Tonal noise from mechanical plant is not a new engineering problem. It has been present in HVAC and industrial equipment for as long as rotating machinery has been used to move air and fluid. What has changed is the prevalence of tonal noise in modern equipment and the degree to which that prevalence is creating compliance and community acceptance problems that were less common in previous generations of plant.

The shift toward variable-speed drives, electronically commutated fans, and high-efficiency heat rejection equipment has changed the tonal noise profile of mechanical plant in ways that are not always apparent at the time of equipment selection. Equipment that delivers measurable energy savings in operation can introduce tonal noise components that are perceptually intrusive, attract penalty in acoustic assessments, and are technically difficult to mitigate after installation.

For acoustic engineers, consultants, and project teams working on data centres, BESS installations, and industrial facilities, the tonal noise characteristics of specified equipment require the same level of attention at the design stage as sound power levels and overall noise output.

Where tonal noise in mechanical plant comes from

Blade pass frequency is one of the dominant sources of tonal fan noise, arising from aerodynamic interaction between the fan rotor and stationary components upstream or downstream of the impeller. It is the product of the number of fan blades and the rotational speed, expressed in revolutions per second. For a twelve-blade fan running at 1,000 rpm, the blade pass frequency is 200 Hz. Harmonics at 400 Hz, 600 Hz, and higher are also present, typically at progressively lower amplitudes.

Variable-speed drives change the rotational speed of the fan in response to thermal load, which means the blade pass frequency shifts continuously rather than remaining fixed. At low load, the rotational speed decreases and the tonal frequency moves down. At high load, the opposite occurs. The tonal frequency at any given operating point depends on the speed the drive selects for that load condition. In some configurations, the changing excitation frequency may coincide with a structural natural frequency, producing transient resonance during acceleration or deceleration that is distinct from the steady-state tonal noise at stable operating conditions.

Power electronic equipment, including inverters used in BESS installations and uninterruptible power supplies, can generate audible tonal characteristics associated with switching harmonics, magnetic components, and cooling systems. These tonal characteristics may not be apparent when equipment is assessed only by overall A-weighted sound levels, despite having a disproportionate influence on perceived noise and compliance assessments.

How tonal penalties affect acoustic compliance

Environmental noise guidelines in most Australian jurisdictions provide for an adjustment to the assessed noise level where a tonal component is audible at a receiver. The adjustment, commonly described as a tonality correction or tonal penalty, adds to the measured or predicted level to reflect the increased annoyance associated with tonal noise relative to broadband noise at the same overall level. The magnitude of the penalty varies between jurisdictions and assessment methodologies, but a correction of 5dB is common where tonality is clearly present and identifiable.

The practical consequence is that a mechanical plant installation that would comply with an applicable criterion at its overall A-weighted level can be assessed as non-compliant when a tonal component is identified across the frequency spectrum and the penalty is applied. The source does not need to be particularly loud. If the broadband background noise at the receiver is low, as is commonly the case in quiet residential or rural locations where BESS and data centre developments are sited, even a modest tonal component can be audible above background and attract the full penalty.

For acoustic modelling conducted at the planning stage, the absence of confirmed tonal data from equipment suppliers means that tonal penalties are often not included in the predictive assessment. The assumption that proposed plant will not generate audible tonality at the receiver is frequently not tested until post-commissioning measurement identifies tonality that was not predicted. The resulting non-compliance requires mitigation that was not included in the capital estimate.

Why tonal noise is hard to mitigate

Broadband noise is attenuated by absorptive materials that dissipate acoustic energy across a wide frequency range. The same mechanism that attenuates a 500 Hz component also attenuates components at 250 Hz, 1,000 Hz, and beyond, subject to the frequency-dependent efficiency of the absorber. Conventional absorptive splitter attenuators provide useful broadband attenuation across the frequency range of most HVAC noise.

Tonal noise at a specific frequency responds to attenuation that is effective at that frequency, but not necessarily to broadband attenuators that happen to include that frequency in their performance range. The tonal component may be driven by resonance or interference effects that require a treatment approach different from simple absorption. Where the tonal frequency shifts with operating speed, the treatment must be effective across the full range of frequencies the source generates, not just the frequency observed at a single operating point.

Reactive silencer design, tuned to attenuate specific frequency ranges, can address tonal noise where the frequency is stable and known in advance. For variable-speed equipment where the tonal frequency shifts across a range, the reactive silencer design may require treatment that addresses the entire range of interest, which can impose considerable size and pressure drop implications.

AcousTech’s Sonic acoustic attenuators include configurations where octave-band performance is selected to address specific low-frequency and tonal noise requirements, informed by the frequency analysis of the source rather than generic broadband specifications.

Addressing tonal risk before equipment is purchased

The most productive point at which to address tonal noise risk is before equipment is purchased. Equipment suppliers for large fans, cooling systems, and power electronics products typically hold acoustic data, including octave-band sound power levels and, in some cases, narrowband spectral analysis identifying tonal components. Requesting that data at the time of equipment enquiry, and reviewing it against the acoustic requirements of the project before purchase orders are placed, allows tonal problems to be identified while options are still available.

Identifying tonal characteristics early generally requires narrowband frequency analysis rather than relying solely on octave-band or overall A-weighted data. Where tonal noise data is not available from suppliers at the time of selection, specifying post-purchase acoustic testing as a condition of supply provides a mechanism to identify problems early enough to address them. Testing at the supplier’s facility before delivery is preferable to discovering tonal issues on site after installation, when the options for source-level treatment are more limited.

For data centre operators and energy developers procuring cooling and power systems at scale, the acoustic performance of specified equipment is a legitimate procurement criterion that deserves the same attention as energy efficiency and thermal performance ratings. AcousTech’s data centre acoustics expertise includes support at the equipment procurement stage on projects where acoustic performance is a primary selection criterion, including review of supplier acoustic data and specification of post-purchase verification testing. The Sonic Series acoustic louvres also contribute to managing the external noise profile of facilities where tonal fan noise is a concern at air intake and discharge openings.

Managing tonal risk across the project lifecycle

Tonal noise risk on modern mechanical plant projects spans the full project lifecycle, from equipment selection through installation, commissioning, and operation. Managing it requires acoustic input at each stage: specification of tonal performance requirements in procurement documents, verification of supplier data before purchase, post-installation testing to confirm compliance before community exposure begins, and operational monitoring protocols that identify emerging tonal issues before they generate complaints.

This is a more intensive acoustic management programme than many project teams currently implement. The justification is the cost and difficulty of retrospective tonal mitigation compared to proactive risk management at each project stage. Tonal noise identified before community exposure allows remediation to be implemented on a managed schedule. Tonal noise identified through a complaint triggers regulatory attention, reputational risk, and remediation work under pressure.

The equipment generating tonal problems on many current projects was selected without acoustic scrutiny at the procurement stage. Adding that scrutiny costs very little relative to the projects it protects.

Talk to the AcousTech team about your project.