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danny

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Post By：2013-4-4 22:57:02 [显示全部帖子]

The story behind - 'Acoustics, soundfields and transducers'

If you ask any engineer or academic working in the field of electro acoustics which is their favourite text book, the reply will nearly always be ACOUSTICS by Leo Beranek. Why is a book published in 1954 without revisions still so popular? There are many reasons: It deals mainly with fundamental principles which have not changed. It is well structured. The author’s passion for the subject is infectious – the opening sentence is “Acoustics is a most fascinating subject”. Wave propagation is explained pictorially before diving into mathematics. Electrical circuit analogies are used to provide insight into the operation of transducers. Formulas are given to help readers to work out their own designs. It is hardly surprising that ACOUSTICS has become one of the most cited books on the subject.

My background was in electrical engineering, so naturally Leo’s circuit analogies sparked my interest in acoustics and his book was my bible for many years. I was always fascinated by the plots of directivity patters and radiation impedances for pistons with and without baffles. In fact Lord Rayleigh (John W. Strutt) had derived an elegant closed-form formula for the radiation impedance of a piston in an infinite baffle in 1894, long before the direct radiator loudspeaker had been invented.

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danny

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Post By：2013-4-4 22:58:05 [显示全部帖子]

The story behind - 'Acoustics, soundfields and transducers'

However, I had much more difficulty in reproducing the impedance of a piston without a baffle. In 2002, I looked up the paper that Leo cited in the footnotes expecting to find an equation that I could simply enter into my computer and use to plot the result. It wasn’t there. I had to go right back to Christoffel Bouwkamp’s 1941 PhD thesis, which is 25 pages long, but contains no final equation that can be used. Every step is needed. At the end, the author states “The time-consuming computation of Eigenvalues, Eigen functions, and other quantities important for the physical interpretation was done on a small hand-driven Brunsviga desk calculator”. This was certainly a heroic achievement at the time and I imagine that he burnt much midnight oil.

At about the same time, I discovered a paper by another Dutch researcher, Hans Streng, which used more powerful boundary integral methods to determine the sound radiated from a circular electrostatic loudspeaker membrane. I then met a Finnish colleague (coincidentally with the same forename as the author of my favourite book), Leo K?rkk?inen, who was working at Nokia Research Centre in Helsinki. When I showed him Streng’s paper, he was quite excited about it because he could see that it could be used to describe any surface velocity distribution such as that of a piston in a circular baffle.

This led to our first published paper. With his physics background, Leo became my mentor in wave theory and mathematical methods. He even modified the Streng method in order to avoid the use of collocation and thus calculate everything directly. In my second paper I applied this method to the Bouwkamp problem of waves diffracted through a circular aperture. Meanwhile, I had been busy creating my own document of everything I had learnt about wave theory starting from first principles until one day, when I showed it to Leo K?rkk?inen, he said “This would make a nice book. Have you thought of publishing it?”

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danny

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Post By：2013-4-4 22:59:09 [显示全部帖子]

The story behind - 'Acoustics, soundfields and transducers'

A few years later, on 23rd April 2007 to be exact, I was thinking that what the world of engineering acoustics needed was a text book that covered everything from lumped-element theory using circuit analogies, as covered in Leo Beranek’s book, to wave theory and sound radiation/scattering problems.

However, I could not cover the fundamental principles any better than Leo had already done and I did not want to plagiarize anything. The idea of writing an updated version of ACOUSTICS suddenly hit me like a thunderbolt. It seemed like completely the right thing to do even if it would involve a huge amount of work. The reason I know the date so exactly is that in my excitement I immediately fired off an email to Leo K?rkk?inen!

I wrote to Leo Beranek who amazingly wrote back informing me that he held the copyright and that I could use anything I liked. However, he was not keen on being a co-author because he was too busy contributing to the current literature on concert hall acoustics, even if I wrote all the new material and he simply reviewed it. This was hardly surprising since at the time since I was completely unknown with only a handful of published papers to my name. However, as time went on, Leo became more enthusiastic about the project and contributed much new material, including two whole chapters on sound in enclosures and rooms for loudspeaker listening.

I was certainly delighted when he eventually decided he would like to have his name on it as that was the best possible endorsement of all the hard work involved.

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danny

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Post By：2013-4-4 23:01:36 [显示全部帖子]

The story behind - 'Acoustics, soundfields and transducers'

The first task was to obtain an optical scan of the original book as a Word document. Unfortunately it then took me a year to correct it because, in addition to numerous scanning errors, the software did not recognize any mathematical symbols or Greek letters. I also had the formidable task of reproducing the figures by drawing over PDFs of them in Word, rather like virtual tracing paper. I first met Leo Beranek face to face at the 2007 ICA meeting in Madrid. We also met a few times in London, Boston and at the 2008 ASA meeting in Miami but, because of the distance between Surrey and Boston, most of our collaboration was done via email. While working on the new text, we were answering all of the questions that I had collected in my mind over the years.

For example:

? What are the independent constituent variables that determine the efficiency of a loudspeaker?

? How is the radiated sound pressure of an unbaffled loudspeaker determined from its equivalent circuit?

? How does a finite open or closed baffle affect the frequency response?

? Can we design a simple crossover which does not produce time-delay waveform distortion?

? What are the 2-port networks for horns of different profiles?

? How much radiated sound power is needed to reproduce music or speech in an auditorium of a given size?

? How does the shape of the radiator affect the response?

? Is there a difference between flat, convex or concave radiators?

? Where does the Kirchhoff-Helmholtz boundary integral come from and what does it mean?

? Can we have a unified approach to sound radiation/scattering instead of the current patchwork of different methods?

? What is the equivalent circuit of a very narrow tube with viscous and thermal losses and a slip boundary condition?

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danny

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Post By：2013-4-4 23:02:44 [显示全部帖子]

The story behind - 'Acoustics, soundfields and transducers'

Leo paid particular attention to the ordering of contents and specifically asked me to include a new section on transmission-line loudspeakers because so many had asked him about how the Bose Wave? system worked. He also requested a new chapter on cell phone acoustics because he was curious as to how so much sound could be produced by something the size of a deck of cards and felt that it would bring the book right up to date. Because I had worked for Nokia for so many years, I was too close to the subject to explain it clearly to the lay person and Leo’s input here proved invaluable. I am also indebted to my colleague Enrico Pascucci for his marvellous photographs and many useful suggestions for the chapter.

Leo wanted a new title which indicated direct lineage with the original while reflecting the change in emphasis and suggested “Acoustics: Transducers and airborne acoustics”. Although “airborne” was intended to indicate “non-structural”, I suggested replacing it with “sound fields” in order to sound less “outdoors”. Also, the term sound field is fairly general as it can mean either a free field or sound in an enclosure. He then suggested putting “sound fields” before “transducers” because that is the order in which the two subjects are introduced and we settled for that.

A new version of ACOUSTICS had to include the work of Neville Thiele and Richard Small. They proposed just six parameters to completely describe the low-frequency behaviour of a loudspeaker, which are now commonly known as the Thiele-Small parameters, and Small showed how to obtain them from the input impedance.

Also, they produced tables/charts which enable anybody to choose a frequency response shape for a given drive unit and engineer the cabinet and bass-reflex port accordingly. Leo is rightly proud of the fact that these authors used his book as their starting point and that it led to the development of smaller loudspeakers using the acoustic suspension principle.

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danny

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Post By：2013-4-4 23:03:39 [显示全部帖子]

The story behind - 'Acoustics, soundfields and transducers'

During my previous experience as an analogue filter designer, the Thiele/Small approach was standard. One would never design a filter by messing around with component values until it “looked about right”. If standard tables/charts were not available for the element values of a particular circuit, I had to derive the transfer function by hand and solve for the polynomial coefficients in terms of the circuit element labels. This was a very laborious process which often involved many pages of algebra, so I fully understood the significance of Thiele and Small’s work. Virtually no circuit simulation tools use the transfer function. Instead they calculate all the node voltages at every frequency step. I had been interested in deriving transfer functions back in the 1980s for studying the transient responses and stability of amplifiers. Back then, computers could only do such calculations numerically. In order to solve for circuit element values, symbolic computation was needed, but at the time, Maple was the only software which did this and it was only available in universities. It was shortly after joining Nokia in 1999 that Noel Lobo introduced me to the numerical and symbolic power of Mathematica and taught me how to use it effectively.

Another Nokia colleague, Andrew Bright, had the foresight to ask me to write a program that could derive a polynomial transfer function from the net list of a circuit. I started with the method described in a recent book by Robert Boyd, but extended it to include ideal voltage sources, current sources, transformers and gyrators. Using Mathematica, I was able to create two versions: one numeric and the other symbolic. This proved invaluable for creating complicated acoustical designs such as a combined call and hands-free loudspeaker. I have described this computation method in the final chapter of the book. If one day someone were to use it to create a proper software tool with a nice GUI, it would really make my day.

I should also mention Juha Backman who has provided much support and encouragement over the years as well as introducing me to many people and important literature. It has certainly been rewarding working with someone as eminent as Leo Beranek. What both Leo’s share is a generosity of nature, energy and passion for acoustics as a subject that makes it seem more like fun than work. I hope that the new book inspires future generations of students, engineers and academics as the original inspired me. It really is a most fascinating subject and there is still plenty to explore.

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