Determining sound power of fixed machinery

How to measure sound emission in an acoustically complex environment by using a reference sound power source

Whilst small and infrastructure independent machinery can be brought into specialized measurements chambers to measure their sound power emission, for larger machines or fixed infrastructure dependent machines this is not possible. A “machine” could also be a pump, an invertor, a cleaning robot, a ventilation, production machine, etc.

In many cases, on-site measurements in their natural conditions is required in which case the measurement conditions can become acoustically complex.

Efficient sound power determination

The reference sound power method is interesting for on-site determination, because it is fast, has low complexity and does not require elaborate equipment.
There is a clear ISO standard 3747 to guide the measurement and processing and provides estimates of worst case accuracy.

The alternative methods include detailed sound intensity scanning and/or inverse techniques. These also allow measurement of the sound power emission. These methods however require a significant time investment and complexity.

The reference power method

The reference source method provides fast and low complexity measurements of sound power on-site in arbitrary acoustic environments.

The reference based procedure includes following steps:

  • Run the “machine” or product in its natural condition and measure the sound pressure spectra at minimum 5 locations.
  • If possible it would be advised to stop the “machine”
  • Place the Qsources reference source against the machine surface, activate, and measure the sound pressure at the same locations
  • Place the reference source against various surfaces of the “machine” if desired to increase accuracy
  • Calculate the sound power level from the averaged measured sound pressure level differences between machine and reference source

Ideally the measurement surface is as indicated in the illustration, following ISO 7473.  Often this is not possible because the microphones have to be closer; in which case, it is advised to repeat the procedure with another combination of reference source and microphone locations to determine the stability of the sound power.

After defining the reference and measuring surface, the microphone can be placed in position. there will be a total of 5 measurement positions. The details of the various positions is described in the application note which can be downloaded at the end of this page. The Reference sound source is placed in position on the upper surface of the machine and/ or under certain conditions against the sides of the machine.

If desired, the accuracy of the method can be improved beyond the ISO 3747 standard by:

  • Increasing the number of reference source locations on the faces of the machinery, and the number of microphone positions
  • Placing reflecting panels on around the machinery on the low refection sides

Qref Sound Power Reference Source

Another way of improving the accuracy of the Sound power reference method is by using the Qsources Qref:
a small and extreme lightweight sound power reference source

  • Qref is a well defined monopole source with a high acoustic impedance
  • driver and integrated sensor technology allowing the application of different signals (e.g. equalized white/pink noise, chirps, sine sweeps, MLS)
  • exceeds the ISO 6926 requirements in the 50-8000 Hz bands
  • incomparably lighter and smaller than any other sound power reference of similar output level on the market
  • electronic control is sensing, correcting, steering the driver, and steering the real time power proportional sound power signal

Qref allows a very controlled sound power emission, and even a spherical wave up to 3.6 kHz
The more traditional ventilator based solutions do not have the flexibility of placement agains any “machine” surface, have stronger directionality and can cause can an interference pattern, all increasing meausrement uncertainty.

The Qsources Qref set includes a dedicated Infra-Qsources QamR amplifier and is delivered with an individual certification according to ISO 6926

Unique to Qref compared to currently marketed sound power reference sources is that it can be placed in various positions on the test object, allowing precision and improving the measurement results compared to the Juxtaposition method.

Next to the substitution method or the improved juxtaposition method, Qref also allows measurements in direction of the inversion techniques.

Qref is also extremely suitable in high background noise environments or when the machinery cannot be stopped, the Qref can be used with special signals to obtain stable reference sound power data.

Next to the sound power identification on-site of for example installed machinery or larger installations, Qref can be used for:

  • Regular verification of acoustic rooms for product testing, machinery testing, anechoic rooms, reverberant rooms, including free field decay measurements.
  • Statistical Energy Analysis (SEA) type of measurements of rooms, halls, other acoustically coupled systems and other airborne excitation applications.

The reference method was applied in practice, using an early Qref prototype, on an electric saw.
This study describes the complete procedure including required infrastructure and test set-up such as:

Different microphone positions
Reference Sound Source positions
Testing procedure including background reducing FRFs
Sound power calculation in 1/3 octaves


Marius Morariu – Morariu Consultancy Limited
Peter J. G. van der Linden – Qsources bvba