Structure-borne sources

A machine fixed to a building radiates sound twice: directly from its own vibrating surface, and indirectly by injecting structure-borne power into the structure, which carries it away and re-radiates it in distant rooms. Six pages cover both paths, three of them here and three elsewhere on the site: one estimates the direct radiation from the surface vibration itself, and the other five characterise the second, sneakier structure-borne path end to end, from describing the vibration and characterising the isolators to quantifying the power and predicting the level a listener finally hears.

The language comes first. Mechanical mobility and the FRF family (ISO 7626-1) defines the motion-per-force frequency-response functions (receptance, mobility, accelerance and their reciprocals) that every later standard speaks, with the closed-form SDOF resonator as the reference and the ISO 7626-2 measurement acceptance criteria. Source and receiver mobilities are what decide how much power actually couples across an interface, which is why this vocabulary matters.

Three pages then characterise the path elements. Bending-wave transmission at plate junctions (Cremer/Craik/Hopkins) follows the power across the structure itself, with the wave-approach transmission coefficients for rigid X, T, L and in-line junctions, their diffuse-field angular average, and the coupling loss factor and vibration reduction index Kij they yield — and, for joints no wave model describes, the experimental route that inverts the same coupling loss factors from measured subsystem energies. Transfer stiffness of resilient elements (ISO 10846) measures the dynamic transfer stiffness of the isolators, mounts and hoses inserted precisely to break the transmission path, by the direct and indirect (transmissibility) methods. Sound power from surface vibration (ISO/TS 7849) handles the direct radiation: the airborne power estimated from surface velocity and a radiation factor, without any acoustic measurement.

The last two pages close the chain on the source and the receiver. Structure-borne sound power of equipment (EN 15657) measures what a machine injects, via the reception-plate method, and derives the plate-independent source quantities (blocked force, characteristic power level, free velocity). Installed structure-borne sound (EN 12354-5) consumes exactly those quantities, couples them through source and receiver mobilities, and predicts the sound pressure level in the receiving room, which is where this section meets the sound insulation models.

Pages in this section

• Mechanical mobility and the FRF family (ISO 7626-1): the FRF family, conversions and the SDOF reference resonator.
• Bending-wave transmission at plate junctions (Cremer/Craik/Hopkins): the wave-approach transmission coefficients for rigid X, T, L and in-line junctions, their angular average and the derived coupling loss factor and Kij, plus experimental SEA: coupling loss factors inverted from measured energies by power injection, with the modal densities they need.
• Transfer stiffness of resilient elements (ISO 10846): dynamic transfer stiffness of isolators by the direct and indirect methods.

See also

Pages elsewhere on the site that this section leans on:

• Sound power from surface vibration (ISO/TS 7849): radiated airborne power from surface velocity and a radiation factor.
• Structure-borne sound power of equipment (EN 15657): the reception-plate method and plate-independent source quantities.
• Installed structure-borne sound (EN 12354-5): the predicted receiving-room level from installed equipment.

What this section does not cover

• The junction coefficients are a closed-form idealisation for a rigid, simply supported junction between homogeneous plates, not a measurement: the empirical vibration reduction index obtained from a direction-averaged velocity level difference is ISO 10848, in Laboratory flanking transmission. The straight-section coefficient is undefined for the T and L geometries, which have no collinear third plate, so only the corner path applies there.

• The FRF page implements ISO 7626-1 and the ISO 7626-2 acceptance criteria for an attached exciter; impact-hammer excitation (ISO 7626-5) is named for context only, with nothing that synthesises or processes an impact spectrum, and the conversions return element-wise free reciprocals — correct for a driving-point or single-path use, not for a full FRF matrix, whose blocked matrix quantities are not built.

• On the isolator page, parts 4 and 5 of ISO 10846 are not implemented, and two of the standard’s own validity checks are described rather than computed: the rigid blocking-mass inequality, and the clause 7.6 linearity criterion (two input spectra 10 dB apart agreeing within 1.5 dB). Finally, nothing here designs an isolator or a floating base: the pages characterise elements and predict transmission, and the sizing decision stays yours.