TL Tool - Sound Transmission Loss Measurement

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

• 📐 Acoustic Formulas Reference — all equations used in the software
• NVGate — OROS analysis software
• FFT Analysis in NVGate
• Octave Analysis
• OROS FFT Analyzer Hardware

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