Low frequency noise – situation in Portugal

Low frequency noise – situation in Portugal

Low frequency noise – situation in Portugal

Although situations related with low frequency noise discomfort regularly arise in Portugal, the applicable legislation does not establish adequate evaluation criteria for low frequency noise.

This situation often presents a difficult challenge to the noise assessment technicians, especially when noise complaints arise, as they must decide on the existence of possible discomfort, without defined low frequency noise legal limits and without international consensus on the best evaluation practices.

Here, is presented the situation in Portugal concerning this theme.

Low frequency noise – Introduction

Low-frequency noise propagation, typically associated with the operation of industrial machinery, is a phenomenon currently recognized by the international community.

The respective effects, observed in different receptors, namely humans, have determined the increasing and systematic search for methodologies of measurement and analysis, since the presence of low frequency noise has become a problem for the society of different countries.

Within the buildings and at a certain distance from the noise generating source, the corresponding medium and high frequency emissions are in most cases attenuated / dissipated by the ground and / or the facade of the building and are also absorbed by the air that between the source and receiver.

In these cases, it is the low frequency noise, due to the corresponding high wavelength that characterizes it, which is heard / “sensed” inside the dwellings, generating consequent complaints due to discomfort. These complaints mostly arise at night, since local residual noise usually has less amplitude.

As can be seen from the curves of equal auditory perception, referred to in the international standard ISO 226, in the lowest frequencies, between 20 and 50 Hz, the growth of human auditory perception is much faster than in the higher frequencies, for example, above 1000 Hz.

as isofónicas - low frequency noise

In fact, when the sound level increases by 20 dB in the low frequency zone, the auditory perception increases by about 40 phones, while the same increase of the sound level of 20 dB in the 1000 Hz zone, corresponds to a growth of the auditory perception of , only, 20 phones.

This simple observation allows us to conclude that the low frequency noise measurements cannot be performed with A weighting, as it follows the curves of equal auditory perception, and can therefore lead to a masking of the received levels at low frequency measured at the highest frequencies.

This situation of possible masking is even more likely the more evident the tonal characteristics exhibited by the emitted / received noise.

It is for this reason that, on several occasions, measurements of complaints of discomfort due to noise result in dB (A) values which meet the applicable legal limits, but the discomfort felt by the complainants is manifestly real.

It is therefore clear that there is a need to update the existing noise assessment criteria in order to highlight the discomfort generated by the different types of noise emissions, in particular those with low frequency but without that those responsible for generating this noise are confronted with completely misaligned and often unfairly penalizing mitigating measures.

Low frequency noise – on the need of an assessment criteria

In Portugal, there are frequent legal proceedings due to complaints of discomfort caused by noise emissions. In most of these cases, the courts have decided in favor of the complainants, although the values obtained in their acoustic assessment show that the applicable legal limits are met.

This leads to increasing insecurity in economic operators, since even though they guarantee sound emissions in accordance with the legislation in force, they are surprised by the demand for additional requirements from the licensing / auditing entities, often difficult and / or very costly implementation.

Complainants often describe that they hear / “feel” disturbing noises with appreciable intensity, and in some of these cases the measurements made in accordance with the regulations in force do not show the occurrence of discomfort.

The situation described above, which is the subject of analysis in several research studies, has in some cases originated in the emission of low frequency noise, whose auditory perception can be identified by individuals with greater sensitivity, but that at the time, the criteria of evaluation in force in Portugal do not contemplate.

Resolving these cases is usually difficult and can be dragged over a long period of time, often several years, leading to tension between those responsible for the noise emissions and the complainants.

Even though it is the final resort to appeal to the courts, it is well-known that the solutions found do not entirely satisfy any of the parties involved.

The above makes clear the need to define legal measurement and noise analysis criteria that include low frequency emissions / reception, so that all those involved in the respective evaluation are benefited, namely:

  • The receivers, which will be better protected
  • The noise generating entities that can act in accordance with the correct identification of the problem and thus implement effective and timely mitigating solutions
  • Courts, which will decide in accordance with appropriate regulations, reducing conflict situations
  • Regulators, who will have their decisions less confronted by the judiciary
  • Noise assessment technicians, who will no longer be involved in situations where the results of the acoustic tests performed do not correspond to what the complainants say they hear / feel

Low frequency noise – Suggested assessment criteria

Based on the above and considering the current Portuguese reality, it is considered that it is urgent to legislate in a way that is possible and feasible the evaluation of low frequency noise.

To this end, a criterion should be established that obeys the following premises:

  • Widely tried and verified as appropriate
  • Conservative, not to sin for excess
  • Easy to apply, using existing instrumentation and measurement of noise

These are, as an example, the methods used in Denmark, Sweden, Germany or Spain.

The suggested criterion should only be used for indoor measurements and whenever there are complaints of discomfort due to noise that can be identified as low frequency noise.


The frequency with which the Portuguese courts decide in favor of the complainants, while complying with the legal limits established in the current national legislation on noise pollution, indicates that it is out of alignment with the current reality and therefore urges that it be amended , in particular as regards the emission / reception of low frequency noise.


Aroeira; Bicker; Leão; Conde; Rodrigues; Avaliação de ruído de baixa frequência – situação em Portugal; TECNIACÚSTICA 2017

A Ruido de Baixa Frequência Engenharia pode colaborar em:

1. Através de ensaios acústicos, identificar a existência de ruído de baixa frequência de acordo com a metodologia da norma Alemã DIN 45680:2013 – Medição e avaliação de imissões de ruído de baixa frequência;

2. Identificar as fontes de ruído de ruído de baixa frequência;

3. Definir as ações necessárias para eliminar o ruído de baixa frequência;

4. Seguir a implementação dessas medidas.

Caso pretenda alguma informação adicional, por favor contacte-nos.