Occlusion is often described as similar to talking with your head in a barrel. Barrels really aren’t that common these days and it’s easier to listen to occlusion and come up with your own description. Use your fingers to plug up your ears, then say the following: “Shhhhh”, “Aaaaaah” and then “Eeeee”. You’ll hear each sound progressively louder in your blocked ears – that’s occlusion. When you talk, the vibrations from your voice box (larynx) travel through your head to your ears. These vibrations generate sound which gets trapped in your ears when they’re plugged, making the sound pressure stronger and louder. Lower frequency sounds, like “Eeeee”, travel through the body tissue with more strength than high frequency sounds, like “Shhhhh”, and that’s why plugging your ears produces an annoying muffled version of your voice when you talk. Occlusion also raises the volume of chewing and swallowing noises.
“Shhhh”, “Ahhhh”, “Eeeeee”
Hearing aids block the ear canal, causing many users to complain about the sound of their own voice from the resulting occlusion. Researchers have estimated that your own voice can sound 15 to 25 dB louder to you due to this effect (Vasil, 2006). Occlusion depends a lot on the style of hearing aid and one of the reasons for the popularity of open fit devices over the last 20 years is the way they keep the ear canal open. Smaller eartips result in lower perceived occlusion, as would be expected (Conrad, 2012).
Traditional in-the-ear style hearing aids block the ear canal more than open fit devices.
Annoyance
Because of occlusion, one of the major complaints of hearing aid users, especially new users, is the sound of their own voice. One study determined that only 41% of users were satisfied with the sound of their own voice (Powers et al, 2018). Hearing aid manufacturers traditionally have two basic approaches to reduce the occlusion effect: large vents or deep insertion in the ear canal.
Vents are used in earmolds and in-the-ear devices to allow sound to escape from the ear canal. The larger the vent, the more sound escapes and occlusion is reduced. Open fit devices can be thought of as devices with extremely large vents. When a vent is added, the acoustics of the ear canal and device are altered, resulting in a change in the frequency response of the system. There has been a tremendous amount of research done on the acoustics of vent geometries and how this affects device performance (Studebaker, 1977; Stuart, 1999; Kuk, Nov. 2005; Stevenson, 2014;).
The other technique to reduce occlusion, deep insertion, takes advantage of an interesting phenomenon first noted by Bézésky then studied further by Zwislocki in 1953 (Zwislocki, 1953). If the ear canal is blocked deep enough in the bony region of the head, occlusion is greatly reduced.
The eartip inserted deeper into the ear canal actually produces less occlusion.
It follows then that hearing aids designed to fit deep in the ear canal that make contact with the bony region have been shown to reduce occlusion (Staab 2004). Making a deep fit custom device is tricky and this style currently represents a relatively small percentage of hearing aids sold every year.
If opening up the ear canal solves occlusion problems, why not design devices with wide open ear canal fittings for everyone and be done with it? An open ear canal device has two problems: a lack of low frequencies and increased feedback. With the canal open, the resulting acoustics make it difficult to deliver low frequency energy from a receiver (hearing aid speaker) to the tympanic membrane (ear drum). This is why recorded music doesn’t sound that great when played through open fit devices – there’s not much bass. For hearing enhancement applications, this lack of low frequencies is less of a problem because most people have hearing loss mainly in the high frequencies. Of course, if you need help at lower frequencies, an open fit device might not work for you.
Feedback in a hearing aid happens when the sound from the receiver (speaker) feeds back to the device microphone. This happens much more easily when there’s an open path between the receiver and microphone. Open fit devices have the advantage that their microphones are farther away from the receiver than in-the-ear devices, but feedback is still a significant problem. (This explains why an open device like the Eargo with microphones close to the receiver AND a very open fit have a particularly difficult challenge.) Feedback reduction algorithms help with the problem, but there’s only so much they can do. Many times to prevent feedback the audiologist will reduce the gain of the device in specific frequency ranges that cause squealing. This helps solve the problem, but at the expense of lower device performance (Kuk, 2005). Different ear tip sizes are used to help with fitting problems, with the trade-off that larger tips reduce feedback but increase occlusion (Groth, 2012).
Bluetooth earbuds
Recently Bluetooth earbuds have started adding hearing enhancement functionality. Sometimes this feature is advertised to provide ‘environmental awareness’ for safety or brief conversations. Occlusion is particularly relevant to the discussion of these devices when they’re used as low end hearing aids. Earbuds are mainly designed and used for listening to music. Additionally, many newer models include Adaptive Noise Cancellation (ANC) for reducing environmental noise. Both music playback and ANC work best when the ear canal is completely sealed. These earbuds actually try to maximize occlusion, which is a problem when they wander into the hearing enhancement field because of the annoyance this causes in hearing aid mode.
Some Bluetooth earbud manufacturers seem to be consigned to the problem and explain that occlusion with their devices is like a new pair of tight shoes that the user just has to get used to. Other manufacturers acknowledge the issue and are at work on technological solutions. Given the popularity of open fit hearing aids, it will be interesting to see how Bluetooth earbuds with strong occlusion are received as they add hearing aid functionality. As a known problem that’s been meticulously studied and documented by the hearing aid research community for decades, it would be very surprising if it could be swept under the rug at this point.
References
Conrad, S.A.; (2012) Perceived occlusion and comfort in receiver-in-the-ear hearing aids; 2012, Dissertations. 48.
Groth, J., et al; (2012) Open-Fit Hearing Instruments: Practical Fitting Tips; Hearing Review November, 2012.
Kuk, F., et al; (Feb. 2005) Efficacy of an Open-Fitting Hearing Aid ; Hearing Review (February 2005).
Kuk F., Keenan D., Lau C. C.; (Nov. 2005) Vent configurations on subjective and objective occlusion effect. Journal of the American Academy of Audiology 16(9), November 2005.
Powers, T., et al; (2018) Clinical Study Shows Significant Benefit of Own Voice Processing; Hearing Review, February 2018.
Staab, W., et al; (2004) Measuring the Occlusion Effect in a Deep-Fitting Hearing Device; Hearing Review, December 2004.
Stevenson, D., et al; (2014) Spatial Design of Hearing Aids Incorporating Multiple Vents;Trends in Hearing, May 2014.
Stuart, A., Allen R., Downs, C. R., Carpenter, M.; (1999) The effects of venting on in-the-ear, in-the-canal, and completely-in-the-canal hearing aid shell frequency responses: Real-ear measures.; Journal of Speech, Language, and Hearing Research 42(4), August 1999.
Studebaker, G.A., Cox, R.M.; (1977) Side branch and parallel vent effects in real ears and in acoustical and electrical models; Journal of the American Auditory Society 3(2),1977.
Vasil, K.A.,Cienkowsk, K.M.; (2006) “Subjective and Objective Measures of the Occlusion Effect for Open-Fit Hearing Aids”; Journal of the Academy of Rehabilitative Audiology, Vol. 39, 2006.
Zwislocki,J.; (1953) “Acoustic Attenuation between the Ears”; The Journal of the Acoustical Society of America, Volume 25, Number 4, July 1953.