Should Spain revise its motorcycle noise measurement protocols?

By César Asensio, Universidad Politecnica de Madrid

In 2006 the Instrumentation and Applied Acoustics research group (I2A2) was part of the acoustics division of the Automobile Research Institute (INSIA) of the Technical University of Madrid (UPM). The regional authorities required our advice regarding the feasibility of performing vehicle noise measurements inside the workshops during the technical inspection of vehicles (ITV). Noise tests basically referred to the procedure described in the ISO 5130 standard, which, at that time, was still in draft form.

A bibliographic search was carried out, were two fundamental references quickly appeared. Firstly, the draft of ISO 5130 (1), which at the draft stage clearly stated the need to carry out the tests outdoors, on a test site that had to meet a series of very demanding requirements. Alternatively, the draft suggested the possibility of measuring indoors, in a semi-anechoic chamber with a cut-off frequency of 100 Hz. Given the measurement protocol, where low-frequency total components drive the measurement result, it seemed clear that performing an acoustics test inside the inspection workshop could lead to deviations and uncertainties greater than the 2 dB requirements of the regulation’s test protocol. In addition, there was a report from the Transportation Research Laboratory (2) where they addressed the same issue and showed by means of comparative tests, the problems and deviations that could arise from taking measurements inside workshops, especially in small ones.

However, the regulatory document that covers the whole ITV does not specify that the test site must be outdoors, it only specifies that there must be a distance of 3m to any reflective surface, except the ground. This poses serious problems of consistency and comparability.

Since 2007, from time to time, there have been several inspection entities that have requested our collaboration to optimize the space of their workshops, since maintaining distances of 3 meters is an important limitation for their performance. These same entities have an outdoor space reserved for measurements, but on rainy days (for example) they take the measurements indoors, even though they have noticed that the results are several decibels higher inside the workshop. But the protocol in Spain allows them to do so. They do not even consider the possibility of dispersion due to diffraction or eigenmodes (not their business). They merely apply the protocols at their convenience, without taking into account the technical quality of the measurements.

This has opened up a new business opportunity for some acoustic consultants who offer secret correction factors, drawn up by means of secret protocols that maintain the deviations of the workshop, for all types of motorcycles, within a deviation of less than 2 dB with respect to the optimal conditions (ISO 5130).

On the occasion of this special issue of Noise News International, I have been able to verify that after such a long time period the situation remains the same. The latest protocol (3), from 2020, still does not expressly mention that the tests must be performed in an open space. There are no studies to prove the validity of these huge deviations from the standard. Where are the rights of motorcycle owners? What is the point of these tests if they are not carried out rigorously?

  1. ISO. ISO/FDIS 5130. Final Draft. Acoustics — Measurements of sound pressure level emitted by stationary road vehicles. 2007
  2. Harris G.J., Nelson P.M., In-service noise testing – Motorcycles fitted with non-standard and defective exhaust silencers. Transportation Research Laboratory. 1996
  3. Manual de procedimiento de inspección de estaciones ITV, versión 7.4.1. 2020