HEARING MUSICAL ACOUSTICS Science of Sound Chapter 5 Further reading: “Physiological Acoustics” Chap. 12 in Springer Handbook of Acoustics, ed. T. Rossing (2007) “Effect of Noise on People” Chap. 31 in Science of Sound.

MUSICAL ACOUSTICS

The human ear is a remarkably sensitive (and delicate) organ. It responds over an intensity range of more than (1,000,000,000,000) and a frequency range of nearly 1000 (or more than 9 octaves)!

Human vision is remarkable, too, but the frequency range doesn’t begin to compare with that of human hearing. The frequency range of vision is about one octave (4x10 14 to 7x10 14 Hz).

RANGE OF HEARING

STRUCTURE OF THE EAR Outer, middle and inner ear

THE EAR MIDDLE EAR: THE OSSICLES

EAR FUNCTION

THE COCHLEA

ORGAN OF CORTI Resting on the basilar membrane is the delicate organ of Corti, a gelatinous mass about 1 ½ inch long. This “seat of hearing” has several rows of tiny hair cells, to which are attached nerve fibers.

OUTER AND INNER HAIR CELLS RESTING ON THE BASILAR MEMBRANE IS THE DELICATE ORGAN OF CORTI, WHICH INCLUDES SEVERAL ROWS OF TINY HAIR CELLS TO WHICH ARE ATTCHED NERVE FIBERS: ABOUT 4000 INNER HAIR CELLS IN A SINGLE ROW AND ABOUT 12,000 OUTER HAIR CELLS IN SEVERAL ROWS. EACH HAIR CELL HAS MANY HAIRS OR STEREOCILIA. BENDING OF THE STEREOCILIA STIMULATES THE HAIR CELLS, WHICH IN TURN EXCITE NEURONS IN THE AUDITORY NERVE.

OUTER AND INNER HAIR CELLS RESTING ON THE BASILAR MEMBRANE IS THE DELICATE ORGAN OF CORTI, WHICH INCLUDES SEVERAL ROWS OF TINY HAIR CELLS TO WHICH ARE ATTCHED NERVE FIBERS: ABOUT 4000 INNER HAIR CELLS IN A SINGLE ROW AND ABOUT 12,000 OUTER HAIR CELLS IN SEVERAL ROWS. EACH HAIR CELL HAS MANY HAIRS OR STEREOCILIA. BENDING OF THE STEREOCILIA STIMULATES THE HAIR CELLS, WHICH IN TURN EXCITE NEURONS IN THE AUDITORY NERVE. INNER HAIR CELLS TRANSMIT SIGNALS TO THE AUDITORY NERVE FIBERS

OUTER AND INNER HAIR CELLS RESTING ON THE BASILAR MEMBRANE IS THE DELICATE ORGAN OF CORTI, WHICH INCLUDES SEVERAL ROWS OF TINY HAIR CELLS TO WHICH ARE ATTCHED NERVE FIBERS: ABOUT 4000 INNER HAIR CELLS IN A SINGLE ROW AND ABOUT 12,000 OUTER HAIR CELLS IN SEVERAL ROWS. EACH HAIR CELL HAS MANY HAIRS OR STEREOCILIA. BENDING OF THE STEREOCILIA STIMULATES THE HAIR CELLS, WHICH IN TURN EXCITE NEURONS IN THE AUDITORY NERVE. INNER HAIR CELLS TRANSMIT SIGNALS TO THE AUDITORY NERVE FIBERS OUTER HAIR CELLS ACT AS BIOLOGICAL AMPLIFIERS. WHEN THEIR STEREOCILIA ARE BENT, THE CELL CHANGES ITS LENGTH AND PUSHES AGAINST THE TECTORIAL MEMBRANE, AMPLIFYING THE SOUND. OUTER HAIR CELLS ADD ABOUT 40 dB OF AMPLIFICATION.

In addition to the air conduction, sound can reach the inner ear by bone conduction. The principle of bone conduction is used in some hearing aids and other listening devices. Sing “aa-oo” with fingers stopping your ears.

In addition to the air conduction, sound can reach the inner ear by bone conduction. The principle of bone conduction is used in some hearing aids and other listening devices. The vibrations in compressional bone conduction are created by stimulation of sensory cells by high-frequency sounds as they compress the bony case around the inner ear. Inertial bone conduction involves the vibration of the entire skull as it reacts to low-frequency sound waves

Bone conduction is one reason why a person’s voice sounds different to him/her when it is recorded and played back (or on the telephone). Because the skull conducts lower frequencies better than air, people perceive their own voices to be lower and fuller than others do, and a recording of one’s own voice frequently sounds higher than one expects it to sound.

RESPONSE OF THE BASILAR MEMBRANE TO A PAIR OF TONES AS THE INTERVAL DECREASES THEIR RESPONSE CURVES SHOW INCREASING OVERLAP

CRITICAL BANDWIDTH CONSTANT AT LOW FREQUENCY BUT PROPORTIONAL TO f AT HIGH FREQUENCY

SOUND SOURCE LOCALIZATION

PRECEDENCE EFFECT (Also called “Haas effect” or “Law of the first wave front”) THE SOURCE IS PERCEIVED TO BE IN THE DIRECTION FROM WHICH THE FIRST SOUND ARRIVES, PROVIDED THAT: 1.Successive sounds arrive within ~ 35 ms; 2.Successive sounds have spectra and time envelopes similar to the first sound; 3. Successive sounds are not too much louder than the first sound.

LOGARITHMS Logarithms are so important in understanding sound and music that it is worthwhile to take time to review or introduce them. The logarithm to the base 10 of a number x is the power to which 10 must be raised to equal x. For example, 100=10 2 so the logarithm of 100 (to base 10) is 2 (log 100 = 2).

LINEAR LOGARITHMIC

LOGARITHMS AND POWERS OF TEN

PRACTICE PROBLEMS (no calculator, please) 2.1x x10 -2 = 3.0x10 8 /300 = log 8 = log 2x10 7 = log ½ = log 1/5 = log 200 = log 400 = log 500 =

SOLUTIONS TO PRACTICE PROBLEMS 2.1x x10 -2 = = x10 8 /300 = 10 6 log 8 = log 2 3 = 3 log 2 = log 2x10 7 = log 2 + log 10 7 = log ½ = -log 2 = log 1/5 = log 2/10 = = log 200 = log 2 + log 100 = log 400 = log 2 + log 2 + log 100 = log 500 = log 1000 – log 2 = 3 – = 2.699

DEPENDENCE OF SUBJECTIVE QUALITIES OF SOUND ON PHYSICAL PARAMETERS

ASSIGNMENT FOR MONDAY, JAN. 25 Exercises 1-9 (p ) Read chapter 6 Sound Pressure, Power, and Loudness Plot hearing curve using the UNSW website (do it twice)