency range, the primary distortions are reverberation and reflections. The dividing line (actual frequency) between these two regions is called the Room's Transition Frequency or Schroeder Frequency. (There is a third frequency region called the transition region, the first 2 octaves above the Transition Frequency where both room modes and reverberation are significant). The transition frequency depends on room size (and reverberation time); a typical home theater-sized room is roughly 200-300Hz.  

 

During the room design phase, the tools available to address audio issues are:

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1. Sound absorption—Material placed on walls that absorbs reflections, such as fiberglass or mineral wool.

2. Sound diffusion—An object (usually a panel) affixed to a wall to break up a reflection and scatter it in many directions.

3. Speaker placement—Typically most effective for the subwoofer because the positions of all other speakers are well-defined (e.g., Dolby).

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Addressing Reverberation

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An initial concern to address is controlling (reducing) the overall reverberation in this room. The human auditory system anticipates some reflections; thus, completely removing them is undesirable. Reverberation time reduction can typically be accomplished by applying absorptive materials to the room's walls and ceiling. Ideally, these materials should absorb the entire frequency spectrum. However, addressing the lower frequencies requires thicker and more expensive absorbers. Another acoustical treatment, diffusion, can break up and disperse reflections and improve the sound. As discussed later, diffusion has not been used in this room.

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Reflection delay time (RdT), similar to RT60, is a specification used to quantify the amount of reverberation. It is used in acoustics and home theater design to quantify reverberation time (see Figure 1). It specifically refers to the time it takes for sound in a room to decay by 60 decibels after the source has stopped.

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Studies have been conducted to determine the optimal amount of reverberation. (although there is some conflicting information on this). An RdT between 0.2 to 0.5 seconds is recommended in a typical home theater for good audio intelligibility and generally pleasing sound. Accurately calculating the number of absorbent materials needed to achieve this value can be difficult. It is possible to calculate RdT approximately based on the room's surface area and the amount of absorption. This is a complex, approximate calculation, so I relied on general guidelines to estimate the overall absorption needed for this room to achieve a reasonable RdT.   From this, the number of panels can be calculated.  

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