Overview
Whether designed by experts or novices, the acoustic performance characteristics are critical to delivering a total theater environment. The artful combination of aesthetic design with performance-quality acoustics provides home theater users with the most realistic theater experience possible. The last thing any home theater owner or theater designer wants is having a great audio system compromised by its environment which has not be optimized for ideal playback.  
Areas of acoustic design significance include:
- room sizing and shaping
- room mode distribution
- early reflection control
- average room absorption
- spatial enhancement location of diffusive treatments
- isolation of sound between the theater and adjacent spaces
- ambient noise criteria
- control of noise from HVAC (heating, ventilating & air-conditioning systems)

Acoustic Primer
The interior design of a theater can be augmented with acoustic treatments that provide a high-quality stereo and surround sound imaging, enhanced spaciousness and localization of sound. Described below are the acoustic factors necessary to address in the design of a home theater. In order to explain the affect that the room size and shape has on the acoustic performance of the theater, we must first review the principles of room modes and modal distribution.

What are the basic definitions of sound?
 
Sound is made up of one or more pressure waves that cycle between high and low amplitude regions. The time period of each cycle is known as its frequency, expressed in terms of Hertz (Hz.). The physical length of each cycle is known as its wavelength. Lower frequencies have longer wavelengths. For instance, a sound with a frequency of 1000 Hz has a wavelength of one foot, while a 60 Hz sound has a wavelength of approximately 18 feet. The effect of the longer wavelengths in a small room (like a home theater) is a potential variation of loudness level dependant upon listener position in the room.

Do we have to worry about these frequencies at all?
 
Actually, this potential acoustic problem only occurs for frequencies below about the 250-300 Hz range, where the distance between the high and low amplitude portions of an individual wave are significant enough to produce the perceived variation in sound level. In order to minimize this effect, one must analyze the distribution of modal frequencies (modes) in the room.

What is a mode?
 
Given a distance, L, between any two planar surfaces (two walls or a floor & ceiling), there is a fundamental frequency equal to c/(2*L), where c is the speed of sound in air - 1170 feet per second. For example, a room with a length of 32 feet will have a fundamental frequency of about 18 Hz. The modes for this pair of surfaces are then defined as the fundamental and all of its multiples (e.g., 18 Hz, 36 Hz, 54 Hz, etc.). The modes between just two surfaces are referred to as axial modes. There are other types of modes that interact with four and even all six boundary surfaces of the room, known as tangential and oblique modes, respectively. Since the energy level of these types of modes, however, is much lower than that of the axials (through the impact on more surfaces), it is often necessary to evaluate only the axial modes
 
Once all of the modes between the three pairs of surfaces (front/back wall, side walls and floor/ceiling) are calculated and listed together in ascending order, they must be evaluated to ensure that a: there are no two adjacent modes that occur at the same or within a 2 Hz frequency of each other and b: that the average spacing between all modes is no greater than 15 Hz.

Why are these criteria important?
 
The first criterion is important for two reasons. First, if two or more modes occur at the same frequency, the peaks and dips in the wave amplitude will sum together - thus resulting in even greater variation of sound at different listening positions. Second, if the modes are not exactly at the same frequency but still within a small differential, a phenomenon called beating occurs, where a listener can actually perceive a note sustained at one modal frequency shifting back and forth between that frequency and another closely spaced modal frequency.

What is the difference between a mode and a standing wave?
 
There is none. The term standing wave was developed due to the fact that, in rooms with strong individual modes or multiple modes occurring at the same frequency, the high pressure region of the wave (or note) almost appears to the ear like it is "standing" in place.

How does this apply to room shaping and sizing?
 
The size and proportions of the room directly influence the modal distribution. If either length, width or height are the same dimension or multiples of each other, than there will be modal overlaps.

Should a theater design incorporate non-parallel walls to help eliminate the modal problems/standing waves?
 
This is a common misconception. Because of the long wavelengths in the low frequencies, slightly angling the walls will not break up, or diffuse, the room modes. Rather, it will take a modal condition that can be calculated, and turn it into something much less predictable.

Early Reflection Control
In any typical room, sound arrives at the listeners’ ears via two main paths:
- Direct, in (more or less) a straight line between the source and the listener.
- Reflected, off of a multitude of surfaces in the room.
 
Because any reflected sound wave arrives at the listener's ear slightly delayed in time from the direct sound, there is another phenomenon that occurs known as phase cancellation, or sometimes called comb filtering.

What is comb filtering?
 
Based on the delay time of the reflected sound which, in turn, is dependant upon the path length difference between the direct and any individual reflected path, the high amplitude region of the direct wave could arrive exactly at the same time as the low amplitude region of the reflected wave - thus canceling the frequency at the listener's ears. This would occur at the frequency (and all its harmonics) whose wavelength relates to one-half of the path length difference.

Why is this important in the home theater?
 
The fundamental goal in designing a room for motion picture or music is to achieve the sound at the listeners ears that the producer intends. Assuming a flat response from the loudspeakers, this assumes that the room itself does not distort or otherwise alter the frequency response, the relative amplitude or the time arrival of any sound events. The comb filtering effect, if left untreated, would in fact cause a significant distortion of sound. The distortion would be different as well at every seat (or at least all seats on one side of the centerline of the room, if the room is completely symmetrical) due to varying reflection patterns and path length differences.
 
Another important reason to eliminate the early-arriving reflections is that a strong reflection can also cause a shifting of an image in the frontal sound-scape. For example, a strong reflection of a speaking voice playing through the center channel loudspeaker off of either side wall can make the voice seem like it coming more from the side of the screen rather than the center. This would be an obvious concern if the film director's intent was to have the voice be located in line with the actor's position on screen.

How to address these issues?
 
In order to avoid the distortion, comb filtering effects and image shift problems, establish which surfaces could reflect sound off and reach the listeners' ears within a very short time after the direct sound. Treat these surfaces with sound absorbing materials that are highly efficient at all frequencies above 125 Hz. These materials serve to significantly reduce the energy level of the reflected sound so that it is well below the level of the direct sound. Some comb filtering might occur, but the impact will be so small that no distortion would be heard.

Average Room Absorption and Reverberation Time
While evaluating surfaces on which to apply sound absorbing materials, we also evaluated the reverberation time, or decay time of the room. Often people make the mistake of applying sound absorbing materials to every possible surface in the room. Not only does this create an unnatural listening environment, but it also eliminates non-absorptive surfaces that can be beneficial for surround sound speakers to interact with (see discussion below on spatial enhancement).

What is reverberation time?
 
The reverberation time, abbreviated as RT60, is defined as the time it takes in seconds for a sound to decay 60 decibels (dB) in level once the sound source has been abruptly shut off. In most cases, the 60 dB reduction is equivalent to the time it takes for one to no longer hear the sound. To experience the RT of a room, clap you hands in a small office or conference room and hear how quickly the sound decays. Then go into a large atrium or lobby with all hard materials and do the same thing.
 
It is important to maintain a relatively low reverberation time in a home theater or listening room since this has a direct relation to the clarity and intelligibility of sound. It also allows any type of artificial environmental sound that is recorded onto the motion picture to be translated to the audience, and not masked by a longer decay in the listening room.

What is a usable reverberation time criteria for home theater design?
 
The amount and location of sound absorbing materials should be very carefully selected to arrive at a balance between reducing early sound reflections and achieving a target reverberation time goal of around 0.30-0.35 seconds at all frequencies between 250 Hz and 4 kHz, and 0.40-0.50 seconds at frequencies between 60 Hz and 250 Hz.

Spatial Enhancements and Location of Difussion Materials
As stated above, having too much sound-absorption in the room can create a difficult listening experience. Yet, having exposed flat and hard surfaces like drywall or wood in upper side wall or rear wall areas can result in several acoustic problems, including:
- flutter echoes, or short repetitive echoes of mid- to high-frequencies between two parallel hard walls.
- long delayed echoes off the rear wall that can actually shift localization of an image from in front of a listener to behind a listener.
- harshness of the reflected surround sound.

What can be incorporated into the home theater design to alleviate these concerns?
 
On relevant areas of the walls, incorporate materials that reflect the sound in a diffused manner. That is, the sound is scattered in multiple directions. This eliminates any perceived echoes as well as enhances the surround sound quality by widening the apparent width of the theater.

Sound Isolation
To ensure that the listeners in the theater do not hear outside noise that could interfere with their enjoyment of movies, or that others who might be in the house are not disturbed at times by the incredibly loud sound that can be generated in the theater, construction details can be developed to achieve a high level of sound isolation at most frequencies. These details deal not just with the basic wall, floor and ceiling partitions, but also all the critical details involving the sealing of penetrations, partition intersections and edge conditions that are essential to achieving maximum performance.
 
Companies claim to have materials or details that can achieve complete isolation of sound. These claims are almost never realized in the real world, since the materials do not significantly address low-frequency energy such as that from a subwoofer. Also, since the amount of sound heard on the other side of a room is dependant upon the level of the sound source in addition to the construction, it is impossible to say that no sound will be heard outside of the theater, especially at the bass frequencies.

Ambient Noise Criteria
Having a noise free environment is extremely important to being able to experience a wide dynamic range of sound. There are many instances where an action scene in a movie builds to a climax of visual and sound level and then suddenly stops with dead silence in the film. To have this immediate contrast in sound level appreciated to its fullest, the theater needs to have a reasonably low background noise level.

How is background noise measured and rated?
 
The noise in the theater is measured with a special sound level meter that records the average decibel level over a short period of time throughout the seating area. The level is measured in ranges of frequencies call octave bands to allow for a more detailed analysis of the quality of noise in the theater. The measured octave bands are then compared to a set of plots that rate decibel level versus frequency. These are known as the Noise Criteria, or NC, curves.

Can you tell me more about the NC curves?
 
The NC curves were developed as a comparison of noise levels for different types of spaces. Each curve shown below has a number attached to it. The lower the number, the quieter the space. For any NC curve, the lower frequencies have a higher decibel level than the mid- and high-frequencies. This is because the human ear is less annoyed by the presence of low frequencies - thus, the low frequency level can be higher and still match the tolerance factor of the upper frequencies.
 
The lowest curves around NC 15 and NC 20 represent the goals for extremely critical spaces like concert halls and recording studios, while a higher level like NC 35 or NC 40 is typically found in office buildings.

What would be a good target NC level?
 
A target goal of NC 20 is an ideal balance between making a reasonable effort to address acoustics and achieving a respectable Noise Criteria level.
 
The NC 20 curve corresponds to the following target noise levels in decibels:

63Hz	125Hz	250Hz	500Hz	1kHz	2kHz	4kHz	8kHz
51	40	33	26	22	19	17	16

Noise Control From HVAC, A/V, Electric, and Lighting Systems
The ambient noise in the theater is influenced by two factors
- outside noise sources infiltrating the theater's boundary partitions
- noise generated by systems that serve the theater.
 
The first group is addressed through the sound isolation construction described above. The second group is further broken down into categories of HVAC, audio/visual and electrical/lighting systems.
 
For the purposes of this article, we will only touch on the major areas of concern as each could be addressed in separate articles. For HVAC, there are various factors including proper selection of equipment/systems, location of equipment rooms, ductwork sizing & routing, internal attenuation treatments, vibration isolation and proper specifying of registers and grilles. For A/V systems, there are various factors including containing noise and vibration of projectors, amplifiers & other fan-cooled equipment; vibration isolation and mounting of loudspeakers; and shielding of electrical noise interference into the audio-visual system. For electrical and lighting systems, there are various factors including locating and attenuating noise of low-voltage lighting transformers and fixtures, electrical noise interference issues, dealing with refrigeration equipment from in-room bars, and selection of quiet dimming systems.

Back to Top