Office Acoustics: STC, NIC and ISO 23351-1 Explained

Table of Contents

Key Takeaways:

• There are three main acoustic performance ratings used for office noise solutions: STC (Sound Transmission Class), NIC (Noise Isolation Class), and ISO 23351-1 DS,A — and they measure fundamentally different things
• STC and NIC are wall/partition-level lab measurements that do not account for the acoustic behaviour of a complete freestanding enclosure; they are not appropriate for comparing office pods
• ISO 23351-1 DS,A is the only standard specifically developed to measure the acoustic performance of complete enclosed office furniture systems — it is the correct standard for pod-to-pod comparison and procurement specification
• HIGHKA’s DS,A = 29.4 dB (independently tested by SGS under ISO 23351-1) places HIGHKA in Class B (DS,A ≥ 25 dB), approaching Class A (DS,A ≥ 30 dB)
• When evaluating any acoustic pod, always require ISO 23351-1 DS,A from a named independently accredited laboratory — not a self-assessed claim, not an STC equivalent, and not a “dB” figure without a stated standard and testing body

When you are evaluating acoustic office pods, you will encounter a range of performance claims: “40 dB reduction,” “STC 38,” “ISO Class A,” “25 dB DS,A.” Each of these figures sounds authoritative. Some of them are. Others are not comparable — they measure different things, in different ways, under different conditions, for different types of products.

The misuse of acoustic standards in acoustic pod marketing is systematic. Suppliers compare STC figures with DS,A figures as if they were equivalent. Products describe their acoustic performance in “dB” without specifying which standard was used or which laboratory performed the testing. Self-assessed claims appear alongside independently verified specifications without distinction.

For procurement teams, HR directors, facility managers, and architects specifying acoustic pods, this confusion has a direct cost: selecting a pod on the basis of a misleading acoustic claim and discovering post-deployment that the product does not actually provide the speech privacy or focus isolation your team needs.

This guide cuts through the confusion. It explains exactly what STC, NIC, and ISO 23351-1 DS,A measure — and why only one of them is the correct specification for office pod acoustic performance.

Before the standards can be understood, three physical fundamentals must be clear.

1. Sound Is Pressure Waves in Air

Sound is created when a vibrating object (vocal cords, a phone speaker, a keyboard) creates pressure waves in the surrounding air. These waves propagate outward in all directions until they encounter a surface or dissipate. When they reach your ears, the pressure variations are interpreted as sound.

In the context of acoustic standards, the question being measured is: how much of that pressure energy reaches a listener in a different location from the sound source?

2. Sound Is Measured in Decibels — A Logarithmic Scale

The decibel (dB) scale measures the intensity of a sound relative to a reference level. Crucially, the scale is logarithmic, not linear:

• Every 10 dB increase represents approximately a doubling of perceived loudness
• Every 10 dB decrease represents approximately a halving of perceived loudness
• A 3 dB difference is the smallest change most people can reliably detect
• A 10 dB difference is perceived as roughly twice or half as loud

This logarithmic relationship is why the difference between DS,A = 29.4 dB and DS,A = 25 dB is not merely 4 dB — it is a perceptually meaningful difference in isolation performance. The 29.4 dB figure provides approximately 2.6 times more effective isolation than 25 dB when expressed in terms of perceived loudness difference.

Reference levels for office environments:

3. Sound Has Frequency — and Frequency Matters for Speech

Sound is not just about intensity (dB) — it also has frequency, measured in Hertz (Hz). The human voice spans approximately 100 Hz (low vowels) to 8,000 Hz (fricatives: s, sh, f, th). The speech frequency range most relevant to intelligibility — where the ISE is most active and where speech privacy matters most — is approximately 500 Hz to 4,000 Hz.

Acoustic standards that A-weight their measurements (as ISO 23351-1 DS,A does) apply a filter that matches the frequency sensitivity of human hearing, prioritising the speech range. This is essential: an acoustic measurement that treats all frequencies equally does not accurately predict whether speech will be intelligible through an enclosure.

What STC measures

STC is an American National Standards Institute (ANSI) rating that quantifies how much a building element (wall, floor, ceiling, door, window) reduces sound transmission between two spaces. It was developed by ASTM International (American Society for Testing and Materials) under ASTM E90 (laboratory test) and ASTM E413 (calculation of STC value).

The STC figure is derived by testing a partition at 16 specific frequencies between 125 Hz and 4,000 Hz in a standardised laboratory, then comparing the resulting transmission loss curve against a standard reference curve. The STC value is the reference curve shift (in dB) that best fits the measured data.

What STC does NOT measure

STC does not measure the acoustic performance of a freestanding three-dimensional enclosure. It measures flat partitions, walls, and similar two-dimensional building elements. An office pod is not a partition — it is a six-sided enclosure with a door, ventilation gaps, panel joints, and acoustic interactions between the enclosure’s interior and exterior that a wall panel test cannot capture.

Applying an STC rating to an office pod — or comparing an STC figure for an acoustic partition with a DS,A figure for a pod — is a category error. The measurements test different things and cannot be directly compared.

Typical STC values for reference

When STC is the right standard

STC is appropriate when specifying the acoustic performance of building partitions, walls, and structural elements — construction components designed to separate rooms within a building. It is the correct standard for: office fit-out construction, partition system specification, conference room wall specification, and floor-ceiling assembly acoustic performance.

It is not the correct standard for specifying freestanding acoustic furniture, including office pods.

What NIC measures

NIC (Noise Isolation Class) is an in-field (rather than laboratory) measurement of the noise isolation between two spaces that have already been constructed or installed. While STC is a laboratory test of a partition specimen, NIC is measured after installation in the actual space, accounting for flanking paths (sound transmission through floors, ceilings, and adjacent structures), gaps, and other real-world installation variables.

NIC is derived from ASTM E336 (measurement of airborne sound attenuation between rooms in buildings). Like STC, the NIC value is a single number that summarises acoustic performance across a range of frequencies.

NIC vs STC: The Practical Difference

The key distinction: STC is what you specify before construction; NIC is what you measure after construction. A partition with STC 40 may achieve NIC 33 in the actual installed environment — because real installations have flanking paths, gaps, and construction imperfections that the laboratory test does not include.

This STC-to-NIC gap is an important caution for procurement: a claimed STC performance figure for a partition system may not reflect the NIC (actual in-field isolation) you will achieve after installation.

When NIC is the right standard

NIC is appropriate for verifying acoustic performance of installed rooms and partition systems — confirming that a completed construction achieves its specification. Like STC, it is a wall/partition-oriented measurement and is not appropriate for specifying or comparing freestanding acoustic furniture including office pods.

What ISO 23351-1 was designed for

ISO 23351-1 is the International Organisation for Standardisation standard specifically developed for measuring the acoustic performance of enclosed office furniture — including acoustic pods, phone booths, meeting booths, and similar freestanding enclosed workspace products.

It was published specifically because neither STC nor NIC appropriately measured the acoustic performance of freestanding three-dimensional enclosures. The measurement method must account for the acoustic behaviour of the complete enclosure system — including the interaction between all six panels, the door and its seals, the ventilation system openings, and the sound field within the enclosed space.

What DS,A means

DS,A is the measurement result of ISO 23351-1: the A-weighted Speech Level Difference in decibels. It measures how much a complete enclosure reduces the A-weighted speech sound pressure level.

Two key technical points:

A-weighting: The “A” in DS,A indicates that the measurement applies an A-weighting filter — a frequency filter that matches the sensitivity of the human ear to the speech frequency range (approximately 500 Hz–4 kHz). A-weighting ensures that the DS,A figure accurately reflects how well the enclosure prevents speech intelligibility, not just how it performs at arbitrary frequencies.

Complete system measurement: ISO 23351-1 tests the complete enclosure as a unified acoustic system — not individual components. A pod’s DS,A reflects the acoustic performance of all panels, the door, the seals, the ventilation openings, and the interior acoustic materials working together. This is why DS,A from ISO 23351-1 is the only meaningful specification for a freestanding acoustic pod.

The ISO 23351-1 Classification System

Why independent accredited laboratory testing matters

ISO 23351-1 DS,A must be produced by an independently accredited testing laboratory following the standardised procedure. Self-assessment — where a manufacturer tests their own product using their own equipment — does not produce a verifiable ISO 23351-1 DS,A figure. Neither does applying STC test methods to a pod and expressing the result as “dB.”

The question that immediately follows any DS,A claim: “Which laboratory tested this product, and do they hold accreditation for ISO 23351-1 testing?”

HIGHKA’s DS,A = 29.4 dB was tested by SGS — one of the world’s leading independently accredited testing, inspection, and certification organisations. SGS accreditation is internationally recognised and verifiable.

The fifth principle is operationally the most important, because it is the dimension most frequently promised but not delivered: the quiet option must be genuinely available when employees need it, not theoretically available when it happens to be unbooked.

The quiet availability gap

Most offices nominally provide quiet space — a conference room, a designated quiet area, a handful of focus booths. In practice, these spaces are:

• Chronically overbooked — the most common complaint about focus space is that it is unavailable when needed
• Inconveniently located — placed at the perimeter of the floor, requiring a journey that creates friction around access
• Socially loaded — occupying the only quiet room in a department can feel like a social statement, particularly if the space is scarce

The result: employees who need quiet for their most demanding work do not reliably access it. They work in the open-floor ambient for tasks that require near-silence, accept the productivity cost, and eventually prefer home working for their most important days.

The data confirms this: CBRE’s 2026 data shows that the primary driver of voluntary office attendance is the quality of the workspace experience — and that the primary quality gap is insufficient enclosed focused workspace. Employees who choose home for focused work are not choosing home because they prefer isolation; they are choosing home because the home acoustic environment is better for focused work than the open-plan office.

What genuine availability looks like

Sufficient quantity: Research supports a baseline of one enclosed acoustic space per 10–15 knowledge workers for teams with high cognitive work content. At this ratio, peak-hour utilisation stays below the saturation point at which employees regularly cannot access a pod when they need one.

Right-sized distribution: A mix of single-person pods (HIGHKA Model S, M) and small-group pods (HIGHKA Model L) positioned across the floor in proximity to the desk clusters they serve — not clustered in a single remote zone.

Responsive capacity: HIGHKA’s 1–4 hour assembly time means that if 30-day utilisation data shows that existing pod capacity is consistently above 70% during peak hours, additional pods can be deployed without construction delays. Capacity can be matched to demonstrated demand rather than estimated at the start.

Booking accessibility: Whether first-come-first-served or via a lightweight booking system, the access protocol must be frictionless and equitable — pods should not be de facto reserved for senior employees or specific teams.

Why STC is often misapplied to pods — and why it misleads

A common marketing approach: a pod manufacturer tests components of their pod (individual panels) to ASTM E90/STC and reports the STC value as the pod’s acoustic performance. This systematically overstates actual performance because:

1. Panel STC does not account for the pod’s door, seals, or ventilation gaps — which are acoustically weaker than the panels themselves
2. Panel STC does not account for the acoustic interaction of the enclosure as a complete system
3. A-weighting is not applied — so the figure does not accurately predict speech intelligibility

The result is an STC figure that appears high but does not translate to the actual speech privacy you will experience in the pod. An STC figure for a partition cannot be directly compared with a DS,A figure for a pod: they measure different things.

The “40 dB” marketing description problem

Some acoustic pod marketing describes performance as a “40 dB hum” or “reduces noise by 40 dB” without specifying which standard was applied, which frequencies were measured, or which laboratory performed the test. These descriptions are qualitative characterisations, not verifiable specifications.

Without a specified standard (ISO 23351-1), a specified metric (DS,A), and a named accredited testing laboratory (e.g., SGS), a “40 dB” claim cannot be verified, cannot be compared with other products, and should not be treated as a procurement specification.

HIGHKA soundproof office pods achieve DS,A = 29.4 dB under ISO 23351-1, independently tested by SGS. This places HIGHKA firmly in ISO 23351-1 Class B (DS,A ≥ 25 dB), approaching Class A performance (DS,A ≥ 30 dB).

The full frequency attenuation data, measured at the frequencies specified by ISO 23351-1:

The ★ frequencies (2,000–8,000 Hz) are the range where speech consonants and formants are most distinct — the acoustic detail that determines whether background conversation is intelligible. HIGHKA’s strongest attenuation is precisely in this range, which is why the ISE (Irrelevant Speech Effect) is eliminated: the frequencies that carry intelligible language content are most attenuated, making background speech non-parseable by the brain’s language system.

The structural basis of HIGHKA’s acoustic performance

HIGHKA achieves DS,A = 29.4 dB through a six-layer hollow composite structure, patent-protected and specifically engineered for the 500 Hz–4 kHz speech frequency range. This structural approach — rather than simple mass or consumable acoustic fill materials — produces consistent acoustic performance across the pod’s 8–12 year design lifespan without material degradation or replacement requirements.

In-use acoustic environment

In a typical active open-plan office at 60–65 dB ambient, HIGHKA’s DS,A = 29.4 dB brings the pod interior to approximately 31–36 dB — the acoustic equivalent of a quiet library or a residential room with distant street traffic. At this level:

• Background speech from the surrounding open floor reaches the pod interior as a diffuse, toneless murmur — not as intelligible language
• The ISE cannot be activated by background speech (below intelligibility threshold)
• Working memory is fully available for the primary cognitive task

The five-question procurement protocol

When evaluating any acoustic pod, ask these five questions in order:

Question 1: “What acoustic standard was used to measure this product’s performance?”

• Accept: ISO 23351-1
• Challenge: STC, NIC, “proprietary standard,” or no standard specified

Question 2: “What is the DS,A figure?”

• Accept: A specific dB figure from ISO 23351-1 (e.g., DS,A = 29.4 dB)
• Challenge: Ranges (“25–35 dB”), unspecified “dB” figures, qualitative descriptions

Question 3: “Which laboratory independently tested this product?”

• Accept: A named internationally accredited laboratory (e.g., SGS, Intertek, TÜV, Bureau Veritas)
• Challenge: Self-tested, tested by the manufacturer, tested “in-house,” or no laboratory named

Question 4: “Can you provide the full ISO 23351-1 test certificate?”

• Accept: A test certificate from the named laboratory showing the testing conditions, frequency data, and DS,A result
• Challenge: Summary data, marketing materials, or refusal to provide documentation

Question 5: “What ISO 23351-1 Class does this product achieve?”

• Accept: Class B (DS,A ≥ 25 dB) as minimum; Class A (DS,A ≥ 30 dB) preferred for high-demand use cases
• Challenge: Classifications that do not correspond to ISO 23351-1 Class A or B thresholds

Minimum performance specifications by use case

HIGHKA’s DS,A = 29.4 dB meets or exceeds every use case in this table.

Why frequency data matters beyond DS,A

DS,A is the headline figure — but the full frequency data tells you where the pod performs well and where it may have limitations. Specifically:

High mid-frequency performance (2,000–4,000 Hz) is what determines whether speech is intelligible or not. A pod with strong DS,A but weak performance at 2,000–4,000 Hz may still allow speech consonants and formants to pass — making background conversation partially intelligible even if the overall A-weighted level is reduced.

HIGHKA’s frequency data shows 39.3 dB at 2,000 Hz and 41.1 dB at 4,000 Hz — strong attenuation precisely in the range where speech intelligibility and ISE activation are highest. This frequency profile explains why the ISE is eliminated inside a HIGHKA pod, not just reduced.

HIGHKA’s DS,A = 29.4 dB meets or exceeds every use case in this table.

Why frequency data matters beyond DS,A

DS,A is the headline figure — but the full frequency data tells you where the pod performs well and where it may have limitations. Specifically:

High mid-frequency performance (2,000–4,000 Hz) is what determines whether speech is intelligible or not. A pod with strong DS,A but weak performance at 2,000–4,000 Hz may still allow speech consonants and formants to pass — making background conversation partially intelligible even if the overall A-weighted level is reduced.

HIGHKA’s frequency data shows 39.3 dB at 2,000 Hz and 41.1 dB at 4,000 Hz — strong attenuation precisely in the range where speech intelligibility and ISE activation are highest. This frequency profile explains why the ISE is eliminated inside a HIGHKA pod, not just reduced.

STC (Sound Transmission Class) measures the transmission loss of flat building partitions in a laboratory. It is the correct standard for specifying walls, ceilings, and construction elements — not freestanding acoustic pods.

NIC (Noise Isolation Class) measures actual in-field isolation between installed rooms. It accounts for real-world installation variables that STC does not — but it is still a partition/room-oriented measurement, not appropriate for pod specification.

ISO 23351-1 DS,A is the only standard specifically developed for measuring the acoustic performance of complete freestanding acoustic furniture enclosures. It measures the A-weighted speech level difference produced by the complete enclosure system — accounting for all panels, door, seals, ventilation, and interior acoustic elements as a unified system. Only DS,A from ISO 23351-1, from a named independently accredited laboratory, is an appropriate procurement specification for acoustic pods.

HIGHKA’s DS,A = 29.4 dB (SGS-verified, ISO 23351-1) is the independently verified performance figure: 25.1/24.1/28.8/33.4/39.3/41.1/43.9 dB across 125/250/500/1,000/2,000/4,000/8,000 Hz. 1–4 hour assembly, no permits, five models (S/M/SL/L/XL), 8 colours, 50+ countries and territories, 8–12 year lifespan, 95% recyclable, CE, UL, ISO 9001, SGS certified.

Need the full ISO 23351-1 test certificate for procurement due diligence?

👉 Request HIGHKA’s complete acoustic test documentation

We provide the SGS test certificate, full frequency data, and ISO 23351-1 classification documentation — at no obligation.