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.

5 Reasons Off-center Room Positioning is a Bad Idea

By Norman Varney

   Symmetry of the audio scene, especially the front horizontal plane, is important to accurate reproduction. We want to place ourselves in the middle of the left and right image in order to hear the proper soundstage. If we don't balance levels correctly, spatial cues, frequency response, low-level details, etc. become skewed because the energy on one side of us is louder than the other. This is true with headphones, but is even more problematic when sound is introduced to a room. Lately, I've been seeing a lot of designs that are incorporating off-center speaker/listener arrangements in the room. The idea is to avoid the room's fundamental width cancellation node by moving away from it. This is not practical for high fidelity.


  Axial room modes in a rectangular room are fairly predictable using simple math. Since room modes are dictated by room dimensions, we can calculate what frequencies will live where in the room. We want to avoid coupling the woofers and listeners with the existing first, second and third-order modes whenever possible, as they are the most energetic. Avoid placing woofers in antinodes (pressure peaks), and listeners in both nodes and antinodes. Placing speakers in an antinode will excite it, resulting in that frequency (and its harmonics) sounding louder than they should.  Placing a speaker in a node (null) will attenuate that mode, which at times can be useful. Placing a listener in an antinode results in the mode sounding too loud. Placing a listener in a node results in the mode sounding too soft. There is always an optimum position for the speakers/listeners in a room to deliver the best soundstage and bass response. 

  Symmetry is important. Placing the speakers and listeners off-center in the room to avoid the fundamental width mode is not a good idea. Here are some reasons why off-center positioning is not good practice:

1. The fundamental (f1) mode wavelengths are too large to move away from. The fundamental wave supported in a 15' wide room is about 30' long (38 Hz.). The longer the dimensions, the longer the lowest wavelength. You would have to move off-center about 3.75' to smooth out a 38 Hz. null.
2. By doing so, you’ll just end up in another mode. In a 15' wide room, 3.75' off-center, you'll find f3 (113Hz.) at its peak.
3. By doing so, you’ll end up too close to the side wall, which will cause timing differences between your left and right ears, resulting in severe spatial skewing. 
4. By doing so, you’ll end up too close to the side wall, which will cause energy differences between your left and right ears, resulting in resolution loss and inequality.
5. By doing so, you’ll end up too close to the side wall, which will cause frequency differences between your left and right ears, resulting in severe timbre skewing and inequality.

  Let's look at what happens at these low frequency room modes. If we took an instantaneous time snapshot (1/75th of a second) of the first-order (f1) width mode in a room 15’ wide (38 Hz.), we would see a positive pressure point to our left, and null in the middle of the room, and a negative pressure point to our right. At the same instant, the second-order (f2) mode (75 Hz.), which is half the length of the first, would show a positive peak at the left wall followed by a null (located about 3.75’ from the left wall), a positive peak in the middle of the room, and a null (located 3.75’ from the right wall), followed by a positive peak at the right wall. We want to avoid the third-order (f3) mode as well (113 Hz.). You would have to move 3-4’ to one side before you would notice any appreciable frequency smoothing of the first-order mode, which moves us into to the f3 antinode at 113 Hz. This particular frequency is contained in nearly all music and dialog recordings. Not a good move (see Fig. 1).

In summary, we must place the audio footprint center of the side walls and settle for the rare, problematic bass note, over distorting all frequencies, all the time. Placing the auditory scene symmetrically between the left and right walls provides optimum dynamics, tonality, imaging, spacial cues and low-level resolution.

10 Reasons Why Frequency Averaging is NOT a Good Idea

By Norman Varney

The idea of averaging multi-channel sound for all listener positions is not a good one. There are many audio myths out there. This one has many professionals fooled. It is a popular practice that should be better understood before it is applied, especially to small, critical listening spaces such as home cinemas, production suites, etc.

 

I am not dissing the use of equalizers. They are a powerful tool and can be used for many reasons, but in this instance, I am cautioning their use to average the frequency response to be received over the entire listening zone. This practice is almost a given in the home theater arena and should be reexamined. There are more reasons why frequency response spatial averaging is not a good solution. I am only going to cover the main reasons:

 

1) It won't average the time domain at more than one location.

2) It won't average energy levels at more than one location.

3) It won't average the reverberation times at more than one location.

4) Each seat will still have a unique sound even with frequency averaging.

5) Only one seat can be calibrated for audio nirvana at a time. This is because multi-channel sound can only converge at the same time, energy and frequency response, at a single point in space (See Fig. 5).  This location should be the primary seat, which should be located in the middle of the listening zone, so that more listeners are closer to the target.

6) If we average the frequency response for all seats, all seats suffer (See fig. 6).

7) Equalizers can only change the the frequency response of the signal being fed to the speaker(s). They do not respond to room interaction. Once the signal leaves the speaker, the room takes control of the sound energies.

8) Equalizers are good for reducing peaks and poor at increasing dips.

9) The differences between seats is not small. They can easily differ by 30 dB SPL.  As the number of seats increase, so does the variance, as do rooms with seats close to boundaries and/or rooms with particularly bad modes.

10) Selecting a "room curve" for spatial averaging, guarantees the original signal is lost.

 

The point in the room where time, energy and frequency converges should be the center of the listening space. Any other seat will be compromised to a degree regardless. Why compromise every seat? As depicted in Fig. 5 & 6, more seats will exhibit better sound without averaging frequency response. In addition, audio nirvana can be experienced in the primary seat.

 

The idea that "global equalization" equates to "great sound at every seat", or even that the sweet spot becomes bigger is not accurate. Applying this scheme means degrading the sound for everyone and lessening the experience otherwise possible for the money seat.  

 

Sometimes poor room mode distribution may call for some electronic equalization. A 1/20th octave band parametric equalizer could be introduced to smooth these low frequencies (around 300 Hz. and below). Remember, we are changing the frequency response of the signal driving the speaker, and are trying to compensate for how the room is reacting to it from our personal point of view. 

 

Room mode anomalies can mean a difference of 30 dB between a peak and valley. An electronic equalizer can do a good job of reducing peaks, but can not offer more than about 6 dB of gain for a coincident dip. 

 

Note that most of the equalizers used for this process use steady-state measurements with a microphone collecting sounds from every direction in the room. It sums them together for analysis and then a programed solution is applied. This is not the way humans process sound with two ears and a brain. Even fancy digital signal processors (DSP) with time domain correction cannot compensate for room reflections, etc. because they cannot separate where the sound is coming from. As a side note, they also sacrifice frequency resolution in order to analyze time.

 

In summary, electronic room equalization, ideally, is used when passive corrective means are not possible, or in conjunction with passive means. Ideally, this equalization should be of the high resolution, high Q, parametric type, and only be used to address room mode problems around 300 Hz. and below. This is assuming that the speakers used are accurate to begin with. If they are, global equalization across the audible spectrum will likely do more harm than good for all seats. 

 

I've listed ten reasons why spatial frequency averaging is not a good idea. There are many more. Can you think of some of them? Please comment in the box below.