Raised Floor

Figure 21.4 An Elevation View of the NRC/IEC Listening Room Showing Possible Listener and Woofer Locations (Toole, 1990)

Figure 21.4 An Elevation View of the NRC/IEC Listening Room Showing Possible Listener and Woofer Locations (Toole, 1990)

close (within 1/6 wavelength) to a hard reflecting surface will be more efficient, because the geometrical directivity due to the reflections from the nearby surfaces increases the power output for the same voltage input. Single-layer drywall partitions do not provide much increase in sound pressure but solid floors can be helpful. Woofers placed in the corners of a rectangular room will excite the greatest number of room modes since they all have pressure maxima there. However it may not be desirable to drive all the modes, since the listener may be sitting at a major node. In this case, it may be better to position the subwoofers at an intermediate point along the floor-wall junction to avoid pouring energy into the fundamental axial modes. By using two (or more) loudspeakers that can be moved around, optimum positions may be determined experimentally. The listener position can be controlled with a knowledge of the locations of the bass nodes. Figure 21.4 gives an example of possible loudspeaker and listener positions in a rectangular room relative to the locations of the low-frequency maxima and minima.

Audible Reflections

One design objective in recording and listening spaces is to minimize the negative effects of the room on the acoustic environment. In an ideal listening room the sound is transmitted from the loudspeaker to the listener with little or no coloration. Similarly, in a studio the sound of the performer should be clearly transmitted to the microphones. Since room reflections are unavoidable, experiments have been undertaken to determine the audibility of an individual reflection. Early experiments by Haas (1951) used two loudspeakers in front of the listener at an included angle of 45° with speech as the test signal. One loudspeaker was delayed with respect to the other and the listener was instructed to adjust the level of the reference loudspeaker until the two sources were perceived to be of equal loudness. The upper curve shown in Fig. 21.5 (Toole, 1990) was the result.

Figure 21.6 contains the results of later experiments that reexamined human reaction to a combination of delay and level, again using speech as the input signal. These yielded a variety of results, indeed a nearly continuous range of reactions, including equal-loudness lines significantly different from that found by Haas. Based on these experiments it can be said that there is no one single point at which the two signals were perceived as unified, since even when the second signal was 20 dB below the first it was still perceptible. As a

Figure 21.5 Various Measures of Significance of a Lateral Reflection (Toole, 1990)

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