Brian C Moore, PhD from Cambridge in the UK recently had a paper published in the International Journal of Audiology (IJA) with the title “A review of the perceptual effects of hearing loss for frequencies above 3kHz”. The IJA is the official journal of the International Society of Audiology and the Society holds biennial international conventions. The most recent one was in Vancouver, Canada and brought together over 1300 participants from 26 countries. The next will be in Capetown, South Africa in 2018.

But, back to the paper. This is an interesting article for several reasons:  it’s a nice review of those studies from the last century where speech was low pass filtered below 3000 Hz with a subsequent degradation of speech intelligibility; more recent work showing that speech intelligibility improved with a wider bandwidth extending above 3000 Hz; speech intelligibility continues to improve with bandwidths extending beyond 5000 Hz; and an improved ability to separate speech from noise if they are separated when information is provided in the 6000-10,000 Hz region.

Figure courtesy of www.audiometrictesting.co.uk

Figure courtesy of www.audiometrictesting.co.uk

This paper is a reminder that degradation of speech intelligibility is a multi-factorial issue that depends on the nature of the noise (steady state vs. fluctuation), spatial separation of the signal and the noise, and required pure tone threshold bandwidth (amplified or not).  And in addition Dr. Moore reminds us of more recent research from Dr. Sharon Kujawa and her colleagues showing that cochlear pure tone sensitivity is only part of the story- neural degradation is also a major factor even in regions of normal cochlear function (at least for pure tone detection such as on the audiogram).

This paper is also interesting since it functions as a review of the relevant research to demonstrate the simplistic and ad hoc nature of compensation for hearing loss caused by occupational noise (industrial or music).

In Canada, and many other jurisdictions, noise exposure can be compensable based on an arithmetic calculation of several pure tone thresholds.  Some countries like Canada have this legislation implemented on a provincial (a local level) and other countries have federal mandates such as the United States and England.  Depending on where a worker may live in Canada, a hearing loss of 25 dB (with any combination of pure tones ranging from 500 Hz to 3000 Hz) may (or may not be) compensable.  Some provinces (e.g. Quebec) require exposure to 90 dBA for at least 5 years, whereas others (e.g. Ontario) require exposure to 85 dBA for 5 years.

Some countries make age adjustments (and even adjustments for gender) according to published tables, and other countries and jurisdictions merely subtract 0.5 dB from the average for every year older than 60.

There is limited science underlying these decisions made for the wide range of compensation approaches as evidenced by the variability around the world.  A worker in a mine in England is still subject to the identical exposure as a worker in a mine in Canada, yet compensation calculations are widely divergent.

Courtesy of www.quickanddirtytips.com

Courtesy of www.quickanddirtytips.com

I don’t necessarily see a simple solution to this problem.  Compensation levels and action levels around the world are as much about politics, public policy, and economics as they are about science.  But I wonder if advocating for inclusion of pure tone frequencies above 3000 Hz may get a bit closer to the real situation?

Realistically I don’t think that measuring the wave I delay in an ABR or calculating the SP/AP ratio would be clinically feasible.  Both of these approaches are useful for determining what the world is starting to call “hidden hearing loss”.  Hidden hearing loss is communication breakdown somewhere in the auditory system despite having normal pure tone thresholds on an audiogram.

But perhaps the inclusion of 4000 Hz and 6000 Hz in a compensation calculation may move us one step closer to reality.

I just returned from the Association of Independent Hearing Healthcare Professionals (AIHHP) conference in Nottingham, England, but I didn’t see Robin Hood, or had any time to go to Sherwood Forest.  I did however give several talks about musicians, and so did our HearingHealthMatters.org editor Brian Taylor; he spoke first and by the time I arrived in Nottingham, Brian was off having dinner with Friar Tuck or Maid Marion.

The gala event of the AIHHP conference included the Golden Lobes presentations and there were a number of them given out to AAHIP members and industry personnel who have gone far beyond the scope of their duties.  And one was given out for the best talk:  I lost and Brian won.

Incidentally the award was accepted by Dr. David Baguely on behalf of Brian (who was still off somewhere with the good Friar).  David Baguely was last year’s winner who could not be present to accept his award.  The awards were given out by the honorary president of the AIHHP- Dr. Brian C. Moore.

Nevertheless here is a picture of the loser (me) in front of two large golden pinnae cupping my ears and faking the benefits of the pinna effect.  Two things come to mind: who stores these gigantic pinnae over the year and brings them to the conference, and what would the effect be on hearing for such large sized pinnae?

marshall-and-golden-lobes

 

Well, it turns out that these pinnae are not really made of gold and are actually a fraction of an inch thick.  Given the low density and small diameter, the effect is actually quite low- cupping your hands behind the ears would result in more of an effect. But let’s take a step back.

The pinna effect is an enhancement of the shorter wavelength (higher frequency) sounds because of the constructive interference between the incident sound entering the external ear canal and the early reflection off of the pinna (or the cupped hand).  Only the higher frequency sounds (typically above 1000 Hz) are re-enforced and enhanced because the pinnae only “see” high frequency sounds – sounds that have relatively short wavelengths.

As a rule of thumb, a sound is obstructed (and subsequently reflected) if the obstruction is on the order of ½ the wavelength of the sound.  Low frequency sounds would require obstructions that are closer to a meter, whereas higher frequency sounds reflect in the cm or mm thickness range.  The diameter of the obstruction such as the pinnae or one’s hand will define the start frequency for which all sounds above that point are enhanced, and how much depends on the density of the obstruction. 

A solid gold pinna would indeed be a wonderful reflector whereas a thin cardboard replica of a pinna would only be good for having pictures taken in front of.

In some sense this could have been another solution for Archimedes when asked by his King to determine whether an irregularly shaped crown was indeed made of gold as claimed by the royal jeweler.  Archimedes immersed the crown in a vat of water and found out how much water was displaced.  Given the known weight of the crown, Archimedes was able to determine the density (volume/weight).  I won’t tell you how the story ends other than that the royal jeweler found himself in jail later that day.

Archimedes could have held the crown up to his ear and using a probe tube microphone (with the reference microphone disabled) he could have determined the density by its reflectivity.  Somehow the crown in a vat of water story sounds better though.  I guess real ear measurement is still too new to make it into the history of science.

So, the take home message is that if you stand in front of a large pair of golden colored ears, if they do indeed enhance the sound, then they may actually be made of gold (or at least a denser material than cardboard).