The subject of this book is aircraft noise. It is a collection of notes from a graduate-level course taught by the author over the past few decades. He assumes no previous exposure to the study of acoustics by the students. Even though the text is brief, the scope is very broad, going from the fundamentals of acoustic theory to computational aeroacoustics (CAA).
The introduction, chapter 1, addresses acoustic propagation and noise metrics, then moves on to very brief discussions of the nature of aircraft noise, types of vehicular horns, and music theory. The next chapter covers the fundamental types of noise sources: monopoles, dipoles, and quadrupoles. Chapter 3 discusses Lighthill's 1952 paper including his “equation of sound” and his acoustic analogy. The brief chapters 4 and 5 are concerned with subsonic jet noise while the next chapter, the longest in the book (46 pages), is an exploration of various CAA schemes, concluding with short discussions of propeller and helicopter noise prediction. Supersonic jet noise, “sound at solid boundaries” (airframe noise?), combustion noise, and sonic booms are dealt with in 11 pages in chapter 7 before it concludes with 8 pages on measurement techniques and noise reduction.
Clearly, the author wishes to share the information he collected throughout his career which is related to aircraft noise, plus a few other aspects of acoustics. As the title indicates, the book is intended to be an introduction to the subject, and that is a reasonable description of the contents. It could be used to show the student a sampling of work done in the area, but not to provide a foundation for further study. While the density of equations suggests rigorous treatment, the breadth of coverage and the brevity of the book necessitate cursory discussions of the many aspects of aircraft acoustics. Anyone seriously interested in the field would be well advised to first take a course in acoustics and then to study a systematic survey of the field such as that compiled by Hubbard (Aeroacoustics of Flight Vehicles, Theory and Practice–Volume 1: Noise Sources; Volume 2: Noise Control, edited by Harvey H. Hubbard, Acoustical Society of America, 1994) which was stringently reviewed.
The work chosen for presentation by Bose is curious indeed. Many of the references are non-refereed conference papers and theses or dissertations. For example, the only references for the extensively researched topic of propeller noise are a 1970 Jet Propulsion Laboratory report and an unspecified 1996 “time-domain calculation.” Lengthy quotations from the references are given throughout the book, but they are usually taken out of context so that the reader has no basis for establishing the limitations or generality of the quote.
Nomenclature and symbology are not only unconventional, but often contradictory and confusing. For example, instead of using f (Hz) for frequency in cycles per second, ν is used and given the dimension of 1/s when it is clearly intended to be cycles/s. Further, even though the traditional ω is used for radial frequency, it is given the units of radians/s rather than 1/s. In the discussion on standing waves in tubes, α is given two different definitions. The second is called “spatial radian frequency” and defined as ω/c, which is traditionally called the wave number and given the symbol k. In fact, in the same discussion, ω/c is set equal to ν, α, and finally k in the space of little more than a page. To compound the confusion, the accompanying Fig. 1.4 erroneously shows pressure antinodes and velocity nodes at the open ends of a tube. All of this discussion and pointless collection of equations clouds one of the simplest concepts in acoustics, resonant frequencies of tubes. One need only note that a closed-closed tube will resonate at frequencies having an integral number of half wavelengths in the tube due to the requirement for a velocity node at each end. Similarly, an open-closed tube resonates with an odd number of quarter wave lengths due to the requirement for a pressure node at the open end and a velocity node at the closed end. Such obfuscation is typical of the entire book.
Nonsensical passages are found throughout the text. For example, here is a run-on sentence from section 1.8 talking about ducted fan noise: “It has been found that pressure noise distribution is less effective can decay [sic] exponentially in their passage through the duct, whereas in a supersonically spinning mode the noise distribution is less effective upstream and is zero under choking conditions.” Unfortunately, this and a short, equally cloudy discussion near the end of the book are the only inclusions of duct acoustics material, a very important consideration in turbofan noise prediction and control. For large, modern turbofan-powered transport aircraft, fan noise often is larger than jet noise, especially during landing approach.
Unfortunately, organization is also a major problem. For example, the brief section on “Measurement Techniques” includes an unrelated discussion of airframe noise sources as well as a paragraph on the increasing role of air traffic controllers in airport community noise reduction, including the imposition and enforcement of curfews.
So, this introduction to aerodynamic noise may have an audience, but it should not include anyone with a serious interest in establishing a foundation for the practice of aircraft noise control engineering.
