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.

 

How Bad Can a 1% Air Gap Be to Noise Control?

by Harry Alter and Norman Varney 

Noise control is a two way street. You may have spent considerable expense on the design and materials of a wall, ceiling or floor system to keep noise from escaping or entering the space. However, you may not realize the impact a tiny hole can have on the entire partition's performance. Most people know that the door is the weak spot in a wall system. You can have a high sound transmission class (STC) rated wall system cut to half the rating if you don't incorporate an acoustical door, or not have one properly installed.

There are two primary means that sound energy can travel through walls, floors and ceilings; vibrations traveling through solid materials such as gypsum, sheathing, studs or joists are called structure-borne vibrations, and vibrations traveling through air, framing cavities and unsealed penetrations, seams or gaps are called air-borne vibrations. Both vibration paths play an important role in determining how well the partition assembly will reduce the transmission of sound through it, and a "systems" approach must be in its design to appropriately address the associated sound energies. A systems approach would incorporate a combination of blocking, breaking, absorbing and/or isolating the energy at the source, along its path(s) and/or at the receiver. We are covering just one of these aspects of noise control in this particular blog.

Air filtration and sound penetration through walls, ceilings and floors occur as one in the same. If air can penetrate a partition, then so can sound. In fact, it takes very little air leakage to cause significant sound leakage. For example, an opening or crack 1/100th of 1% of a total wall's surface area can reduce the sound transmission loss (TL) of a wall from 50 to 39 dB. That's an 11 dB drop in noise control performance. Likewise, a partition designed to achieve a TL of 40 dB would be reduced to approximately 30 dB (a 10 dB drop) with only 1/10th of 1% air leakage area to wall area. Note that the 10 dB drop in the poorer assembly would be perceived by the average person as twice as loud as the better assembly.

The above graph illustrates how openings and cracks can affect the TL (and subsequent STC) of a partition assembly. The horizontal axis indicates the design or desired performance of the assembly. The vertical axis indicates the resultant TL based on the % of surface area air/sound leakage. From the graph, the level of noise control performance will not increase beyond a certain level based on the size of the unsealed air gap. As a result, sealing air gaps reduces sound (noise) transmission through partitions. Less air penetration equals less sound penetration. The beauty is in the details.