How To Get Started in Acoustics Analysis

Tyrrell, R J

First Published: 1998
Softback, 48 pages

The purpose of this book is to present an overview of the current state of the art in predictive acoustics work. It is intended to form both an introduction to the subject, and also to present a set of guidelines for good practice.

Predictions of acoustic performance are of importance in many areas. These are not limited to just those projects whose objectives are noise reduction, but include aspects of quality and durability. Examples of the need to reduce noise include almost all consumer goods, plus requirements of good acoustic design in the architectural and civil engineering fields. The noise shields being mounted alongside many major roads today are not cheap: they reflect the growing unacceptability of noise pollution by the general public.

Some applications are concerned with the development of transducers to radiate sound more effectively than erstwhile – e.g. underwater sonar devices and loudspeakers. Others require investigation of mechanisms of excitation of structures by acoustic phenomena. The author’s own company was involved, for example, in the investigation of fatigue problems of space rocket nozzles, when fatigue arose from vibrations induced acoustically from the rocket exhaust.

The reasons for performing predictive acoustics are of course limitless, but the following points are probably the most important:

predictions may be carried out in advance of the prototype. As such, they promote the cost effective development of acceptable products. (This does mean that work may be done without supportive experimentation; validation of methodologies is vital, as are acceptance tests).

Predictive work provides insight into the mechanisms of noise generation and propagation. The insight is a pre-requisite for the successful improvement of designs.

Up to approximately the mid-1980’s it was common practice to use predictive techniques based either on empirical rules, or on extrapolations of relatively simple analytic examples. The prediction of acoustic performance of complex geometries or combinations of fluid media beyond the scope of most groups. For most engineers, the prediction of acoustical behaviour appeared to rely on some mixture of acoustic theory, many years of experience, intuition, and preparedness to stretch analytic examples well beyond their true application.

Since the mid 1980’s, advances in computer hardware and software systems have made the accurate prediction of acoustics both possible and available to all at reasonable cost. As with many advances in predictive methods, this availability of capability may lead to problems. Acoustics is a field where intuition is sometimes misleading. It is essential for predictions of system performance to be made by qualified staff, who have a least have an understanding of the mathematical principles of acoustics, and who understand the capabilities and limitations of the various computer tools available.

One of the objectives of this booklet is to provide some guidelines for new workers in acoustic prediction. The booklet makes no attempt to be an exhaustive treatment of any aspect of acoustics; there are many excellent texts available, some of which are referenced here. If this booklet achieves nothing else, it may at least help practitioners be aware of some of the questions which need to be answered if results are to be trusted.

It is assumed that the reader has some engineering training, is familiar with fundamentals of engineering mathematics, and also understands basic concepts of linear vibration theory, such as the concept of working in the frequency domain. The most important pre-requisite for reliable predictive work is a preparedness to question whether or not results obtained ‘make sense’, and to be prepared to invest a little time to understand the principles behind the work in hand.

Contents

Introduction

Fundamentals of Acoustics

Solution Approaches

• • Analytical vs Numerical Mehods
  • Rayleigh Methods
  • Finite Element Solutions
  • Boundary Elements Formulations
  • Ray Tracing Methods

Vibration Analysis

Acoustic Prediction Guidelines

• • General
  • Cavity Modes
    • Approach
    • Modelling
    • Results

• Transfer Function Calculations

  • • Approach
    • Modelling
    • Results

• Radiation Calculations

  • • Approach
    • Modelling
    • Results

• Coupled Analysis

  • • Approach
    • Modelling
    • Results

• Architectural Work (Large Geometries)

  • • Approach
    • Modelling
    • Results

Results Interpretation

Advanced Analyses

• • Foam Finite Elements
  • Energy Finite Elements
  • Acoustic Holography

Computer Systems

References

Glossary of Acoustic Terms

• • Nomenclature
  • Definitions