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Machine Noise Control by the Numbers

Decibels, sound power, rooms, barriers, enclosures, silencers and isolation, worked through one compressor in one workshop

Control machine noise with numbers: decibel arithmetic, sound power, octave bands, exposure, room absorption, barriers, enclosures, silenced openings, vibration isolation and measurement.

10 chapters · About 100 minutes · Free to read · Intermediate engineering acoustics and workplace noise control

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KEVOS® LibraryENGINEERINGIMPACTLIKELIHOODMachine NoiseControl by theNumbersKEVOS® ORIGINAL BOOK10 CHAPTERS

About this book

A noisy machine is a problem with a source, a path and a receiver. The source makes sound power; the path carries it, through the air of the room, over or around obstacles, through walls and panels, and along the structure the machine stands on; the receiver is a person at a work position, whose ears accumulate a daily dose. Every noise control measure acts on one of those three, and every one can be estimated with a few numbers before money is spent: how much sound power, how far away, how much absorption, how heavy a panel, how large a gap, how soft a mount.

This book works through machine noise control in the order an engineer meets it: decibels and how they combine, sound power and sound pressure, frequency and weighting, daily exposure, sound in the room, barriers, enclosures, ventilation openings and silencers, vibration isolation, and measuring and verifying the result. Each chapter ends with an exercise and a worked answer.

It suits mechanical and facilities engineers, workshop managers, safety advisers and designers of machines. It assumes school algebra and logarithms. Welding Health and Safety by the Numbers covers noise as one hazard among many in a welding shop; this book is about designing the controls.

The running case

Every chapter follows one fictional installation. This is an illustration. CP-30 is a 30 kW rotary screw air compressor, supplied without an acoustic canopy, standing on the floor, clear of the walls, at one end of a fabrication workshop 24 m long, 16 m wide and 7 m high, with bare steel walls, a concrete floor and a metal roof. A fitter’s workbench is 4 m away, where one worker spends six hours of most days. The compressor’s sound power spectrum, the room’s absorption and every level in the book are illustrations chosen to show the method.

The installation and its numbers are invented; none describes KEVOS® work or a real workplace. Prices, where they appear, are illustrations in Australian dollars excluding GST.

Source limits and professional responsibility

This book is educational. The estimates in it are the simplified relationships engineers use for a first design, and real installations differ from them by several decibels. A real noise assessment is made by a competent person with calibrated instruments, under the work health and safety laws and codes of practice of the jurisdiction concerned, and its controls are verified by measurement. Where this book names a regulation or standard, the current adopted version governs, not this book’s summary.

Who this book is for

Mechanical and facilities engineers, workshop managers, safety advisers and designers who must predict, reduce and verify the noise of machines in workshops and plant rooms.

What you will learn

  • Add, subtract and average decibels, and rank the sources that set the level at a work position.
  • Predict sound pressure at a work position from a machine's sound power, its distance and the room.
  • Work out a worker's daily noise exposure and compare it with the exposure standard.
  • Estimate what absorption, barriers, enclosures, silencers and isolators will achieve, and where each fails.
  • Measure before and after a change, correct for background noise and write a noise control plan.

Inside this book

Chapters

  1. 01Decibels and how they combineRead sound levels in decibels, add and subtract sources correctly, rank what sets the level at a work position, and catch a noise budget spent on the source that did not matter.
  2. 02Sound power and sound pressureSeparate what a machine emits from what a person hears: sound power level, the free-field fall with distance, directivity near floors and walls, and catch a brochure's pressure read as power.
  3. 03Frequency, octave bands and weightingBreak machine noise into octave bands, see what A and C weighting do, read a low-frequency problem from the gap between them, and catch a high-frequency absorber bought to stop a rumble.
  4. 04Daily exposure and the duty to controlWork out a worker's eight-hour noise exposure and dose, compare it with the exposure standard, apply the hierarchy of control, set a design target, and catch a shift averaged in plain decibels.
  5. 05Sound in the room: absorption and reverberationSplit a room's sound into direct and reverberant parts, find the room constant, reverberation time and critical distance, size absorption, and catch an absorber sold on a 10 dB promise.
  6. 06Barriers and screensEstimate a screen's attenuation from its path difference and Fresnel number, see why it works outdoors and fails in a reverberant room, and catch a barrier sold on an 18.8 dB free-field figure.
  7. 07Enclosures: mass, lining and leaksEstimate a panel's transmission loss by the mass law, turn it into an enclosure's insertion loss with an absorbent lining, see what gaps cost, and catch an enclosure built without a lining.
  8. 08Cooling air, openings and silencersSize the air openings an enclosed machine needs, see how they limit the enclosure, estimate a lined silencer's attenuation, and catch an enclosure quoted at its sealed figure with plain louvres.
  9. 09Vibration isolation and structure-borne noiseStop machine vibration reaching the building: natural frequency from static deflection, transmissibility, mounts, flexible connections, and catch hard pads tuned to the motor's speed.
  10. 10Measuring, verifying and the noise planMeasure noise with a calibrated meter, correct for background, verify that controls worked, buy quiet, keep a noise plan, and catch a result taken from two readings 0.3 dB apart.

Apply This in Practice

Use these KEVOS® tools to try the method from this reading with your own numbers.

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 fundamental acoustics, sound in enclosures, transmission between rooms, noise control procedures and measurement. The text is in copyright; no text, figures, tables or problems are reproduced, and this book's treatment, examples and figures are its own.
  • Safe Work Australia. Model Code of Practice: Managing noise and preventing hearing loss at work (Current version). The national model code on managing workplace noise, used for the duty to manage noise, the hierarchy of control and the exposure standard described in Chapter 4. Each state and territory adopts its own version; the adopted code and regulations govern.
  • Z. Maekawa, Applied Acoustics, vol. 1, no. 3, pp. 157–173. Noise reduction by screens (1968). The measured relationship between a screen's attenuation and its Fresnel number, which underlies the barrier estimates of Chapter 6. The simple formula used there is a common engineering approximation to that chart, not a quotation from the paper.
  • International Electrotechnical Commission; International Organization for Standardization. IEC 61672-1 Electroacoustics — Sound level meters — Part 1: Specifications; ISO 3744 Acoustics — Determination of sound power levels of noise sources using sound pressure — Engineering methods (Current editions). Named by number and scope only, as the standards for sound level meters and for measuring a machine's sound power, referred to in Chapters 2 and 10. No values or methods are quoted from them.

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