DD ISO TS 07849 — Machinery Airborne Sound Power by Vibration Measurement Standard Series

📐 DD ISO/TS 7849 is a British Standards Institution (BSI) “Draft for Development” (DD) series adopting the ISO/TS 7849 (Acoustics — Determination of airborne sound power levels emitted by machinery using vibration measurement) technical specifications. Prepared under BSI Technical Committee EH/1/4, Machinery noise, the series provides an alternative noise assessment method based on vibration measurement for situations where anechoic or reverberation rooms are unavailable, or acoustic measurement is impractical. This batch contains two parts: Part 1 is the survey method using a fixed radiation factor, and Part 2 is the engineering method including determination of the adequate radiation factor.

💡 In many industrial sites, machinery noise cannot be assessed by conventional sound-power methods (high background noise, restricted sound field). The vibration method measures surface vibration velocity of the machine and combines it with a “radiation factor” to infer airborne sound power, enabling noise assessment in complex environments.

1. Series Overview and Positioning 📌

The ISO/TS 7849 series is developed by the International Organization for Standardization (ISO), where TS denotes a Technical Specification — a technical document that may not yet have the full consensus required for a formal international standard but is still published publicly. BSI adopts it as a DD (Draft for Development) for UK trial use and feedback, reporting UK experience to ISO to drive conversion into an international standard.

This batch contains the following two parts:

Document	Subject	Year	International source	Core content
DD ISO/TS 7849-1:2009	Acoustics — Determination of airborne sound power levels by vibration measurement — Part 1: Survey method using a fixed radiation factor	2009	ISO/TS 7849-1:2009	Rapid survey method using a fixed radiation factor, suitable for low-precision screening evaluation
DD ISO/TS 7849-2:2009	Acoustics — Determination of airborne sound power levels by vibration measurement — Part 2: Engineering method including determination of the adequate radiation factor	2009	ISO/TS 7849-2:2009	Engineering method determining the adequate radiation factor by measurement and calculation, with higher accuracy

⚠️ Both parts follow the same “vibration to sound power” conversion route but differ in accuracy: Part 1 is survey grade, Part 2 is engineering grade. Where conformity determination or precise comparison is required, Part 2 should be preferred.

2. Basic Principle of Vibration-Based Measurement 🔭

The structural vibration of a machine is the main excitation of its radiated airborne sound. A vibrating machine surface converts vibration energy into acoustic energy radiated outwards, with radiation efficiency depending on the surface vibration amplitude, frequency and coupling with the acoustic environment. The core idea of DD ISO/TS 7849 is:

Sound power ≈ surface vibration energy × radiation factor

The measurement steps are:

Place accelerometers on a regular grid over the machine surface and measure vibration velocity levels at each point;
Integrate over the measurement area to obtain the total vibration energy of the surface;
Multiply by the radiation factor (reflecting the efficiency of converting surface energy to sound) and sum over frequency bands to obtain the airborne sound power level.

The accuracy of the radiation factor directly determines the final sound-power result, which is exactly the methodological difference between Parts 1 and 2.

3. Part 1: Survey Method Using a Fixed Radiation Factor 🔧

Part 1 suits cases requiring rapid, low-cost screening of machinery noise. Its characteristics include:

Fixed radiation factor: directly adopts the conservative/typical radiation factor given in the specification, without separate determination for the particular machine — simple to operate.
Survey-grade accuracy: results suit noise-source ranking, trend monitoring and preliminary judgement, not high-precision conformity determination.
Applicability: suited to simple structures with near-planar surfaces and approximately uniform radiating vibration.

Because the radiation factor uses a fixed assumed value, the systematic uncertainty is relatively high; results need careful interpretation when the machine is structurally complex or its radiation differs significantly from the assumption.

4. Part 2: Engineering Method (Determining the Adequate Radiation Factor) 🛠️

Part 2 provides an engineering-grade method, the core improvement being determination of the adequate radiation factor by measurement and calculation rather than using a fixed value:

Radiation factor determination: can be derived by comparing vibration with acoustic measurement results, making the conversion closer to the actual machine.
Engineering-grade accuracy: lower uncertainty than the survey method; usable for engineering acceptance, checking noise contract limits and source-contribution analysis.
Data organisation: measurement and calculation are organised in frequency bands (e.g. octave or 1/3-octave), enabling alignment with standard acoustic-method results.

💡 Engineering advice: when a machine is installed on site and cannot be moved into an acoustic laboratory, Part 2 is almost the only viable route to assess its sound power level. Points must be placed strictly per the specification, accelerometer mounting quality controlled, and environmental conditions recorded.

5. Engineering Practice Points ⚙️

🔍 Site assessment: in high-background-noise or restricted sound fields, the vibration method avoids systematic acoustic bias and predicts machine contribution to workplace noise.
🔧 Sensor mounting: accelerometers must be securely attached (e.g. adhesive or magnetic) to avoid contact resonance affecting vibration velocity measurement.
📊 Band consistency: vibration measurement and radiation-factor conversion must use the same frequency-band system, otherwise the combined sound-power level will be in error.
📋 Link to standard acoustic methods: results can be cross-validated with ISO 3744 (engineering-grade pressure method) and ISO 3746 (survey-grade pressure method).

6. Frequently Asked Questions ❓

❓ Q: Why can vibration measurement replace acoustic measurement for sound power?
A: The machine surface vibration is the main excitation of its sound source; measuring vibration velocity and multiplying by the radiation factor gives the radiated sound power. The method suits industrial sites with restricted acoustic environments.

❓ Q: How to choose between Parts 1 and 2?
A: Choose Part 1 (fixed radiation factor, survey grade) for rapid screening and noise ranking; choose Part 2 (determining adequate radiation factor, engineering grade) for engineering acceptance, high-precision evaluation or complex structures.

❓ Q: What is the effect of an inaccurate radiation factor?
A: The radiation factor directly affects the sound-power conversion result. The larger the difference between the fixed assumption and the real machine, the larger the result deviation, which is why Part 2 determines it by measurement for higher accuracy.

🔍 Q: Is this series mandatory?
A: DD/TS documents are provisional technical documents and are not themselves mandatory. However, EU Machinery Directive and similar regulations often reference sound-power measurement methods, and using methods conforming to this series helps demonstrate compliance.

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