Independent Acoustic Test Data
What is Riverbank Acoustical Laboratories?
Riverbank Acoustical Laboratories [RAL] is the birthplace of architectural acoustics. RAL was founded in 1918 by Wallace Clement Sabine, the man for whom the unit of measure for acoustic absorption is named [the sabin]. RAL has been a center for research ever since – in divers fields of scientific inquiry from acoustics to cryptanalysis.
Explicit scientific standards for absorption testing exist, but these standards are not designed to address the difficulties encountered when one wishes to measure absorption at very low frequencies, or in corner mountings. The fact that the measurement of absorption properties is an art invented by the founder of RAL, and advanced through the years by its staff, was attractive to us, and gave us confidence we could work with RAL to conduct accurate and scientifically valid measurement methodologies which addressed the challenges presented by corner absorption devices at low frequencies.
How are reverb chambers used to test absorption?
The measurement of the intrinsic absorptive properties of a material or item is not possible in a normal room, as the reverb field is not diffuse, and the reverb time is too short. The reverb chamber at RAL is a specially designed test facility designed to overcome these limitations and is comprised of a large volume room [about 10,000 cubic feet] with highly reflective surfaces. There are also several diffusion panels in the test chamber including a set of large rotating vanes hung from the ceiling. These vanes are slowly rotated when the chamber is in use taking measurements. The result is a room in which a highly diffuse reverberant field is created [sounds comes from all directions – more or less evenly distributed], and in which, once a sound occurs, it will reverberate [echo] for a long time. Reverb chambers such as the RAL “Room 0” are capable of taking accurate absorption measurements because they have long reverb times and highly diffuse reverb fields.
When conducting standard absorption tests, and seeking to provide for the repetition of such tests, one must define a position for the measurement microphone. In standard tests RAL uses a defined microphone position and sample area. The microphone is mounted on a tilted boom that is slowly rotated during measurements. The boom length can be altered, Mic Boomand the entire rotating system can be tilted, and these factors when maximized assure that the microphone varies its position to the largest degree possible during the measurements. The longer the boom radius is, and the greater the tilt, then the greater the variance in mic position during rotation and the more accurate the measurement. These factors are limited by the fact that the microphone cannot be allowed during its revolutions to come too close to any room surface [including the diffusion panels and vanes].
The final key variable in absorption testing is sample placement. There are defined locations on the test floor typically used for all absorption testing and chosen based on sample size. One of these defined areas is marked permanently by a white square painted on the RAL test chamber floor and which you can see in the picture below. These defined areas have been established from the strictures of testing standards which require a minimum amount of material to used in each test, and from the experience of the laboratory staff in arriving at placements which achieve consistent measurement results.
Once the parameters of microphone and sample placement are fixed, then the chamber is sealed, the lab technicians repair to a control room and introduce a white noise signal into the room and measure the decay of the sound impulse. EngineeringThis process is repeated many times – RAL does the procedure 80 times and then correlates the results to produce a measurement. These results are then compared to similar measurements taken of the chamber while empty. By comparing the rate of the decay of the diffuse reverberant sound field in the empty chamber with the rate of decay that occurs with the specimens in place, the lab is able to measure the absorptive properties of the sample.
The measurements taken in this manner are reported per one-third octave frequency band. The accuracy of the measurements vary per band depending on several factors including the modal structure of the test chamber, the acoustic reflectiveness of the various room surfaces and the reverb time [how long the echo lasts] as to that band of frequencies. In calculating the measurement for each section of the frequency band, the lab can also calculate the accuracy of the measurement – expressed as “% Uncertainty”.
Absorption measurement standards such as ASTM C423 do not address taking measurement at very low frequencies. As a result taking absorption measurements below 100 Hz is an “out of spec” proposition. Therefore, in order to rationally review and understand low frequency absorption measurements you must take careful note of the sample placement and give consideration to the % Uncertainty of the measurements of each frequency band. Incomplete absorption measurement reports which do not provide this information should not be taken seriously in the lower frequency levels, as sample placement can have a large effect on measurement results, and it is in the very lowest frequency bands that the inaccuracies of measurement maximize. The RAL reports Ready Acoustics has published on corner mount absorption test includes appendixes which provide all this information. Note that only those frequency bands which have some reasonable accuracy are reported by Ready Acoustics in its product descriptions, and the calculated % Uncertainty is provided in the lab reports we publish, so you can know all the facts related to the measurement.
How where the RT424 corner mount absorption tests conducted?
Corner mounting of absorbers causes a large increase in device efficiency, and for this reason corner mounting is the preferred mounting method when using Ready Acoustics Bass Traps. Acousticians and savvy audiophiles have used corner mounting to enhance the effectiveness of acoustic treatments for years [right-click and “save as” to learn more – absorber placement and measurement], but the physics behind corner mounted absorber efficiency is not well understood to science. Thus any test of low frequency absorbers in a corner mount must be carefully conducted, and exhaustively detailed in its publication in order to provide reliable measurements.
The first task in setting up the Ready Acoustics corner mounting testing was to establish locations for the specimens. In choosing how to place the specimens, we sought to spread the samples out as much as the dimensions of the chamber would allow so that each unit was exposed to sound from all directions.
Placing the specimens edge-to-edge could have been used as a means of increasing the apparent absorption measurement at lower frequencies by shifting the low frequency absorption peak common to corner mounted absorption devices lower into the band, but this would not be the best way to measure the intrinsic absorption properties of a single unit.
Next we set the microphone position, boom radius and tilt so as to create as large a variance in mic position during rotation as physically possible while still maintaining the required distance from the chamber’s walls. The exact specifications for mic and sample placement are included in the below linked full lab report or can be viewed here. This information is provided so that users can understand how the tests were conducted, and so that the tests can be repeated on other items with full knowledge that the lab conditions are precisely similar to the ones used for this measurement. Ready Acoustics is the ONLY acoustics treatment manufacturer that publishes full details of its absorption testing procedures. We invite scientifically valid comparison of our products to any other.
Ready Acoustics Bass Traps Corner Mounted Testing Results
In a series of tests focused on the acoustic properties of porous absorbers in corner mountings, Ready Acoustics tested the RT426 SubBass Trap and the RT424 Broadband Acoustic Bass Trap in the reverb chamber at Riverbank Acoustical Laboratories [RAL] to establish the performance of these devices when mounted across the diagonal of a corner – this type of mounting being the preferred mounting position for these device.
- Certified Absorption Report – RT426 SubBass Trap
- Certified Absorption Report – RT424 Broadband Bass Trap
These tests were conducted and the techniques of measurement were logged in such a way that all the information necessary to reproduce the tests could be reported with both clarity and precision. Complete information on the test, the location of the test specimens, and the measurement techniques are included in the above linked official RAL reports on these measurements. The sabins per unit as measured in frequency bands from 63 through 10k Hz are shown in the graphs above. Lower frequencies were measured [both 40 and 50 Hz bands] but the “% Uncertainty” of these measurements disallows publication with reasonable basis for reliance. Nonetheless, these lower certainty measurements are included in the linked full reports.
Nested System Data
The test method conformed explicitly with the requirements of the ASTM Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method: ASTM C42302a and E795-05. Riverbank Acoustical Laboratories has been accredited by the U.S. Department of Commerce, National Institute of Standards and Technology (NIST) under the National Voluntary Laboratory Accreditation Program (NVLAP) for this test procedure (NVLAP Lab Code: 100227-0). A description of the measuring procedure and room qualifications is available separately.
DESCRIPTION OF THE SPECIMEN
The test specimen was designated by the manufacturer as RT426 – 6 inch thick Ready Traps with nested panel. The specimen consisted of sixteen (16) units. Eight units as measured were nominally 610 mm (24 in.) wide by 1.22 m (48 in.) long and 152 mm (6 in.) thick. Eight units as measured were nominally 305 mm (12 in.) wide by 1.22 m (48 in.) long and 102 mm (4 in.) thick. The specimen was tested in the laboratory’s 292 m3 (10,311 ft3) test chamber.
The manufacturer’s description of the specimen was as follows: The (24 in.) wide units consisted of 3 pieces of 2 inch thick Owens Corning insulation 703, 3 PCF wrapped in polyester acoustical stretched suede. The (12 in.) wide panel consisted of 2 pieces of 2 inch thick Owens Corning insulation 703, 3 PCF wrapped in cotton. A visual inspection verified the manufacturer’s description of the specimen.
The weight of the entire specimen as measured was 62.8 kg (138.5 lbs), an average of 3.9 kg (8.7 lbs.) per panel. The room temperature at the time of the test was 22ºC (71ºF) and 64% relative humidity.
Fourteen panels were distributed about the test chamber as follows: Six units each on the south and north walls and two units on the west wall, “nested” smaller dimension panels placed directly behind larger dimension panels. Respective to each wall surface the panels were located at a 45º angle supported at both the wall and floor with the long dimension of the panel parallel to the wall-floor intersection. The panels were located nominally 203 mm (8 in.) apart. The fifteenth and sixteenth panels “nested” were located at a nominal 45º angle in the southwest corner of the room. The panel was suspended vertically with bottom edge hanging 622 mm (24.5 in.) from the test room floor. The sides of the face of the panel were butted against the south and west wall surfaces.
|1/3 Octave Center Frequency (Hz)
|Total Absorption In Sabins
SOUND ABSORPTION REPORT RAL-A06-140
Appendix to Sound Absorption Test: RAL™-A06-140
|No. of Pages
|Specimen Mounting Details
|Specimen Mounting Room Layout
TEST NUMBER: RAL™-A06-140
TEST DATE: May 25, 2006
CLIENT: Ready Acoustics LLC
DESIGNATION: RT426-6 Inch Thick Ready Traps with Nested Panel
DIMENSIONS: Two Part Panel System, Facing Panel = 24″ x 48″ x 6″, Nested Panel = 12″ x 48″ x 4″
NUMBER OF UNITS: 8
3 specimens each on the south and north wall with nominal 8” spacing between specimens. 1 specimen on west wall. 1 specimen in the southwest corner vertically oriented with bottom edge a nominal 24.5” above test room floor [nominal 18” above 6.5” floor ledge]. All specimens mounted @ 45 degree angle to wall with a nested panel behind each facing panel. All panels are mounted with the backside long axis edges resting against the test room wall / floor. See figures 1 & 2 below.
ADDITIONAL INFORMATION IS PROVIDED ON THE ATTACHED FLOOR DIAGRAM PAGE
Additional Frequency Data for Absorption Testing
As requested by the client, sound absorption values were calculated at additional test frequencies. Although the measurements were made in accordance with the procedures described in ASTM C423-02a, they do not qualify as part of the standard. Since the results are representative of the test environment only, they are unofficial and intended for research and development guidelines rather than for commercial purposes. The sound absorption values at additional frequencies were as follows:
|1/3 Octave Center (Hz)
Additional Measurement Uncertainty Data
As requested by the client, the uncertainties of the measured sound absorption values were calculated and are presented below. The absorption of the test specimen contains the combined uncertainties of the two room absorptions: the absorption of the empty room and the absorption of the room containing the test specimen. For the total number of 80 decays conducted for both the empty room and the room containing the test specimen, distribution factor and the mean to derive the 95% confidence limits. The uncertainties were combined using the usual “root-sum-of-squares” method. Room uncertainties are used internally for quality control purposes although these uncertainties are not established as criteria for conducting the test. More information regarding room qualifications, reproducibility, R, and repeatability, r, can be found in the current ASTM C423 standard.
|1/3 Octave Center Frequency (Hz)
With 95% Confidence