Chapter 3

Subjective Preferences for Sound Fields

Preferences are the most primitive responses of subjective attributes, because preferences are the evaluative judgments that actually steer an organism’s behavior so that it can survive and propagate. Preferences guide the organism in the direction of maintaining life. In humans, these preferences are deeply related to a esthetic issues. The experiments described throughout this book have mainly used the paired-comparison test (PCT) to assess subjective preferences for sound fields (Thurstone, 1927; Mosteller, 1951; Gullikson, 1956; Torgerson, 1958; for assessment of individual preferences, see Ando, 1998). The paired comparison is the simplest and most accurate method. It permits both inexperienced and experienced listeners to participate, making the method appropriate for a wide range of applications.

Our experimental observations of listener-preferred conditions for temporal and spatial features of the sound field are first presented, and then a theory of subjective preference is derived based on four orthogonal factors [Table 3.1, (Ando, 1983, 1985, 1998)]. These factors are (1) binaural listening level (LL), (2) delay time of the first reflection ( t1), (3) subsequent reverberation time (Tsub), and (4) magnitude of the interaural crosscorrelation function (IACC). The calculation of subjective preference at each listener’s position in Symphony Hall in Boston, Massachusetts is

Table 3.1 Four orthogonal factors of the sound field identified (Ando, 1983, 1985, 1998)

shown as an illustrative example (Section 3.4). In Chapter 6, we discuss temporal sensations of sounds, and in Chapter 7 we discuss spatial sensations of sound fields.

3.1Preferred Properties for Sound Fields with Multiple Reflections

We first discuss the temporal and spatial percepts associated with the simplest sound field, which has a single reflection. The next Section 3.2 extends this analysis to more complex sound fields with multiple reflections. Preferences for particular sound field parameters were obtained by means of paired-comparison tests (PCT).

3.1.1 Preferred Delay Time of a Single Reflection

In these experiments the sound field consisted of the frontal direct sound and a single reflection from a fixed direction. The reflection’s horizontal angle from the front is ξ = 36◦ and its elevation is η = 9◦. The delay time t1 was adjusted within the range of 6–256 ms. Using two different music motifs, A and B (Table 2.1), as stimuli, the paired-comparison test was performed in an anechoic chamber in Goettingen by subjects of normal hearing ability using the sound field simulation system outlined in Section 2.3 using the three front speakers. All pairs of stimulus sound field conditions were tested.

In order to ascertain the most preferred listening conditions, the total score was simply obtained by accumulating scores, giving +1 or −1 for positive and negative preference judgments, respectively. In order to obtain a normalized score, the total score was then divided by the number of stimulus sound field pairs S(F–1), S being the number of subjects and F the number of sound fields. Normalized scores and percentage of preference of the sound field as a function of first reflection delay time ( t1) are shown in Fig. 3.1 (Ando, 1977; Ando and Morioka, 1981; Kang and Ando, 1985).

Obviously, the most preferred first reflection time, i.e., that delay with the maximum score, differs greatly between the two music motifs and continuous speech. With an amplitude of reflection, A1 = 1, the most preferred delays are around 130 ms for music motif A, 35 ms for music motif B (Fig. 3.1a), and 16 ms for speech (Fig. 3.1b). It was found that these values correspond well to the effective duration of the ACF of the respective source signals: 127 ms (motif A), 35 ms (motif B), and 12 ms (speech), as indicated in Table 2.1. These music and speech programs sound best when the first reflection delays of the sound field are matched to the effective durations of the program material. The experimental data for preferred values of single reflections collected as a function of the duration τ p are shown in Fig. 3.2, where data from continuous speech signal of τ e = 12 ms are also plotted.

and reflected sound in the coherent time region can produce tone coloration effects that are the result of periodicities in the pitch range that have been introduced. If the reflection times are very short, less than 1 ms, the delay can produce binaural effects that increase the interaural crosscorrelation magnitude (IACC), and thus decrease listening satisfaction.

The preferred first reflection time also changes as a function of the relative amplitude of the reflected sound vis-à-vis the direct path (Fig. 3.1b). The value of the ACF at zero delay is A1, so its value at τ e becomes 0.1A1. Thus, τp = τe only when A1=1 (0 dB re:direct level). When the envelope of the ACF is exponential, then the preferred delay is expressed approximately by (Ando, 1985)

It is worth noting that the amplitude of reflection A1 relative to that of the direct sound in Equation (3.1) should be measured by the most accurate method, i.e., the square root of the ACF at the zero-delay origin: [Φ(0)]1/2 of the reflection relative to that of the direct sound.

3.1.2 Preferred Horizontal Direction of a Single Reflection

In the experiment that estimated the preferred direction of a single reflection, the delay time of the single reflection was fixed at 32 ms. The direct sound was presented by a loudspeaker positioned at ξ = 0◦ (η = 27◦) and the second, reflected sound was presented by the loudspeaker located at five equally spaced angular positions that ranged from ξ = 18◦–90◦, with (η = 9◦).

Results of the preference test are shown in Fig. 3.4. No fundamental differences were observed between the curves for two music motifs with respect to changes in horizontal direction in spite of the great difference between their values of τ e. The preferred score increases roughly with a decreasing IACC. The correlation coefficient between the preference score and the IACC is 0.8 (p < 0.01). The score with motif A at ξ = 90◦ drops to a negative value, indicating that the lateral reflections, coming only from around 90◦ are not always preferred. The figure shows that there is a preference for angles greater than ξ = 30◦, and on an average there may be an optimum range centered on about ξ = 55◦ (see also, Fig. 7.14 for the frequency range around 1 kHz). Similar results can be seen in the data from speech signals (Ando and Kageyama, 1977). The general conclusion is that dissimilarity between the signals arriving at the two ears is the preferred condition (Damaske and Ando, 1972; Schroeder et al., 1974).