Tonal components of noise and Czech legislation

What is the tonal component of noise

The tonal component is a pronounced frequency component in sound that people often perceive as whistling, humming, squealing, buzzing, hissing, or a monotonous tone. It is therefore not only a question of how loud the noise is, but also what character it has.

In practice, noise with a tonal component can be annoying even at a lower overall level. A typical example is a fan with audible blade tones, a compressor with a pronounced frequency, a transformer with hum, a cooling unit on a building roof, or technology that at certain speeds creates a narrow-band tone.

Czech Government Regulation No. 272/2011 Coll. defines noise with tonal components technically. In simplified terms it is a situation where the sound pressure level in a given one-third octave band, or in two adjacent bands, is more than 5 dB higher than levels in both neighbouring one-third octave bands. At low frequencies of 10 to 160 Hz the relationship to the threshold of hearing is also assessed.

For the operator a simple conclusion follows: it is not enough to commission routine measurement of overall noise level. If there is suspicion of tonal character, frequency spectrum must also be measured and evaluated.

Why the tonal component is a problem in permitting and in complaints

The tonal component is a frequent reason for complaints. People often do not describe noise as a high level but as unpleasant whistling, humming, or a constant tone heard mainly in the evening and at night. The operator may meanwhile feel that the equipment is relatively quiet.

The problem is that Czech legislation takes tonal character directly into account in the hygiene limit. For noise with tonal components, with the exception of noise from transport on roads, railways, and from air traffic, an additional correction of −5 dB is applied. In practice this means the limit is stricter.

Typical example for a stationary source in the protected outdoor area of structures:

This is a fundamental difference for the operator. Equipment that would comply without a tonal component, for example at 47 dB during the day, may no longer comply if a tonal component is demonstrated.

Practical note: If complaints in the surroundings of the operation concern whistling, humming, or a monotonous tone, it is not enough to demonstrate only the overall LAeq level. It is necessary to verify whether the noise has a tonal component. That can decide the outcome of assessment.

Where tonal components most often occur

Tonal components are typical for equipment with rotating parts, air flow, electromagnetic hum, or a regular operating cycle. Sources that run for long periods and have stable speeds are mainly at risk.

In operations we encounter them mainly with this equipment:

For these sources it is advisable to address the tonal component already at design or technology change. Subsequent solution after a complaint is more difficult because the equipment is already installed, connected, and operationally needed.

Daytime and night-time assessment: why night is most at risk

For stationary sources in the protected outdoor area of structures, daytime is assessed for the 8 consecutive and adjoining noisiest hours and night-time for the noisiest 1 hour. The night-time limit for routine stationary sources in the protected outdoor area of structures is typically 40 dB. With a tonal component it is in practice tightened to 35 dB.

This is a very low value. Operators often underestimate equipment that runs automatically at night: cooling, ventilation, compressors, pumping stations, heat exchanger stations, boiler houses, cogeneration units, fans, or heat pumps. During the day equipment may be masked by traffic and surrounding activity, but at night the tonal component becomes pronounced.

For permitting a new source or operation change it is therefore advisable to verify not only daytime but also night-time regime. If technology does not run at night, this must be clearly stated in operating conditions. If it runs at night, the design of acoustic measures must be stricter.

How the tonal component is measured and demonstrated

The tonal component cannot be reliably assessed by listening alone. Listening can give suspicion, but for assessment measurement with frequency analysis is needed. One-third octave analysis is usually used, or narrow-band FFT analysis if one-third octave evaluation is not unambiguous.

The Ministry of Health (MZ) methodological guideline states that sound pressure level in one-third octave bands with centre frequency 20 Hz to 20 kHz is measured preferentially. For identification of tonal component, fractional octave filters or narrow-band analysis may be used. Narrow-band analysis is suitable especially where the tone is narrow, stable, or lost in a wider band.

The operator should count on measurement taking place in a regime when the assessed source is actually in operation and the tonal component is manifested. If measurement takes place at different speeds, under different load, or with the source switched off, the result may not correspond to the complaint or the real problem.

What a noise study or report should contain

If the tonal component is addressed, the measurement report or noise study should not contain only the resulting LAeq level. It must be clear whether tonal character was verified, by what method, and under what operating regime.

In practice it is important that the output contains description of the source, operating regime, measurement point, measurement period, meteorological conditions, LAeq level, frequency spectrum, evaluation of tonal component, residual noise, uncertainty, and conclusion in relation to the hygiene limit.

For a noise study before implementation caution is needed. Tonal components often cannot be reliably evaluated from catalogue data if the supplier does not provide frequency spectrum or data on sound power in bands. If only overall sound power level is used in the study, the risk of tonal component may not be correctly captured.

What to prepare before measurement

Before measurement it is necessary to define clearly which source is to be verified. If there are several fans, cooling units, compressors, or exhausts in the site, their operation must be distinguishable during measurement.

The most useful supporting materials are site plan, list of noise sources, operating regimes, equipment data sheets, photographs, information on complaints, and location of nearest protected structures. For fans and cooling it is important to know speeds, control, day/night regime, and possible output changes according to temperature.

If a complaint exists, it is advisable to find out when noise is heard most. It often turns out that the problem occurs only at a certain combination of regimes: night cooling, low speeds with resonance, operation of several units simultaneously, specific wind direction, or a specific phase of the production cycle.

Most common mistakes in practice

A common mistake is to commission only routine noise measurement without a requirement to assess tonal component. The result may then be formally correct for overall level but not correspond to the actual problem perceived by the surroundings.

A second mistake is reliance on catalogue noise level. The manufacturer often states a value under standard conditions, but actual equipment location, reflections from facades, ducting, exhausts, silencers, control, and building penetrations can significantly change noise character.

A third problem is measurement in the wrong regime. If measurement takes place during the day but the complaint concerns night-time, or if the source does not run in the regime when the tonal component arises, the result cannot safely be used to assess the problem.

A frequent mistake is also in designing measures. The operator installs general noise enclosure, but the tonal component remains because the real cause is duct resonance, fan, unsuitable silencer, mechanical fault, frequency converter, or structure-borne transmission.

What measures can help

For tonal components the cause must first be found. General reduction of level by a few dB may not suffice if a pronounced tone remains. Measures must target the specific frequency and specific mechanism of generation.

For fans help may come from speed change, impeller replacement, correctly designed silencer, removal of turbulence, adjustment of exhaust or intake, flexible mounting, and removal of duct resonance. For cooling and heat pumps night-time regime, speed limitation, acoustic screens, relocation, or replacement with a unit with a more suitable acoustic spectrum may help.

For compressors, cogeneration units, and boiler houses exhaust, intake, machine room ventilation, building penetrations, and vibration are often important to address. For transformers the problem may be location, building separation, structure-borne transmission, and hum at specific frequencies.

In some cases the most effective measure is organisational: limitation of night-time operation, switching equipment to another regime, operation only during daytime, or excluding simultaneous operation of several sources. Such a measure must however be realistically achievable in operation.

When to address a noise study and when measurement

A noise study is suitable before installation of new technology, during operation change, when permitting a source, or when designing noise protection measures. If there is suspicion of tonal component, work must be done with frequency data, not only with overall sound power level.

Measurement is suitable for complaints, after equipment installation, at building completion, during regional public health authority (KHS) inspection, or when verifying effectiveness of measures. For an existing operation measurement is often the fastest way to find out whether tonal component is actually present and whether it affects the hygiene limit.

For new equipment we recommend requiring from the supplier not only overall noise level but also sound power in octave or one-third octave bands and information on whether the equipment has pronounced tonal components. Without these data acoustic assessment is less certain.

What you can send us for assessment

Send us a site plan, description of noise sources, equipment data sheets, operating regimes, day and night operation, location of nearest houses or other protected structures, and any complaints. If you have an older noise study or measurement report, send those as well.

We will verify whether measurement with frequency analysis, noise study, inspection of a specific source, or design of noise protection measure is appropriate. For existing operations we recommend first verifying actual noise character by measurement. For new technologies it is advisable to verify tonal risk already from project and catalogue supporting materials.

Brief summary

The tonal component of noise can decide whether operation complies with the hygiene limit. For routine stationary sources, if it is demonstrated, a −5 dB correction applies, i.e. in practice a stricter limit. This can typically mean the difference between a limit of 50 dB and 45 dB during the day or between 40 dB and 35 dB at night.

The operator should address tonal components especially for fans, cooling, compressors, cogeneration, heat pumps, transformers, ventilation, and technologies with rotating parts. If complaints about whistling, humming, or monotonous tone arise, frequency spectrum must be measured, not only overall noise level.

Send us equipment data sheets, site situation, operating regime, and information on complaints. We will propose whether tonal component measurement, noise study, or specific noise protection measure is needed.

Factual basis of the article

From these sources it follows that tonal components are not only a subjective feeling of neighbours. They are a legislatively defined character of noise that can tighten assessment by 5 dB. The operator should therefore address frequency character of noise already at design, permitting, building completion, and when handling complaints for equipment at risk.