A 2-inch fiberglass panel that carries an NRC rating of 1.05 absorbs only 0.31 of incident sound at 125 Hz, and reads 0.04 at 63 Hz in the same reverberation room, according to an accredited 2021 test and its non-accredited extended appendix (Riverbank Acoustical Laboratories, RAL-A21-302 2021). The industry’s favorite single number simply does not look at bass: NRC averages the 250 Hz to 2 kHz bands and stops there. Meanwhile, the standard lab method behind every published coefficient, ISO 354, is only defined from 100 Hz to 5 kHz (PTB, Forum Acusticum 2025), and two labs can disagree on a weighted coefficient by up to 0.1 (Peutz, ICA 2019). On the listening side, test tones revealed modal decays as short as 0.3 seconds at 63 Hz (Fazenda et al., JASA 2015). We aggregated data from Riverbank Acoustical Laboratories, ISO, ASTM, the EBU, ITU-R, Germany’s PTB, the RPS-Qualitas proficiency scheme, Harman International, peer-reviewed JASA and MDPI papers, and manufacturer data sheets that publish third-party lab numbers. If you want the measurement-lab side of the story, our anechoic chamber testing statistics cover the free-field counterpart to the reverberation rooms discussed here.
TL;DR
- ISO 354 sound absorption tests are standardized only between 100 Hz and 5 kHz, in rooms of at least 150 m³ (PTB, Forum Acusticum 2025)
- NRC is the average of 250, 500, 1,000 and 2,000 Hz, so 125 Hz never counts (Riverbank Acoustical Laboratories, RAL-A06-219 2006)
- A 2-inch, 6 lb/ft³ fiberglass panel rated NRC 1.05 measured 0.31 at 125 Hz and 0.22 at 100 Hz (Riverbank, RAL-A21-302 2021)
- Adding a 25 mm air gap behind a 2-inch panel lifted 125 Hz absorption from 0.45 to 0.51 (Riverbank, RAL-A06-219 and RAL-A06-220 2006)
- Below 100 Hz, non-accredited extended data gave the same 2-inch panel 0.04 at 63 Hz and -0.08 at 80 Hz (Riverbank, RAL-A21-302 2021)
- A 4.25-inch damped-plate bass absorber measured 1.05 at 63 Hz but has an NRC of only 0.30 (RPG Acoustical Systems, Modex Plate 2.0 data sheet 2024)
- Lab reproducibility for near-perfect absorbers is 0.255 at 100 Hz versus 0.065 at 500 Hz (Peutz, ICA 2019)
- Seven ISO 354 proficiency rounds from 2012 to 2024 drew 13 to 17 accredited labs each (RPS-Qualitas, Forum Acusticum 2025)
- Listeners detect modal decays of 0.3 seconds at 63 Hz with test tones and 0.51 seconds with music (Fazenda et al., JASA 2015)
- Two subwoofers at opposing wall midpoints cut seat-to-seat bass spread from 17.2 dB to 4.6 dB (Harman International, Welti)
- EBU listening rooms target a nominal reverberation time between 0.2 and 0.4 seconds (EBU Tech 3276)
1. What ASTM C423 and ISO 354 Actually Measure
Every absorption coefficient printed on a bass trap box traces back to a reverberation room, and the rules of that room shape what the number can tell you. The standardized window is 100 Hz to 5 kHz (PTB, Forum Acusticum 2025), which means the bands below 100 Hz, where room modes take longest to decay (see section 6), sit outside the formal test.
The specimen is also large and flat: 10 to 12 m² of material on the floor of a room of 150 m³ or more. That geometry suits ceiling tiles and wall panels. It does not resemble a single corner trap in a 30 m³ bedroom studio, a gap the ISO 354 scope itself hints at by excluding weakly damped resonators.
| Metric | Value | Source |
|---|---|---|
| Standardized ISO 354 frequency range and room size | 100 Hz to 5 kHz, in rooms of at least 150 m³ (200 m³ for newly built rooms) | PTB, Forum Acusticum 2025 |
| Required test specimen area | 10 to 12 m² | Peutz, ICA 2019 |
| Current ISO 354 edition | 2003 edition, last reviewed and confirmed in 2024 | ISO, ISO 354:2003 record |
| Current ASTM method | ASTM C423-23 (active version C423-23e1) | ASTM International, C423 record |
| Riverbank test chamber volume | 292 m³ (10,311 ft³) | Riverbank Acoustical Laboratories, RAL-A15-277 2015 |
| Bands averaged into NRC | 250, 500, 1,000 and 2,000 Hz, rounded to 0.05 | Riverbank Acoustical Laboratories, RAL-A06-219 2006 |
| Bands averaged into SAA | 12 one-third octave bands from 200 to 2,500 Hz | Riverbank Acoustical Laboratories, RAL-A06-219 2006 |
Context note: neither single-number rating includes any band below 200 Hz. A bass trap buyer has to read the one-third octave table, not the headline. The ASTM method is listed on the ASTM C423 record.
2. Porous Absorbers: Thickness, Density and Air Gap at 125 Hz
The clearest finding in third-party lab data is that NRC barely moves while bass absorption swings by more than 60%. Three 2-inch, 6 lb/ft³ fiberglass test configurations scored NRC 1.00, 1.05 and 1.10, yet their 125 Hz coefficients ranged from 0.31 to 0.51 (0.51 / 0.31 = 1.65), per the three Riverbank reports in the table below. The difference came from mounting and edge exposure, not from the material.
The air gap effect is real but modest at this depth. The same Model F102-1248 panel, tested on the same day in the same chamber, gained 0.06 at 125 Hz (0.45 to 0.51) and 0.05 at 100 Hz (0.34 to 0.39) when lifted onto 25 mm spacers (Riverbank, RAL-A06-220). Real bass control needs real depth: a German workplace-safety study found the corner bass trap effect only appeared at a depth of at least 400 mm.
| Metric | Value | Source |
|---|---|---|
| 2-inch, 6 lb/ft³ fiberglass, Mounting A, sealed edges | 0.34 at 100 Hz, 0.45 at 125 Hz, 0.83 at 250 Hz, NRC 1.00 | Riverbank Acoustical Laboratories, RAL-A06-219 2006 |
| Same panel on 25 mm spacers (76 mm total depth) | 0.39 at 100 Hz, 0.51 at 125 Hz, 0.90 at 250 Hz, NRC 1.10 | Riverbank Acoustical Laboratories, RAL-A06-220 2006 |
| 2-inch, 96.1 kg/m³ fiberglass panel, edges exposed | 0.22 at 100 Hz, 0.31 at 125 Hz, NRC 1.05, SAA 1.07 | Riverbank Acoustical Laboratories, RAL-A21-302 2021 |
| 4-inch quilted fiberglass absorber | 0.38 at 125 Hz, 0.98 at 250 Hz, NRC 1.05 | Sound Seal, QFA-20 data sheet 2024 |
| 12-inch open-cell foam corner wedges, laid on floor | 1.05 at 100 Hz, 1.30 at 125 Hz, NRC 1.50 (sabins per ft² of footprint) | Riverbank Acoustical Laboratories, RAL-A15-277 2015 |
| Depth needed before a corner trap outperforms spread-out material | At least 400 mm | IFA (DGUV), project 4185, completed 2011 |
| Same porous sample placed in a corner instead of mid-room | Sabine coefficients over 20% lower at all frequencies | Acoustics (MDPI), absorber placement study 2019 |
Outlier note: coefficients above 1.0, such as the 1.30 reading for 12-inch foam wedges (Riverbank, RAL-A15-277), are an edge and area artifact of small, tall specimens, not proof that a trap absorbs more energy than reaches it. Most recent available data for the air gap comparison: Riverbank, 2006.
3. Below 100 Hz: The Blind Spot in Lab Data
This is where room treatment matters most and where published numbers are weakest. Riverbank’s own reports label every value below 100 Hz as non-accredited, “representative of the RAL test environment only” (Riverbank, RAL-A21-302). In that extended appendix, a 2-inch panel scored 0.04 at 63 Hz, then -0.08 at 80 Hz and -0.02 at 31.5 Hz. A negative absorption coefficient is physically meaningless, which tells you the reverberation room itself, not the specimen, dominates those bands.
Deep traps fare better but erratically. The 12-inch foam wedge array read 0.56 at 63 Hz, then 0.07 at 50 Hz and 0.20 at 40 Hz (Riverbank, RAL-A15-277 2015). A swing from 0.56 to 0.07 across one-third of an octave is consistent with individual modes of the test chamber, not the trap, setting the result.
| Metric | Value | Source |
|---|---|---|
| Accredited frequency range in Riverbank reports | 100 Hz to 5,000 Hz; lower bands reported as unofficial | Riverbank Acoustical Laboratories, RAL-A15-277 2015 |
| 2-inch fiberglass panel, extended data | 0.04 at 63 Hz, -0.08 at 80 Hz, -0.02 at 31.5 Hz | Riverbank Acoustical Laboratories, RAL-A21-302 2021 |
| 12-inch foam wedges, extended data | 0.56 at 63 Hz, 0.07 at 50 Hz, 0.20 at 40 Hz | Riverbank Acoustical Laboratories, RAL-A15-277 2015 |
| Small (about 50 m³) test rooms | Deliver reasonable coefficients but in most cases fail below 250 Hz | PTB, Forum Acusticum 2025 |
| Low-frequency uncertainty shift from 200 m³ to 50 m³ room | Two one-third octave bands upward (factor 2^(2/3)) | PTB, Forum Acusticum 2025 |
| Reverberation time measurement at low frequency | Standard notes difficulties in measuring small reverberation times | ITU-R BS.1116-3 2015 |
| Early reflection measurement | May be difficult below about 500 Hz | EBU Tech 3276 1998 |
Context note: the PTB team tested five rooms of 49 to 58 m³ against two full-size chambers of 204 and 237 m³ (PTB, Forum Acusticum 2025). A typical home studio is smaller than even their “small” rooms, so in-situ measurement of modal decay, not catalog coefficients, is the honest test of a bass trap below 100 Hz.
4. Reproducibility: How Much Labs Disagree
The low-frequency problem is not only about what one lab can measure. It is about whether two labs agree. For a near-perfect absorber, the ISO 354:2003 reproducibility standard deviation is 0.255 at 100 Hz, almost 4 times the 0.065 at 500 Hz (0.255 / 0.065 = 3.9), according to data compiled for the ISO 354 revision (Peutz, ICA 2019). A draft correction method using a 200 mm glass wool reference absorber cut the 125 Hz figure from 0.195 to 0.142 but left 100 Hz at 0.267.
Proficiency testing tells a subtler story. Pooled reproducibility looks smallest at 100 Hz (0.023) largely because the test materials absorb so little there. In the 2021 round, the 100 Hz standard deviation was 0.018 on an assigned value of 0.03, a spread equal to 60% of the value itself (0.018 / 0.03). At 1 kHz in 2024 it was 0.048 on 1.03, under 5%.
| Metric | Value | Source |
|---|---|---|
| Difference in weighted coefficient for one product between two labs | Up to 0.1, equal to the gap between absorption classes A and B | Peutz, ICA 2019 |
| Reproducibility SD for absorbers near 1.0, ISO 354:2003 | 0.255 at 100 Hz, 0.195 at 125 Hz, 0.065 at 500 Hz, 0.055 at 1 kHz | Peutz, ICA 2019 |
| Same, with draft reference-absorber correction | 0.267 at 100 Hz, 0.142 at 125 Hz, 0.028 at 500 Hz | Peutz, ICA 2019 |
| ISO 354 working draft (circulated 2018) | 103 comments received; room volume range widened from 200-300 m³ to 190-350 m³ | Peutz, ICA 2019 |
| ISO 354 proficiency testing rounds | 7 rounds, 2012 to 2024, 13 to 17 labs per round | RPS-Qualitas, Forum Acusticum 2025 |
| Pooled reproducibility SD | 0.023 at 100 Hz, 0.035 at 125 Hz, 0.062 at 1 kHz, 0.070 at 4 kHz | RPS-Qualitas, Forum Acusticum 2025 |
| 2021 round at 100 Hz | SD 0.018 on an assigned coefficient of 0.03 | RPS-Qualitas, Forum Acusticum 2025 |
Context note: the RPS-Qualitas paper points out that the 2024 confirmation of ISO 354 still contains no precision data, so labs have no normative reproducibility target. Its next round, AQUS-8, is scheduled for 2026.
5. Membrane, Plate and Resonant Bass Absorbers
Resonant absorbers invert the porous profile. A 4.25-inch damped-plate panel measured 1.05 at 63 Hz and 0.95 at 50 Hz, yet its NRC is just 0.30 (RPG Acoustical Systems, Modex Plate 2.0 data sheet). Put it next to the 2-inch porous panel from section 3 (0.04 at 63 Hz, NRC 1.05) and the single-number rating ranks them exactly backwards for bass work. The comparison crosses labs and mountings, so treat it as directional.
Tuned units trade bandwidth for depth efficiency. RPG’s corner membrane traps come in 40, 63 and 80 Hz versions, each covering about one-third of an octave above and below its tuning (RPG, Modex Corner). Cavity depth is the main design lever: in a 2021 study from National Cheng Kung University, the 125 Hz coefficient of a panel membrane absorber rose from 0.14 to 0.33 as the air layer grew inside a 10 cm frame, and from 0.35 to 0.80 inside a 20 cm frame (Applied Sciences, 2021). Metamaterial absorbers push the same idea further, as covered in our acoustic metamaterials statistics.
| Metric | Value | Source |
|---|---|---|
| Damped-plate absorber, corner load | 0.95 at 50 Hz, 1.05 at 63 Hz, 1.06 at 80 Hz, 0.93 at 100 Hz, 0.57 at 125 Hz | RPG Acoustical Systems, Modex Plate 2.0 data sheet 2024 |
| Same panel single-number ratings | NRC 0.30, SAA 0.29 | RPG Acoustical Systems, Modex Plate 2.0 data sheet 2024 |
| Same panel depth and mass | 4-1/4 in. deep, about 5.00 lb/ft² | RPG Acoustical Systems, Modex Plate 2.0 data sheet 2024 |
| Corner membrane trap tunings | 40, 63 and 80 Hz, each about 1/3 octave either side | RPG Acoustical Systems, Modex Corner product page |
| Panel membrane absorber, 125 Hz, 20 cm frame | Rose from 0.35 to 0.80 as cavity layout changed | Applied Sciences (MDPI), panel membrane absorber study 2021 |
| Same, 10 cm frame | Rose from 0.14 to 0.33 | Applied Sciences (MDPI), panel membrane absorber study 2021 |
| Field result in a 1,600 m³ hall | 125 Hz reverberation time fell from 1.74 s to 1.55 s; C50 rose 2.07 dB | Applied Sciences (MDPI), panel membrane absorber study 2021 |
| Steel foil cover over porous absorber | No notable absorption loss below about 200 Hz | IFA (DGUV), project 4185, completed 2011 |
Outlier note: the RPG data sheet does not name the test laboratory, and ISO 354 states it is not intended for weakly damped resonators, so resonant-absorber coefficients deserve extra caution.
6. Room Modes: When Bass Ringing Becomes Audible
In small listening spaces, room modes dominate the low-frequency response at the listening position, so reverberation in the textbook sense is the wrong yardstick. Listeners detected modal decays of 0.9 seconds at 32 Hz, 0.3 seconds at 63 Hz and 0.17 seconds at 200 Hz with test tones (Fazenda, Stephenson and Goldberg, JASA 2015). With music, the thresholds relaxed to 0.51 seconds at 63 Hz and 0.12 seconds at 250 Hz. Those figures give a bass trap a measurable job: pull each mode’s decay below its threshold, not chase a generic reverberation target.
Source placement is the other half of the problem. In Harman simulations of a 24 x 20 x 9 ft room with a 16-seat grid, one optimized subwoofer left a 17.2 dB max-min spread across seats; two at opposing wall midpoints cut it to 4.6 dB, a 12.6 dB improvement (Harman International, Welti). Structural paths are a separate problem, covered in our subwoofer and low-frequency HVAC noise statistics.
| Metric | Value | Source |
|---|---|---|
| Modal decay threshold, test tones at 85 dB | 0.9 s at 32 Hz, 0.3 s at 63 Hz, 0.27 s at 100 Hz, 0.17 s at 200 Hz | Fazenda et al., JASA 2015 |
| Modal decay threshold, music | 0.51 s at 63 Hz, 0.3 s at 125 Hz, 0.12 s at 250 Hz | Fazenda et al., JASA 2015 |
| Earlier single-mode study | Threshold rose from about 0.3 s at 200 Hz to 0.4 s at 100 Hz; no difference heard at 50 Hz up to 2 s | Karjalainen et al., AES 116th Convention 2004 |
| Modal Q audibility threshold | Q = 16, equivalent to about 1.1, 0.5 and 0.2 s at 32, 63 and 125 Hz | Avis, Fazenda and Davies, JAES 2007 |
| One optimized subwoofer, 16 seats | Std 3.9 dB, max-min 17.2 dB | Harman International, Welti, Subwoofers: Optimum Number and Locations |
| Two subwoofers at opposing wall midpoints | Std 1.3 dB, max-min 4.6 dB | Harman International, Welti, Subwoofers: Optimum Number and Locations |
| Four subwoofers, one per wall midpoint | Std 0.9 dB, max-min 4.2 dB; no justification found for more than four | Harman International, Welti, Subwoofers: Optimum Number and Locations |
Context note: modal decay relates to Q through T = 2.2Q / f, so a mode with the same Q rings twice as long at 50 Hz as at 100 Hz. The JASA thresholds came from panels of 17 listeners for test tones (16 analyzed) and 10 for music, and the Karjalainen and Avis figures above are as summarized in that paper. Most recent available data: Fazenda et al., 2015; the Harman study is older still (AES 112th Convention preprint 5602).
7. Small-Room Reverberation Time and Listening Room Targets
The broadcast standards give small rooms a target that shrinks with volume. EBU Tech 3276 sets the nominal reverberation time between 0.2 and 0.4 seconds, averaged over the one-third octave bands from 200 Hz to 4 kHz (EBU Tech 3276). Both the EBU and ITU-R BS.1116-3 scale it as Tm = 0.25 x (V / 100 m³)^(1/3) seconds. Worked through, a 200 m³ room gets 0.25 x 1.26 = 0.31 s, a 50 m³ room gets 0.25 x 0.79 = 0.20 s, and a 30 m³ home studio gets 0.25 x 0.67 = 0.17 s, below the EBU’s 0.2-second floor.
The low end is where the tolerance opens up. The ITU-R BS.1116-3 tolerance mask lets reverberation time sit up to 0.3 s above Tm at 63 Hz, narrowing to 0.05 s by 200 Hz. That allowance is generous next to the 0.3-second modal decay threshold at 63 Hz in section 6. Small creator rooms used for vocals and streamer voice effects face the same physics, and most mixing decisions, including the ones covered in our AI audio mastering adoption statistics, still start from what the room lets you hear.
| Metric | Value | Source |
|---|---|---|
| Nominal reverberation time, 200 Hz to 4 kHz average | 0.2 s < Tm < 0.4 s | EBU Tech 3276 1998 |
| Volume scaling formula | Tm = 0.25 x (V / 100 m³)^(1/3) s | ITU-R BS.1116-3 2015 |
| Reverberation tolerance at 63 Hz | Up to +0.3 s above Tm, narrowing to +0.05 s at 200 Hz | ITU-R BS.1116-3 2015 |
| Allowed change between adjacent 1/3 octave bands | Under 0.05 s from 200 Hz to 8 kHz; under 25% of the longer time at or below 200 Hz | EBU Tech 3276 1998 |
| Minimum floor area and maximum volume | 40 m² reference room, 30 m² control room, volume no more than 300 m³ | EBU Tech 3276 1998 |
| ITU floor area range | 20 to 60 m² for stereo; 30 to 70 m² for multichannel | ITU-R BS.1116-3 2015 |
| Background noise limit | NR 10 preferred (30.7 dB at 125 Hz, 43.4 dB at 63 Hz), NR 15 maximum | EBU Tech 3276 1998 |
| Early reflections | At least 10 dB below the direct sound within 15 ms, 1 to 8 kHz | ITU-R BS.1116-3 2015 |
Context note: the EBU also asks designers to avoid dimension ratios within 5% of whole numbers and to restrict electrical room EQ to below 300 Hz. Most recent available data: EBU Tech 3276, 2nd edition 1998; ITU-R BS.1116-3, 2015 with editorial amendments in 2023.
Summary: Recording Studio Bass Traps by the Numbers
| Metric | Value | Source |
|---|---|---|
| ISO 354 frequency range and room size | 100 Hz to 5 kHz, rooms of at least 150 m³ | PTB, Forum Acusticum 2025 |
| Bands included in NRC | 250 to 2,000 Hz only | Riverbank, RAL-A06-219 2006 |
| 2-inch fiberglass panel rated NRC 1.05 | 0.31 at 125 Hz, 0.04 at 63 Hz (extended data) | Riverbank, RAL-A21-302 2021 |
| 25 mm air gap effect on 2-inch panel | 0.45 to 0.51 at 125 Hz | Riverbank, RAL-A06-219 and RAL-A06-220 2006 |
| 4-inch quilted fiberglass | 0.38 at 125 Hz, NRC 1.05 | Sound Seal, QFA-20 data sheet 2024 |
| Depth for corner trap effect | At least 400 mm | IFA (DGUV), project 4185 2011 |
| Damped-plate bass absorber | 1.05 at 63 Hz, NRC 0.30 | RPG, Modex Plate 2.0 data sheet 2024 |
| Membrane absorber, 20 cm frame | 0.35 to 0.80 at 125 Hz | Applied Sciences (MDPI) 2021 |
| Inter-lab spread | Up to 0.1 in weighted coefficient; SD 0.255 at 100 Hz vs 0.065 at 500 Hz | Peutz, ICA 2019 |
| ISO 354 proficiency rounds | 7 rounds, 13 to 17 labs each | RPS-Qualitas, Forum Acusticum 2025 |
| Small 50 m³ test rooms | Fail below 250 Hz in most cases | PTB, Forum Acusticum 2025 |
| Modal decay threshold, test tones | 0.3 s at 63 Hz | Fazenda et al., JASA 2015 |
| Modal decay threshold, music | 0.51 s at 63 Hz | Fazenda et al., JASA 2015 |
| Seat-to-seat spread, 1 vs 2 subwoofers | 17.2 dB vs 4.6 dB | Harman International, Welti |
| EBU nominal reverberation time | 0.2 to 0.4 s | EBU Tech 3276 1998 |
| Reverberation tolerance at 63 Hz | Up to +0.3 s above Tm | ITU-R BS.1116-3 2015 |
Methodology and Sources
All figures come from primary documents, standards records and lab reports read during research for this roundup. Coefficients are quoted exactly as the reports print them, and every derived figure shows its arithmetic in the text.
- ISO: ISO 354:2003, Measurement of sound absorption in a reverberation room (edition status, 2024 confirmation, exclusion of weakly damped resonators).
- ASTM International: ASTM C423-23, Sound Absorption by the Reverberation Room Method (current designation and scope).
- Physikalisch-Technische Bundesanstalt (PTB): Bietz and Wittstock, Combining absorption coefficient measurements in reverberation rooms with different volumes, Forum Acusticum 2025 (ISO 354 range and volume, 50 m³ room results).
- Peutz: Vercammen, On the revision of ISO 354, ICA 2019 (inter-lab spread, reproducibility by frequency, draft revision details).
- RPS-Qualitas: Rosario and Martinez, Reproducibility precision in sound absorption measurements, Forum Acusticum 2025 (AQUS proficiency testing 2012 to 2024).
- Riverbank Acoustical Laboratories, RAL-A06-219: 2-inch fiberglass panel, Mounting A, 2006.
- Riverbank Acoustical Laboratories, RAL-A06-220: 2-inch panel on spacers, 2006.
- Riverbank Acoustical Laboratories, RAL-A21-302: 2-inch ProPanel with extended low-frequency appendix, 2021.
- Riverbank Acoustical Laboratories, RAL-A15-277: 12-inch foam corner wedges with extended appendix, 2015.
- Sound Seal: QFA-20 quilted fiberglass absorber data sheet, 2024.
- RPG Acoustical Systems: Modex Plate 2.0 data sheet, 03/2024 and Modex Corner product page.
- Applied Sciences (MDPI): Development of a Panel Membrane Resonant Absorber, 2021.
- Acoustics (MDPI): Influence of Sound-Absorbing Material Placement on Room Acoustical Parameters, 2019.
- IFA (DGUV): In-development measurements on innovative sound absorbers, project 4185.
- University of Salford / University of Huddersfield: Fazenda, Stephenson and Goldberg, Perceptual thresholds for the effects of room modes as a function of modal decay, JASA 2015.
- Harman International: Welti, Subwoofers: Optimum Number and Locations.
- EBU: Tech 3276, Listening conditions for the assessment of sound programme material, 2nd edition 1998.
- ITU-R: Recommendation BS.1116-3, 2015.
- Data watch: much of the core data is old (EBU 1998, Riverbank 2006 and 2015, Fazenda 2015, and an early Harman study), so treat it as most recent available rather than current. Report RAL-A06-220 is titled “2 Inch Spacers” but its body describes 25 mm (1 in.) spacers and 76 mm total depth; we quote the body. All sub-100 Hz coefficients are non-accredited, and the RPG and Sound Seal data sheets do not name their test lab. The 12-inch foam result is expressed in sabins per square foot of footprint, so values above 1.0 are not comparable with flat panels. EBU sets minimum floor areas of 30 to 40 m² while ITU-R BS.1116-3 accepts rooms from 20 m², a real conflict for small-room designers, although both documents use the same Tm = 0.25 x (V / 100 m³)^(1/3) formula. The ISO 354 revision with a reference absorber has circulated as a working draft since 2018 and has not replaced the 2003 edition.
Last updated: October 18, 2026. We update this roundup quarterly, and the next refresh is expected when RPS-Qualitas publishes results of its AQUS-8 ISO 354 proficiency test round (scheduled for 2026).