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Product Acoustics by the Numbers
Acoustic circuits, resonators, small enclosures, drivers, microphone ports and fan noise, worked through one room air purifier
Design the sound of a product with numbers: acoustic circuits, ports and resonators, small speaker enclosures, drivers, air paths, microphone ports, directivity, fan noise and declared levels.
Start readingAbout this book
Many products have an acoustic job, whether their designers think of it that way or not. An appliance must be quiet enough to live with; a tool must not be so loud that its user needs ear muffs; a connected device must speak clearly and hear its user over its own noise. Each of those jobs depends on parts that are small compared with the wavelengths of sound: cavities, ports, grilles, membranes, drivers and the gaps between panels. At that scale, acoustics becomes a matter of circuits, of masses, springs and resistances that can be calculated on a page before a prototype is made.
This book works through product acoustics in the order a designer meets it: writing the acoustic requirements, acoustic circuits, resonators and ports, small sealed enclosures for drivers, the driver itself, air paths through a product, microphone ports, radiation and directivity, fans and structure-borne noise, and measuring and declaring the result. Each chapter ends with an exercise and a worked answer.
It suits product designers, mechanical and electronics engineers and product managers. It assumes school algebra and some familiarity with simple electrical circuits, which the acoustic analogies borrow. Machine Noise Control by the Numbers deals with noise from installed machines in workshops; this book is about designing the sound of the product itself.
The running case
Every chapter follows one fictional product. This is an illustration. AP-40 is a floor-standing room air purifier, a cabinet about 250 × 380 mm in plan and 600 mm high, with a backward-curved fan that runs from 500 rpm on its night setting to 1,200 rpm at maximum, a 40 mm loudspeaker for voice prompts, and a microphone near its top for voice commands. Its sound powers, driver parameters and every level in the book are illustrations chosen to show the method.
The product and its numbers are invented; none describes KEVOS® work or a real product. Prices, where they appear, are illustrations in Australian dollars excluding GST.
Source limits and professional responsibility
This book is educational. The relationships in it are the simplified models designers use for first estimates, and real products differ from them by several decibels, particularly at high frequencies where parts stop being small compared with the wavelength. Real designs are refined with measurement and, where needed, numerical simulation, and declared noise values are measured to the applicable standards. Products sold in Australia must meet the safety, electrical and labelling requirements that apply to them; this book does not describe those obligations.
Who this book is for
Product designers, mechanical and electronics engineers and product managers who design devices that make noise, play sound or listen, from appliances and tools to connected products.
What you will learn
- Write a product's acoustic requirements as measurable quantities: sound power, sound pressure at a distance and signal-to-noise at a microphone.
- Model ports, cavities and grilles as acoustic masses, springs and resistances, and find their resonances.
- Size a small sealed enclosure for a driver from its Thiele–Small parameters, and check its excursion limit.
- Predict how fan speed, air paths and mountings set a product's noise, and where its tones come from.
- Measure and declare a product's noise emission and check production units against the declaration.
Inside this book
Chapters
- 01Writing a product's acoustic requirementsTurn quiet, clear and able to hear into numbers: sound power by setting, sound pressure at a listener, levels in a real room, and catch a prompt speaker's level measured 2 cm from its grille.
- 02Acoustic circuits: masses, springs and resistancesModel the small air spaces in a product as acoustic masses, compliances and resistances with an electrical analogy, know when that model holds, and catch a long duct treated as a lumped mass.
- 03Resonators, ports and grillesFind a Helmholtz resonance from a cavity and its neck, add the end corrections that short holes depend on, design a speaker grille, and catch a front-cavity peak predicted above the band.
- 04Small sealed enclosures for driversPut a driver in a small sealed chamber: the air spring, the system resonance and Q from Thiele–Small parameters, the low-frequency roll-off, and catch a chamber shrunk to fit a circuit board.
- 05The driver: sensitivity, output and excursionWork out how far a small cone must move to make a given level at a given frequency, read sensitivity and power, see why low notes run out of excursion, and catch an alarm chime pitched too low.
- 06Air paths, grilles and lined passagesFollow a product's noise out through its air inlet and outlet, see what linings, plenums, grilles and ports do, weigh the fan's extra work, and catch a lining that made the product louder.
- 07Microphone ports and hearing over the fanDesign a microphone's acoustic path as a resonator kept above the speech band, place it where speech beats the product's own noise, and catch a gasket channel that put a peak at 6.4 kHz.
- 08Radiation, baffles and directivitySee when a small source radiates evenly and when it beams, how a product's face acts as a baffle, how walls change the output, and catch a prompt speaker verified only on its axis.
- 09Fans, motors and panel resonancesTrace a product's noise to its fan and motor: broadband and blade-pass noise, inlet flow, motor whine, mounts, panels that resonate, and catch a panel tuned to the fan's top-speed tone.
- 10Measuring and declaring product noiseMeasure a product's sound power, set declared values that allow for spread and uncertainty, test every unit at the end of the line, and catch a declaration set at the pilot units' average.
References and source notes
The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 26 October 2026.
- Leo L. Beranek, Acoustical Society of America. Acoustics (1954 edition, Acoustical Society of America reprint) (1954, reprinted 1986–1996). Supplied source, read as a subject map of electro-mechano-acoustical circuits, radiation, acoustic components, microphones, direct-radiator loudspeakers and their enclosures. The text is in copyright; no text, figures, tables or problems are reproduced, and this book's treatment, examples and figures are its own.
- A. N. Thiele, Proceedings of the IRE Australia, vol. 22, no. 8; reprinted in the Journal of the Audio Engineering Society, vol. 19, nos. 5 and 6. Loudspeakers in Vented Boxes (1961, reprinted 1971). The Australian paper that framed loudspeaker and enclosure design as filter design with a small set of driver parameters, the basis of the parameters used in Chapters 4 to 6. No values or tables are reproduced.
- R. H. Small, Journal of the Audio Engineering Society, vol. 20. Closed-Box Loudspeaker Systems, Part I: Analysis (1972). The analysis of drivers in sealed boxes behind the closed-box relationships used in Chapter 4. Only the widely published relationships are used, not the paper's text or data.
- International Organization for Standardization. ISO 3744 and ISO 3745 (determination of sound power levels using sound pressure); ISO 4871 Acoustics — Declaration and verification of noise emission values of machinery and equipment (Current editions). Named by number and scope only, as the standards for measuring a product's sound power and for declaring and verifying noise emission values, referred to in Chapter 10. No values or methods are quoted from them.