Software-Based Acoustical Measurements

This textbook provides a detailed introduction to the use of software in combination with simple and economical hardware (a sound level meter with calibrated AC output and a digital recording system) to obtain sophisticated measurements usually requiring expensive equipment. It emphasizes the use of free, open source, and multiplatform software. Many commercial acoustical measurement systems use software algorithms as an integral component; however the methods are not disclosed. This book enables the reader to develop useful algorithms and provides insight into the use of digital audio editing tools to document features in the signal. Topics covered include acoustical measurement principles, in-depth critical study of uncertainty applied to acoustical measurements, digital signal processing from the basics, and metrologically-oriented spectral and statistical analysis of signals. The student will gain a deep understanding of the use of software for measurement purposes; the ability to implement software-based measurement systems; familiarity with the hardware necessary to acquire and store signals; an appreciation for the key issue of long-term preservation of signals; and a full grasp of the often neglected issue of uncertainty in acoustical measurements. Pedagogical features include in-text worked-out examples, end-of-chapter problems, a glossary of metrology terms, and extensive appendices covering statistics, proofs, additional examples, file formats, and underlying theory.

• Provides a cost-effective, primarily open source solution to obtaining and analyzing acoustical measurements
• Presents methods which are applicable to embedded systems running any system using free GPL software
• Equips readers with a deep understanding of the central challenges in acoustical measurements including a discussion of uncertainty
• Features comprehensive description of hardware, from the hard disk to flash memory, and from the digital recorder to the microphone and audiometric earphone
• Includes a chapter on testing hardware to ensure standard compliance

Auteur

Federico Miyara graduated as an Electronic Engineer from the Universidad Nacional de Rosario (UNR) in 1984. He then joined the faculty of his alma mater, first as an assistant teacher and later as an associate professor. There, he founded the Laboratory of Acoustics and Electroacoustics and has served as its director for more than 20 years. Over the years, he has led several postgraduate courses at the UNR, as well as at other universities in Argentina, Spain, Chile, Uruguay and Bolivia. He has conducted research in the field of noise assessment, particularly in noise mapping and in noise at the work place, as well as in psychoacoustics, musical acoustics, and architectural acoustics. His publications include four previous books on the subject of Acoustics, including the original Spanish-language version of this book. As a pianist and composer, the author attributes his initial interest in Acoustics to his curiosity about the science of music.

Contenu

Chapter 1. INTRODUCTION

1.1 Acoustical measurements

1.2 Instantaneous and effective (RMS) value

1.3 Sound pressure level

1.4 Equivalent level

1.4.1 Stabilization time for Leq

1.5 Weighted levels

1.6 Exceedance levels

1.6.1 Stabilization time for exceedance levels

1.7 Spectrum analysis

1.7.1 Constant percentage spectrum analyzer

1.7.2 Line spectrum analyzer

1.7.3 Line spectrum and time-frequency uncertainty 1.7.4 Line spectrum and power spectral density

1.7.5 Band filters

1.7.6 Transient response of band filters

1.7.7 Parseval's identity

1.7.8 Effect of spectrum tolerance on Parseval's identity

1.7.9 Weighted levels computed from one-third octave bands

Chapter 2 UNCERTAINTY

2.1 Introduction

2.2 Resolution, precision, accuracy

2.3 Measurement method and procedure 2.3.1 Direct and indirect measurement methods

2.4 Measurement model

2.5 Uncertainty

2.5.1 Type A Uncertainty

2.5.2 Type B uncertainty

2.5.3 Expanded uncertainty

2.5.4 Combined standard uncertainty

2.6 Examples

2.6.1 Relationship between uncertainties in level and pressure

2.6.2 Uncertainty in the correction of environmental conditions

2.6.3 Uncertainty in the calculation of the equivalent level

2.6.4 Uncertainty in A-weighting computed from octave spectrum

2.6.5 Uncertainty in the measurement of sound transmission loss

2.7 Uncertainty and resolution

2.8 Uncertainty and systematic error

2.8.1 Additive systematic error 2.8.2 Multiplicative systematic error

2.8.3 Non linear systematic error

2.8.4 Uncorrected systematic errors

2.9 Chain calculation of uncertainty

Chapter 3 DIGITAL RECORDING

3.1 Introduction

3.2 Digital audio

3.3 Sampling

3.4 Digitization

3.5 Signal-to-noise ratio

3.6 Low-level distorsion

3.7 Dither

3.8 Jitter

3.9 D/A and A/D conversion

3.9.1 Digital / analog conversion

3.9.2 Analog / digital conversion

3.10 Pulse code modulation (PCM)

3.11 Differential pulse code modulation (DPCM)

3.12 Delta modulation (DM)

3.13 Sigma-delta modulation (SDM)

3.14 Digital recording devices

3.15 Sound file formats

3.15.1 WAV format

3.15.2 FLAC format

3.16 Project files

3.16.1 AUP format

3.16.2 AU format

3.16.3 AUF format

3.16.4 External access to audio data 3.17 Recording and storage media

3.17.1 Hard disk

3.17.2 Flash memory

3.17.3 Optical discs

3.17.4 Digital audio tape (DAT)

3.18 File systems

3.18.1 FAT 32

3.18.2 NTFS

3.18.3 EXT4

3.18.4 HFS+

3.18.5 UDF

3.19 Long-term preservation

3.20 Conclusion

Chapter 4 DIGITAL AUDIO EDITING

4.1 Introduction

4.2 Audacity

4.2.1 Opening an existing file

4.2.2 Recoding sounds

4.2.3 Generating signals

4.2.4 Adding new tracks

4.2.5 Saving a project

4.2.6 Selection

4.2.7 Labels

4.2.8 Selection in the presence of labels

4.2.9 Calibration tone

4.2.10 FFT filters

4.2.11 Spectrogram and spectrum analysis

4.2.12 Noise removal

Chapter 5 TRANSDUCERS

5.1 Microphones

5.2 Polarization

5.3 Preamplifier

5.4 Sound fields

5.4.1 Free field

5.4.2 Diffuse field

5.4.3 Pressure field

5.4.4 Stationary field

5.5 Microphones and sound fields 5.6 Frequency response

5.7 Directional response pattern

5.8 Noise

5.9 Distortion

5.10 Micromachined microphones

5.10.1 Frequency response

5.11 Audiometric earphones

5.12 Omnidirectional sources

Chapter 6 DIGITAL SIGNAL PROCESSING 6.1 Discrete signals

6.2 Discrete impulse

6.3 A signal as its convolution with a discrete impulse

6.4 Discrete systems

6.5 Finite- and infinite-impulse-response systems

6.6 Difference equation of a discrete system

6.7 Frequency response of a discrete system

6.8 Z transform of a discrete signal 6.9 Z transform of a difference equation

6.10 Z transform of a convolution

6.11 Z transform and frequency response

6.12 Solution of a difference equation

6.13 Poles and stability of a discrete system

6.14 Inversion of a rational z transform

6.15 Continuous- to discrete-system bilinear conversion

Chapter 7 BASIC ALGORITHMS FOR ACOUSTICAL MEASUREMENTS

7.1 Introduction

7.2 Opening a .wav file

7.3 Energy average and equivalent level

7.4 Calibration

7.5 Energy envelope

7.6 A weighting

7.7 Statistical analysis

Chapter 8 SPECTRUM ANALYSIS

8.1 Introduction

8.2 Spectrum analysis paradigms

8.3 Digital filters

8.4 Discrete Fourier transform (DFT)

8.5 Fast Fourier transform (FFT)

8.6 Spectrum analysis with the FFT

8.6.1 Line spectrum analysis

8.6.2 The problem of frequencies close to Fs/2 8.6.3 The problem of subharmonic frequencies

8.6.4 Precise detection of pure tones

8.6.5 Windows

8.6.6 FFT band spectrum analysis

8.6.7 Spectral density and spectrum averaging

8.7 …