Imaging

In the last article we talked about setting the timing for delay or front fill systems. In this article we’ll talk about imaging.

I’m sure we’ve all heard of the “precedence effect” or “Hass effect,” perhaps one of the most misunderstood principles in audio. It goes something like this: “I timed the arrival time between the mains and delays – and then added 10 ms for precedence/Haas effect.”

If our goal is to achieve high speech intelligibility, this “extra” delay is exactly the wrong thing to do. However, if we are making artistic judgments with regard to music or realism then there might be a valid case for additional delay but not just some arbitrary 10 ms.

Quick History Lesson…

The “precedence effect” was described and named in 1949 by Wallach et al. They showed when two identical sounds are presented in close succession they will be heard as a single fused sound. In their experiments, fusion occurred when the lag between the two sounds was in the range of 1 to 5 ms for clicks, and up to 40 ms for more complex sounds such as speech or piano music. When the lag was longer, the second sound was heard as an echo.

Additionally, Wallach et al. demonstrated when successive sounds coming from sources at different locations were heard as fused, the apparent location of the perceived sound was dominated by the location of the sound that reached the ears first (i.e. the first-arriving wave front). The second-arriving sound had only a very small (albeit measurable) effect on the perceived location of the fused sound. They designated this phenomenon as the “precedence effect,” and noted it explains why sound localization is possible in the typical situation where sounds reverberate from walls, furniture and the like, thus providing multiple, successive stimuli. They also noted that the “precedence effect” is an important factor in the perception of stereophonic sound.

Wallach et al. did not systematically vary the intensities of the two sounds, although they cited research by Langmuir et al. Which suggested if the second-arriving sound is at least 15 dB louder than the first, the “precedence effect” breaks down.

The “Haas effect” derives from a 1951 paper by Helmut Haas.

In 1951, Haas examined how the perception of speech is affected in the presence of a single, coherent sound reflection.  To create anechoic conditions, the experiment was carried out on the rooftop of a freestanding building. Another test was carried out in a room with a reverberation time of 1.6 ms.  The test signal (recorded speech) was emitted from two similar loudspeakers at locations 45° to the left and to the right in 3 m distance to the listener.

Haas found humans localize sound sources in the direction of the first arriving sound despite the presence of a single reflection from a different direction. A single auditory event is perceived. A reflection arriving later than 1 ms after the direct sound increases the perceived level and spaciousness (more precisely the perceived width of the sound source). A single reflection arriving within 5 to 30 ms can be up to 10 dB louder than the direct sound without being perceived as a secondary auditory event (echo). This time span varies with the reflection level. If the direct sound is coming from the same direction the listener is facing, the reflection’s direction has no significant effect on the results. A reflection with attenuated higher frequencies expands the time span that echo suppression is active.Increased room reverberation time also expands the time span of echo suppression.

The “precedence effect” appears if the subsequent wave fronts arrive between 2 ms and about 50 ms later than the first wave front. This range is signal dependent. For speech, the “precedence effect” disappears for delays above 50 ms, but for music the precedence effect can also appear for delays of some 100 ms.

Now the important thing to take from this is these results were obtained in the horizontal plane where our hearing resolution has an average sensitivity of one degree. In the vertical domain our hearing has a resolution of about four to five degrees. (One of the reasons Vertical Line sources are successful).

If we place a signal into two sources separated horizontally we get a “phantom” or ghost center image. This rapidly collapses to a mono source as we shift position or change levels or delay times. The same can happen between vertical separated sources but it’s much coarser.

Knowing time offsets between sources can affect imaging but at the expense of ripple (comb filter) or smoothness is the tradeoff of Art vs. Science.

With wireless tablet control of these sub-systems it’s relatively easy to change these delay offsets and hear what’s happening in real time. We can “pull” or “push” the image around but remember the tradeoffs.

tt2_imaging_fig1

Fig. 1

Here in Fig 1, you can see how relative dB offsets and gains can be used to shift the image around for music. Just try to stay out of the yellow area. “One man’s comb-filter is another man’s stereo enhancement.”

In the next series of Articles, I’m going to touch on “Subs – The Final Frontier”

About Martyn “Ferritt” Rowe

Industry veteran and OSA’s Director of Engineering Services Martyn “Ferrit” Rowe brings nearly three decades of real-world experience in live event technical services. Ferrit most recently came from Martin-Audio as the technical training manager for MLA, and uses his vast knowledge and expertise of the multi-cellular technology to support client projects as well as support and train engineers and technicians.

Ferrit began his career running cables on a Thin Lizzy “Live and Dangerous Tour,” and then taking on the roles of running monitors, front of house, and system technician for some of the most popular acts in music, such Judas Priest, Ozzy Osborne, Black Sabbath, The Police, KISS, The Who, Elton John, Poison, Bon Jovi and Van Halen.

Ferrit’s training career began in 2000 when the first of the line array’s from V-DOSC emerged and became an instructor on the line array theory and continued that path with various systems over the years before joining the Martin-Audio MLA division, and then bringing his knowledge and expertise to OSA International, Inc.