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Speaker Cabinet Damping Materials: A Practical Guide to Acoustic Cotton
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posted by Vincent Zhang
Polyester fill, fiberglass, rock wool, long-fiber wool, cotton batting, polyurethane and melamine foam — how to read absorption data and choose damping for a sealed, vented or transmission-line enclosure.
Table of Contents
“Acoustic cotton” is a trade term used for several kinds of porous speaker damping. Check the composition before buying. For example, IWISTAO lists WSP-POLYFC-001 as polyester fiber. In a loudspeaker, a liner can attenuate cavity resonances, and loose fill can also change the low-frequency behavior of a sealed box.[19][1]
1. What Damping Material Actually Does Inside a Cabinet
1.1 Absorption through viscous and thermal losses
The rear of the cone drives the air inside the box. As that air oscillates through interconnected pores, viscous friction and thermal exchange dissipate part of the acoustic energy. Thickness and airflow resistivity help determine the frequencies affected.[6]
1.2 Standing waves inside the enclosure
Between two approximately rigid, parallel walls separated by L, the first axial mode occurs at f = c / (2L), when half a wavelength fits between the walls. Higher axial modes occur at integer multiples of that frequency. For a cabinet with 178 mm of internal width, the first width mode is about 964 Hz at a sound speed of 343 m/s. A longer internal dimension can produce a lower mode. A porous lining can reduce the strength of these resonances.[5][3]
1.3 The thermodynamic effect and apparent box volume
In the usual small-signal model, air compression in an empty sealed box is approximately adiabatic. Fibrous fill can exchange heat with the air during each cycle, moving compression toward isothermal behavior and making the air spring more compliant. The simple ideal-air comparison gives an upper ratio of 1.4× for compliance. Eargle reports a limit closer to 1.31× from Leach’s more detailed model and cites about 1.2× as an engineering observation. Dickason gives a practical range of about 15–25% for suitable applications. The actual change depends on the material, placement and enclosure.[1][2][3][4]
Surface absorption, acoustic compliance and resistive damping describe different effects. Compare cabinet measurements when choosing fill for a target response.
2. Reading Absorption Coefficients Correctly
ISO 354 and ASTM C423 describe reverberation-room measurements. The examples below use published ASTM C423 data for three specific products. The mounting and frequency-band details are part of each result. Coefficients can exceed 1.00 when edge diffraction makes the effective absorbing area larger than the specimen’s geometric area.[7][8]
| Product | Mounting and bands | 125 Hz | 250 Hz | 500 Hz | 1,000 Hz | 2,000 Hz | 4,000 Hz |
|---|---|---|---|---|---|---|---|
| Owens Corning 703 Unfaced glass wool[9] | Type A, octave bands | 0.10 | 0.71 | 1.14 | 1.14 | 1.03 | 0.95 |
| ROCKWOOL Rockboard 60 stone wool[10] | Mounting unspecified in sheet, published band values | 0.28 | 0.75 | 1.11 | 1.08 | 1.05 | 1.07 |
| Auralex 2-inch Studiofoam Wedges[11] | Type A, one-third-octave center bands | 0.11 | 0.30 | 0.91 | 1.05 | 0.99 | 1.00 |
These specimens show lower reported coefficients at 125 Hz than in much of the midrange. Even at similar nominal thickness, the 125 Hz values vary, from 0.10 for the listed glass wool board to 0.28 for the listed stone wool board. Use thickness, mounting, facing and band width when interpreting any comparison. NRC is a single-number room-acoustics rating and does not include 125 Hz. For a loudspeaker, measure the finished cabinet’s response to a liner or fill.[8][9][10][11][4]
- 703 glass wool
- Rockboard 60 stone wool
- Studiofoam Wedges
3. The Materials, Compared One by One
| Material | Typical role | What to check |
|---|---|---|
| Loose polyester fiberfill | Adjustable fill for sealed boxes | Weigh each increment and measure the result. A bonded polyester sheet behaves differently from loose fiber. |
| Glass wool and stone wool | Glass wool for some sealed fills or linings. Stone wool boards are candidates for fixed lining. | Choose a suitable product, secure the material and control fiber exposure.[4][10][17] |
| Long-fiber sheep’s wool | Loose fill where specified by the enclosure design[20] | Account for moisture and pests during storage and use. |
| Bonded acetate fiber (BAF) | Bonded wadding for designs that call for it[20] | Match the specified form and placement. |
| Acousta-Stuf | Crimped synthetic fill for sealed or transmission-line designs | Set the density and placement by measurement.[14] |
| Cotton batting or recycled denim | Sheet lining when the product is suitable for the application | Check composition, fire performance and how the sheet will be secured. |
| Open-cell polyurethane or melamine foam | Cut-to-fit wall lining | Check thickness, backing and fire classification for the exact product.[11][15] |
3.1 Polyester fiberfill
Loose polyester is easy to weigh and add in stages. Dickason’s Dacron sample produced smaller changes in sealed-box resonance and system Q than several other fills in his test. Use that comparison as a result for those samples and that enclosure. IWISTAO’s polyester damping is sold in sheets. Measure the enclosure with the sheet in place before assigning it the same effect as loose fill.[4][19]
3.2 Fiberglass
Dickason tested an 8-inch driver in a 0.95 ft³ sealed box. Its resonance was 84.54 Hz empty, 76.79 Hz with a full fill of 1 lb/ft³ fiberglass, 75.91 Hz with a full fill of the tested acoustic foam, and 73.55 Hz with that foam as a lining and 1 lb/ft³ fiberglass filling the rest. The lowest reported system Q was 0.75 with two other fiberglass combinations. Each number describes one test configuration. The results show that resonance, Q and the −3 dB cutoff can change by different amounts.[4]
3.3 Stone wool, natural wool and cotton
A rigid stone-wool board takes up interior space, and a loose fiber or bonded batt distributes differently through the cabinet. For a vintage restoration or a transmission-line kit, use the original material specification, mass and placement where available. Cotton batting, wool, bonded acetate and crimped synthetic fills should be identified by their actual composition and form when comparing results.[4][13][14]
3.4 Foams and other lining materials
Open-cell polyurethane and melamine foam are available as liners. Profiles such as wedges and pyramids change the thickness across the panel. Compare published absorption for the profile, thickness and mounting you plan to use. For newer composites, ask for product-level data and check the effect in the actual enclosure before assigning a damping benefit.[11][6][15]
4. Sealed, Vented and Transmission Line Placement
Sealed box. Loose, evenly distributed fill is one way to adjust the alignment. A wall liner can target cavity resonances. Leave clearance around the back of the cone and any pole vent. Record the amount and placement so you can repeat the setup.[1][3][4]
Bass reflex. Start with lining if the design calls for it. Some vented systems can also use loose, distributed damping, as Visaton’s measurements show. Keep the port passage and both mouths clear, and secure the material so it cannot migrate. Check port tuning and output after changing the amount or placement.[12][3]
Open quarter-wave transmission line. Place fill to suppress unwanted higher modes while preserving the intended bass contribution from the open terminus. Position and density affect both outputs. Follow the design’s loading schedule, or compare the woofer and terminus responses while adjusting it.[13]
5. How Much to Use and How to Measure It
For loose polyester in a sealed box, 1.0–1.5 lb of fill per cubic foot of net internal box volume is about 16–24 g/L. Treat it as a DIY reference range and start with the light end or less, especially if the design already has a target Q. Subtract the driver, bracing and other solid displacements when calculating net volume. Weigh sheet lining separately. A lining specified at 0.5 lb/ft³ (about 8 kg/m³) describes the bulk density of that material, so multiply by the volume of the sheet to get its mass.[21][4][3]
To tune a sealed box:
- Measure the box with the driver installed, seams sealed and damping removed. Save the impedance trace and a reference frequency response.
- Add a small, weighed amount of fill. Record where it goes and repeat the measurements at the same drive level and microphone position.
- Use the impedance sweep to estimate system resonance Fc and total system Q (Qtc). Choose the fill that gives the desired response and Q. A lower resonance alone can reflect increased moving mass as well as increased compliance.
For a vented design, check the tuning region on the impedance sweep and compare the port and woofer responses before and after the change.[12]
Keep a record of the final mass and placement for a matched pair or a restoration. Panel treatments such as bracing and constrained-layer damping address the cabinet walls’ mechanical vibration. Use cavity absorption to address airborne resonances.[1][4][12][18]
6. Handling, Health and Environmental Notes
IARC’s evaluation, published in 2002, places insulation glass wool, rock wool, slag wool and continuous glass filament in Group 3, meaning the evidence does not allow classification as to carcinogenicity in humans. Continuous glass filament was already in Group 3 in the 1988 evaluation. Certain special-purpose glass fibers and refractory ceramic fibers were evaluated separately and placed in Group 2B. Fibers can irritate the skin, eyes and airways during handling.[16][17]
Follow the material’s safety data sheet for handling, ventilation and protective equipment. Where fiber containment matters, use a product and facing designed for that purpose. Ordinary speaker grille cloth gives no assurance of containment. Secure any lining and keep exposed fibers away from the port and driver ventilation. For fire ratings or recycled content, consult documentation for the exact product and facing. BASF’s Basotect literature, for example, lists grades and test classifications separately.[17][15][10]
7. A Short Selection Guide
| Situation | First step | Check after installation |
|---|---|---|
| New sealed box | Add loose polyester in measured increments. | Match the design Q and frequency response.[4][21] |
| Bass reflex | Start with a secure lining or follow the specified distribution of fill. | Verify port clearance, tuning and output.[12] |
| Open transmission line | Follow the design’s placement and density for each section. | Compare higher-mode suppression and terminus bass output.[13] |
| Vintage restoration | Match the original material, mass and placement where documented. | Check impedance and response against the design or a matching unit. |
8. FAQ
Which lining absorbs the most?
Compare specific products at the frequencies you need to treat and under the same test conditions. The three published samples in this guide have different 125 Hz results even at similar thickness. Use cabinet measurements to check the lining in your design.[9][10][11]
Can stuffing make a sealed box behave like a larger one?
Yes. Suitable fill can increase the compliance of the enclosed air. The published 15–25% range is a design reference. For a specific build, the fill and the enclosure determine the result.[1][3][4]
Can a ported speaker use loose fill?
Some designs can. Keep the passage and both port mouths open, secure the material, and recheck tuning and port output after any change.[12]
Find More
References
- Richard H. Small, “Closed-Box Loudspeaker Systems—Part I: Analysis,” JAES, 1972. See the sections on compliance and mass loading. View source
- W. Marshall Leach Jr., “Electroacoustic-Analogous Circuit Models for Filled Enclosures,” JAES, 1989. View source
- John Eargle, Loudspeaker Handbook, 2nd ed., 2003, §4.2.1, pp. 70–72. View source
- Vance Dickason, Loudspeaker Design Cookbook, 7th ed., 2006, §§1.81–1.82 and Table 1.21. View source
- COMSOL, “Eigenmodes of a Room.” Analytic modes of a rigid rectangular cavity. View source
- COMSOL, “Porous Absorber.” Viscous and thermal loss in open-cell absorbers. View source
- ISO 354:2003, Acoustics — Measurement of Sound Absorption in a Reverberation Room. View source
- ASTM C423, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. View source
- Owens Corning, Fiberglas 703 and 705 Series Insulation Product Data Sheet, February 2024. Use the 2-inch 703 Unfaced row. View source
- ROCKWOOL, Rockboard 60 Technical Data Sheet, April 2026. Use the 2-inch row. View source
- Auralex, Acoustical Testing Data Master Table. Use the 2-inch Studiofoam Wedges row and the listed one-third-octave center bands. View source
- Visaton, “Bedämpfung von Gehäusen Teil II.” Measured effects of damping amount and placement in vented enclosures. View source
- Martin J. King, The Two-Year Transmission Line Speaker Design, 2023. See the driver and terminus measurements. View source
- Parts Express, Acousta-Stuf product description. Identifies the fill as a crimped synthetic fiber. View source
- BASF, Basotect technical brochure. Product grades and fire-test classifications. View source
- IARC, Man-made Vitreous Fibres, Vol. 81, 2002. See the overall evaluation and the table of previous evaluations. View source
- ATSDR, ToxFAQs for Synthetic Vitreous Fibers. Exposure, irritation and protective measures. View source
- H. D. Harwood and R. Mathews, “The Mechanical Design of Loudspeaker Cabinets,” AES, 1975. View source
- IWISTAO, WSP-POLYFC-001 product page. The seller identifies the material as polyester fiber. View source
- Martin Colloms, High Performance Loudspeakers, 6th ed., “The Enclosure,” pp. 378–379. View source
- IWISTAO, From Specs to Sound: A DIY Guide to Designing the Perfect Sealed Speaker Enclosure. Starting fill quantities and measurement guidance. View source
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