THE SCIENCE OF SOUND RECORDING JAY KADIS Amsterdam • Boston • Heidelberg • London New York • Oxford • Paris • San Diego San Francisco • Singapore • Sydney • Tokyo Focal Press is an imprint of Elsevier Focal Press is an imprint of Elsevier 225 Wyman Street, Waltham, MA 02451, USA The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK Ó 2012 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data Application submitted British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN: 978-0-240-82154-2 For information on all Focal Press publications visit our website at www.elsevierdirect.com 1213141516 54321 Printed in the United States of America INTRODUCTION It is hard to believe that we have been able to record sounds for only a little over one hundred years. Those born after the digital revolution take the ability to make sound recordings entirely for granted. The widespread availability of inexpensive, high-quality recording equipment has greatly increased the number of audio recording practitioners. Although it is easy to get started, many soon discover that attaining the quality of recordings they expect is not as simple as sticking a microphone in front of an instrument and pressing the record button. Many beginning audio devotees fail to consider the scientific principles behind the techniques professionals use to create the recordings they wish to duplicate. Long internships – once a staple of entry into the recording industry – are vanishing, while more beginners simply start on their own with little or no instruction. Once the difficulty of making great recordings becomes evident, those serious about a career in recording seek out more information. Many schools promise to make the student into a world-class engineer in a few simple lessons, often for a hefty tuition. Simply learning the current techniques without an understanding of why these particular approaches work as they do limits one’s ability to deal with the challenges presented by new recording situations and with the continuing rapid development of recording hardware and software. Science is a framework used to organize our observations of how the world works. It makes use of mathematics to quantify the behavior of physical objects. It relies on repeatable experiments to verify the theories we formulate about how things work. Although science is applicable to any endeavor that involves physical objects, it is often ignored when we try to figure out how something works. The complexity of mathematics can scare us away from its fundamental usefulness in analyzing the processes we wish to understand. Even without being able to solve differential equations, we can derive a useful understanding of how a system behaves from how the equations are set up and what variables are included. The goal of this book is to show how science explains the techniques we have at our disposal for making and manipulating sound recordings. Though science is not a static collection of observations, it is a stable source of knowledge. As new discoveries are made, previously accepted ideas are altered to fit both the old and new information. Fortunately, most ixj x Introduction of the science applied to recording technology is old enough to have been time-tested and proven. It is well worth the effort to learn the scientific principles on which sound recording relies. Areas such as acoustics and electronics are well described and considered settled. Psychoacoustics, on the other hand, leaves much to be explained about how we translate physical air vibrations into musical sound that excites the emotions. The application of the scientific method in our quest to understand the intricacies of recording brings us ever closer to mastering the incredible power we have available to create any sound environment we can imagine. We will start with a review of the mathematics used in the scientific explanations of acoustics and electronics. The science of measurement is also important because microphones essentially make a measurement of air pressure or velocity that becomes our recorded signal. The ability to quantify physical behaviors is central to building a structured understanding of the systems we use to record sound. The physics of mechanical systems and gases helps understand how microphones transfer sound wave infor- mation into electric signals. We also want to understand how we perceive sound, as our ability to hear determines what we must preserve in the processes involved in recording and playing back sound recordings. Of course, sound is at the center of the recording process, and understanding its behavior will begin to explain why microphone placement can be such a tricky proposition. The tools available to process the electronic repre- sentation of sounds are at the heart of studio practices. We will look at the passive and active electronic devices, often considered “black boxes,” that make up our devices. Then we will look at the audio processing devices themselves. Digital recording is the popular technology of the day, but the analog tape recorder was the dominant recording technology throughout the later 20th century and is still popular with rock music studios. Analog recorders, once the mainstay of the recording business, are being discovered by a new generation that appreciates the sound of tape enough to master the care and maintenance of these machines. New magnetic tape is being designed and manufactured to meet the continuing demand. Learning how a mature technology dealt with the limitations of the medium is enlightening. Digital recording systems have been refined from the days of PCM (pulse-code modulation) to the video converters that began the digital audio age. Digital data is significantly different from the original analog signals that are produced by microphones, so examining the theories underlying digital audio will explain the limitations of analog-to-digital conversion as well as Introduction xi reveal the immense power of digital signal processing. The final chapter involves using all the technical information to accomplish the artistic goals of mixing sounds into the product: recordings for others to enjoy. This book is not intended to give the reader a mastery of science but rather to reveal how scientific principles apply to the processes we use every day without a second thought. When we understand more of how the systems work, we become better at using them. The reader will find rec- ommended readings that go further into the science of selected subjects to continue the quest we are only beginning here. One of the great aspects of sound recording is that one may enjoy learning the craft for a lifetime. May this book set you on that path. CHAPTER ONE Mathematics and the Measurement of Sound Contents Mathematics 2 Sinusoids 4 Logarithms and Exponents 8 Vectors 10 Polar Coordinates 11 Complex Numbers 11 Calculus 13 Statistics 14 Units of Measure 15 Sound is inherently a transient phenomenon. Natural sounds fade quickly, and we can’t just bottle up the air as it vibrates with sound waves and expect to hear the sounds again when we open the bottle. Although this behavior keeps the world from becoming a total cacophony, it also complicates the process of recording sounds. We need a method of converting such air vibrations into a form of energy that we can preserve over time. Metrology, the science of measurement, is a most important consideration in the recording of sound. To record and manipulate audio signals, we must first measure characteristics of the sound – generally pressure amplitude as a function of time – and in some manner preserve those measurements. Analog magnetic tape stores a continuous magnetic pattern representation of the measured signal. Digital audio systems store the signal as a list of discrete measurements at regular times that can then be treated like any other computer data. Because these measurements are a function of time, we must have a way of measuring time as well. Once we have a reliable record of time and the signal amplitude is stored, we can manipulate the signal electronically as analog audio or numerically as digital audio and then make changes to suit our artistic desires; we can process and mix sounds into sonic landscapes that never before existed. Appreciating that the quality of a recording depends on how well we are able to make a measurement is an important first step in the study of sound recording.