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Sound Image

Feb 11, 2013 8:34 AM, By Bob McCarthy

The costs and benefits

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Figure 1: Horizontal localization: a) localization of a centered and off-
centered source.

Figure 1: Horizontal localization: a) localization of a centered and off- centered source. Click here to see a larger image.

In an ideal world, sound systems would be invisible and inaudible. It is easy for us to comprehend the invisible part since we are constantly told by architects and set designers that they can’t stand the sight of a stationary rectangular box. Amazingly, lights moving around changing color and brightness, spilling light out of their sides and backs do not bother these folks. But a black box, God forbid one with a tiny LED on it, is an abomination. Why the prejudice against seeing speakers? This ties in to the secondary desire for the speakers to be inaudible, and by that I mean that we strive to create the illusion that the sound is magically coming directly from the stage performers rather than the rectangular boxes. Visible boxes break the magician’s illusion, resulting in a strong desire to hide them. The troubling part of all of this is that hiding the speakers visually can actually make hiding them audibly much more challenging.

This is one of the tradeoffs in the game of sound image control. There are more, most notably in the categories of intelligibility, tonal modification, and uniformity. Maintaining a realistic sound image is a balancing act between relative level, time, distance, and angle. The first installment of this two-part article (part two will appear in a later issue) will explore how we perceive sound image and how we can control its placement with multiple speakers. The second part will cover examples of image placement and control in typical sound systems.

Figure 1: Horizontal localization: b) localization of direct vs. reflected sound.

Figure 1: Horizontal localization: b) localization of direct vs. reflected sound. Click here to see a larger image.

Sound Image Perception
Our sound image experience is comprised of two primary aspects and a variety of secondary ones. The dominant features are source direction and range, which give us the source location relative to our ears. The source angular relationship (its bearing) is subdivided into vertical and horizontal planes, which are decoded separately by the ear-brain system. These will be described momentarily. Our range perception is more complex, relying on a memory map that compares what we are hearing to our expectations regarding the particular sound source material. Our expectations are influenced highly by a secondary sense: sight, which gives us a framework with which to normalize what we are hearing. If we see a violin across the room, we compare what we hear to our memory of a distant violin sound, rather than what we experience when we are playing it ourselves (which, in my case, would sound like a nearby cat being tortured).

This is not at all to say that a blind person lacks range-finder capability. They will have mapped the range clues much more finely than sighted folks since they lack the secondary sense backup that the eyes provide. The secondary range clues include the sound level, frequency response, and direct/reverberant ratio. Seeing the source distance and the shape and materials of a room give context to the range expectations. We adjust our sonic expectations when we see that we are in a large reflective environment. In such a context, it can be difficult to carry on a conversation even at a fairly close distance. “Objects may be closer than they sonically appear” would be a fair warning in a highly reverberant environment. By contrast, if we are blindfolded in an anechoic chamber, we will find it much harder to determine the range. Adjustments of level and frequency response can, in fact, alter our range perception without moving the speaker.

Figure 2: Horizontal localization with a pair of speakers: a) Level offset favors
left speaker.

Figure 2: Horizontal localization with a pair of speakers: a) Level offset favors left speaker. Click here to see a larger image.

Let’s set up an experiment to illustrate your sound image detection system in action. You are blindfolded in a room with a continuously moving sound source. How accurately will you be able to track the moving source’s bearing and range? The easiest aspect to localize is the horizontal position. This is because we have a two-channel detection system: our binaural hearing. The source location is double-checked by a pair of two-channel comparisons between the arrivals at our ears: relative time and relative level. As the sound source moves off of the horizontal center, it arrives first and louder at one ear. These two findings confirm each other to provide the localization clue.

The vertical location is found by each ear individually using a memory mapped signature. This is unique for each ear and for each person (and animal) because it is derived from memorizing the comb-filtered frequency response created by the reflections of our outer ear as the sound enters the ear canal. We have never heard sound that was not reflected off of this structure and therefore we have normalized our hearing to this response. Each vertical orientation of the sound source creates a slightly different set of reflections into the canal. These microscopic differences are recognized by the ear and linked to the memory of the vertical position of sound sources previously localized in our life.

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