TL Tool - Sound Transmission Loss Measurement

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TL Tool — Sound Transmission Loss

Parameter Value
Delivery download here version from 22/05/2026 - Beta version - download at your own risk !
Measurement methods 4-mic transfer matrix (ASTM E2611) • 2-mic standing wave (ISO 10534-2)
Results TL [dB], absorption α, ISO 11654 class (α_w, NRC, SAA)
Octave resolution 1/3 • 1/6 • 1/12 • 1/24 octave
NVGate integration Live acquisition • automatic result injection
Export CSV (frequency, TL, α)

Standards

ASTM E2611

Transfer matrix method — 4-microphone impedance tube.
Recommended for Transmission Loss.

ISO 10534-2 / ASTM E1050

Two-microphone standing wave method.
Absorption coefficient only.

ISO 11654

Weighted sound absorption coefficient α_w and absorption class (A–E).

ISO 9613-1

Speed of sound and air density from temperature and pressure.

Tube Setup

Geometry

  [SP]    x1      x2              x3      x4    [ Sample ]
  ||||----o-------o---------------o-------o-----[=========]
  Source   \____Source side_____/  \___Trans. side___/


 
  • SP — Sound source (loudspeaker)
  • x1, x2 — Upstream microphones (source side)
  • x3, x4 — Downstream microphones (transmission side)
  • Sample — Material under test, placed between x2 and x3

Default Parameters

Parameter Default Notes
x1 50 mm Configurable in Tube Setup tab
x2 150 mm
x3 350 mm
x4 450 mm
Tube diameter D 100 mm Determines f_max
Temperature 20 °C Affects speed of sound
Pressure 1013.25 hPa Affects air density

ℹ️ Valid frequency range is computed automatically from tube geometry and air properties. Measurements outside [f_min, f_max] are masked and excluded from results.

Software Interface

The application is organized in five tabs:

📋 Material — Material name, notes, measurement mode (2-mic / 4-mic)

📐 Tube Setup — Microphone positions, diameter, temperature, pressure

🎤 Acquisition — NVGate channel config, FFT settings, run/stop, phase calibration

📈 TL Result — Transmission Loss curves (fine band + octave bands)

🔊 Absorption — Absorption coefficient α + ISO 11654 rating

Top Toolbar

Button Action
Calculate Run TL / absorption computation from acquired data
Fine band ☐ Show / hide fine-frequency-resolution plots
1/N octave selector Select octave resolution: 1/3 • 1/6 • 1/12 • 1/24
Send to NVGate Inject octave result into NVGate display window
Send fine band Inject fine-band result into NVGate
Export CSV Save results to CSV file

Measurement Modes

4-Microphone Mode (Recommended) — ASTM E2611

✓ Two-Load Method

Recommended — ASTM E2611 §8

Two measurements with different tube terminations.
The software builds the full transfer matrix [T] of the sample and extracts TL from T⊂12;.

● Load 1: anechoic termination
● Load 2: rigid cap

⚠ Single-Load Method

One measurement only, anechoic termination assumed.

Less accurate — use only when Load 2 cannot be measured.

2-Microphone Mode — ISO 10534-2

Uses CH1 and CH2 only (source-side microphones):

  • Reflection coefficient R(f) from standing wave decomposition
  • Absorption coefficient α(f) = 1 − |R|²
 

Step-by-Step Measurement Procedure

1

Configure Channels
In the Acquisition tab: set coupling (ICP), label and sensitivity for each microphone.
Click Configure NVGate → channels are enabled, FRF results are registered.

2

Phase Calibration (recommended)
Compensates microphone phase mismatch:
● Place Mic 1 & Mic 2 at the same port → Calibrate CH1/CH2
● Physically swap microphones → Measure (swapped)
● Repeat for CH1/CH3 and CH1/CH4
● Save calibration — applied automatically during calculation.

3

Load 1 Measurement
Insert the sample with anechoic termination.
Click Run Load 1 → NVGate acquires and stops automatically.

4

Load 2 Measurement (two-load mode)
Change termination to rigid cap.
Click Run Load 2 → NVGate acquires and stops automatically.

5

Calculate
Click Calculate. The software:
● Retrieves FRFs and auto-spectrum from NVGate
● Applies phase calibration
● Computes TL (fine band) and absorption coefficient
● Synthesizes octave bands
● Displays results and injects them into NVGate

Results

TL Result Tab

Panel Content
Fine band (left) TL in dB vs. frequency — valid range highlighted, singularities masked
Octave bands (right) TL per 1/N octave band, color-coded
Status bar c [m/s], ρ [kg/m³], f_min, f_max, calculation time


 

Absorption Tab

Panel Content
Fine band (left, optional) α(f) from 0 to 1 — valid range highlighted
Octave bands (right) α per 1/3 octave band
ISO 11654 table α_w • Class (A–E) • SAA • NRC • α at 250/500/1k/2k/4k Hz


 

CSV Export

Click Export CSV (bottom-left). Header includes: material name, date, tube geometry, air properties, valid frequency range.

4-mic mode:
Frequency_Hz, TL_dB, Alpha
100.0000, 18.423, 0.123
125.0000, 20.115, 0.145
...

2-mic mode:
Frequency_Hz, Alpha
100.0000, 0.452
125.0000, 0.481
...

NVGate Display

After calculation, results are automatically injected into NVGate:

  • Octave bar chart (1/3, 1/6, 1/12 or 1/24 depending on toolbar selection)
  • Fine-band curve (optional)
  • Each resolution uses a dedicated NVGate window to avoid conflicts

Delany-Bazley-Miki Model

The software includes a DBM fitting tool for porous absorbers.
From the measured α(f) or TL(f), it extracts the flow resistivity σ [Pa·s/m²] by minimizing the RMS error between measurement and model.
σ can then be used to predict material performance at any frequency or thickness.

→ See the Formulas Reference for full DBM equations.


 

Multi-Tube Merge

For a wide frequency range, measurements from a large-diameter tube (low frequencies) and a small-diameter tube (high frequencies) can be merged into a single spectrum.

The blend zone [f_blend_lo, f_blend_hi] uses a cosine cross-fade, ensuring a smooth transition.


 

Configuration

Settings are saved automatically in config_tl.json next to TL_Tool.exe:

Parameter Default Description
x1 – x4 50/150/350/450 mm Microphone positions from source
Tube diameter 100 mm Determines f_max
Temperature 20 °C Air temperature
Pressure 1013.25 hPa Atmospheric pressure
Octave resolution 1/12 Default resolution (3/6/12/24)
TL method Two-load Calculation method
Phase calibration (none) Path to .npz calibration file


See Also

References

  • ASTM E2611 — Normal Incidence Determination of Porous Material Acoustical Properties Based on the Transfer Matrix Method
  • ISO 10534-2 — Determination of sound absorption coefficient and impedance in impedance tubes
  • ISO 9613-1 — Attenuation of sound during propagation outdoors — Calculation of the absorption of sound by the atmosphere
  • ISO 11654 — Sound absorbers for use in buildings — Rating of sound absorption
  • Miki Y. (1990) — Acoustical properties of porous materials — modifications of Delany-Bazley models
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