The Importance of Speaker/Listener Locations

By Norman Varney

The corner stone for high fidelity playback is positioning the speakers and listener at the optimal locations in the room. The idea is to avoid as much room boundary interference as possible, while providing an accurate soundstage. In very basic terms, let's find out why this is so important to the end result. 

Room walls, floors and ceilings react to sound energy with reflections and resonances from the make up of their construction surfaces and cavities. These interferences compete with, and distort, the direct signal sent by the loudspeakers. As speakers are located further away from boundaries, less energy is transferred in which to move room surfaces. In addition, as listeners are distanced further away from boundaries, less energy from room surface resonances and reflections is received by listeners. This mitigation of non-original signal information means improved low-level resolution, dynamic range, spatial cues and timbre accuracy.

There are three types of boundary interferences:

1. Cavity resonances. Try stomping on a wood floor and pounding on a framed wall and listen for them to sound like a drum. This adiabatic compression of a low frequency note is dictated by the mass-air-mass construction of the partition itself. When a loudspeaker plays the frequency in question, the partition will move sympathetically, resulting in that note being returned to the listener from the room surface, after the original event. 

2. Room resonances. Like any kind of enclosed space or musical instrument, a room has resonances defined by its dimensions, mass, compliance and friction. Each axis; length, width and height, has its own frequency in which the lowest (longest) wavelength can fit. Resonance, or room modes, are "standing waves". They are formed when the distance is a multiple of one-half the wavelength. When this occurs, the resonant frequency (and its harmonics) will sound louder than normal in some locations, and quieter in others. Think of the waveform with its pressure peaks and valleys traversing from one surface to the opposite parallel surface, and then reflecting back into the oncoming waves, etc. As they collide,  peaks from one surface run into the valleys from the other, resulting in a cancellation of energy. On the other hand, some peaks will run into other peaks, causing an increase in energy level.

3. Reflections.  Obviously, if we position ourselves and/or speakers near a large surface, will will hear the effects of sound energy being reflected to our ears later in time than the direct signal. The distance between the loudspeaker, the surface, and our ears will determine how much interference will be perceived. Basically, if the reflection is within about 15 dB SPL of the direct, it will be audible. In addition, the construction of the reflecting surface will determine what extent and what frequencies are absorbed and reflected by it.

In rooms of rectangular shape (preferred), simple math will predict what frequencies will resonate. It is correct to think that certain dimensions will offer better results than others. For example, rooms with dimensions divisible by each other will tend to exaggerate those resonant frequencies because they are similar in musical relationship. Once you determine the fundamental resonant frequencies, you can figure out where the peaks and valleys are located in the room along each axis. It is important to figure out the second and third order resonant frequencies for each axis as well because their energy levels are also likely audible. With this information you can avoid placing your speakers and listener in locations that will exasperate the room's unique modes and offer the most linear bass response.

Positioning for Room Modes

All rooms have room modes. Larger rooms have more of them. This means that there is less of a gap between one and the next, which is a good thing. Fewer modes mean that they draw more attention to themselves. Because all modes start and end at the boundaries with high pressure peaks, you have lots of bass there. Essentially, about 3 dB (sounds twice as loud) more bass at a single surface boundary, 6 dB (sounds three times louder) where corners meet, and 12 dB (sounds four times louder) in a tri-corner. People can use this for passive acoustical bass gain, but at the sacrifice of accurate, linear bass response. Same results for listener locations.

Ideally, you want to avoid placing a speaker or listener in a mode peak. Doing either will result in certain frequencies being discernibly louder than all the others. Though it's best to place speakers and listener between these primary room modes, you must always compromise. With speakers, avoid the peaks over the valleys. With the listener, avoid both, with one exception. It is very important to place the speaker/listener footprint exactly between the side walls to allow for symmetry in the horizontal plane. Without this established, the timing, energy levels and frequency response will be different for the left ear than for the right. As you can imagine, this means that you will be sitting in a spot that is a null for the first order resonance frequency of the width mode. This position is also a peak for the second and a null for the third width modes. This is a compromise that must be taken. It will suffer the fewest anomalies; only in the low frequency range and only at certain instances. Any other position will compromise all time arrivals, all energy levels and all frequencies, all of the time.(See Symmetrical vs. Non-symmetrical Layouts)

Positioning for Soundstage

By soundstage, I mean the accuracy in sound representation of the recorded space for width, depth and even height. Once mapping of the room modes is complete, either by modeling or with test instruments, the soundstage must be considered. The relationship of separation between the two speakers and the listener must be precise.  If the speakers are much closer to each other than the distance between them and the listener, there will be a small, narrow soundstage and sound will appear to originate from the speakers. On the other hand, if the speakers are too far apart, you'll have a hole in the middle of the soundstage and again, the sound will seem to come from the speakers. When the speaker/listener positions are correct, the soundstage will become three dimensionally large and solid, well beyond the speaker's edge. There will be a sense of true sound development beyond where the speakers reside and the recorded space will be realized. 

Fine tuning the soundstage is beyond the scope of this article. I will mention that are ways to precisely adjust the toe-in of the speaker angle using the ears and laser alignment tools. You can also adjust for personal preference of soundstage perspective, meaning if you prefer an intimate, front row perspective, or one more laid-back from say row T.  Note that toe-in not only controls balance, spaciousness, focus and intimacy, but also tonal brightness. It is speaker/room specific, due to the unique interactions of the speaker's energy dispersion pattern and the make up of the room.

The drawing above is an indicator of how positioning the speaker/listener footprint off center causes havoc on all signals, all of the time. The point that should be understood is how important it is keep things symmetrical, especially in the horizontal plane. Construction, even furnishings can impact how sound energy is absorbed, reflected and diffused.

Summary

Optimal speaker/listener location within the room is paramount to high fidelity playback. Keeping the speakers and listener footprint centered between side walls, away from boundaries, and room modes is the first priority in setting up a sound system. I would prioritize stereo separation as second, toe-in as third, and symmetry of furnishings in the horizontal plane as fourth. Without optimizing this footprint for the specific room, the full potential of the recorded experience cannot be realized. Avoiding room modes and optimizing soundstage go hand in hand. They are the foundation for optimal bass response, dynamic range & low-level detail, and accurate tonality & imaging. Getting this right is the most important aspect of the system. Regardless of the quality of the equipment, the quality of the sound will depend on how well the speaker/listener locations are set up in the room. A/V RoomService offers both modeling and onsite testing (voicing) services. Visit avroomservice.com for more information.

16 Common Partition Considerations for Noise Control

By Harry Alter

This is an excerpt from a much more in-depth article Harry wrote regarding noise control, which will be available (along with many others) on our website www.avroomservice.com in the near future. Though the article's target audience was home theater enthusiasts, it certainly applies to any small room environment such as: project studios, conference rooms, class rooms, condo's, etc.

You may have heard during some investigative discussions about building a home cinema, that noise control is something not to be overlooked, especially if you’re looking to create a truly awesome home theater experience. But what really is noise control, how much do you need and do all those noise control products out there really make a difference? How do I choose a product and when I do … will it really work? 

Well, we’re getting to the bottom of all that in this article and while we’re at it you’ll learn a little about what to look for in noise control products, what questions to ask, and what to be cautious of. So without further a due let’s clear the smoke from the room, work our way through the maze of technical jargon, and remove the mirrors so everyone can clearly see and hear what a really great home theater experience is all about.

Where to start? What better place than “Why”. Why do we need home theater noise control in the first place? Two reasons: a) noise reduction means improved sound quality, and b) we don’t want to disturb others. The most important reason to design for control noise in the home cinema environment is to create conditions that will first and foremost allow for the recreation of the cinematic experience intended by the artist(s). Pretty obvious right? “Creating conditions” is really what home cinema noise control is all about. If we fail at creating desirable room conditions, the result can quickly go from disappointing to disastrous. The common aphorism “garbage in, garbage out” holds true throughout the structural and electronic design stages of home cinema. Noise is distortions and/or distractions that are not original to the audio signal.

One of the most important reasons we approve or disapprove of any listening or home cinema experience is the result of our own ability to listen and experience sound with a critical ear. This includes all of us who enjoy the home cinema experience. The desire to recreate and understand this experience is probably why you’re reading this article. We love it, because we know when the experience is right and like-wise, we know when the experience just isn’t right. We quickly become a discerning audience that knows the difference between awesome and awful, and as a result, become “critical” about our expectations and how we “listen to our surroundings” during the home cinema experience. I emphasize, “listening to our surroundings”, because what we hear within the shell of a home cinema is largely influenced by how the walls, floor, door and ceiling treat the sound energy generated within, around, and through the space. 

So let’s begin by taking a closer look at how walls, floors, doors and ceilings influence your listening experience.

 

There are three basic ways that rooms (walls, floors, doors, and ceiling partitions) influence sound energy: 

1.                  The partition will absorb sound energy. 

2.                  The partition will transmit sound energy through it.  

3.                  The partition will reflect sound energy back into the listening space.

How sound energy reacts with its surrounding room envelope can vary immensely depending on how much sound energy travels via each energy path. Changing or varying the energy path for better or for worse depends on a complex array of products, their material properties, and how they are integrated together to form an assembly.

To better illustrate how the flow of sound energy effects the room’s listening environment lets bundle items 1 and 2 (absorption & transmission) together as all the sound energy that potentially “leaves” the room, and item 3 (reflection) as all the sound energy that remains in or is reflected back into the room. Let’s call the sound energy that leaves the room a  (alpha) and that sound energy reflected back into the room r  (sigma). My high school physics tells me that Newton once said that energy can neither be created nor destroyed. So all the sound energy that is incident to your room’s shell, before any reflection or absorption takes place, is equal to 100% of a partition’s incident sound energy.  The following equation describes how these principles come together. 

r + a  =  1.0  (100%)

Pictorially let’s look at how different wall partitions can treat sound energy. 

As you can see Figure 3 provides the best results by utilizing a number of sound absorption characteristics to limit the amount of energy flowing back into the listening space as well as into adjacent rooms. Unfortunately, achieving this is easier said than done. Often the use of too much mass and too little panel absorption provides good sound transmission loss results, at the expense of interior room sound quality. i.e.; way too much energy is being pumped back into the room from the un-optimized partition assembly design. 

By combining various construction elements and effective products, one can greatly reduce potential design problems or failures.

The following is a list of elements often considered to optimize partition absorption, transmission, and reflection. 

1.      Increase stud/joist spacing

2.      Change stud/joist type (wood vs. metal)

3.      Increase depth of cavity

4.      Fill cavity with acoustical insulation

5.      Increase mass of surface boards

6.      Introduce multiple layers of surface board

7.      Reduce thickness of surface boards while maintaining overall thickness

8.      Vary thickness of surface boards

9.      Introduce resilient isolation between surface boards and studs/joists

10.  Introduce damping compounds between layers of surface boards (See RoomDamp2)

11.  Change the material and/or component properties of the surface boards

12.  Introduce vibration breaks wherever possible

13.  Reduce hard surface-to-surface connections between floors and walls

14.  Seal any and all gaps or penetrations to reduce air movement through the partition

15.  Introduce a noise-rated door or double door assembly

16.  Refrain from introducing regions with little air space available (i.e. Center septums or resilient channels fastened over existing gypsum board. These often make things worse instead of better.) 

Reflected Room Energy:

An item I would like to speak about before the close of this article is the potential sound energy that walls, floors, and ceilings can reflect back into the listening room due to poor partition design. As I noted at the beginning of this article, the best assemblies are those that gain the most sound absorption over a broad frequency range using a variety of noise control options & techniques. A frequent problem is relying too much on mass. A good example of this are the reverberation times below that show how a wall can push energy back into the room based on its construction. Two walls which both have very similar STC performances, but very different contributions to the reverberation times within the room. One promotes the control of low frequency energy from being reflected back into the room, while the other pumps too much low frequency sound back into the listening environment, which destroys the sound quality.

In closing, this is a basic start which I hope you have found valuable toward understanding more about the importance and science of noise control. I’m sure you have many questions: like how many dB will each of the items listed above provide to my home theater design and how many is enough? I hope that future articles will delve deeper into questions like these, as well as addressing the importance of controlling flanking noise, impact insulation, HVAC noise and other design issues. Remember that noise control is a two way street: sound that leaves the space and sound that enters it. Noise control partitions are system approaches to principals incorporating block, break, isolation and/or absorption of sound waves and vibrations. These systems must adhere to the unique governing weight, thickness, décor, budgetary and/or even “green” requirements of the project. These systems must be designed to address each unique noise control issue; for example, maybe there is going to be a water pump for a pool adjacent to the cinema, or a child’s bedroom above. Different sound energy levels and their frequency ranges must be understood in order for noise mitigation to be designed appropriately. Means of acoustic computer modeling (if new construction) or testing and modeling (if existing) will increase the likelihood of solving problems through proper acoustic design, resulting in a higher performance cinema and a greater experience.