What can explain differences between experimental tests and MANATEE simulation results?

Sound pressure versus sound power level

First be sure to compare two comparable figures: MANATEE acoustic noise level can be given as a sound power level (SWL), or as a sound pressure level (SPL). SPL measurements are easy to carry but can include significant uncertainties (e.g. background noise, reverberant field, directivity). Sound power level measurements run according to acoustic standards are recommended for a valid comparison.

When comparing measured and simulated sound pressure levels, be sure to input in MANATEE

  • the right distance from the outer surface of the machine to the microphone in Input.Simu.type_micro_distance
  • the right directivity coefficient in Input.Simu.directivity_coeff
  • the right room constant for reverberant field in Input.Simu.room_cstt

Electrical machines are usually tested at the factory where the directivity coefficient is not ideal and reverberation field is uncertain, in that case simulation results may differ from experiments with up to 10 dB difference.

If you want to estimate your room constant to improve quantitative noise calculations with MANATEE, EOMYS can run a specific measurement campaign to quantify reverberation effects in your test facilities.

Damping

Besides that, a key value to have a good estimation of the absolute SPL value is structural damping (from 0.5 to 2% in electrical machines) which depends on many parameters (temperature, resin type, winding type, etc), so you can only expect MANATEE to calculate the sound level in absolute value with high accuracy (<3 dB) if you have run a detailed experimental modal analysis to quantifiy the damping involved in your application.

Mechanical and aerodynamic parasitic sources

When comparing measured and simulated SWL, be sure that you have also included the non-magnetic acoustic noise sources (aerodynamic noise, mechanical noise). MANATEE allows to add an external source of noise function of speed.

Eccentricities and asymmetries

It is known that eccentricities and geometrical / magnetic asymmetries can introduce new resonances due to magnetic forces. If you have simulated a symmetrical machine in MANATEE some resonances may be missing compared to experiments.

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