Acoustical Focus and Envelopment

One of the necessities of acoustical design and calibration is that of defining just what good sound quality means.  We can make numerous measurements but ultimately, our own ears are the final arbiter.  We judge the sound quality of a reproduction system by the satisfaction and enjoyment we feel when listening.  How can we quantify the essential metrics of creating a quality experience.  Often, its easier to identify the elements of poor sound quality, however that does not drive us to the ultimate goal.  The list of sonic metrics includes Clarity, Focus, Envelopment, Smooth Response, Wide Dynamics, and Seat-to-Seat Consistency.  Understanding the design and calibration elements that govern these is essential to achieving high performance sound.  In this article we’ll discuss two of these basic concepts; Focus and Envelopment.

Focus

The ability to precisely locate each reproduced sonic cue or image in a three-dimensional space is defined as acoustical focus. Recordings contain many such images superimposed side to side and front to back in every direction for 360 degrees around the listener. A system is said to have pin-point focus if, from the perspective of the listener, each of these images is properly sized, precisely located, and not wandering. Good focus also provides that individual images be easily distinguishable from among others within the limits of the recordings quality.

Envelopment

An audio system should reproduce virtual images of each recorded sound presenting the listener with its apparent source location in a three-dimensional space. Each sonic image relates a part of the recorded event and together these sounds compose a wrap- around sound stage that envelopes the listener. Proper envelopment requires that the sound stage be seamless for 360 degrees without interruption by holes or hot spots caused by speaker level imbalance or poor placement.  While envelopment requires three-dimensional imaging of all sonic cues, of pivotal importance is the realistic recreation of the ambient sound field of the recorded venue. Focused sounds become more realistic as they move side to side and front to back with the backdrop of the ambient sounds of the intended venue.

These metrics are not unique to the HAA. Focus is simply an updated way of saying “stereo imaging”.  Also, who among the throngs of audiophiles has not gasped at an impressively immersive sound stage otherwise known as envelopment? For the certifiable Gurus, well I mean the certified HAA Gurus among us, we further expand these definitions using the following terminology found on the HAA Critical Listening Criteria. For focus we judge three sub-categories of sonic quality:

Precision Localization: The sound stage is composed of a series of stereo images superimposed on each other to recreate a sonic experience. Instruments microphoned or miked some distance away should be precisely positioned. Instruments or effects closely miked should reveal the location of individual part of the instrument i.e. low parts of a piano vs. high parts, the sound of the pick vs. the chord squeaks etc. Keep in mind this is a three dimensional space within which each sound is placed.

Image Stability: Once you locate an image in the sound stage, its position can appear to shift or blend into the mass of sound. Try to listen to an instrument and keep focused even in the midst of other sounds.

Instrumental Dimension: In addition to position in the sound the apparent size of an instruments image is a valuable clue. This also relates closely to how the instrument was miked. A close miked voice may occupy a larger space than a distant miked one. Does the instrument appear to be properly sized or is blown out of proportion?

Again this hearkens back to stereo imaging. The question arises,” Why do we care about stereo imaging in a Home Theater”? The answer is simple we are still using stereo but now instead of the two channel version first developed by Alan Dower Blumlein we are using Dr. Harvey Fletcher’s three channel version with a few updates thanks to many in the film industry.

The bottom line is that the front of a well designed home theater should be occupied by a focused sound stage where voices, instruments and anything else should occupy a specific location in a three dimensional sonic space. I should add that with the advent of immersive sound technology we are now asking our systems to produce images in virtually all directions. This sonic space of which I speak plays a particularly important role in the grand scheme of things as well. It is against the backdrop of the ambient space of a specific venue, church, the beach, a small room, that the focused stereo images are presented. The space of your small home theater room is not exactly the most impressive setting for a church organ or the roar of an F-14 streaking across the sky so we hope to recreate the space of the recorded venue. Proper envelopment creates a three-dimensional spaciousness that enhances the realistic placement of focused images.  In the HAA we describe this sense of space as envelopment. Like focus we can further define sub-categories to help us refine our impression of the quality of envelopment. Again from the critical listening worksheet, here are the sub-categories:

Depth: The sound stage is composed of a series of stereo images superimposed on each other to recreate a sonic experience. The backdrop for this is the ambient sound field. The farthest sense of depth in any recording is this ambiance and the instruments farthest away from the microphone.  In addition, proper envelopment creates a sense of spaciousness bringing focused images into the room; they seem to be floating in front of the speakers.

Continuity: For multi-channel systems the surround field should be seamless without interruption. Panning from front to back or side to side should appear smooth and without interruption.

Surround Field Cohesiveness: This is an element related to any frequency shift that is observed as sound pan around the sound stage. Timbre shifting is difficult to combat since our ears create their own timbre shifting as sounds change elevation or azimuth.

So now the table is set with these two elements. Why are they related? This is the fulcrum of decision in calibrating the soundstage of a home theater. The acoustical properties which heighten focus are embedded in the recording itself. That is, stereo imaging is recorded and assuming we have our front LCR speakers positioned reasonably close to that of the original monitoring studios’, we should hear the sonic images of the recording very precisely located in this front soundstage. (NOTE: There are many speaker born factors which can limit focus; I will limit this discussion to the acoustical side of the equation.) Our acoustical axiom on focus is to design a system that allows all listeners to clearly and without obstruction hear the direct sound of these front three speakers. All you calibrators rush out there to make sure that your LCRs are not blocked.

If it’s that simple, why do so many systems still lack scary good focus? I’ve already disavowed myself of identifying speakers with good focus and for that matter the specific problems poor design or orientation of the speaker can have on focus, let’s talk room acoustics. The acoustical distortions on the direct sound from a speaker are fairly negligible. After all, between speaker and listener the only impact on direct sound transmission is the normal decay of sound intensity. This is where envelopment comes in. What creates great envelopment? It’s simple, it’s largely a function of the “indirect” sound. That’s right; those dreaded reflections in a room actually assist in creating the desirable effect of immersing us in sound. The sense of immersion we call envelopment requires a diffusive reflected sound field reinforcing the sense of space recorded in the music or movie. The operative effect of diffusion means no particular directional source or apparent location. There’s much we don’t understand about how this works however we can make some observations.

First, the direct-sound from the speakers gives us the highly focused and clearly non-diffusive stereo imaging. Such localizable recorded sounds such as from a person speaking or a solo instrument require unfettered direct sound to be a focused stereo image. Second, the reflected sound, if it is sufficiently diffusive, emulates another key portion of a sonic experience; the ambient sound field. This is that sphere of space which is in the background of every real-world sonic experience. Since we don’t listen for articulation and location from the ambient sound field, we can more easily duplicate it in a home theater. There is a catch though; it requires a truly diffusive sound field. As the list of HAA Acoustic Design Review elements holds testament to, there are many accidental ways to make sound field non-diffusive. Any gaps in the reverberant sound field (at least azimuthally) degrade the experience and potentially pop us out of the “suspension of disbelief”. Gaps in this enveloping soundstage are caused by missing or overly damped walls which is governed by the acoustical design of the space. The significance of the interaction between focus and envelopment is that each property tends to benefit from opposing acoustical treatment techniques; something that benefits focus tends to be a detriment to envelopment and likewise.
During the HAA Advanced training there are often disagreements over preferences for better focus versus better envelopment. Many listeners have been raised without significant exposure to live music. For some, the razor sharp focus they have grown accustomed to in headphones or near field listening like in a car has become a new hallmark of good sound. The haunting sound of a live soloist reverberating in a concert hall seems too distant and ill defined to be considered impressive. It’s equally true that the preference for close miking or even electronic transfer of instruments is prevalent for popular music recording techniques. The preference in a listening room becomes very personal and dictated by one’s experience.

I find that live music goers tend to prefer a better balance between envelopment and focus. Also the emphasis on movies as opposed to studio music in home theaters has further advanced the need for a healthy serving of reverberation in a home theater even in an Atmos or DTS:X equipped room. Movies are generally mixed in a reasonably reverberant space designed to sound much like a large theater (movie soundstages often are large theaters). In these cases, the need for reverberation to enhance envelopment is integral to a successful listening room design. In the meantime, the balancing act between strong focused imaging and rich immersive envelopment remains a design and calibration element able to be tweaked by an experienced Acoustical Designer.

More later.

I am sometimes confronted, during friendly discussions with clients, about the relative importance of one or another “minor” acoustic or setup flaws in their home theater. Does proper setup and calibration really matter that much balanced against other priorities? In the final analysis, just what is missing due to any of these seemingly innocuous misalignments? I think the answer is best explained from my perspective as a music lover relating how I am occasionally teleported in time and space by the majesty of an amazing recorded performance. If never on a quiet evening in your home, a recording has astonished you with its realism and moved you emotionally, which among these few subtle acoustical flaws has robbed you of the experience? If you have not been surprised recently by your sound system, perhaps ACR Elements 3 or 4 could be a contributing factor, of course, don’t forget about the many other Design and Calibration elements in the mix.