High Quality Acoustic Time History Measurements of Rotor Harmonic Noise in Confined Spaces

A methodology has been developed and presented to enable the use of small to medium scale acoustic hover facilities for the quantitative measurement of rotor impulsive noise. The methodology was applied to the University of Maryland Acoustic Chamber resulting in accurate measurements of High Speed Impulsive (HSI) noise for rotors running at tip Mach numbers between 0.65 and 0.85 – with accuracy increasing as the tip Mach number was increased. Several factors contributed to the success of this methodology including: • High Speed Impulsive (HSI) noise is characterized by very distinct pulses radiated from the rotor. The pulses radiate high frequency energy – but the energy is contained in short duration time pulses. • The first reflections from these pulses can be tracked (using ray theory) and, through adjustment of the microphone position and suitably applied acoustic treatment at the reflected surface, reduced to small levels. A computer code was developed that automates this process. The code also tracks first bounce reflection timing, making it possible to position the first bounce reflections outside of a measurement window. • Using a rotor with a small number of blades (preferably one) reduces the number of interfering first bounce reflections and generally improves the measured signal fidelity. The methodology will help the gathering of quantitative hovering rotor noise data in less than optimal acoustic facilities and thus enable basic rotorcraft research and rotor blade acoustic design.

SELECTED VARIATIONS BY COMPOSERS IN GERMANY AND FRANCE

The Haward of Dictionary of Music (1983), defines variation as "a technique modifying a given musical idea." From the Baroque period on, the form and the techniques of variation were developed and enriched in Germany and France. Therefore, I presented the works of composers from these two nations. Even though there was a vast number of possibilities, I wanted to be scholastically fair and interesting in making my selections by choosing well-known pieces along with lesser-known ones. Haydn's well-known Variations in F minor consist of two sets of double variations which break into an improvisation fantasy. The first movement of Beethoven Sonata in A flat major, Op. 26, is a set of five variations on the composer's original theme. The variations are positioned in the first movement instead of Sonata-Allegro form. In 1861 Brahrns composed the Variations and Fugue, Op. 24, on the theme of Handel. Brahms displays a wealth of rhythmic, harmonic and textural contrasts in the variations. Chopin's E Major Variations without opus number are written on a Swiss influenced German folksong. Faure's Theme and Variations in C sharp minor, Op. 73, includes eleven variations. The work displays the composer's subtlety, grace and reticence. 12 Variationen iiber ein eigenes Thema were written by Alban Berg as a composition study with Schonberg. The Finale of Dutilleux's Piano Sonata, titled "Chorale with Variations", is written in an impressionistic style. A rich expressiveness is well blended in a classical form. In 1742, the remarkable Aria and thirty variations known as the Goldberg Variations were composed by J. S. Bach. The thirty Variations are unified by the bass line, which forms the foundation of the Aria. The pieces discussed above were presented in three recitals. Compact disc recordings of these recitals are available in the Michelle Smith Performing Arts Library of the Clarice Smith Performing Arts Center at the University of Maryland.