6788280 Home Theatre Acoustical Design

TECHNICAL BULLETIN Home Theater Room Acoustical Design

CinemaSource Technical Bulletins. Copyright 2001 by CinemaSource, Inc. All rights reserved. Printed in the United States of America. No part of this bulletin may be used or reproduced in any manner whatsoever without written permission, except in brief quotations embodied in critical reviews. CinemaSource is a registered federal trademark. For information contact: The CinemaSource Press, 18 Denbow Rd. Durham, NH 03824

CinemaSource Technical Bulletin

Home Theater Room Acoustical Design

ore and more home theater rooms are being designed from the ground up. Whether it be a spare bedroom, an unfinished room in the basement, or even a completely new addition, many home owners are deciding to start "with a blank slate". The reason for this trend is simple. When you start from scratch, you have complete control over almost all construction details and thus have the ability to optimize your room for the highest level of performance. However, there's a small catch. When you start from scratch, some of the more esoteric construction details needed to bring your home theater into the realm of world-class performance are difficult to get information on. Getting opinions on these matters isn't hard to do; what's difficult is finding objective, proven information that you can count on in your design. In this section, we will tackle just one small, but important, construction detail you should know about; the optimum physical dimensions for a home theater room. We will toss aside a lot of the conjecture about the subject and focus on known research data. As you will see, the dimensions of the room, and seating placement within, can have a dramatic effect on the resultant home theater beast.

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Room Dimensions - Audio Performance Considerations The laws of nature often provide us with limiting criteria and we have to deal with them in real world applications. Determining the ideal acoustic dimensions of a listening room is one such process. Fortunately, however, with regards to audio performance, the subject material has been well researched with the result that the acoustical behavior of listening rooms is reasonably well understood. As a means of introduction, let's look at the character of sound itself. Sound is the propagation of energy that occurs longitudinally among air molecules. When a sound generator, for example, a speaker driver, pushes air molecules nearby, local molecules push other molecules and the resultant wave of excitation travels throughout airspace. You are probably familiar with sound waves being compared visually to ocean waves. This is done for pedagogical purposes. The reason for this is that water waves are easy to visualize, and represent a similar phenomena. It's just that water waves are lateral in nature and sound is air molecules excited in longitudinal form. Both of them propagate with similar effects.

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For the purposes of analysis, professional acousticans divide up the sonic behavior of listening rooms into four intervals. Each interval is characterized by a frequency range of sound. As it turns out, the physical dimensions of a listening room have the profoundest effect on the behavior of the lowest frequency range, the one that bass notes (200 hz and lower) occupy. The behavior of this interval, in acoustic terms, is dominated by "standing wave resonance effects". Standing waves are the phenomena that results when waves in an elastic medium, such as air, travel back and forth within an enclosure. At certain critical frequencies, the waves traveling in one direction combine with those bouncing back, with the result that some areas multiply in strength and others actually cancel out. In our diagram below, we show low frequency sound waves being generated by a speaker bouncing back and forth in a tubular enclosure. The energy in the sound is actually a pressure wave and the combination of the waves traveling in opposite directions yields high pressure zones and low pressure zones. If you were to step inside this tube and walk to the areas of low pressure, you would actually hear the sound disappear. At other points, the sound would sound twice as loud as normal! A home theater room behaves much the same way as our "resonance tube" but with the added complexity of more dimensions. Instead of just two surfaces, we have six (four walls, the floor and the ceiling). Sound waves can, and will, bounce off not only just two surfaces (axial modes), but four surfaces, (tangential modes) and even all six (oblique modes) at once. A complete analysis of the sound field in a room must account for all these mode

When a speaker drives a column of air in an enclosed space, standing waves occur at specific frequencies

interactions to understand the resultant pattern of high pressure and cancellations. Our diagram shows the simple combination of sound energy of two axial modes into tangential mode form. As you can see, complex disturbances in the room's sound field emerge. This is exactly what we want to avoid in a well designed home theater room. As it turns out, if you design a listening room with exactly the right dimensions, you can spread out the high and low pressure areas so that an optimum sound field is obtained. Roy Allison, founder of RA labs and Allison Acoustics, has written extensively on this, and in his paper "The Loudspeaker/Room System" he examines some of the "golden room ratios" developed by researchers L. Louden and L.W. Sepmeyer. These golden room ratios are the result of calculating the modal distributions created by low frequency sound waves, and striving for the most uniform distribution. L.W. Sepmeyer's ratios, in particular, have been highly utilized. Our table illustrates the three famous Sepmeyer room ratios that have formed the basis for hundreds of listening room designs over the last 30 years. It should be noted that if you are starting from scratch in a home theater room project, it isn't absolutely necessary to design your room around these golden ratios. Often these golden ratios may not fit one's aesthetic needs or other constraints. Other options concern modifying the sonic behavior of the room with acoustical treatments. This is especially true in existing rooms. The general principle is to tame low frequency behavior by disturbing standing wave formation with either physical obstructions (furniture, bookcases, etc) or via absorbing objects (furniture, tube traps, etc). (This subject was examined in greater detail in

SOUND PRESSURE GRAPH

MAXIMUM SOUND PRESSURE

MINIMUM SOUND PRESSURE

MAXIMUM SOUND PRESSURE

MINIMUM SOUND PRESSURE

MAXIMUM SOUND PRESSURE

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ROOM SOUND PRESSURE

ROOM SOUND PRESSURE

=

+

ROOM SOUND PRESSURE

AXIAL STANDING WAVES PRODUCED ACROSS THE LENGTH OF A ROOM

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AXIAL STANDING WAVES PRODUCED ACROSS THE WIDTH OF A ROOM

PRODUCES COMPLEX TANGENTIAL RESONANCE MODES THAT CANCEL SOUNDS AT MANY POINTS THROUGHOUT THE ROOM

our last issue.) However, having a good understanding of the effect that room dimensions can have on the distribution of resonance modes can provide you with a starting point if you are designing from scratch.

SEPMEYER'S THREE GOLDEN ROOM RATIOS ROOM TYPE

CEILING HEIGHT

ROOM WIDTH

ROOM LENGTH

ROOM A

C

1.14C

1.39C

ROOM B

C

1.28C

1.54C

ROOM C

C

1.60C

2.33C

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1.54C

1.28C

C

EXAMPLE: SEPMEYER ROOM B

Software designed to help you with your home theater room project Ovation Software offers a wonderful software package designed not only to help you plan your home theater room design, but also to fully calibrate the audio and video equipment after installation. The package is called AVIA (Audio Visual Interactive Aid) and is available for Windows 3.1/95/NT, MAC OS and DVD. A feature we found especially interesting is a real time resonance mode calculator that allows you try out different combinations of room dimension ratios and see the resultant resonance modes plotted out Ovation Software (614-373-6212)

AVIA’s Resonance Mode Calculator

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Home Theater Room Acoustical Design

acoustical makeup of a listening room can play a significant role in the total system of sound reproduction. Roy explains: "The acoustical parameters of a room have a tremendous effect on the quality of sound you hear. The problem is that acoustics as a science is poorly understood by many audio enthusiasts. However, it should be known that one does not need a PhD in the field to properly treat their own listening room, a few modest room treatments can go a long way. The only trick is knowing which ones are best to use". Back to school

our decision is finally made. After weeks of reading reviews and driving from store to store, you finally decide on the speaker system that's going to make your home theater room come alive. It's a little more money than you wanted to spend, but your local specialty retailer has offered you a deal that's hard to refuse. The next thing you know you're back home pulling speakers out of cardboard boxes, and draping speaker cables all over the place. Finally, when all the connections are made, you plop yourself in the front row center seat, hit the play button and....Hey, wait, what's going on. Where's the center channel detail I heard in the store! And where's that thundering bass!

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Welcome to the mysterious world of room acoustics! The reason that your speakers sounded different in the dealer's demo room D most likely had to do with the difference between the acoustical characteristics of his room vs. yours. Audio experts have known for years that the physical LISTENING ROOM attributes of listening SOUND COMPONENTS: rooms are every bit as important as the audio D = Direct Sounds equipment involved. As F = First Reflections matter of fact, some would R = Reverberations argue that room acoustics are far more important than popular equipment features such as gold connectors, polypropylene capacitors and the like. Now don't get us wrong, it's not that these details are unimportant; it's just that room acoustics are far more important. And often overlooked. One of the first scientists to investigate the role of room acoustics in high quality sound reproduction was Roy Allison. Roy's seminal research in this area was revolutionary and led to a new way of looking at speaker and equipment design. The basic premise is that the

Before we can dig deeper into the subject of listening room acoustics, a basic review of the physics of sound is prudent. By understanding the way that sound waves travel in listening environments, we can more fully understand how to keep them out of trouble. Although the exact acoustical sound field that occurs in an enclosed space can be quite complex, there are three components that predominate. The first is the direct sound from the speakers themselves. These are the sound waves that travel in a straight line directly from the speaker drivers to the listener’s ears. These direct sounds are considered the most significant component of sound reproduction because of their relatively R large amplitude and direct transmission F characteristics. Their successful propagation merely requires an unencumbered line-ofsight path from the speakers to the listeners. Next are the first reflections. These are the sound waves that bounce off surfaces flanking the R speakers and the listeners. As our diagram illustrates, these sound paths typically bounce off nearby walls, ceiling and floor surfaces. Acousticans feel that these early reflections are important to the perception of the “sound stage" and a failure to properly attenuate them can result is a marked reduction in “breadth” of the recording. The last component is reverberation. Reverberation consists of the countless random reflections that bounce off other surfaces in the room and eventually arrive at the listener's ears. These sound reflections reinforce the feeling of room size and ambiance. When you are in a

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Some of the soundwave is absorbed

All of the soundwave bounces off

All of the soundwave is diffused

How soundwaves reflect off room surfaces ACOUSTICALLY REFLECTIVE SURFACE (WALLBOARD, WOOD)

large room with hard surfaces, such as a gothic stone church, the reverberatory echoes bouncing off the stone walls are the components that give you that sonically cavernous feeling. Listening Room Physics So how do these sound components effect your listening room? When you fired up your new home theater speaker system, all the sounds bouncing around the room combined to form a unique acoustical room signature. This acoustical signature effectively becomes superimposed on the primary recording and modifies it. It is almost as if there are two separate sound systems in the room playing at the same time. Acousticans agree that the best way to design a listening environment is to strive for sonic balance of these “systems”. For example, you do not want an overemphasis on reverberatory energy components or it sounds like you are listening in a tiled bathroom. On the other hand, you do not want a room void of reflections or the room sounds dead and lifeless. The ideal listening room allows some of the first reflections and some of the reverberatory components to appear at the listeners ears. Just enough of each to balance the resultant sound and make it sound natural. There are two ways to control the first reflections and room reverberations to achieve sonic balance; absorption and diffusion. Absorptive surfaces consist of materials that dampen sound energy so that only a fraction of the energy is reflected. The portion that isn't reflected is actually converted into a tiny amount of thermal energy which dissipates into the air. Acousticans use "Absorption Coefficients" to indicate how well a material absorbs sound. The scale ranges in value from 1 to 0. A material with a absorption coefficient of “1” means it absorbs sound energy completely. A material with a “0” coefficient reflects it entirely. As you might imagine, real world materials lie somewhere in between. The chart on this page shows the different absorption coefficients of typical surfaces found in

ACOUSTICALLY ABSORBING SURFACE (CURTAINS, CARPETS)

ACOUSTICALLY DIFFUSING SURFACE (BOOKCASES, ACOUSTICAL PANELS

residential structures. The other technique used to control sound is diffusion. The principal here is to take the sound waves directed at the diffusive surface and break them into many small components. This resultant sound field is then scattered around the room at greatly reduced magnitude. This technique can also be augmented in a listening room via natural room components like bookcases, furniture or with specially engineered diffusion panels. How to do it There are dozens of methods and hundreds of products available to whip your listening room into shape. Rather than catalog all the products and techniques available, we will take a more pragmatic approach. We will consider each room surface individually and suggest appropriate treatments. First, lets look at the floor and ceiling surfaces. These can be the worst offenders in a listening room because they are often constructed of extremely reflective materials. The absorption coefficient, for example, of your standard plaster/gypsum board ceiling is approximately .05 (at 1kHz). In other words, sounds bounce right off this stuff. And hardwood and tile floors aren't much better with absorption coefficients of .01 and .07 (at 1kHz), respectively. Fortunately, though, there is a standard household building material that does a wonderful job of soaking up sound waves; carpeting. Both wall to wall carpeting and area rugs do a wonderful job of absorbing incident sound energy. And fortunately carpeting goes quite well with most listening room decors. Ceilings, however, can be more problematic. One could glue carpeting to the ceiling but we doubt your interior decorator (read: spouse! ) would approve. The next best thing is to install acoustic ceiling tiles. The least expensive option is to use the standard 1' x 1', fiber-based, tongue and groove tiles sold by building supply stores. Typically, these are installed 12" on-center, on 1"x 3" strapping that

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is firmly screwed into the ceiling joists. Proper installed, these fiber-based tiles actually do a pretty good job of absorbing incident sound waves, particular in the higher frequencies. Even better sound absorption can be obtained via drop ceilings, and most building supply stores offer a wide variety of acoustical tiles designed for this purpose. The standard "worm-hole" pattern tile is common in commercial applications, because it provides a good mix of absorptive and diffusing properties. One thing to note: if you plan to listen at very high sound levels, drop ceilings can rattle. This could be quite annoying but several companies offer small rubber hangers to isolate the drop ceiling structure and thus reduce this problem.

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If these conventional wall taming approaches are inconvenient, you can consider professional products. RPG Diffuser Systems, for example, offers their Acoustic Tools For Home Theater™ line. This line contains several panels that can be used to attenuate wall reflections. Armstrong also offers a line of decorator styled acoustical wall panels designed for easy installation. Their SoundSoak™ panels are available in many woven fabric colors and are constructed from 79% recycled materials.

The remainder of the surfaces in the room contribute to the reverberatory ambiance. Amateur acousticans often test the reverberatory tendencies of a room by standing near the listener's position and clapping their hands You can also consider the professional acoustical ceiling together loudly. The resultant echo can tell you a lot about materials. Various companies sell materials that are decor reflected sound signature of the room. If the clap friendly and specifically designed for ceiling use. IIlbruck, produces a distinct echo, you probably have some for example, offers their famous Sonex™ panels made surfaces that need to be toned down. Generally, the main from a new, Melamine™ absorptive foam. These panels offender is the rear wall. If it is flat and open you should are available in a number of consider different surface textures, some and in a variety of colors. treatments. For those who are opposed As in the Room Diffusers to the commercial "anechoic" other look of Sonex, there are surfaces, plenty of flat-faced products this does available. These are typically not always constructed of fiberglass or necessitate foam materials. Acoustical professional Solutions, for example, products. offers a number of panels Anything to that can be applied to the reduce ceiling. Their Alphasorb™ reflections panels are constructed of is fiberglass sheets wrapped appropriate. with woven decorator fabrics Bookcases, and are offered in 65 pictures, different colors. Several drapes, etc. companies also offer special can absorb ceiling diffusing panels. These panels come in a multitude and diffuse the sound energy. of shapes and sizes and designed with intricate diffusing surfaces. The last issue to consider is "standing waves". Standing waves occur when lower frequency sounds bounce off Listening room walls often promote sonic misbehavior opposing walls and combine to produce unnatural because they are typically constructed of hard substances sounding peaks or dips in the bass response. First of all, such as gypsum board or wood. Since the wall surfaces unless your room has large areas of opposing surfaces, located near the listener are directly involved in first this may not be a problem. However, if your room is stark, reflections, it is especially important that they be standing waves could occur. Absorption is the technique of addressed. For the economy approach, ordinary drapes choice here and one of the best ways to soak up standing can go a long way. You can also place furniture, plants or waves is with furniture. Plushly upholstered seating does a other items in the way to diffuse the sound. For example: great job. Several manufacturers also offer high tech it is hard to beat an upholstered couch or chair for sound professional approaches. Acoustic Sciences Corporation absorption. If an object like this is placed right at the spot offers their Tube Traps™ to reduce standing waves. These on the wall where first reflections occur, a remarkable devices are tall free standing cylinders that are designed improvement in sound can result. to absorb specific sound frequencies. Half sections are

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SOUND TRANSMISSION CLASS (STC) RATINGS STC RATING

CHARACTERISTICS

50-60

Loud sounds do not pass through

40-50

Loud speech heard faintly

35-40

Loud speech is heard but is not understood

30-35

Loud speech is heard and understood

25-30

Normal speech is heard and understood

20-25

Faint speech is heard and understood

Wall Construction Methods

2x4 Wall Studs

5/8" Drywall

STANDARD WALL, STC= 33 (empty cavity)

Keeping The Bad Sounds Out So far we have concentrated on the sounds inside the home theater room. Often overlooked are sounds that originate from outside the room. These extraneous sounds typically travel in through windows, walls and ceilings and can range from quiet conversations in adjoining rooms to fire trucks outside with sirens going full tilt. Whatever the source, these noises are unwelcome sonic intruders.

1/4"- 1/2" Drywall

By isolating the wall surfaces from each other, STC ratings can be dramatically improved. The second wall surface diagram shows how a second sheet of drywall can be isolated from the wall structure with VE/VO™ damping material from American Acoustical Products. The next diagram shows a similar isolation but with metal resilient channel used to “float” the drywall. Resilient channel is available at most large building supply stores. The last diagram shows one of the most secure ways to inhibit sound transmission; build two independent, uncoupled walls. This structure can have an STC rating up to 63 and will impede sound transmission about the same as a 8” reinforced concrete wall (!) A home theater room constructed of walls like this should be dead quiet.

5/8" Drywall

DOUBLE WALL with SANDWICHED DAMPING MATERIAL, STC= 36 (empty), STC= 44 (with insulation)

The solution : Acoustic barriers. Acoustic barriers do exactly what they indicate. They stop sound from traveling through the structure and entering the home theater room. On the right we illustrate four common wall construction techniques and their Sound Transmission Class Ratings (STC) ratings. The standard wall, with 5/8” wallboard screwed to 2x4 wooden stands, is a relatively poor sound barrier with a STC rating of 33. As the chart above shows, this will allow loud speech to easily pass from one side to the other.

VE/VO Adhesive Damping Material

Resilient Channel

5/8" Drywall

Air Space

DOUBLE WALLBOARD with RESILIENT CHANNEL, STC= 40 (empty), STC= 48 (with insulation)

1"

Doubled 5/8" Drywall

DOUBLE WALL, DOUBLED WALL BOARD, plates separated by 1", STC= 58 (empty), STC= 63 (with insulation)