ASTM C384 vs C423
For years, technically decades, there has been an ongoing debate of the veracity of ASTM C384 impedance tube testing vs. full scale reverberation room testing aka ASTM C423.
As a voting member of the ASTM E33 acoustics committee I have been in sessions where this very topic has been discussed ad nauseum. The problem lies in the fact that C384 does not mate up with C423 when the same product is tested in the 2 different test methods.
At RealAcoustix we made a decision long ago not to rely on, or use, C384 as a standard test method. The problem lies in the fact that it is a confined tube that does not mimic real world conditions. This is especially true as we get into low frequency spectrums, where its efficacy has proven to be inadequate, at best.
While we maintain a full-size C423 reverberation chamber in our facilities, we don't test low frequency products in there except for our own R&D purposes. This is because the lab has a design frequency of 100Hz. Every test data you will see regarding low frequency units is done at NWAA labs in Washinton, which is tested and accurate down to 40Hz.
An impedance tube, by it's very nature, cannot fully develop or mimic real-world conditions. It is a confined space in which a signal is shot down the tube and multiple microphones are used to attempt to diagnose the signal. While it comes close to matching C423 room testing at frequencies above 150Hz, it is woefully lacking in frequencies below that.
Some manufacturers will use impedance tube testing as a way around having to use an outside lab or spend the time and money it takes to ensure a precise outcome. At RealAcoustix, we don't play that game or accept anything less than real-world test data.
As an additional note, absorption tests for low-frequency products must be preformed in a qualified lab that meets the criteria and design required by ASTN C423 for full-frequency testing at the lowest frequency desired. Currently, there is only one commercial lab in the U.S. that can accurately test down to 40Hz, or even 70Hz. That is NWAA Labs in Elma, Washington. Everyone else is providing compensated information that, again, is calculated, not actual.
In short, no one in the acoustics industry provides more accurate information than RealAcoustix. If you would like more information, please call or email me, I would be glad to send you more data.