AcousPlan vs INSUL: Sound Insulation Prediction Software Compared (2026)

Sound insulation is the other half of building acoustics — the counterpart to room acoustics. While RT60 and speech intelligibility address how sound behaves within a space, STC (Sound Transmission Class) and Rw (weighted sound reduction index) describe how well a partition keeps sound out. INSUL and AcousPlan both address sound insulation, but through fundamentally different approaches.

Understanding the Approaches

INSUL (developed by Marshall Day Acoustics, New Zealand) is a physics-based sound transmission prediction tool. It uses the mass-spring-mass model and transfer matrix method to predict the transmission loss spectrum of wall, floor, and ceiling assemblies from their physical properties — mass per unit area, Young's modulus, cavity width, damping characteristics. From the predicted transmission loss spectrum, it calculates STC (ASTM E413) and Rw (ISO 717-1) single-number ratings.

AcousPlan takes a database approach to sound insulation. Its Sound Insulation Calculator contains 52 tested wall and floor assemblies with laboratory-measured STC and Rw values from ASTM E90 and ISO 140 test reports. Users select assembly types, compare performance, and check compliance against building code requirements. There is no physics prediction — all values are measured and validated.

The implications of this difference are significant.

INSUL's Approach: Physics-Based Prediction

How INSUL Works

INSUL accepts construction layer inputs: material type, thickness, density. For a 140mm concrete slab, INSUL calculates the transmission loss spectrum from concrete's mass per unit area and bending stiffness. For a double-leaf stud wall with 92mm framing, insulation fill, and two layers of gypsum wallboard, INSUL applies the mass-spring-mass model to predict cavity resonance and the critical frequency dip.

The result is a predicted third-octave band transmission loss curve from 100 Hz to 3150 Hz, from which STC and Rw are calculated per ASTM E413 and ISO 717-1 respectively.

INSUL Strengths

Novel assembly prediction: If you are specifying a non-standard assembly — a proprietary composite panel, a heritage masonry wall of unusual thickness, a novel insulated cladding system — INSUL can predict performance from physical properties before any laboratory testing exists. This is the tool's core value proposition.

Parametric study: Changing the thickness of an inner leaf by 10mm, or comparing 50mm vs 100mm mineral wool fill, produces instant comparative predictions. This supports early-stage design optimisation where physical intuition needs numerical confirmation.

Material customisation: Users can enter custom material properties (density, Young's modulus, loss factor) for materials not in INSUL's built-in library. Research-oriented users can model experimental assemblies.

Third-octave band detail: INSUL outputs transmission loss at each third-octave band, not just the single-number STC or Rw rating. This detail is needed for situations where low-frequency performance (the critical range below 250 Hz for bass noise from mechanical equipment or music) is the design concern.

INSUL Limitations

Flanking is separate: INSUL predicts direct transmission through the partition. Flanking transmission — sound traveling via connected flanking paths (floor-to-wall junction, wall-to-ceiling junction) — is modelled in a separate INSUL module. In practice, flanking often reduces the in-situ measured level difference by 3–8 dB compared to the laboratory direct transmission value. Consultants must separately assess flanking paths.

Prediction uncertainty for complex assemblies: For assemblies with multiple cavity layers, resilient mounting, or irregular geometry, INSUL's predictions carry larger uncertainty. Published INSUL validation studies show ±2 dB accuracy for simple assemblies; complex assemblies can exceed ±5 dB.

No compliance reporting: INSUL generates transmission loss spectra and STC/Rw values. Checking whether those values satisfy IBC, Part E, or NCC requirements, writing the compliance report, and documenting the calculation method are all manual steps outside INSUL.

Pricing and platform: INSUL is a Windows desktop application. Pricing (single-user license) is approximately NZD $1,000–1,500 (~$600–900 USD), with maintenance fees. No free tier.

AcousPlan's Approach: Validated Assembly Database

How AcousPlan Works for Sound Insulation

AcousPlan's Sound Insulation Calculator provides 52 wall, floor, and ceiling assemblies with laboratory-measured performance data. Each assembly includes:

• STC (ASTM E413) rating from ASTM E90 test report
• Rw (ISO 717-1) from ISO 140 laboratory measurement
• Third-octave band transmission loss data where available
• Assembly composition (layers, materials, thicknesses)
• Compliance status against selected building codes

• IBC 2021 §1207 (STC 50 minimum for dwelling units)
• Part E of UK Building Regulations (Approved Document E)
• NCC 2022 (Australia) — Deemed-to-Satisfy provisions
• DIN 4109 (Germany) — Rw,R requirements for residential

AcousPlan Sound Insulation Strengths

Compliance integration: Sound insulation compliance exists within the same platform as room acoustics compliance. A project requiring both RT60 compliance and party wall compliance can be handled in one tool, producing a combined compliance report.

Verified measured values: Laboratory test values are not predictions — they are measurements. For standard assemblies (stud walls, concrete floors, typical residential constructions), the measured values are the most reliable data available.

Speed for standard specifications: Selecting from 52 assemblies with compliance checking is faster than modelling physical properties in INSUL for assemblies that already exist in the database.

No Windows dependency: Web-based, no installation.

Combined acoustic reporting: A single AcousPlan project report can include both room acoustics results and sound insulation compliance — useful for residential developers who need both for building control submission.

AcousPlan Sound Insulation Limitations

No novel assembly prediction: If the project uses a non-standard assembly not in the 52-assembly database, AcousPlan cannot predict its performance. INSUL is required for this case.

52 assemblies may not cover every scenario: The database covers the most common construction types but does not cover every regional variant, proprietary system, or heritage construction.

No third-octave detail in all cases: AcousPlan focuses on STC/Rw single-number ratings for compliance; full third-octave band analysis for complex noise source problems may require INSUL.

Feature Comparison

Use Case Decision Framework

Use INSUL when:

• You are specifying or evaluating a non-standard assembly with no published test data
• You need to optimise assembly thickness or layer composition to hit a target STC without testing every variant
• Low-frequency transmission loss detail (below 250 Hz) is critical — mechanical plant noise, music noise, HVAC
• You need to model a heritage or existing construction and predict likely performance from visual survey data
• Your practice involves significant research or novel construction types

Use AcousPlan Sound Insulation Calculator when:

• You are specifying from standard construction types and need compliance documentation
• The project requires combined room acoustics + sound insulation compliance in a single report
• You need compliance against IBC, Part E, NCC, or DIN 4109 with automated checking
• WELL v2 or LEED acoustic credit documentation is required alongside insulation compliance
• Workflow speed matters — 52 standard assemblies cover the majority of typical residential and commercial compliance scenarios

Use both when:

• The project includes both standard assemblies (AcousPlan for compliance documentation) and non-standard assemblies (INSUL for prediction)
• Early-stage design uses INSUL for parametric optimisation; documentation phase uses AcousPlan for compliance report generation

Practical Scenario: Residential Apartment Block

Party wall specification for 24 apartments in a concrete-frame residential building. Requirements: IBC §1207 STC 50 minimum for dwelling unit separations.

INSUL workflow:

• For each proposed partition type, enter layer properties, run prediction
• If predicted STC ≥ 50, proceed
• Write compliance memo manually citing predicted vs required value
• Note: flanking paths require separate assessment
• Time per partition type: 15–20 minutes (experienced user)

• Select assembly type from 52-assembly database
• Check IBC 2021 compliance: pass/fail generated automatically
• Export compliance PDF with assembly description, STC value, code reference
• Time per partition type: 3–5 minutes

Low-Frequency Sound Insulation: Where INSUL's Detail Matters More

For standard residential party wall compliance checking against IBC STC 50 or Part E Rw 45, the single-number rating is the compliance metric and assembly database lookup is sufficient. But sound insulation problems in real buildings often involve specific frequency ranges rather than broadband performance.

Gym-to-apartment interfaces: Fitness centres in mixed-use developments generate significant energy at 63–125 Hz from impact exercise and bass music. A wall achieving STC 55 may still transmit bass energy that creates residential complaints because the STC rating weights mid-frequencies. INSUL's octave-band transmission loss output makes the low-frequency gap visible.

Mechanical plant noise: Chiller plants, generators, and lift motor rooms transmit tonal noise at specific frequencies determined by equipment rpm. A wall design optimised for broadband STC may have a resonant dip at the problem frequency. Identifying and correcting this requires octave-band analysis — INSUL provides this; AcousPlan's assembly database does not.

Music rehearsal studios and recording facilities: Low-frequency containment (63–125 Hz) is the critical design parameter for music applications. No standard residential assembly achieves adequate low-frequency isolation. INSUL is the appropriate tool for designing the double-leaf, floating floor constructions required in music applications.

For these specialised noise control applications, INSUL's physics-based approach and third-octave band detail are genuinely necessary. AcousPlan's assembly database reflects the typical building code compliance scenario, not the specialist noise control design scenario.

Multi-National Projects: Standards Coverage

AcousPlan's sound insulation compliance checking covers IBC 2021, UK Part E, NCC 2022 Australia, and DIN 4109 Germany within its current database. For projects in other markets — France (NRA), Scandinavia (various), Japan (JIS A 1416), or Middle East (local codes) — the coverage is incomplete, and consultants may need to verify compliance manually against the relevant standard.

INSUL's physics-based prediction is standard-agnostic: it calculates transmission loss spectrum, from which any single-number rating system can be derived. International practitioners can use INSUL outputs to check compliance against any regional standard that specifies an STC, Rw, Dn,T,w, or equivalent metric.

For international consulting firms working across multiple regulatory regimes, this flexibility is a meaningful INSUL advantage.

Sound Flanking: The Missing Variable

Neither INSUL standard mode nor AcousPlan's assembly database addresses flanking transmission comprehensively. This is important context for interpreting both tools' outputs.

Flanking occurs when sound travels between spaces via indirect paths — through the floor-wall junction, via the ceiling plenum, or through connected structural elements. In laboratory conditions (ASTM E90, ISO 140), flanking is eliminated by the test methodology. In real buildings, flanking reduces the apparent sound reduction index by 3–10 dB depending on construction type and junction details.

INSUL offers a flanking module that estimates the flanking contributions from junction paths using the EN 12354-1 calculation method. This provides a more realistic prediction of in-situ performance than direct transmission alone.

AcousPlan's assembly database reflects laboratory test values. The practical in-situ performance will generally be 3–8 dB lower than the database STC/Rw value, depending on flanking path quality. Consultants should account for this offset when setting specification targets — specify STC 55 where the code requires STC 50, to account for the flanking penalty.

Verdict

INSUL and AcousPlan address the same problem domain through complementary approaches. INSUL's physics-based prediction is genuinely superior for novel assemblies, non-standard constructions, low-frequency performance problems, and multi-national projects requiring standard-agnostic calculation. AcousPlan's validated assembly database is faster and more automation-capable for standard compliance work in its supported jurisdictions.

The most effective workflow combines both: INSUL for the design exploration phase where novel assemblies need performance prediction and for specialised noise control problems requiring octave-band detail; AcousPlan for the compliance documentation phase where standard assemblies need code compliance verified and reported.

For practitioners who work primarily with standard construction types and need sound insulation compliance as part of broader acoustic compliance reports, AcousPlan's free tier provides immediate value with no software purchase required. Try the Sound Insulation Calculator alongside your next residential or commercial project.