Ramani Ramakrishnan

Design curves for rectangular splitter silencers

Abstract Passive silencers with acoustic fill such as glass fiber, rock wool or foam are commonly used in conventional heating, ventilation and air conditioning systems. Acoustic performance can be estimated for a few basic silencers through the use of design curves available in the literature. Recently, a large microcomputer data base using design curves generated to cover the entire range of manufactured rectangular silencers was made available. Details of the design curves and the mathematical model are presented. Insertion loss of the silencers is estimated from attenuation rates calculated from a finite element method. Unlike existing models, the present method considers multimodal acoustic propagation and can be extended to account for the effects of shearing airflows in the airway. The sound-absorbing material is considered to be bulk reacting. Wave propagation in the material is thus included. Design curves are grouped by using three nondimensional parameters, thereby covering the entire line of conventional rectangular duct silencers. Results from the model are compared to least attenuated mode predictions and to actual test data. The results show that the present model gives better predictions than least attenuated mode models. Good comparison between the current model and the test data was also observed. Development of a computer program for a quick estimation of the insertion loss is also described.

High Intensity Noise Generation for Extremely Large Reverberant Room Test Applications

A recent operational need for the development of a large (101,000 ft3) reverberant acoustic chamber at the Space Power Facility of NASA Glenn Research Center’s Plum Brook Station with the requirement of generating sound pressure levels (SPL) as high as 163 dB has resulted in the need to re-examine the generation of noise in reverberant rooms. Early in the design stage, it was realized that the acoustic power level capability (10-30 kW) of conventional electrodynamic air modulators, such as those supplied by the Wyle Corporation, would be required in unprecedented numbers to meet the test spectra requirements. The design team then turned to a lesser known modulator, the hydraulically driven air modulator supplied by the Team Corporation, which has 150-200 kW acoustic power capability. The advantage to the project was a significant reduction in the number of modulators required to meet the requirements. However, since only limited characterization of Team modulator’s performance has been reported, a test program was required in order to mitigate the risk of the design of the RATF. Aiolos Corporation, which is responsible for the acoustic design of the RATF, and the Institute of Aerospace Research (IAR) of the National Research Council of Canada (NRC), entered into a collaborative agreement with the objective of characterizing, optimizing and investigating the controllability of the Team modulators. The test program was performed at the NRC-IAR reverberant chamber, a 19,000 ft3 facility located in Ottawa, Ontario, Canada. The current paper provides details of the principle of operation of the Team modulators, including their servo control loops and provides of a summary of the characterization and controllability test program.

Concave surfaces and acoustics of performance spaces—Part I— Hybrid ray-image analysis

Current acoustic practices deem that concave surfaces do not provide good acoustical performance. However, old cathedrals, churches, and enclosed performance spaces with concave interiors seem to perform well. Part I of the current investigation analyzes the acoustical performance of spaces with curved surfaces. The main focus of the current investigation was to research the uniformity of the sound field produced by curved surfaces by analyzing sound pressure level distribution throughout the audience space. It studied the impact of the focal plane on the overall sound distribution within an enclosed space. To analyze the effect of curved surfaces at different frequencies, three enclosed rooms with curved surfaces were used to measure the sound pressure levels throughout an audience space: the Paul Cocker Gallery in the Ryerson Architecture Building, Toronto; St. Martin-in-the-fields Anglican Church, Toronto; and Wigmore Hall, United Kingdom. The evaluations were achieved with both experimental methods, and computer simulations using hybrid-ray-image methods. Computer simulations were validated by the initial on-site measurements in the Toronto locations. After these evaluations were performed, results showed that in these conditions, the curved surfaces had minimal negative impact as perceived by the audience. The results of the investigation will be presented in this paper.

Concave surfaces and acoustics of performance spaces—Part II —Wave analysis

Conventional wisdom states that having concave surfaces as the envelope of any occupied space does not produce good sound. The focussing effect of concave surfaces can cause high sound pressure levels, coloration, and echoes. However, throughout history there have been many enclosed rooms with large curved surfaces as envelopes that seem to produce good acoustics. Recent research suggested that wave analysis must be undertaken to establish the impact of concave surfaces. In contrast to Part I of the current investigation, evaluation of the sound pressure level distribution, in rooms with concave surfaces, was performed by solving the governing wave equation. The main reason is that the image-ray theory is valid only at frequencies greater than the Schroeder cut-off frequency. The wave theory is used for frequencies lower than 100 Hz. Finite element modelling was applied to solve for the sound pressure level distribution within rooms with concave surfaces. Three spaces, the Paul Crocker Gallery in Ryerson University, Toronto, St. Pauls Anglican Church in Toronto and Wigmore Hall in London were investigated in this study. The results for three low frequencies (25 Hz, 50 Hz, and 100 Hz) as well as their combination will be presented in this paper.Conventional wisdom states that having concave surfaces as the envelope of any occupied space does not produce good sound. The focussing effect of concave surfaces can cause high sound pressure levels, coloration, and echoes. However, throughout history there have been many enclosed rooms with large curved surfaces as envelopes that seem to produce good acoustics. Recent research suggested that wave analysis must be undertaken to establish the impact of concave surfaces. In contrast to Part I of the current investigation, evaluation of the sound pressure level distribution, in rooms with concave surfaces, was performed by solving the governing wave equation. The main reason is that the image-ray theory is valid only at frequencies greater than the Schroeder cut-off frequency. The wave theory is used for frequencies lower than 100 Hz. Finite element modelling was applied to solve for the sound pressure level distribution within rooms with concave surfaces. Three spaces, the Paul Crocker Gallery in Ryerson ...

Reactive acoustic liner design

Acoustic treatment to reduce fan noise levels in a wind tunnel circuit consists of fibrous materials such as fibreglass or rockwool. Open cell foam materials are also used as acoustic treatments. The acoustic treatments are conventionally applied at fan tail cone regions, tunnel walls along fan diffuser section, cross-legs, test section diffuser, and nozzle contraction areas. However, conventional treatments are not possible in cryogenic wind tunnels, since bulk absorber materials with required resistivity, when operating at cryogenic temperatures, are not available. One possible solution is to design reactive silencers tuned to dominant frequencies. One such approach was used as noise control technique so as to satisfy test section noise specifications. The sound power spectrum of the compressor at different speeds were evaluated. The estimated test section sound pressure levels showed noise reduction at two dominant frequencies were required. The acoustic treatment, therefore, resulted in a double layer reactive design tuned to the two dominant frequencies. The design process will be highlighted in the presentation. The final treatment details will also be presented.Acoustic treatment to reduce fan noise levels in a wind tunnel circuit consists of fibrous materials such as fibreglass or rockwool. Open cell foam materials are also used as acoustic treatments. The acoustic treatments are conventionally applied at fan tail cone regions, tunnel walls along fan diffuser section, cross-legs, test section diffuser, and nozzle contraction areas. However, conventional treatments are not possible in cryogenic wind tunnels, since bulk absorber materials with required resistivity, when operating at cryogenic temperatures, are not available. One possible solution is to design reactive silencers tuned to dominant frequencies. One such approach was used as noise control technique so as to satisfy test section noise specifications. The sound power spectrum of the compressor at different speeds were evaluated. The estimated test section sound pressure levels showed noise reduction at two dominant frequencies were required. The acoustic treatment, therefore, resulted in a double layer...

Multi‐variate error analysis of beam‐forming acoustic measurements in a wind‐tunnel

Source localization has been in the forefront in acoustics, since quantification of the source power and its exact location is de rigueur for successful noise control. Starting from single microphone measurements, many different methods, progressively more successful and complex, have been attempted for source localization. Recently, beam‐forming methods, due to their success in underwater acoustics and architectural acoustics, have been used in wind tunnel tests. These tests have been quite extensively utilized both in automotive and aircraft tests. Beamforming techniques use an array of microphones, arranged in different patterns, to detect the source location and source power. In beamforming, the array in effect beams, not in the physical sense, towards the source to determine its position. Various parameters determine the success of the beamforming techniques. Some of these parameters are: number of microphones, microphone spacing, array pattern, source frequency and signal analysis procedures. A seri...

Numerical methods for gas turbine silencer design

Gas turbines are becoming common power producers in small to medium scale power plants. However, the environmental noise impact due to the proximity of these plants to urban centers has become a critical concern and large amounts of silencing are required. Passive and/or reactive elements are commonly used to silence the noise propagating from the turbine exhaust stack. Two factors, namely temperature and low‐frequency dominance of the sound spectrum, must be properly accounted for in the silencer design. Simple conventional designs must include the effects of the above two parameters so that appropriate modifications can be incorporated to provide the necessary insertion loss. In this paper, performance evaluation of a passive silencer as well as a reactive silencer using numerical methods will be presented. Finite‐element methods (FEM) have been successfully used to predict the performance of passive silencers. Details of the FEM procedure and the necessary temperature modifications will be highlighted....