One of the pillars of audiology is the notion of “normal hearing” as shown by pure tone audiometry. Yet, hearing health care professionals know that audiograms are not the best predictors of some aspects of hearing, especially the common task of listening to speech in a challenging environment (Hearing Journal. 2000;53:46 http://bit.ly/2lcL58G). It seems likely that simple pure tone detection is unable to predict the complexity of supra-threshold speech perception. But there is another possibility. In this article, we ask: Why do we test hearing using pure tones only up to 4 or 8 kHz when we know very well that healthy young people can hear extended high frequencies (EHFs) up to 20 kHz?
EHF HEARING MYTHBUSTERS
The audiogram has always been the workhorse of audiology, and it has served us well. So why should we bother with EHF? Here are some common myths:
- “There is little signal energy above 8 kHz.” Most mammals make extensive use of EHF. We can use EHF to distinguish speech sounds, and may employ that energy under extreme challenges, such as in noisy, highly reverberant rooms (J Acoust Soc Am. 2015;137:EL65 http://bit.ly/2jVGHWt).
- “EHF thresholds are unrealiable.” Reliability measures, even in young children, show that listeners perform as well in EHF threshold judgements as they do in judgements within the conventional range of frequencies (Ear Hear.1996;17:1 http://bit.ly/2lh1mph).
- “Our audiometers/earphones only work up to 8 kHz.” Time to update your equipment. Modern audiometers allow testing up to at least 12.5 kHz. When updating tools, insist on getting an audiometer with a capability of up to 16 kHz (20 kHz for pediatrics). Norms for EHF hearing are available for professional headphones (e.g., Interacoustics DD 45; Sennheisser HD 280; Int J Audiol. 2010;49:850 http://bit.ly/2lgO0JE).
- “There is no evidence that EHF is useful.” There is substantial evidence, old and new, that when EHF hearing is lost, so is optimal hearing (Audiology. 1981;20:347 http://bit.ly/2kknJfH;Ear Hear. 1996 http://bit.ly/2lh1mph). In fact, EHF may be the single most important source and the most easily measured index of hidden hearing loss (including cochlear synaptopathy, which are hearing difficulties that occur despite having normal audiograms (PLoS One. 2016;11:e0162726 http://bit.ly/2jV9Tll;Ear Hear. 2015;36:24 http://bit.ly/2kkibC2).
- “This will extend our testing time.” Not necessarily. Here are two ways we could make up time for EHF testing. First, eliminate bone conduction at 4 kHz. The data are of limited value and false air-bone gaps occur (Int J Audiol. 2013;52:526 http://bit.ly/2kGnU5Z). Second, eliminate speech reception thresholds. They provide little information in a reliable patient. If the schedule is tight, you could simply add one EHF (e.g., 12 kHz) to the normal test frequencies.
GOOD REASONS FOR TESTING EHF
EHF testing can open doors to further diagnostic understanding. Be bold and think not about the challenges, but about the opportunity.
- Early warning. Knowing that you have EHF loss would be a great wake-up call, for active monitoring, prevention (e.g., ear protection), and intervention (below).
- Understanding unexplained difficulty. Middle-aged people frequently have listening difficulties that are unexplained by current audiological practice. A finding of EHF loss would provide at least a partial explanation.
- Professional service. Clinics and manufacturers could specialize in delivering these exacting measurements and follow-up management, including interventions.
- Research opportunity. There are many questions to be addressed, for example: What are its specific mechanisms? Can we develop simple, self-administered ways to screen for or measure EHF loss? What is the impact of EHF loss generally and in specific populations (e.g., older people, ototoxicity, tinnitus, chronic OME, APD, pediatric)?
Hearing aids provide amplification that is only up to about 6 kHz. Squeezing very high frequency energy from conventional hearing aids into the ear is not possible and standing waves are difficult to control. Can we address these and other EHF management challenges?
- Is this a real hearing loss? Yes, definitely. But we could end up with nearly everyone having a hearing loss (e.g., if testing to 20 kHz or above). So what do we do? Keep calm and carry on–and consider the following:
- Amplification. In principle, amplification could be useful in providing additional energy. Conventional hearing aids do not provide EHF amplification, but some other devices do. For example, the Earlens light-driven hearing aid provides energy in excess of 10 kHz.
- Active monitoring. Knowing that you have EHF loss could mark the beginning of regular reviews, say every six to 12 months. It may be possible to conduct these “test and talk” sessions remotely.
- Ear protection. An early finding of EHF loss could signal a specific and robust recommendation to use physical protection and avoid noisy environments.
- Behavioral intervention. EHF loss could be especially suited to novel forms of therapeutic experience, such as frequency transposition of EHF.
- Reposition the headphones. Standing waves can be an issue when using very high-frequency signals. A simple cross-check is to remove, then reposition the headphones before making verification measures.
WHAT SHOULD WE DO RIGHT NOW?
Unlike many research developments, this one is easy to implement. Go on, be the first kid on the block.
- Develop strategy. Seek out information, then educate your colleagues about EHF. This article has been written with clinical strategy in mind. Agree on a realistic and proportionate approach to include testing and management.
- Retool your clinic for EHF audiometry. Get the right audiometer, headphones, and calibration for testing up to 12, 16, or 20 kHz (see Notes).
- Contact your regulator and professional society. Respond to local conditions. Educate your referral services and perhaps suggest candidate patients. Ask your business contacts about reimbursement. Does your regulator support EHF? Lobby your professional society to make EHF a priority issue. Ask them to include EHF in revised practice guidelines.
- Get testing! What are you waiting for? See for yourself if any of your puzzling patients has EHF hearing loss, and discuss management options.
Notes:Headphones: As above. Note that the Etymotic research ER-3A insert earphone, popular in clinical practice, is not suitable for delivering EHF stimuli.
Standards: ISO 389-5 provides RETSPL for EHF but most of the earphones (e.g. HDA200) are no longer available. The equivalent ANSI standard for RETSPL up to 20 kHz is S3.6-2010
Revised ISO 389-1 will provide ref values for the whole range of test frequencies including EHF for different earphones including HDA280. Sennheisser provide norms for HDA300.