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Absolute thresholds for a single lateral reflection (anechoic,) using various signals

Pizzicato Violin Schubert 3O0 -e- Handel Concerto Crossso -B- Mozart Barron

— Speech Seraphim

—x- Pink. Noise OliveiToole

-.».- Speech -■- Castanets -A- Clicks (Pulses @ 2/sJ

2 O 3 O AO SO bO TIME DELAY, ms

to s>o consequence Toole argues that the masking of a late reflection by a series of early reflections has probably been overemphasized in small listening rooms.

These results do not negate the effectiveness of delayed loudspeakers in sound reinforcement systems. Because they are A-B comparison tests, a shift in image can trigger a noticeable response. In a delayed loudspeaker system a change in image direction is one of the design objectives. Fixed installations do not feature A-B comparisons except during setup. Second, these tests were done in the horizontal plane where we are most sensitive to perceived direction. Time delays and level changes from loudspeakers located in the medial plane are less perceptible.

Separate experiments by Olive and Toole (1989) tested the influence of the source material as well as the level and delay. It was found that the audibility of reflections using short-duration signals varied greatly from those obtained with music and speech. Figure 21.7 illustrates the differences; although there was forward temporal masking, the nearly horizontal shift in level with delay time measured by Haas was observed only in the case of music. Speech thresholds dropped linearly with delay time and short-duration signals provided very little masking.

The audibility of reflections is also dependent on the reverberation time. Figure 21.8 shows the results of psychoacoustic studies by Nickson, Muncey, and Dubout, (1954). Here the source was speech and the "echo" signal was electronically delayed and presented through the same loudspeaker as the original. Note that the lower line representing 20% disturbed follows the reverberation time line quite closely. These straight lines are drawn based on the assumption of an exponential decay in the room. A reverberation time of 0.5 sec is equivalent to a 10 dB decrease in (0.5/6 = 0.083 sec). This is plotted as the top curve (Everest, 1994). The upshot is that the deader the room, the more noticeable are the individual reflections.

Figure 21.7 The Results of Two Psychoacoustical Listening Tests (Nickson, Muncey, and Dubout, 1954)

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