Volumetric diffusers: pseudorandom cylinder arrays on a periodic lattice.

Most conventional diffusers take the form of a surface based treatment, and as a result can only operate in hemispherical space. Placing a diffuser in the volume of a room might provide greater efficiency by allowing scattering into the whole space. A periodic cylinder array (or sonic crystal) produces periodicity lobes and uneven scattering. Introducing defects into an array, by removing or varying the size of some of the cylinders, can enhance their diffusing abilities. This paper applies number theoretic concepts to create cylinder arrays that have more even scattering. Predictions using a boundary element method are compared to measurements to verify the model, and suitable metrics are adopted to evaluate performance. Arrangements with good aperiodic autocorrelation properties tend to produce the best results. At low frequency power is controlled by object size and at high frequency diffusion is dominated by lattice spacing and structural similarity. Consequently the operational bandwidth is rather small. By using sparse arrays and varying cylinder sizes, a wider bandwidth can be achieved.

Lüke and power residue sequence diffusers.

Conventional Schroeder diffusers have been successfully used for many years. However, their frequency range is limited by the flat plate effect that occurs when all the wells radiate in phase. This occurs at harmonics of p times the design frequency f(0), where p is the small prime that is used to generate the structure. A typical diffuser, using p=7 and f(0)=500 Hz, has an upper frequency limit of only 3.5 kHz. Achieving a first flat plate frequency above 20 kHz requires a prime equal to at least 41 and results in diffusers that are too big to be practical in most applications. This paper suggests an alternative approach using number theoretic sequences that, although short in length, are based on large integers. Two new sequences, Type-II Luke and power residue, have this desired characteristic. They are investigated using both simple models and the more exact boundary element method. The results show the flat plate effect is moved to much higher frequencies as expected. For Luke sequences at certain frequencies, redirection rather than dispersion is achieved. Modulation techniques can be used to mitigate these problems. Power residue sequences perform the best, providing good diffusion and a flat plate frequency outside the audible range.

A spread spectrum technique for the study of outdoor noise propagation.

This paper describes field measurements to assess innovative correlation techniques for the study of meteorological and topographical effects on sound propagation. To take advantage of the properties of coded signals in a time-varying system, the correlation signal is produced by the modulation of a code sequence onto an acoustic carrier. An established method of increasing signal-to-noise ratio is to use correlation techniques with maximum length sequences. However, this standard method is restricted in its use outdoors because of the time-variant nature of the atmosphere. On the other hand, the correlation properties of a directly carrier-modulated code sequence modulation signal may be exploited in a time-varying environment. An experiment is described in which the correlation properties of the spread spectrum signal are demonstrated and are used to calculate accurate times of flight that compare well with sonic anemometer measurements of speed of sound. The results illustrate that an acoustical spread spectrum system can provide significantly improved ways of measuring sound propagation outdoors.