Lecture 2 (Basic Physics of Sound and the Decibe Scale)

8/6/2019 Lecture 2 (Basic Physics of Sound and the Decibe Scale)

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Tahani Alothman

2009

Basic Physics of Sound & the Decibel Scale


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References

Introduction to Audiology Chapter 3 (Sound and its Measurement)

Audiology Diagnosis Chapter 11 (Pure Tone Tests)

Lecture notes


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Topics

Sound waves

Physical & psychological factors affecting sound waves

Physical description of pure tones

The decibel scale

Frequency and intensity function of the human ear


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What is Sound?

If a tree falls in an uninhabited forest,does it make a sound when it hits theground?

Sound = series of changes inmechanical pressure in an elasticmedium, such as air or water


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Humans are accustomed to hearing sound as a wave disturbancepropagated through air

Three prosperities are important to produce sound waves:

Force (i.e. something to set the sound source into vibration as when a tuning fork isstruck against a firm surface)

A vibrating mass (i.e. the tines of a tuning fork)

Elastic medium (i.e. air being the primary medium conveying sounds that thehuman ear hears)

Factors affecting sound waves:

Physical factors

Psychological factors


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Physical Factors:

Frequency

Intensity

Psychological Factors

Human reactions to sound

Subjective experiencesi.e. pitch, loudness, sound quality and ability to tell the direction

of sound source


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Two basic physical measures associated with pure tones:

Frequency

Intensity


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It specifies the number of back-and-forth oscillations or cycles produced by avibrator in a given time as molecular movement occurs and sound is created

The number of times that the object producing sound vibrates each second

The number of cycles that occur in 1 second

Hertz (Hz) OR cycles per second (cps): the term or unit used to describefrequency

The physical measurement of what is psychologically perceived as Pitch


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The normal human ear can hear from about 20 Hz to 20,000 Hz

The human ear is most sensitive to the mid-frequencies around 1000 Hz to1500 Hz

Frequencies that are less than this range are infrasonic

Frequencies that are greater than this range are ultrasonic

In English and French, vowels are essentially low-frequency sounds thatcontrol speech intensity while consonants are essentially high-frequencysounds that control speech intelligibility


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Pure tone: a single frequency of sound

Most sounds are made up of a mixture of frequencies and are called complex sound(i.e. speech)

Each time a given frequency is doubled, the range between the two frequencies is anoctave

Harmonic: is a multiple of frequency

If you go up one octave above a specific sound, that higher sound is the firstharmonic

If the harmonic is lower in frequency than the specific sound, it is called asubharmonic

Fundamental frequency

The most prominent sound in a complex sound

The lowest frequency of a vibrating system

the lowest harmonic produced by a periodic sound, generally corresponding to thesound's pitch

The frequency of each harmonic must be a multiple of the frequency of the lowestharmonic, or fundamental frequency

Pure Tone


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The perceptual equivalent of frequencyHow you subjectively perceive sound

Not a scientific measurement like frequency

Frequency and pitch are related in that as the frequency of a soundincreases, the listener perceives a tone of a higher and higher pitch

Increases in frequency cause increases in pitch

Decreases in frequency cause decreases in pitch


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The perceptual equivalent of intensity

How we subjectively perceive sound

Not a scientific measurement like intensity

Described with words such as "loud", "quite", etc.


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The ability under many conditions it is possible even without seeing thesource of a sound, to tell the direction from which it comes

A complex phenomenon resulting from the interaction of both ears

When we localize, we identify the azimuth of a sound source

How we can localize?

b/c of the relative intensities of sounds and their times of arrival at the twoears (i.e. phase)

Interaural phase differences in the LFs

Intensity differences in the HFs


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Reverberation: when the sound isperceived to come from a directionother than its source (i.e. acousticenvironment with hard surfaces)

Free-field: an area with no hardsurfaces to cause reverberation

Free fields exists only in such

exotic areas as mountaintops andspecially built anechoic chambers


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The way in which one sound can cover up or hide another soundso that we dont actually hear it, despite the fact that it is present,either at the same time, or immediately before or after

When two sounds are heard simultaneously, the intensity of onesound may be sufficient to cause the other to be inaudible

This change in the threshold of a sound caused by a secondsound with which it coexists is called "Masking


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i.e. experiencing masking in noisy situations in the form ofspeech interference

The sound that causes the interference is called: " Masker orNoise

The sound of interest (the sound we want to hear) is often called:Signal or Target

Masking plays an important role in clinical audiology


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A Convenient way of expressing two lengthy numbers is to use theLogarithm

Bel: one unit established in such a way

b/c a Bel may have a rather large value, the Decibel, which is onetenth of a Bel is the unit of measurement of intensity used in

acoustic s and audiometrics

dB: means one tenth of a bell

Named for Alexander Graham Bell

So, Decibel is: the logarithmic ratio between two sound pressuresor two sound powers


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Important aspects of the Decibel:

It involves a ratio

It utilizes a logarithmIt is therefore nonlinear

It may be expressed in terms of various reference levels, which mustbe specified

It is a relative unit of measure


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Intensity Level

Sometimes its useful to express the decibel with an intensityreference (IL)

Unit:watt per meter squared (watt/m) The intensity reference in a given system may be expressed as IR

(the no. of watts of the reference intensity)

The output of a system (e.g. loudspeaker) may be expressed as IO

So the ratio may be set up between the intensity reference and theintensity output by using this formula:

dB = 10 x log (IO/ IR)


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Intensity Level

The usual intensity reference (IR) is 10 watt/m , then the term

Intensity Level (IL) may be used

If IO = IR, the ratio is 1:1, because the log of 1 is 0, then the formula ofdB will give zero number of decibels

0 dB does not mean that sound is absent

Rather, it shows that the intensity output is the same as the intensityreference


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Intensity Level

Remember decibel is a logarithmic expression

If the intensity of a wave is doubled (e.g. by adding a second loudspeakerof a sound of identical intensity)

The number of decibels is not doubled but is increased by three

So:

Every time sound intensity goes up by 3 dB, it has doubled

Every time sound intensity goes down by 3 dB, it has been halved

Why?

b/c the intensity of the outputs of the two signals and not the number ofdecibels are added algebraically according to the principles of waveinterference and the rules for working with logs


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Decibel Reference Levels

10 dB or 20 dB had no specific meaning without specifying thereference for the measure

The Decibel Had Three Reference Levels:

Sound Pressure Level (SPL)

Hearing Level (HL)

Sensation Level (SL)


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dB SPL: Decibel Sound pressure level refers to the absolutereference level for the decibel

dB SPL equals 20 times the log of the ratio of an observed soundpressure level of 20 microPascals (or 0.0002 dyne/cm, 0.0002microbar, 20 micro Newtons/meter)

because dB SPL is a physical measure, it is not affected by the

frequencies present in sound


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dB HL: Decibel hearing level

Decibel notation used on the audiogram that is referenced toaudiometric zero

The human ear is not sensitive to all frequencies at the sameintensity level


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Therefore, 0 dB HL represents an intensity equal to the thresholdof hearing sensitivity* of the normal ear at each frequency

Audiometers are calibrated in dB HL so that any decibel valueabove 0 dB HL represents a deviation from normal hearing levels

Exp. 25 dB HL is 25 decibels above the normal hearing thresholdfor that frequency

* Threshold of hearing sensitivity: lowest intensity level at which an auditory signal isaudible


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dB SL: Decibel sensation level

Decibel notation that refers to the number of decibels above aperson's threshold for a given acoustic signal

Exp. Pt.'s threshold is 45 dB HL, a signal presented at 20 dB SLwould be 20 dB above 45 dB HL or at 65 dB HL

Pts threshold (45 dB HL) + 20 dB = 65 dB HL


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Other Decibel Notations

dB gain

dB nHL

dB peSPL

dB A

dB B

dB C


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Environmental Sounds


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Frequency and Intensity Function of theHuman Ear

The ear responds to different absolute intensities or different SPLsas a function of frequency

The ear is most sensitive in the mid-frequencies (1000-15000)Hz

Because audiometers are calibrated in dB HL, no need to know theabsolute dB SPL/HL difference at each frequency

The audiometer automatically corrects for the dB SPL/HL differenceas the frequency is changed


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Threshold sensitivity of the normal ear as a function of frequencyANSI (1996) reference equivalent threshold sound pressure levels arerequired to reach 0 dB HL

Documents

Lecture 2 (Basic Physics of Sound and the Decibe Scale)