Battery energy storage is often described as quiet technology, and at the cell level that is true. There is no combustion, no reciprocating machinery and no prime mover.

Everything around the cells is a different matter. Thermal management, power conversion and voltage transformation are all noise producing, and they are loudest when the asset is working hardest. On a site with a night-time criterion in the mid thirties dB(A), that equipment is the whole acoustic problem.

This guide is written for project managers, developers, engineers and specifiers who are not acousticians. It covers what each component contributes, why the components differ from one another, and which variables move the numbers.

Start with the architecture, because the source list depends on it

There is no single battery energy storage arrangement, and an acoustic description that assumes one will be wrong on at least half of projects.

Air cooled container designs mount air conditioning units directly on the container, so the heat rejection is distributed across the site and located at the battery units themselves. Liquid cooled designs circulate coolant through the racks to heat rejection equipment, which may be mounted on the container or grouped in a separate skid. Where heat rejection is grouped, the acoustic centre of gravity moves away from the batteries and toward the cooling plant.

Power conversion varies as well. Some projects use integrated blocks that combine converter and transformer on a single skid. Others use central converters serving multiple containers, with separate transformers. The number of units, their spacing and their proximity to the boundary all follow from that choice.

Before anything is assessed, the project should be able to say which arrangement it has, how many of each unit type, and where they sit. Two projects of identical megawatt hour capacity can present entirely different acoustic problems.

Cooling equipment is the continuous source

For most battery energy storage sites, thermal management is the source group that governs.

Fans produce two kinds of sound at once. Broadband energy comes from turbulence over and behind the blades. Tonal energy sits at the blade passage frequency, which ASHRAE guidance gives as fan speed in revolutions per minute multiplied by the number of blades, divided by 60. Every fan produces a tone at that frequency and its multiples. On a site with dozens of identical fan units, those tones land at the same frequency across the whole site.

Fan speed matters more than fan count. ASHRAE gives the change in fan sound power as approximately 50 times the logarithm of the speed ratio, so a fan running at 80 per cent of full speed is roughly 5dB quieter than the same fan at full speed. This is why the governing acoustic case for a battery site is usually a hot night rather than a cold one.

Compressors in direct expansion cooling units and chillers add a further mechanical component, typically with tonal content related to the compressor’s own operating frequency. Pumps in liquid cooled arrangements are usually a minor contributor externally, though they are continuous.

The important behavioural point is that cooling equipment is demand driven. It responds to cell temperature, which responds to charge and discharge rate and to ambient temperature. It is not a fixed source with a single level.

Power conversion equipment produces tonal content by design

Converters change direct current from the batteries into alternating current for the grid, and they do so by switching at high speed.

Two mechanisms generate sound. Electromagnetic forces act on inductors, reactors and filter components, producing vibration at frequencies related to the electrical waveform rather than to any rotating part. And the converter cabinet itself needs cooling, which brings fans back into the picture.

The frequencies involved fall into two groups. Low order content appears at multiples of the fundamental, which on the Australian 50 Hz grid means 100 Hz and its harmonics. Higher frequency content appears at the switching frequency and around it, and the specific frequencies depend on the converter topology, the modulation strategy and the operating point. That content can be clearly audible and distinctly tonal in character.

The switching frequency is not something a project should guess. It is a property of the specific converter and should be obtained from the manufacturer, alongside the associated acoustic data.

Transformers hum, and the hum is predictable

Transformer sound is the most predictable content on a battery energy storage site, which makes it the easiest to plan for and the hardest to treat.

The dominant mechanism is magnetostriction, the small dimensional change in the core steel as magnetic flux density varies. IEC 60076-10-1 describes transformer hum as low in frequency, with fundamental frequencies of 100 Hz or 120 Hz at twice the supply frequency, and notes that the non linear behaviour of magnetostriction produces several even multiples of the exciting frequency. On the 50 Hz Australian grid the expected series is 100 Hz, 200 Hz, 300 Hz and upward.

Two additional points from the same standard are worth carrying into a project. Where a direct current bias is present in the magnetisation, the vibration pattern repeats at the fundamental power frequency rather than twice it, odd harmonics appear, and the sound level can rise significantly. And larger units have their own cooling fans and pumps, which are separate sources with their own duty cycle tied to load and ambient temperature.

Transformer noise is tonal by nature and low in frequency. Low frequency energy is discounted by A-weighting, attenuates slowly with distance, and is poorly screened by barriers. A transformer that looks modest on a headline dB(A) figure can still be the source that reaches the nearest dwelling.

Auxiliary equipment is small individually and continuous collectively

Control room air conditioning, switchroom ventilation, fire protection equipment, site services and communications cabinets are individually minor. Collectively they run continuously, including at times when the batteries are idle.

This matters for the night-time case. An assessment that models only charge and discharge operation may miss the state in which the site is quiet but not silent, and on a low background site the auxiliary load alone can approach an intrusiveness trigger derived from a rating background level plus 5dB.

Four variables move the numbers

The real problem is repetition, not any single unit

The characteristic of a battery energy storage site is that it contains many identical units. That has a direct arithmetic consequence.

Identical sources add logarithmically. The total from a number of equal sources is the level of one source plus ten times the logarithm of the number of sources.

There is a second consequence that is less widely appreciated. Because every unit is identical, the tonal content of each unit lands at the same frequency. Broadband energy and tonal energy therefore rise together by the same ten times the logarithm of the number of units, so the tone does not become less prominent as the site grows. A tonal character present on one container is still present, in the same proportion, across forty of them.

This is why a site can satisfy a level based criterion at the boundary and still attract a modifying factor correction for tonality, and why cumulative noise impacts in clustered installations behave differently from a single large source of the same total output.

What the character means for treatment

Different content calls for different treatment, and this is where an accurate source description pays for itself.

Where treatment has to pass air, the constraint is free area and pressure drop rather than acoustic performance alone. AcousTech publishes free area between 21 and 35 per cent for the Sonic Series acoustic louvres across the models with published datasheets, with Rw values from 18dB for the 100mm SL4-10 to 33dB for the 600mm SL4-60, assessed to AS 1191-2002 and rated to AS/NZS ISO 717.1. Where a path is ducted rather than an open aperture, Sonic acoustic attenuators apply instead.

Where equipment can be screened or enclosed, barrier walls and enclosures built from Sonic System acoustic modular panels provide both a barrier face and an absorptive inner face. The panels publish Rw up to 45dB for the V100SP and NRC up to 1.10 for the V100, assessed to AS 1191-2002 and AS ISO 354.

One caution applies throughout. None of these figures is a site result. A published Rw is a rating for an element, not the reduction achieved at a receiver, which depends on geometry, coverage, openings and flanking paths.

Three questions to answer before the layout is fixed

Battery energy storage noise is not one thing. It is a cooling problem, a power conversion problem and a transformer problem occurring simultaneously, in an arrangement that varies between projects and at levels that vary with temperature and duty.

Three questions will get most project teams a long way. What is the architecture, and therefore what equipment is actually on this site. What does each unit type sound like, in octave or third octave bands, at the operating state that governs. And how many of them are there, since forty identical units are 16dB louder than one.

Answer those before the layout is fixed, and the acoustic design has something to work with. Answer them afterwards, and the options are whatever will fit in the space left over.