Friday, September 30, 2011
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.
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.