HEAR THE MUSIC

Marshall Chasin, Hearing Health Matters Marshall Chasin, Editor
"This is the best of blogs; this is the worst of blogs'. To paraphrase Dickens’s A Tale of Two Cities, it is the two-headed nature of music and hearing aids that a hearing aid can be great for speech yet useless for music, and, conversely, great for music yet less than optimal for speech. What are some tricks that can be used to improve a hearing aid for music? How can we prevent hearing loss from loud music? This blog asks these questions, and with your input, even more. Comment Policy


Whales, dolphins, and porpoises are really neat mammals. They are evidence that our common ancestor exited the primordial sea; some became land dwelling mammals, but a few returned to the sea and re-adapted to their water-based environment. One can think of this as evolution going backwards, or maybe, evolution doesn’t have a direction and we should just be thinking in terms of “change”. I am sure that Chuck Darwin would support the later perspective.
Photo courtesy of www.en.wikipedia.org

Photo courtesy of www.en.wikipedia.org

It is true that whales and their close mammal cousins don’t have to work in a factory, listen to MP3 players, or ride snowmobiles, but it is also true that they can be exposed to very noisy life under the sea. This has led many groups to examine the ecology and effects of under-water noise on marine life, and this includes the Acoustical Society of America.

The effects of environmental noise on marine mammals and marine life in general are poorly understood- a complete understanding would necessitate good models of water surface tension, underwater sound propagation over large distances, and the susceptibility of various species to this (ship based) noise exposure. Coming to grips with these issues and physical phenomena are a life’s work and indeed many dedicated acousticians and biologists are doing just that.

In a recent study in Biology Letters  American and UK scientists examined the feeding and diving behaviors of ten humpbacked whales and found that the whales dove less deeply and had fewer side feeding periods when noisy ships passed overhead. During diving, humpbacked whales roll to the side in order to feed, and the frequency of these rolls was also significantly reduced during shipping traffic.

Photo courtesy of www.dailymail.co.uk

Photo courtesy of www.dailymail.co.uk

The scientists used sensors attached to the whales that simultaneously measured elements of movement as well as the sound level of the noise over time.

The authors found that “These results are among the first [to] support that ship noise can impact humpback whales’ foraging, making this source of disturbance a management concern,” and that “Chronic impacts of even small reductions in foraging efficiency could affect individual fitness and translate to population-level effects on humpback whales exposed to ship noise in critical foraging areas.”

The authors noted that these aberrant feeding and diving behaviors were noted at night as well as in daytime even though there is significantly less night time shipping traffic.

In part 1 of this blog series, we looked at why a clarinet sounded different than a soprano saxophone. Both are “closed” at the mouth piece end and “open” at the end of the bore. Both are identical lengths, yet one functions as a quarter wavelength resonator and has odd numbered multiples of its resonances (clarinet) and the other functions as a one half wavelength resonator (saxophone, and its conical cousins, the oboe and bassoon) with integer multiples of its resonances.

Picture courtesy of www.allmusic.com

Picture courtesy of www.allmusic.com

The difference is that the clarinet is a constant diameter cylinder and the saxophone is a conical instrument that gradually gets wider as one gets closer to the bore. The physics is based on the pressure wave, or equivalently, if you want to shift phase, the volume velocity, and it varies according to sin(x) for a cylinder such as a clarinet, but varies according to sin(x)/x for a cone shaped resonator.

Physicists typically talk in terms of “pressure” and audiologists (and speech scientists) talk in terms of “volume velocity”.  They are the same thing, but 180 degrees out of phase with each other.

Despite the apparent physical similarity, the clarinet and the soprano saxophone are dramatically different animals and indeed sound quite different: one is a cone and the other is a cylinder. A cone functions as a one half wavelength resonator (F = kv/2L) and a cylinder functions as a one quarter wavelength resonator (F = (2k-1)v/4L).

So,… is the human ear canal a cylinder or a cone?

In audiology textbooks and many pretty pictures we have in our clinics, the ear canal is usually drawn as a cylinder with the ear canal, being “open” at one end and “closed” at the tympanic membrane end.  It has its first mode of resonance at 2700 Hz in the adult unoccluded condition. And for those who have real ear measurement systems that can assess up to 10,000 Hz (or for those who have standard clinical real ear measurement systems, and whose patients have a low 2500 Hz resonance), the second mode of resonance is at three times the first one…. So far so good.

A constant diameter cylindrical ear canal.  Figure courtesy of www.slideplayer.com

A pretty picture of a constant diameter cylindrical ear canal. Figure courtesy of www.slideplayer.com

This odd numbered multiple is what would be expected from a quarter wavelength resonator cylinder.

But anatomically the ear canal is not a uniform diameter cylinder. There are parts where it is conical and other parts where it is indeed uniform.

It is not unusual for a resonator to have characteristics of all three types of resonators: a quarter wavelength resonator, a half wave length resonator, AND a Helmholtz resonator- one does not obviate the other.

It is also quite possible that the so called “concha related resonance” at about 4500-5500 Hz is not totally a concha or volume related issue, but actually the first mode of resonance of a half wave length resonator (which would be at exactly 2 x 2700 Hz or 5400 Hz). I have touched on this possibility in previous blogs and have written a more complete article on this .

Anyone can perform this simple experiment- take a wad of putty or slightly chewed chewing gum and use it to fill up the concha of a person, then perform a real ear measurement of their unoccluded ear (REUR). Indeed for some people, there will be a high frequency loss of energy (because you deleted the concha resonance) but for others (such as myself), there is no measurable change.  If indeed, there is no measurable change and the concha is indeed obstructed, then we are measuring the first mode of resonance of the ear canal where it is functioning as a one half wave length resonator.

If one were to perform this experiment on KEMAR (or simply remove KEMAR’s pinna), there would be a significant high frequency loss. KEMAR uses a cylindrical (Zwislocki coupler) and not one whose shape mimics that of the human ear.

Real ear canals can be cylindrical and also conical.  Figure courtesy of www.123rf.com

Real ear canals can be cylindrical in some parts and also conical in other parts.   Figure courtesy of www.123rf.com

This is not a trivial issue. Many manufacturers have come out with products over the years where either the concha was not occluded in hopes of maintaining the 4500-5500 Hz “unoccluded concha resonance” or placing the hearing aid microphone in that portion of the outer ear in order to pick up some pre-amplified high frequency sound energy.

If indeed this second bump we see on the real ear unaided response (REUR) is related to the earcanal and not the concha, this free pre-amplification may not even be there if the ear canal is occluded in any way.

And as a trivia question- what other FFT spectrum, other than a quarter wavelength resonator, has odd numbered multiples of the primary? (Hint- the magnitude of the higher frequency elements decrease by a factor of 1/f).