Editor’s Note: At CYMBACAVUM, we’re constantly trying to bridge science with the wonderful audio experiences we have. @miceblue takes on an scientist’s curiosity to examine Asius Technologies and 1964Ears’ ADEL — how it functions, the scientific principles behind it, and what it might or might not be.
This article was intended to be published on August 15, 2015, but we continually pushed back the publication in an effort to try to understand the science better. Since then, after producing many ADEL-equipped models, 1964Ears has reorganized into 64Audio, and interestingly now has its own version of in-ear pressure equalization in Apex. The difference between Apex and ADEL is yet to be substantiated.
Back in November 2014, 1964 Ears teamed up with Asius Technologies under the name 1964|ADEL to propose a novel line of in-ear earphone products through the Kickstarter crowd-funding platform. About a month later, their campaign ended and they successfully raised more than three times their original funding goal.
1964|ADEL made some pretty big claims about their ADEL technology and even had a research study to reinforce them. I wanted to investigate more into their claims and analyze their methodology. Are the claims they make with their RealLoud technology truly beneficial to in-ear earphone users?
Let us peruse through their published findings, shall we?
Anatomy of the Ear and Auditory Physiology
Having a background in bioengineering, I was naturally curious to delve into how 1964|ADEL‘s RealLoud technology works. To do so, one needs to know about the anatomy of the ear; not just the outer ear’s parts such as the CYMBA and the CAVUM (hah!), but the middle and inner ear systems as well.
Wikipedia is a great resource for learning more about the ear and its anatomy, but for a brief overview, for sound:
- The outer ear guides pressure waves (sound) into the ear canal as well as amplifying certain frequencies of sound
- The middle ear acts as a mechanical amplifier
- The inner ear is a transducer that converts mechanical energy into electrical signals that your brain can then interpret
Two aspects of the middle ear are important to examine for the RealLoud technology: the tympanic membrane (commonly referred to as the eardrum) and the Eustachian tube.
- The tympanic membrane is an actively moving part of the middle ear that responds to pressure waves from the ear canal; it is essentially the input for the amplification process of the middle ear. It is also involved in a process, called the stapedius (or acoustic) reflex, that acts as an acoustic equivalent of a surge protection device. Instead of having a voltage spike threshold (the clamping voltage value) and metal oxide varistors (which short the circuit upon a large voltage spike), muscles near the tympanic membrane and stapes bone contract at certain sound pressure levels (SPLs) to prevent excess movement of the middle ear and, thus, transmission of an excessively large signal to the inner ear and brain. It has been suggested, however, that the stapedius reflex correspondingly only activates the muscles at the stapes and not the tympanic membrane in humans1.
- The Eustachian tube is equally as important for the ear and how we perceive sounds. This tube extends from the base of the middle ear to the cavity behind your nose and mouth. The tube thus acts as a conduit for air flow and it allows for the equalization of pressure between the ear and the surrounding atmosphere. You may be familiar with the phenomenon of your ears “popping” when a large difference of pressure exists, such as in an airplane. Your Eustachian tube is correspondingly opening up and allowing air to flow in and out of the middle ear. Like the stapedius reflex, the equalization of pressure between the middle ear and surrounding atmosphere affects the movement of the tympanic membrane and the rest of the middle ear. An easy analogy to consider would be the venting that is required for dynamic in-ear earphones; many moving coil-based earphones contain vent holes on either side of the transducer to allow for the diaphragm to vibrate more freely.
RealLoud Technology Breakdown
Now that the relevant anatomies of the ear have been covered and explained, let us take a look at the RealLoud technology. RealLoud boils down to what is referred to as the ADEL Module, where ADEL is an acronym for Ambrose Diaphonic Ear Lens. Accordingly, Stephen Ambrose is the pioneer for this membrane, the founder of Asius Technologies, and the original inventor of the in-ear monitor.
The ADEL Module is a mechanically adjustable system that attempts to mimic the same functions as the tympanic membrane and the Eustachian tube through biomimicry.
In the ADEL Module, the ADEL diaphragm acts as a secondary, adjustable tympanic membrane, whereas the tube-like enclosure acts as a Eustachian tube to adjust the pressure and thus tension of the ADEL membrane.
In the 1964|ADEL‘s Control and Ambient series of in-ear earphones, RealLoud is in implemented in a different way. In short, there are multiple disc-like layers containing ADEL membranes and/or airflow ports in an off-axis formation. When the user manually rotates the modules on the earphone itself, the membranes and ports will be aligned in different positions to adjust airflow resistance.
Both this and the ADEL Module are patented by Stephen Ambrose and Asius Technologies (US Patent #8,737,635, published on May 27, 2014). What I believe to be the Control and Ambient‘s implementation of RealLoud can be seen in Figure 1 of the patent (structure 103), whereas the ADEL Module is similar to structure 163 in Figure 3B. An additional patent from Asius Technologies (US Patent #8,774,435, published on July 8, 2014) helps to explain the rotatable airflow ports in Figure 56D.
1964|ADEL‘s Research Study and Claims
In 2011, Stephen Ambrose and Asius Technologies received funding from the National Science Foundation for their proposal of ADEL. At the time, Ambrose was using the membrane as a way to inflate a porous in-ear balloon-like structure to replace a typical silicone in-ear earphone tip (structure 170 in US Patent #8,737,635). It was later developed into the ADEL membrane. I actually first heard about ADEL through the technology blog Engadget, complete with the full press release.
Three years later in 2014, a research poster from Vanderbilt University (poster #40) was displayed at the American Auditory Society Scientific and Technology Meeting. This very same poster is featured on 1964|ADEL‘s website and is claimed to be a “full report.”
For those unfamiliar with what a research poster is, it is a snapshot of a full research project to give others an idea of what the project is about at a 5-10 minute’s glance. It is not a full, or complete, report and the study has not been published in any scientific journals as of the date of this published post.
Note: The implementation of RealLoud in this study was more akin to that used in the ADEL Module, as opposed to the ones found in the Control or Ambient series of in-ear earphones. No data has been presented from the latter.
The methods outlined in the research poster are pretty straightforward and are adopted from a previously published paper in the Journal of the Acoustical Society of America2. In short, participants of the study listened to low-pass filtered music and test tones through an unmodified/stock/occluded in-ear earphone in one ear and were asked to volume-match it to a modified/ADEL/un-occluded in-ear earphone in the other ear. Microphones placed near the ear were used to measure the SPL. Two brands of in-ear earphones were used for this poster. In the published paper, tests were done with a loudspeaker and in-ear hearing aid to represent an occluded ear canal listening situation.
In their poster, 1964|ADEL claims that this research group did not do listening tests with non-low-frequency signals and that they did. However, the research group did in fact test the participants with 3000 Hz signals, and the recorded data suggested that the participants did not hear a difference in volume level between the un-occluded and occluded listening conditions. Because of this, 1964|ADEL‘s claim of:
the level advantage was observed for both low and high frequency stimuli, a result which is in partial contrast to previous findings demonstrating an advantage of the open ear condition, albeit only for the low frequencies.
is not entirely accurate because even they only did listening tests up to 3000 Hz. Test signals between 3000 and 5000 Hz may have been more interesting to observe since the ear canal undergoes resonance at those frequencies3, 4.
In addition, the research paper specifically states that the stapedius reflex would not be a factor for the difference in audibility between the occluded and non-occluded ear because the SPL would not be high enough to cause the reflex to activate. Only in the case of where a user would be listening to their passive noise-isolating in-ear earphones at levels higher than 85 dB SPL (about as loud as a busy highway) would the acoustic reflex be activated1.
1964|ADEL also claims that a previously published paper demonstrated that altering the impedance of the tympanic membrane would affect the sound threshold5. 1964|ADEL claims that altering the impedance of the tympanic membrane may cause the difference in sound threshold between the occluded and non-occluded ear canal. However, this particular paper was exploring the properties of the air-bone gap — a case where there is a difference in the audibility of sounds due to bone-conduction versus air-conduction.
Furthermore, the impedance of the tympanic membrane is not specifically discussed in this paper, but rather the round window shield that is the result of types IV and V tympanoplasty (as suggested in the title of the paper). This is not necessarily indicative of what occurs in people who have normal hearing abilities and have a fully intact tympanic membrane, like those tested in the 1964|ADEL research poster. What can be suggested from the published paper, on the other hand, is that an activation of the stapedius reflex (and thus an increase in impedance of the stapes) would increase the air-bone gap in that particular model of the ear.
Regardless of whether or not the impedance of the tympanic membrane affects the “level advantage,” as suggested by 1964|ADEL, previously published research does show that the reflectance (prevention of sound waves from entering the ear canal), of lower-frequency sounds becomes higher when the stapedius reflex is activated6. The activation of the stapedius reflex in this context could, to some degree, help explain why 1964|ADEL claims that users of in-ear monitors are listening to music at volume levels louder than necessary. However, they do not mention this at all in their poster, let alone mention the the SPLs that the subjects were listening at to demonstrate that the stapedius reflex was in fact being activated.
Finally, one of the biggest claims made by 1964|ADEL is that having the ADEL membrane installed in in-ear devices will minimize the risk of hearing loss because their products will allow you to listen to music at lower volume levels while having the same perceived volume. Although their research poster does seem to suggest this with their “level advantage” data, there are many variables changed in the experiments that may affect the validity of their data. For one, they explicitly state that the frequency response changes between the unmodified and ADEL modified in-ear earphones. This was not accounted for in their Analysis of Variance (ANOVA) tests, which are statistical models to determine which variables are responsible for significant changes in data. It is possible that the alteration of frequency response influenced the “level advantage” data. 1964|ADEL did not show the frequency response for the second modified earphone either, and instead states:
While only data for Model B are presented in this figure, a similar small reduction in low frequency output was evident in Model A.
Secondly, in the “level advantage” data they show, the modified in-ear earphone that best matched the data from previous research is the one containing multiple ADEL membranes, not the other earphone with a single membrane like what is used in the ADEL Module. Again, their ANOVA tests did not account for this and the inclusion of more than one ADEL membrane may have altered the “level advantage” data.
Thirdly, the addition of the ADEL membranes to the modified in-ear earphones could affect the sensitivity and/or impedance of the overall earphones. This factor was not accounted for in their poster and it very well could be the case that the impedance is lower with the added ADEL membranes. This would thus cause the participants to lower the volume knob compared to the unmodified earphone, assuming the sensitivity remains equal. A similar case would occur if the ADEL membrane made the in-ear earphones more sensitive.
Fourthly, the data presented in the poster is in units of decibels, with no reference point and may not necessarily be in decibels SPL, which is important for determining the loudness of sound within the ear canal.
One thing that was not mentioned at all, nor cited, in this poster was the fact that Ambrose had a previous publication in the Audio Engineering Society (AES) regarding “trapped volume insertion gain,” which is the apparent increase in SPL of low-frequency sounds within the ear as a result of sealing the ear canal with a in-ear earphone7. Although this paper outlines some interesting theories, sections 1 and 2 of the paper discuss some very specific claims about the physics of a sealed ear canal, including seemingly unrelated terms such as thermodynamics, without citing any sources for those claims.
In modern times, it seems that Ambrose is the only person investigating this “trapped volume insertion gain” phenomenon, even including sections of the AES paper verbatim in US Patent #8,774,435, which also outlines an otoscope-like device that was used to collect data for that paper. Indeed, the AES paper was published in 2011 and it has only been cited in one other scientific publishing since then8, suggesting that the paper has a low impact factor amongst the scientific community. In comparison, although completely irrelevant to this discussion, Sean Olive et al.’s 2013 AES published paper has been cited in four other published papers since then9.
Despite what has been outlined here in this article, I really do hope that the RealLoud technology can help reduce the risk of hearing damage. I have a lot of respect for Ambrose and the inventions and research he has done. However, from the claims presented in the Vanderbilt University research poster, as well as Ambrose’s AES paper, I am still skeptical about the idea.
The ambiguous data from their research makes me even more skeptical of their claim that:
When applied to personal listening devices specifically engineered to take advantage of these revolutionary ADEL principles and technological breakthroughs, our new products typically allowed listeners to routinely listen at even lower volume levels than were so astonishingly achieved in this 2013 Study. Additionally it is important to note here that persons listening at what would normally be deemed excessive volume levels were able to do so with greater fidelity and dynamic range while also creating significantly less in-ear pressures than they were used to.
Unfortunately, no solid data thus far has been officially published to directly address that claim, let alone quantifying “greater fidelity and dynamic range.” While private companies such as Asius Technologies and 1964Ears are not required to provide full justification of their claims through peer-reviewed journals and widespread acceptance in the scientific community, their claims should nevertheless be testable and rigorous if they wish to state that their technology is somehow scientifically proven.
- “Notes on the Acoustic Middle Ear Reflex,” American Academy of Audiology, Oct. 2012. [Online]. Available: http://www.audiology.org/news/notes-acoustic-middle-ear-reflex. [Accessed: March 15, 2015].
- G. Keidser, R. Katsch, H. Dillon, and F. Grant, “Relative loudness perception of low and high frequency sounds in the open and occluded ear,” J. Acoust. Soc. Am., 107(6): 3351–3357, Jun. 2000.
- Yao Wen-juan, Ma Jian-wei, and Hu Bao-lin, “Numerical Model on Sound-Solid Coupling in Human Ear and Study on Sound Pressure of Tympanic Membrane,” Mathematical Problems in Engineering, vol. 2011, Article ID 282696, 2011.
- Hertsens, Tyll. “Headphone Measurements Explained – Frequency Response Part One,” Innerfidelity, Feb-6-2015. [Online]. Available: http://www.innerfidelity.com/content/headphone-measurements-explained-frequency-response-part-one [Accessed: May 22, 2015].
- Merchant, Saumil, Rosowski, John, and Ravicz, Michael, “MIDDLE EAR MECHANICS OF TYPE IV AND TYPE V TYMPANOPLASTY: I. MODEL ANALYSIS AND PREDICTIONS.,” Otology & Neurotology, vol. 16, no. 5, pp. 555–564, Sep. 1995.
- M. P. Feeney and D. H. Keefe, “Acoustic Reflex Detection Using Wide-Band Acoustic Reflectance, Admittance, and Power Measurements,” Journal of Speech Language and Hearing Research, 42(5): 1029, Oct. 1999.
- S. Gido, R. B. Schulein, and S. D. Ambrose, “Sound Reproduction within a Closed Ear Canal: Acoustical and Physiological Effects,” presented at the Audio Engineering Society Convention 130, 2011.
- Rämö, Jussi, “Equalization Techniques for Headphone Listening,” Aalto University publication series DOCTORAL DISSERTATIONS, no. 147, Oct. 2014.
- S. Olive, T. Welti, and E. McMullin, “Listener Preferences for In-Room Loudspeaker and Headphone Target Responses,” presented at the Audio Engineering Society Convention 135, 2013.