It is shown by elementary arguments that the familiar loudness distribution pattern in the immediate neighborhood of a tuning fork is not, as is often suggested in elementary texts, due to pathlength-dependent phase differences between the waves reaching the ear from the different faces of the prongs, but to pathlength-dependent amplitude differences. At great distances from the fork, however, amplitude variations become negligible and pathlength-dependent phase differences give rise to a quite different loudness distribution pattern. From an idealized model, which represents a tuning fork as four colinear sources of strengths +B, −B, −B, +B, the velocity potential is calculated and the mean-square-pressure and intensity distributions are derived. It is shown that the mean-square-pressure distribution depends on polar angle (θ) and distance (R) as (3 cos2θ − 1)2/R6 close to the fork—the easily audible (amplitude-dependent) nearfield pattern; and as cos4θ/R2 at great distances—the less familiar (phase-dependent) farfield case. The intensity distribution comprises a cos4θ/R2 radial outward flow with, superposed, a circulation of energy from equatorial (θ ≈ 90°) to polar (θ ≈ 0,thinsp;180°) regions, varying with distance as 1/R4. There are two zeroes of intensity: where R = 0.225λ, θ = 0 and 180°. The power radiated is estimated for a typical 520 cps fork, and is found to be 2.5 × 10−15 W when the prongs vibrate with an amplitude of 0.2 mm.

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The construction of a 50- x 50-pixel spatial light modulator based on an active silicon backplane and using the hybrid field effect in nematic liquid crystals as the light modulating process is described. The design and electrical evaluation of the pixel array, which is fabricated in 1.5-microm nMOS and has an individual memory cell within each pixel, are discussed. The performances of a 16 x 16 prototype SLM and the new 50- x 50-pixel device are compared to provide an indication of progress toward high performance spatial light modulators with onboard pixel memory.

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