Ultrasonics – Part 2

Last week’s blog was about acoustic tractor beams and even though only a measly 9 watts was required, it did use a 40,000 Hz signal at very high sound levels.

A stimulus such as 40,000 Hz (40 kHz) is considered an ultrasonic sound because it exceeds the hearing threshold of normal hearing young people. Unfortunately I can only hear up to 15,500 Hz but I am still quite young… age 61 is the new 31!  So I guess for me, an ultrasonic sound is anything above 16,000 Hz.

Ultrasound emitters can be found in many shopping malls and at convenience stores. A device called a Mosquito is a small box that emits a high pitched sound at about 18,000 Hz.  I can’t hear it when I go in to buy potato chips (I mean bread and carrots), but the 15 year old teenager hanging around outside the door is quite uncomfortable.  Presumably a 61 year old is more welcome at a convenience store than a 15 year old.  Whether this is fair or not, it is used commonly.

What, if any, are the effects on the human body of ultrasonic sound? Can high sound level ultrasound affect our hearing?  What about the effect on our body tissues?

The most common use of ultrasound in today’s world is for medical imaging. When it comes to ultrasound and imaging, I always think of the old question- what is the circumference of the British Isles?  While these two subject areas may appear to be quite unrelated, as we will see, they are intimately connected.

If one were to measure the circumference of the British Isles with a yard stick (or a meter stick in Canada) one would start early in the morning on a sunny day in April at Dover on the south coast, and perhaps by the end of the summer, you, and your friends would have a figure, in yards. Next summer you do it again, only this time use a foot ruler instead of a yard stick.  After working all summer and perhaps well into the fall, you would have another figure- a figure that is much greater.  With the foot ruler you were able to get into all of the nooks and crannies up and down the coast and measure those with greater precision than with the yard stick.

If you were to continue that with one inch longer ruler, the circumference would be even greater- the smaller the measuring stick, the more accurate and greater the measured circumference would be.

The same is true of ultrasound or using any sound (or light) to measure something. Low frequency sound has a long wavelength and it simply does not see obstructions that are much smaller than ½ of their total wavelength.  Higher frequencies acoustically “see” these obstructions that were previously missed; and even higher frequencies “see” even more.  When it comes to pathologies or objects that are on the millimeter or even micrometer scale, the frequency needs to be very, very high; perhaps on the order of MHz or GHz.

Acoustically anything over 20,000 Hz can be considered ultrasound, but in medical imaging, the probe frequency needs to be at least 2 MHz, and for very fine resolution can be on the order of 2-3 GHz.

Although there have been many studies over the years, each more modern study uses the latest assessment tools. In an article by Claude Harbarger, Paul Weinberger, Jack Borders, and Charles Hughes, in the Journal of Otolaryngol Head Neck Surg. 2013, the authors retroactively examined the files of 100 neonates who had undergone various prenatal ultrasound screenings, and correlated this with a number of physiological measures, such as otoacoustic emission (OAE) testing.

The short answer is that they found no correlation. The medical imaging ultrasounds that were performed were not found to be damaging in any way.  The authors do admit that there are some limitations to their study, one of which is that there can be a mild sensori-neural hearing loss and still have normal OAE test results.

However, a new Swedish study found that all is not peachy-keen.  While it is probably true that ultrasound is perfectly safe on the hearing, especially while still in utero, a new study based on over 1.4 million children born in Sweden between 1986 and 2008 had an “80% increased risk in noisy occupational environments”.

This noise however is not ultrasound- it is the “routine” occupational noise that every worker is exposed to in industry.

In a comparison of two groups (those working in environments with <75 dBA exposure and those working in environments with >85 dBA) there was a significant increase in hazard.

When interviewed about the study by Audiology World News:  “The Swedish Work Environment Authority recommendation is that pregnant women should avoid noise levels of over 80 dBA, but unfortunately this recommendation is not always followed,” says Jenny Selander, lead author for the study. “Our study shows how imperative it is for employers to observe this recommendation. Even if pregnant women themselves use ear protectors in noisy environments, the babies they’re carrying remain unprotected.”