Overview
ISO 18431-4:2007 - "Mechanical vibration and shock - Signal processing - Part 4: Shock-response spectrum analysis" defines standardized digital methods to calculate a shock-response spectrum (SRS) from an acceleration time record. The standard specifies how to model a set of single-degree-of-freedom (SDOF) oscillators, the transfer functions used to obtain maximum responses versus natural frequency, and the digital-filter approach (including the Ramp Invariant Method) to produce accurate SRS results. It also provides filter coefficient formats and recommendations for adequate sampling frequency.
Key topics and technical requirements
- SDOF modelling: SRS is built by applying the measured base acceleration to an array of SDOF mass–damper–spring systems and recording the peak response per natural frequency.
- Response types: acceleration response (input is acceleration), plus variants such as relative velocity, relative displacement, pseudo‑velocity and equivalent static acceleration.
- Spectrum variants: maximax (maximum absolute response), positive, negative, primary (during input) and residual (after input).
- Transfer functions & parameters: formulation in terms of natural frequency, Q‑factor (resonance gain) and damping ratio (ζ). Guidance on frequency spacing - e.g., for Q = 10 (≈5% damping) a minimum of six frequencies per octave (~20 per decade) is recommended.
- Digital filter design: second‑order digital filters (z‑transform form with numerator/denominator coefficients α, β) are used to approximate the analogue SDOF transfer functions. The standard prescribes algorithms and coefficients and uses the Ramp Invariant Method for filter derivation.
- Sampling considerations: the document gives recommendations on sampling frequency and anti‑aliasing precautions required for valid SRS calculations.
- Normative references: includes ISO 2041 (vibration and shock vocabulary) and cross‑references other parts of ISO 18431.
Practical applications
- Characterizing dynamic mechanical environments (shock and transient loading) with an SRS to quantify peak responses across frequencies.
- Defining and deriving test specifications for shock testing and environmental qualification of components and equipment.
- Converting measured shock records into SRS-based pass/fail criteria for product design, reliability, packaging, and field damage investigation.
- Supporting the development of vibration/shock test procedures (used together with test standards such as IEC 60068‑2‑81).
Who uses this standard
- Vibration and shock engineers, test lab personnel, and reliability / environmental test designers.
- Mechanical, aerospace, automotive and electronics engineers involved in qualification and ruggedization.
- Software developers implementing SRS calculation algorithms in data‑acquisition and signal‑processing tools.
Related standards
- ISO 18431 series (Parts 1 and 2 and other parts in preparation)
- ISO 2041 - Vibration and shock - Vocabulary
- IEC 60068‑2‑81 - Environmental testing: shock test design (example of test design from SRS)
Keywords: ISO 18431-4, shock-response spectrum, SRS, mechanical vibration, digital filters, SDOF, Ramp Invariant Method, sampling frequency, Q-factor, pseudo-velocity.
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