lecture 7 limits of hearing, critical bands sound localization hearing & vision compared ...

Lecture 7Limits of hearing, critical

bandsSound localizationHearing & vision comparedLoudness of a pure tonePitch of a pure tone

Instructor: David Kirkby ([email protected])

Physics of Music, Lecture 7, D. Kirkby 2

MiscellaneousHomework #3 is due on Thursday.

There will be no new homework assigned on Thursday.

The midterm will be held in class Thursday Oct 31 and will cover the material in Lectures 1-8 and Homework 1-3.

We will have a review session during class this coming Thursday (Oct 24).

The final exam is scheduled for Friday Dec 13 from 10:30am-12:30pm.

Conflicts with present office hours? (Wed 9-11am).Replace 9-10am with 12-1? 1-2? 3-4?


Review of Lecture 6In this unit, we are exploring the relationship between the physical and psychological attributes of musical sound.

We took a tour of the human ear by following a sound wave on its path from the pinnae through to the signals sent to the brain on the auditory nerve.


Our hearing capabilities are primarily determined by the mechanical response of the basilar membrane in the cochlea.

The basilar membrane sends signals to the brain that encode where it vibrates (ie, the sound’s wavelength) and the rate at which it vibrates (ie, the sound’s frequency). These two pieces of information are normally redundant.


The Limits of HearingThe conventional wisdom is that our hearing covers the range from 20-20,000 Hz.

In fact, most people’s hearing covers a smaller range.

A quiet sound is harder to hear than a loud sound of the same frequency. Therefore a description of hearing range must specify the sound intensity as well as its frequency.


We are most sensitive to sounds in the range 2000-5000 Hz where the ear canal acts as a resonator.

Speech relies on a relatively small range of frequencies andintensities. Music places greater demands on our hearing.

Frequency (Hz) Sourc

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Hearing range decreases with age. This decrease is generally faster for men than for women, especially at high frequencies.

Source: H. Tremaine, Audio Cyclopedia, Howard Sams and Co., 1978


Demonstration of Effects of Hearing LossHearing range also decreases after long-term exposure to background noise. This loss is usually most pronounced at intermediate frequencies in the range 2000-8000 Hz.

For more information, refer to Chapters 30-31 of yourtextbook.


InfrasoundSounds below the threshold of human hearing are called infrasounds.

At low intensities, infrasoundhas potential for long-rangecommunication and atmosphericmonitoring (in particular,detecting nuclear tests).

At high intensities, infrasound can cause dizziness and nausea (primarily via the semicircular canals of your inner ears) and even internal bleeding due to internal organs rubbing against each other.

CTBT IMS network


UltrasoundHigh frequency sounds above the range of human hearing are called ultrasounds.

Ultrasound is an important medical diagnostic tool:

Obstetric ultrasoundDoppler ultrasound


Hearing Range of AnimalsAnimals have a wide range of hearing capabilities.

Elephants are particularly sensitive toinfrasound.

Mice are particularly sensitiveto ultrasound, but so are whales!

http://shop.store.yahoo.com/animalmedicstore/sildogtrainw.html

See also: http://www.lsu.edu/deafness/HearingRange.html






http://www.lsu.edu/deafness/HearingRange.html








What is Your Range of Hearing?Listen to these pure SHM tones of different frequencies. How low a sound can you hear? How high a sound?

This demonstration needs a big speaker: why?

(We will cover some of the issues of speaker design in Lecture 16.)

The lowest note on a piano has a frequency of about 27 Hz but is easily audible. Is this consistent with the results of the test you just did?


Critical BandsThe basilar membrane in the cochlea responds to a pure (SHM) sound by vibrating at a position along its length that depends on the sound’s wavelength:

400 Hz

4000 Hz

http://www.sissa.it/bp/Cochlea/utils/twhi.htm

http://www.sissa.it/bp/_vti_bin/shtml.exe/Cochlea/utils/twhi.htm/map


Even for a sound of a single wavelength, the vibrations of the basilar membrane are spread out over a length of about 1.3 mm (compared with the total membrane length of 30mm).

Therefore, two sounds of slightly different wavelengths will result in overlapping vibrations (and therefore overlapping signals to the brain).

The correspond range of frequencies (band) is called a critical band.


Two pure sounds are harder to distinguish when their frequencies lie within the critical band since the brain must disentangle their overlapping signals.

Source: “The Science of Sound”, Fig 5.9, Rossing, Moore & Wheeler, 3rd ed.

Position along Basilar membrane


The human range of hearing is divided into about 24 critical bands (but the bands do not have fixed boundaries).

The size of the critical bandis about 100 Hz at lowfrequency and then increasesin proportion with frequencyat higher frequencies.

We will revisit critical bandsseveral times in this unit…

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Critical Bands and HarmonicsThe first six harmonics of C3 (128 Hz) are well separated, but higher harmonics are more difficult to distinguish.

Source: Campbell, M. and Greated, C. (1987). The Musician's Guide to Acoustics. New York: Shirmer Books.

As a result, we can expect that high harmonics contribute less to our subjective impression of a complex musical sound than the lower (1-6) harmonics.


Binaural HearingWe receive signals from two ears. How do these signals differ when listening to a single pure sound?

Listen to this SHM tone with your eyes closed, and try to determine its source.


Most people can locate a sound’s source with an accuracyof 1-10 degrees (1 degree is about the width of your thumb when held at arm’s length).

This ability is primarily based on two cues:•Timing differences•Intensity differences

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Timing Differences Between Each EarYour head is about 20cm wide, so sound arriving from the side takes about 600 microseconds ( = 0.6 msec) longer to reach your far ear than your near ear.

The direct sound arriving from a source located directly in front, above, or behind you reaches both ears at the same time.

Your brain can detecttiming differences assmall as 30 microsecondsbetween the signals fromeach ear.

Source: Mills, "Auditory Localization", in Tobias, ed.,Foundations of Modem Auditory Theory, Academic Press,

1972


Role of Reflections in LocalizationThe signals received by each are more complicated that we would predict based only on the different distances to the source of the sound.

Multiple reflections from the folds of the pinnae play an important role in shaping the sound heard from different directions, and provide additional cues to the brain.

Only sounds arriving from slightly behind you can enter your ear canal directly. Sounds are delayed by reflections the more the source is in front of your.

Sounds arriving from below are also delayed relative to those arriving from above.For more info see http://www.sfu.ca/sonic-studio/handbook/Binaural_Hearing.html

http://www.sfu.ca/sonic-studio/handbook/Binaural_Hearing.html




Intensity Differences Between Each EarTiming differences provide the main cue for localizing pure sounds with frequencies below about 1500 Hz.

For localizing higher frequency sounds, the attenuation of the sound heard by the far ear is the main cue.

Your head shadows high frequency sounds more than low frequency sounds because it absorbs high frequencies more and they diffract less.

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Head Tracking of Sound SourcesThe brain receives complex signals that encode several cues with information about the location of a sound source.

However, making sense of all this information is not easy!

One of the most powerful strategies is to turn your head and see how the timing and intensity of the sound changes.

Even with your eyes closed, you instinctively turn to face a localized source of sound.


Localization and HeadphonesDelivering a sound through headphones defeats most of our mechanisms for localizing the sound’s source.

Sounds all enter the ear at the same angle so there is no variation in the reflections from the pinnae.

The timing and intensity can be different for each ear, but turning your head does not vary these parameters.

How would you record & playback sounds to improve upon these limitations of traditional headphones?


VisionSound and light are both waves. How do our senses of hearing and vision compare?

Wave Sound LightFrequencyAmplitudeSpeed

PitchLoudness345 m/s

ColorBrightness3 x 108 m/s


Pure Waves of Sound and LightA pure sound wave is the result of a Simple Harmonic Motion and has a definite unique frequency. Most musical sounds are not pure, but are combinations of many sounds.

(colors are approximate!)

A pure light wave corresponds to a spectral color, ie, one produced by a prism or visible in a rainbow. Most colors (brown, pink, …) are not pure, but are combinations of spectral colors (more on this in the next Lecture).


Frequency Ranges of Hearing and VisionYou can hear pure sounds over a frequency range of almost 20-20,000 Hz. This corresponds to about 9 octaves of dynamic range.

You can see pure colors over a frequency range of about 400-700 GHz. This corresponds to just under one octave!

Middle C256 Hz

32 Hz 16384 Hz

hearing range

vision range


Physical Ingredients to Perceived Loudness

We can characterize the quantity of sound at each stage:

Sound = Production + Propagation ( + Detection)

The quantity of sound produced by a source is measured by its total power output level (usually given in Watts).

The quantity of sound propagating through the air at some point is measured by the amplitude of the pressure disturbance (usually given in N/m2).

The quantity of sound reaching a detector is measured as the power reaching a unit area (usually given in W/m2).


These three physical measures of the quantity of sound produced are all related.

For example, the detected sound intensity at a distance R from a spherical source source of total power W is given by:

I = W / (4R2)

The detected intensityvaries as the square ofthe pressure level:

I = (p/20)2

(p in N/m2, I in W/m2)


Combining these two relationships, we find that for a spherical source the sound pressure level drops off as 1/R with distance R from the source.

Try this demo in which speech is recorded 25, 50, 100, and 200 cm from a microphone:

The resulting sound pressure levels reaching your ear should (ideally) decrease by 1/2 each time and the intensity levels should decrease by 1/4 each time.(What effects might spoil this expected ideal behavior?)

How did your subjective impression of the loudness change with each step away from the microphone? Would you describe the decrease as more like 1/2 or 1/4?

Source: Auditory Demonstrations, #4,Houtsma, Rossing, Wagenaars (IPO-NIU/ASA)


Perception of Loudness for Pure TonesThe perceived loudness of a pure tone depends primarily on the amplitude of the pressure disturbance that it creates as it travels through air.

The contours in thisplot show thepressure levelsthat are (onaverage) perceivedto have equalloudness, fordifferentfrequencies. so

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What is Your Loudness Threshold?Low-frequency sounds appear quieter than high-frequency sounds of the same intensity.

Try this demonstration to hear this for yourself.

First calibrate the volume so that this 1000 Hz test signal is barely audible:

Next, listen to these 14 sets of repeated tones getting progressively quieter, and count how many steps you can hear:




Each tone is repeated 10 times with the intensity detected by your ear dropping by about 1/3 each time(-5 phons).

The tones are atfrequencies of125, 250, 500,1000, 2000, 4000,8000 Hz.

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We are most sensitive to sounds in the 3000-5000 Hz range where the ear canal responds resonantly.

The threshold of audibility is at a detected sound intensity of about 10-12 W/m2 and the threshold of pain is at an intensity of about 1 W/m2.

This is a huge dynamic range of 1:1,000,000,000,000 !


Degrees of Loudness in MusicWritten music often includes directions for how loud notes should be played (“dynamics”).

These directions are usually based on a scale of 6 different degrees of loudness: pp, p, mp, mf, f, ff.

The range of sound intensities (loudest/quietest)that orchestral instruments are typically capableof ranges from as little as 3x (English horn) up toalmost 100x (French horn).

However, most people need a range of at least 10x in order to identify six distinct loudness levels. Therefore six degrees of musical dynamics are wishful thinking for some instruments!


Perceived Loudness and Sound DurationHow does the perception of the loudness of a sound depend on how long it lasts?

Listen to this demonstration of sounds of decreasing intensity in 7 groups of decreasing duration:

This example uses noise rather than pure tones. Successive bursts of noise decrease in intensity in 8 steps. The duration of the bursts decreases after each group of 8: 1000, 300, 100, 30, 10, 3, 1 ms.



Experiments like these have shown that your brain measures the loudness of a sound during an interval of about 200 ms (1/5 of a second).

This means that sounds of equal intensity lasting 200 ms or longer have the same perceived loudness, but shorter sounds appear quieter.

200ms 200ms

This extra silence is averaged with the shortburst to give the impression of a quieter sound.

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SummaryWhether a sound is audible or not depends mostly on its intensity and frequency.

Critical bands measure the size of the region of your basilar membrane that vibrates in response to a pure tone.

The primary purpose of binaural hearing is to localize the source of a sound. Variations in timing (low frequency) and intensity (high frequency) are the main cues.

The perceived loudness of a pure tone depends mainly on the sound pressure level, but is also affected by the sound frequency and duration.


Review QuestionsWhat role does the response of your Basilar membrane play in your ability to distinguish pure tones of slightly different frequencies?

How do your senses of hearing and vision compare? We saw that hearing has a much larger dynamic range. In what ways does vision exceed the capabilities of hearing?

How much quieter does a sound appear when you move twice as far from its source?

Why can you easily hear the lowest note on the piano (with a fundamental frequency of 27 Hz) but cannot hear a pure tone with the same frequency?

lecture 7 limits of hearing, critical bands sound localization hearing & vision compared ...

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