Psychological Attributes of Sound

We are essentially dealing here with the subjective qualities of sound: pitch, loudness and timbre. We have already noted that:

• frequency is approximately equal to pitch
• loudness is approximately equal to amplitude

It is important to recognize that they are not equal, one is objective, one is subjective and depends on characteristics of our ear and our ability to process and understand sound.

Pitch

Pitch is our perception of frequency. Although we often use the terms interchangeably, there are differences -- and we should be aware of them: frequency is a definite quantity measureable by an electronic instrument in cycles per second (Hz), whereas pitch is also dependent on the loudness, the conplexity of the sound and the relative frequency -- in other words, our perception can be altered by these factors -- we find it harder to distinquish pitches in the lower register, than in the higher. there are certain frequencies that are not heard as pitches. The range of the human ear is approx. 20 - 16000 Hz. Supersonic or ultrasonic frequencies are above our hearing range and usually cannot be perceived at all. Subsonic or infrasonic are below our hearing range and often perceived as rhythmic clicks rather than pitches. we tend to hear:

• pitch drops as loudness increases in a low frequency tone
• pitch rises as loudness increases in a high frequency tone

a sound must last a certain amount of time (depending on the frequency) before it can be perceived as a pitch rather than a click we will sometimes perceive a fundamental that is not present (if the proper overtones are present) motion affects the perception of sound -- this is the doppler effect -- we hear a ptich shift as an object moves past us. pitch acuity: this is the degree to which we can distinquish between pitches that are close together. In midrange we can discriminate at about .03%, at low and high frequencies at about 10% or more -- (it is harder to tune low and very high frequencies)

Loudness

Loudness is the subjective characteristic which deals with the magnitude of the auditory sensation. The physical quantity that is associated with loudness is sound intensity. Although increases in intensity are heard as increases in loudness, the perception also depends upon the frequency (the ear is more sensitive it certain frequency ranges)

Sound Intensity

This is the measurement of the amount of power in a sound at a given location. We don't usually think of sound in terms of usable power, and in fact it takes a large symphony orchestra to produce about 60 watts of power. Sound Intensity is measured in deciBels or dB. Decibels are a logarithmic scale. A sound intensity is expressed as a logarithmic ratio of a given sound to a reference sound (usually the threshold of hearing). For example, a sound 10 times the threshold of hearing would be 1 dB, (N.B. dB is used for many logarithmic meaurements in sound -- sound pressure, volume units etc -- these are not necessarily sound intensity) As a rule of thumb:

• each doubling of distance decreases sound level by 6 dB
• doubling the number of sound sources increase sound level 3 dB
• 0 dbthreshold of hearing
• 10 dbwhisper at 150 cm.
• 30-40 dbaverage background noise
• 80 dbheavy traffic
• 90 dblarge orchestral forte
• 115 dbvery loud thunder
• 120 dbthreshold of pain

Loudness (the perception of sound intensity) depends both on amplitude and frequency. Lower frequncies need more amplitude to appear to be the same loudness (Fletcher and Munson first discussed this is a paper in1933). There is a unit of measurement the phon which takes both elements into consideration and represents a loudness scale. (This is really a modification of a sound pressure scale). To double the loudness, the loudness level must be increased by 10 phons (SIL = 10dB) -- 10 violins = twice as loud as 1 violin.

Sound Quality or Timbre

This is the characteristic that enables us to distinquish between sounds having the smae pitch and loudness -- a piano, trumpet, violin etc. The elements that help us distinquish are:

• the waveshape -- the partials and their strengths
• the formants -- these are fixed areas of resonance in the spectrum -- they do
• not change, and as the fundamental changes, the emphasized partials may change
• envelope -- expecially the attack characteristics
• noise characteristic (the bow, mouthpiece, key clicks)

Perception of Sound - The Ear

The anatomical structure of the ear can be conveniently divided into three parts: the outer ear, the middle ear and the inner ear. The Outer Ear consists of the external portion -- Pinna, the auditory canal (3 cm long) and the membrane at the inner end of the auditory canal called the eardrum.

The Middle Ear consists of a chain of three bones called the ossicles: (malleus,incus,stapes) or the hammer, anvil and stirrup. There is an opening into the middle ear from the throat called the Eustacian Tube, which permits equal pressures to be maintained on either side of the eardrum. The hammer is linked to the anvil which is linked to the stirrup shich is attached to the fenestra ovalis (oval window)

The Inner Ear consists of a coiled cavity called the cochlea (uncoiled it would be about 3.5 cm) filled with a liquid called perilymph. Dividing the cochlea is the basilar membrane. There is a small opening at the end of membrane called the heliocotrema which separates th two chambers. The second chamber is separated from the inner ear by the fenestra rotundas or round window.

Hearing

The sound waves from the outside pass down the auditory canal and exert an alternating pressure on it causing it to vibrate. The vibrations are transmitted through the ossicular chain to the oval window. There is a level system in the middle ear, so the force on the oval window is about 1.3 times that on the ear drum (this is an amplifier). The movements of the oval window cause the liquid (perilymph) and the round window to move. There are some 30,000 nerve endings imbedded in the basilar membrane which sense disturbances in the liquid. Electrical energy is transmitted from the cells to the brain.Different places on the basilar membrane respond to different frequencies -- low frequencies at the far end, and high frequencies near the round window. With continued exposure to certain frequencies of sound or excessive volumes the delicate hair cells become damaged or totally destroyed, and those frequencies are forever lost.