Loudspeakers

For Music Recording and Reproduction

Philip Newell, Keith Holland

• Publisher: Routledge
• Year: 2018
• ISBN:
  9781351371193
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Acoustical Engineering
• Index: Technology & Engineering

Loudspeakers: For Music Recording and Reproduction, Second Edition is a comprehensive guide, offering the tools and understanding needed to cut out the guesswork from loudspeaker choice and set-up.

Philip Newell and Keith Holland, with the assistance of Sergio Castro and Julius Newell, combine their years of experience in the design, application, and use of loudspeakers to cover a range of topics from drivers, cabinets, and crossovers, to amplifiers, cables, and surround sound. Whether using loudspeakers in a recording studio, mastering facility, broadcasting studio, film post-production facility, home, or musician's studio, or if you simply aspire to improve your music-production system this book will help you make the right decisions.

This new edition provides significant updates on the topics of digital control, calibration, and cinema loudspeaker systems.

Chapter One

What Is a Loudspeaker?

[What is sound? The origins of loudspeakers. Sound radiation. Mechanical and acoustic impedances. Directivity of pistonic radiators. Resistance and reactance.]

1.1 A Brief Look at the Concept

Before answering the question posed by the title of this chapter, perhaps we had better begin with the question ‘What is sound?’ According to Fahy, ‘sound may be defined as a time-varying disturbance of the density of a fluid from its equilibrium value, which is accompanied by a proportional local pressure, and is associated with small oscillatory movements of the fluid particles’.¹ The difference between the equilibrium (static) pressure and the local, oscillating pressure is known as the sound pressure.

Normally, for human beings, the fluid in which sound propagates is air, which is heavier than most people think – it has a mass of about 1.2 kg per cubic metre at a temperature of 20° C at sea level. It is also interesting to note that sound propagation in air is by no means typical of its propagation in all substances, especially in that the speed at which sound propagates in air is relatively slow, and is constant for all frequencies. For music lovers, this latter fact is quite fortunate, because it would be hard to enjoy a musical performance at the back of a concert hall if the notes arrived jumbled up, with the harmonics arriving before the fundamentals, or vice versa. Conversely, as we shall see later, most of the materials from which loudspeakers are made do not pass all frequencies at the same speed of sound, a fact which can at times make design work rather complicated.

The speed of sound in air is about 343 metres per second (often abbreviated to m/sec, m/s, or ms−1) at 20° C and varies proportionally with the square root of the temperature. (In fact, the speed of sound in air is only dependent upon temperature, because the changes that would occur due to changes in atmospheric pressure are equal and opposite to the accompanying changes in density, and the two serve to cancel each other out.) Air therefore has some clearly defined characteristic properties, and our perception of sound in general, and music in particular, has developed around these properties.

The job of a loudspeaker is to set up vibrations in the air which are acoustic representations of the waveforms of the electrical signals that are being supplied to the input terminals. A loudspeaker is therefore an electro-mechanico-acoustic transducer. Loudspeakers transform the electrical drive signals into mechanical movements which, normally via a vibrating diaphragm, couple those vibrations to the air and thus propagate acoustic waves. Once these acoustic waves are perceived by the ear, we experience a sensation of sound.

To a casual observer, a typical moving-coil loudspeaker (or ‘driver’ if you wish to restrict the use of ‘loudspeaker’ to an entire system) seems to be a simple enough device. There is a wire ‘voice coil’ in a magnetic field. The coil is wound on a cylindrical former which is connected to a cardboard cone, and the whole thing is held together by a metal or plastic chassis. The varying electrical input gives rise to vibrations in the cone as the electromagnetic field in the voice coil interacts with the static field of the (usually) permanent magnet. The cone thus responds to the electrical input, and there you have it: sound! It is all as simple as that! . . . Or is it?

Well, if the aim is to make a sound from a small, portable radio that fits into your pocket, then maybe that concept will just about suffice, but if full-frequency-range, high-fidelity sound is the object of the exercise, then things become fiendishly complicated at an alarming speed. In reality, in order to be able to reproduce the subtle structures of fine musical instruments, loudspeakers have a very difficult task to perform.

1.2 A Little History and Some Background

When Rice and Kellogg developed the moving-coil cone loudspeaker in the early 1920s, [and no: they did not also invent Kellogg’s Rice Krispies!] they were already well aware of the complexity of radiating an even frequency balance of sound from such a device.² Although Sir Oliver Lodge had patented the concept in 1898 (following on from earlier work by Ernst Werner Siemens in the 1870s at the Siemens company in Germany), it was not until Rice and Kellogg that practical devices began to evolve. Sir Oliver had had no means of electrical amplification – the thermionic valve (or vacuum tube) had still not been invented, and the transistor was not to follow for 50 years. Remarkably, the general concept of loudspeakers was worked out from fundamental principles. It was not a case of people simply experimenting with magnets, wire, and cardboard, and developing things by trial and error. Despite the fact that the inventors lacked the benefit of modern materials and technology, they had the basic principles very well within their understanding, and what Rice and Kellogg developed is still the essence of the modern moving-coil loudspeaker. Nevertheless, their goals at the time were not involved with achieving a flat frequency response from below 20 Hz to above 20 kHz at sound-pressure levels in excess of 110 dB SPL (sound-pressure level). Such responses were not required because they did not even have electrical signal sources of such wide bandwidth or dynamic range, although the requirements of the ‘talking’ cinema industry soon encouraged the development of higher-powered systems. It was not until the 1940s that microphones could capture the full frequency range, and the 1950s before it could be delivered commercially to the public via the microgroove, vinyl record.

Prior to 1925, the maximum output available from a radio set was in the order of milliwatts, normally only used for listening via earphones, so the earliest ‘speakers’ only needed to handle a limited frequency range at low power levels. The 6-inch, rubber surround device of Rice and Kellogg used a powerful electromagnet (not a permanent magnet), and as it could ‘speak’ to a whole room-full of people, as opposed to just one person at a time via an earpiece, it became known as a loud speaker. The inventors were employed by the General Electric Company, in the USA, and they began by building a mains-driven power amplifier which could supply the then huge output of 1 watt. This massive increase in the available drive power meant that they no longer needed to rely on resonances and rudimentary horn-loading to achieve adequate volume, which typically gave very coloured responses. With a whole watt of amplified power, the stage was set to go for a flatter, cleaner response. The result became the Radiola Model 104, which with its built-in power amplifier sold for the then-enormous price of $250. [So there is nothing new about the concept of self-powered loudspeakers: they began that way!] Marconi later patented the idea of passing the direct current supplied to the valves through the energising coil of the loudspeaker, to use it instead of the usual, separate smoothing choke to filter out the mains hum from the amplifier. Therefore right from the early days it made sense to put the amplifier and loudspeaker in the same box.

Concurrently with the work going on at General Electric, Paul Voight was busy developing somewhat similar systems at the Edison Bell company. By 1924 he had developed a huge electromagnet assembly weighing over 35 kg and using 250 watts of energising power. By 1926 he had coupled this to his Tractrix horn, which rejuvenated interest in horn loudspeakers because it enormously improved the sensitivity and acoustic output of the moving-coil loudspeakers, and when properly designed did not produce the ‘honk’ sound associated with the older horns. Voight then moved on to use permanent magnets, with up to 3.5 kg of Ticonal and 9 kg of soft iron, paving the way for the permanent magnet devices and the much higher acoustic outputs that we have today.

Gilbert Briggs, the founder of Wharfedale loudspeakers, wrote in his book of 1955:

It is fairly easy to make a moving-coil loudspeaker to cover 80 to 8000 cycles [Hz] without serious loss, but to extend the range to 30 cycles in the bass and 15,000 cycles in the extreme top presents quite a few problems. Inefficiency in the bass is due mainly to low radiation resistance, whilst the mass of the vibrating system reduces efficiency in the extreme top.³

The problem in the bass was, and still is, that with the cone moving so relatively slowly, the air in contact with it simply keeps moving out of the way, and then returning when the cone direction reverses, so only relatively weak, low-efficiency pressure waves are being propagated. The only way to efficiently couple the air to a cone at low frequencies is to either make the cone very big, so that the air cannot get out of the way so easily, or constrain the air in a gradually flaring horn, mounted directly in front of the diaphragm. Unfortunately, both of these methods can have highly detrimental effects on the high-frequency response of the loudspeakers. For a loudspeaker cone to vibrate at 20 kHz it must change direction 40,000 times a second. If the cone has the mass of a big diaphragm needed for the low frequencies, its momentum would be too great to respond to so many rapid accelerations and decelerations without enormous electrical input power – hence the loss of efficiency alluded to by Briggs. Large surfaces are also problematical in terms of the directivity of the high-frequency response, but we ...

Table of contents

• Cover Page
• Half Title
• Series Page
• Title Page
• Copyright Page
• Contents
• About the Authors
• Acknowledgements
• Preface
• Introduction
• 1 What Is a Loudspeaker?
• 2 Diversity of Design
• 3 Loudspeaker Cabinets
• 4 Horns
• 5 Crossovers
• 6 Amplifiers and Loudspeaker Cables – A General Review
• 7 Loudspeaker Behaviour in Rooms
• 8 Form Follows Function
• 9 Subjective and Objective Assessment
• 10 The Mix, the Music, and the Monitors – The Instability of Perception
• 11 Low-Frequency and Transient-Response Dilemmas
• 12 The Challenges of Surround Sound
• 13 Loudspeakers for Cinema Soundtrack Mixing
• 14 What to Measure: And Why – The Influence of Advanced Measurement and Psychoacoustic Concepts on Loudspeaker Design
• Glossary of Terms
• Index