This 4-part series of blogs overviews the phenomenon of masking and is written for the musician, not the audiologist.  The first three parts (upwards spread of masking, downwards spread of masking, and temporal masking) relate to the function and structure of the cochlea and associated neural structures, whereas this last part (phase) refers to the acoustics of any room.  Strictly speaking, phase issues are not related to “masking” in the typical sense but can be viewed as masking in a more general sense, since it can be responsible for a deletion of important information. In part 1, the characteristics of upwards spread of masking were discussed.  In part 2, the phenomenon of downwards spread of masking was discussed.  In part 3, temporal masking was discussed, and In this part, the beneficial and destructive characteristics of phase, are discussed.

I recall in the early 1990s, I was crossing a street in Stockhom, Sweden.  Stockhom is a beautiful city but very expensive.  At any rate, Stockhom was one of the first cities in the world to have auditory signals for people with low vision to assist in crossing the road. I recall pushing the crossing button and when the light turned green, a loud clicking began.  And interesting it also began at the other side of street for those people with low vision to cross in the other direction.

In this part of this four part blog series, we leave the ear and go into room acoustics

About half way across the street, the clicking stopped.  As I moved another couple of meters forward, the clicking began again.  And of course, as I am an intrepid clinical scientist, I moved back and forth in the intersection, much to the dismay of the motorists who wanted to proceed but couldn’t because some idiot tourist seemed to be doing a dance in the middle of the intersection.

I tried to explain my excitement to a passing motorist but all I go was some unusual hand movements aimed in my direction.  I presumed that this was the Swedish “hello”.  I did this several times just to make sure that I wasn’t imagining things and received similar “hellos” from a range of motorists.

Of course I was experiencing destructive interference caused by the phase in the signal being exactly 180 degrees different than the phase emanating from the other side of the road.  The signal was still there, merely being cancelled at that exact point in the busy intersection.

This was 1991 and despite the initial article about phase cancellation being in the literature since 1933, phase cancellation had not entered into the domain of noise control in cars, mechanical installations, or in hearing aids.

Its all about phase. Courtesy of www.phasecic.co.uk

Destructive interference caused by an unfortunate confluence with phase and intensity (yes- intensity and not sound level) occurs everywhere from the study of room acoustics to the speech production of vowels and nasals, and from musical instruments to Swedish intersections.

Strictly speaking this is not masking in the sense of the first three parts of this blog series but destructive interference has the same effect – it alters the signal of what is being perceived, not by covering it up as in upwards spread of masking or temporal masking, but by reducing (or enhancing) a particular range of frequencies.

One of the most important parameters in the design of concert halls is to achieve the correct balance of incident and reflected waves.  When the incident wave intersects an unfortunate reflected wave that happens to be about 180 out of phase and is of similar intensity, a null or quiet point can occur. I have heard of many cases where a musician can hear their own musical instrument quite well and then merely by tilting their head, creates a null or difference in the signal that enters their ear.

Sound waves interact with each other constructively where they add up and destructively where they can create nulls. Courtesy of www.uaudio.com

Of course these musical venues are well-known and musicians cringe when they are expected to perform there.

There are, however, a number of environmental modifications that can be made such as using a low hanging artificial ceiling (or cloud) over the performers to alter the phase relationship of the sound.  These tend to be rather inexpensive and can significantly improve the musical experience for both the performer and the listener.

And like the various other forms of altering the speech and music that we hear, whether its by masking or by phase destruction, too little can be as bad as too much.  A dead room with no reflections will not have any nulls caused by standing waves, but it also would be useless to listen to music and people talk.

 

The following 4 part series of blogs, overviewing the phenomenon of masking, is written for the musician, and not the audiologist.  The first three parts (upwards spread of masking, downwards spread of masking, and temporal masking) relate to the function and structure of the cochlea and associated neural structures, whereas the last part (phase) refers to the acoustics of any room.  Strictly speaking, phase issues are not related to “masking” in the typical sense but can be viewed as masking in a more general sense, since it can be responsible for a deletion of important information. In part 1, the characteristics of upwards spread of masking were discussed.  In part 2, the phenomenon of downwards spread of masking was discussed.  In this part, the characteristics of temporal masking are discussed.

Temporal masking sounds like something out of a Star Trek episode where some time travelling alien has altered the time line and things have changed.  The one thing about “time” is that it is linear.  Time moves forward at a well-define pace and, short of a black hole or other gravitational threat, cannot be altered.

Despite the great-sounding name of “temporal masking” however, the reality is not so interesting, but it does occur.

Temporal masking is a favorite topic of science fiction…. well maybe temporal infarctions but anything with the word ‘temporal’ in it sounds neat.

The time taken to travel to the local store from your home is about the same- it can however change if you speed (which I am constantly told by my local constabulary that I should not), or if there is some other obstruction that will delay you. 

Despite a given distance, the time of arrival can be quite different and this is what happens in our auditory system.

In the case of “upwards spread of masking” in part 1, or its closely related but distorted cousin, “downwards spread of masking” in part 2, these two phenomena were related to the function and the structure of the cochlea.

In the case of “temporal masking” this phenomenon is related to the structure and the function of nerves- in this case the VIII- auditory nerve that carries electrical impulses from the cochlea to the auditory cortex.

There is a well-researched body of evidence, and in fact a mathematical equation, that relates the diameter of the nerve, and whether it has myelin covering (a sheath, much like wire insulation) that defines these parameters with the neural conduction rate or speed of propagation of the nerve impulses.  Generally the thicker, and the better insulated a nerve is, the quicker rate of propagation.

Courtesy of www.theodessyonline.com

Since all humans are human, this rate should be rather similar from person to person.  There are some pathologies such as Multiple Sclerosis (MS) where there is a gradual de-myelination process that will slow down the speed of the neural impulse and it is therefore no surprise that some of the symptoms of MS are auditory in nature.

Another factor that impinges on neural conduction rate is the sound level of the speech and music- higher sound levels result in quicker neural conduction rates than quieter sounds- vowels and forte musical notes reach the auditory cortex before quieter consonants and quieter pianissimo level musical notes, given the same starting point.

This results in one sound arriving in the brain before another if it is significantly louder than a co-incident sound.  This temporal alteration occurs in the normally functioning ear and like the other forms of masking can serve to cover up elements of temporally adjacent sounds. 

This is a neural form of masking and unlike the spread of masking mentioned in parts 1 and 2 of this blog series, does not derive from the cochlear structure and function.

Like other forms of masking, too much temporal masking may be a deleterious as too little.

In part 4 of this blog series, we will discuss a fourth type of alteration that to a certain extent can be desirable at one level, but disastrous at another.