Night operation of a production site: which sources are often decisive for the hygiene limit

Why night operation is critical in terms of noise

For the purposes of noise protection, under Act No. 258/2000 Coll., on Public Health Protection, night time is the period from 22:00 to 6:00.

For common stationary sources, Government Decree No. 272/2011 Coll. starts from a basic level of 50 dB. In the protected outdoor space of buildings, a correction of −10 dB applies for night time.

For highly impulsive noise, a further correction of −12 dB is used, and its assessment requires a separate evaluation under the Government Decree.

The method of time assessment is also important for production operations. At night, the one noisiest hour is assessed, i.e. L_Aeq,1h. During the day, the eight consecutive noisiest hours are assessed for stationary sources.

A facility therefore cannot "average" an unfavourable noisy hour with several quiet hours during the rest of the night.

Which sources are often decisive at night

When a noise study is being prepared, it often turns out that the main production technology is not the dominant source at the nearest house. The hall may screen its noise fairly well, whereas outdoor equipment operates without any building protection.

For night-time assessment, it is therefore necessary to go through the whole site, including auxiliary technologies. Equipment with a relatively low electrical output may matter more for the resulting noise than a production machine several times more powerful inside a closed hall.

Several below-limit sources may together exceed the limit

Noise levels are not added arithmetically. They are added energetically. The practical consequence is that two equally noisy sources raise the resulting level by approximately 3 dB.

If, for example, one fan causes a level of 37 dB at a protected building and a second similar fan likewise 37 dB, their combined contribution will be approximately 40 dB.

When designing new equipment, it is therefore not appropriate to work on the assumption that each individual source may reach the hygiene limit. A new technology must leave sufficient acoustic reserve for the other sources on the site.

Tonal noise can lower the limit to 35 dB

A tonal component is among the most significant risks of night-time industrial noise. It can typically appear with fans, electric motors, transformers, frequency converters, compressors or some pumps.

If a tonal component is demonstrated by a procedure under the relevant regulations and methodology, a correction of −5 dB is applied to the hygiene limit.

For a stationary source at night, the decisive limit may thus be 35 dB instead of 40 dB.

A tonal component cannot be reliably determined by subjective listening alone. Frequency analysis is used to objectify it. When selecting a fan or other equipment, it is therefore advantageous to also request acoustic data in octave or third-octave bands from the supplier.

Ventilation and cooling often run even without production

A production line may be stopped at 22:00 while extraction, cooling, air conditioning, a compressor or process ventilation continue all night.

For a noise study, we therefore need to know the actual operating regime of every significant piece of equipment.

A significant difference may arise, for example, between running four fans at once and a regime in which only one is left running after production ends. For equipment controlled by a frequency converter, reducing the night-time speed can also be an effective measure, if the process allows it.

The operating regime can be just as important a noise-control measure as a structural modification.

Beware of doors, rooflights and ventilation openings

With noisy production inside a hall, the noise in the surroundings depends on the sound insulation of the building envelope. Its weakest part is usually decisive.

A massive sandwich wall may have good sound insulation, but open doors or a large ventilation louvre create a direct path for noise propagation. Lightweight roof rooflights or leaky penetrations of process piping may be significant in a similar way.

The night regime therefore often requires closing doors, restricting handling in front of the hall or installing acoustic louvres and silencers on ventilation openings.

For new buildings, it is advisable to assess these paths already in the noise study. Retrofitting noise protection to a finished hall tends to be considerably more expensive.

On-site traffic is included in the noise of the facility

The Government Decree includes among stationary noise sources also traffic inside industrial and other sites.

In the noise study of a production plant, the movement of lorries, handling equipment and other traffic on on-site roads is therefore included, depending on the specific situation.

Vehicle traffic on a public road outside the site is assessed under a different regime. It is assessed under the rules for traffic noise.

In logistically intensive operations, it may be precisely the night-time arrival of lorries, reversing to the ramp, handling and departure that make up the decisive part of the noisiest hour.

Loading and short noisy operations cannot be overlooked in a study

The equivalent level over one hour also includes intermittent sources. Loading need not run continuously for the full 60 minutes in order to affect the resulting assessment.

Material falling into a container, handling of metal parts, movement of pallets, impacts when closing trailers or short-term operation of noisy outdoor technology may all be significant.

In operations with pronounced impacts, it is advisable to examine in advance also the possible character of impulsive noise. The Government Decree sets a stricter correction for highly impulsive noise.

The noise study must be based on the least favourable realistic operation

To assess planned night operation, it is necessary to build a model of the operating situation that can actually occur during normal permitted operation.

If three fans, a cooling unit and a compressor can operate at the same time while dispatch is taking place, this combination must be taken into account in the assessment. A study based on the operation of a single piece of equipment may substantially underestimate the resulting load.

For each significant source, it is advisable to know above all the sound power level L_WA, location, height, directivity, operating time and, where applicable, the frequency spectrum.

For existing technology, the input data can be supplemented by measurement directly on the site.

A noise study before night operation begins

We recommend preparing the noise study before a third shift is introduced, new technology is installed or the dispatch regime changes.

The calculation makes it possible to determine the contribution of individual sources at the nearest protected buildings and to identify which of them creates the largest part of the resulting level. As a result, noise-control measures can be targeted at the equipment where they have the greatest effect.

An effective solution may be, for example, a silencer on the vent, an acoustic enclosure, a reduction of night-time fan speed, relocation of the equipment, a noise barrier or a change in the organisation of dispatch.

You will find details on the page Noise studies – NATURCHEM.

When noise measurement is needed

For a site that is already in operation, the actual effect of the sources can be verified by measurement. To assess night operation, the measurement must capture a representative operating state and the noisiest hour between 22:00 and 6:00.

With a more complex combination of sources, it is useful during the measurement to switch individual pieces of equipment off one after another, as far as possible, or to change their operating regime. In this way their individual contributions can be determined and measures proposed where they will bring the greatest noise reduction.

Environmental noise measurement under Act No. 258/2000 Coll. may be carried out only by a holder of an accreditation certificate or a holder of the relevant authorisation.

A complaint about night noise from a facility

Under Section 30 of the Public Health Protection Act, the operator of a source is obliged to ensure, by technical, organisational and other measures, that noise does not exceed the set hygiene limits.

In the case of a repeated complaint, it is advisable first to determine precisely the time and the operating state at which the disturbance occurs. Information that the noise appears, for example, between 3:00 and 4:00 when cooling starts is considerably more valuable for diagnosis than a general statement about a noisy plant.

Regional public health authorities deal with complaints about noise from industrial sites and, when investigating them, may carry out or require an objective assessment of the noise.

What you can send us and what we will verify

For an initial assessment of night operation, you can send us:

• a site plan of the premises, the location of the nearest residential development, photographs and a list of outdoor and indoor noise sources,
• equipment data sheets with LWA values, operating time, the number of simultaneously operating pieces of equipment and a description of night-time traffic and handling,
• an existing noise study, measurement report, project documentation or any complaint or requirement from the regional public health authority.

We will examine which sources may be decisive for the night-time hygiene limit, propose the scope of the noise study or measurement and determine a suitable operating state for the assessment. If the study shows a risk of the limit being exceeded, the effectiveness of specific noise-control measures can also be evaluated in advance.

You will find more information on the page Noise studies – NATURCHEM.

Related legislation and sources

Act No. 258/2000 Coll., on Public Health Protection

Government Decree No. 272/2011 Coll., on the protection of health from the adverse effects of noise and vibration