ISO 9611:1996

Overview

ISO 9611:1996 specifies an approximate method to characterize sources of structure‑borne sound from machinery that is resiliently mounted. The standard defines how to measure one‑third‑octave (or octave where appropriate) free velocity level spectra at machine contact points (supports/isolation interfaces) to estimate the sound radiation transmitted into connected structures. It covers steady noise and describes measurement of all six velocity components (three orthogonal translations and three orthogonal rotations) at each contact point.

Key topics and requirements

• Measured quantities: one‑third‑octave‑band translational and angular velocity level spectra for each support (six degrees of freedom).
• Frequency range: limited by a low frequency f1 (set by isolator/foundation freedom; commonly ~20–40 Hz) and an upper frequency f2 (set by rigid‑body/contact‑point assumption; often 1–4 kHz).
• Mounting and isolators: isolators must be sufficiently soft so contact points vibrate freely; foundations must be heavy and stiff to avoid dynamic loading of machine supports. Soft rubber or air springs are typical.
• Test environment: background vibration at contact points must be at least 10 dB below levels from the machine; airborne sound from the machine must not affect contact point velocities. Flexible connections (pipes, couplings) are required so secondary connections do not stiffen the support.
• Validity and limitations: results represent free vibratory velocities only under stated assumptions. The method is approximate - intended to trade measurement simplicity against complete source description. It does not cover vibration criteria for human response, machine fatigue, or operational vibration standards.
• Uncertainty and repeatability: the standard provides guidance and estimated standard deviations for measurement uncertainty and defines repeatability/reproducibility conditions. Reference velocities used are given in the standard.

Applications and users

ISO 9611 is practical for:

• Acoustic engineers and noise consultants performing structure‑borne noise assessments.
• Test laboratories and OEMs preparing technical specifications and comparative emission data for resiliently mounted machines.
• Shipbuilders and industrial planners needing input data to model structure‑borne sound transmission (e.g., diesel gensets, engines, compressors, fans, motors, presses, lathes). Typical uses include comparing equipment, supplying input for transmission calculations, and defining test procedures for a class of machines.

Related standards

Useful companion references (cited by ISO 9611) include:

• ISO 3740–3747 series (airborne sound measurement)
• ISO 10816‑1 (mechanical vibration evaluation)
• ISO 5348, ISO 5347‑1, IEC 651, IEC 1260 (measurement instrumentation and filters)

Keywords: ISO 9611, structure‑borne sound, resilient mounting, velocity level spectra, one‑third‑octave band, vibration measurement, isolators, machine supports, acoustic source characterization.