Download COMD 3700 Basic Audiology Lesson 3 The measurement of sound

COMD 3700 Basic Audiology
Lesson 3
The measurement of sound
Highlighted information refers to a change between the audio
recording (using 10th edition) and the 11th edition of the textbook
1. COMD 3700 for distance education. This is lesson 3, the
measurement of sound
2. In this section we will be discussing sound and its measurement.
We'll review a few things about the attributes of sound. Most of you
have already taken acoustics and anatomy and physiology of hearing.
But you might not have already taken those classes. You may have
taken another course in your life that talks about acoustics. For some
of you, this is review. For others, it's new. Either way, please review
Page 30-43 in textbook and make sure you understand the concepts
listed. We will build on this base, so it is important that you
understand sound waves and their common attributes and the ways
that they are measured. We will pick up from where you left off in the
acoustics course. In your textbook, we will begin on page 44.
However, you will still be responsible for the information contained
in the beginning of the chapter.
3. Before we discuss the measurement of sound, lets review the
physical attributes of sound. Sound is a wave disturbance that travels
through any medium; humans are accustomed to hearing sound
propagated through air. Three properties are necessary to produce
sound waves: a force, a vibrating mass, and an elastic medium. Air
molecules are the mass undergoing to-and-fro motion (or the
vibration) and air itself has an elastic nature so that the molecules, in
their vibratory motion, seem to be connected by “springs”. Sound
waves are produced by molecular vibration because the pressure
states that they are created when molecules are packed closer
together (the state of compression) or spread further apart (the state
of rarefaction) than normal. As molecules undergo oscillation,
successive compressions, followed by rarefactions, are passed along
the line of particles at the speed of sound. Waves behaving in simple
periodic oscillation are often called sine waves. These waves may be
described in terms of how often they move from maximum
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rarefaction to maximum compression and then return to their point
of origin: this is called the frequency of the wave. The intensity of a
wave is the force that moves it to its maximum amplitude. The
measurement unit for frequency is cycles per second (cps) or hertz
(Hz).
4. Waves of different frequencies may combine to form interactions
called complex waves. Frequency is interpreted psychologically as
pitch, and intensity as loudness; different complex waveforms
produce the quality or timbre of a sound.
5. The measurement unit for intensity is the decibel. It is a ratio
between 2 sound pressures or 2 sound powers. Common units of
measurement, such as the pound or mile, are additive in nature. So as
an example, 10 (1 lb.) weights equal exactly the same as 10 lbs.
However, because the range of human hearing is so great, using such
units result in very large, cumbersome numbers. It is convenient to
discuss one intensity in terms of the number of times it is multiplied
by another intensity—that is, the ratio between the 2. The decibel
(dB) is commonly used for that purpose. Because of the problems
working with such a vast range of values in effective sound pressure,
Alexander Graham Bell developed the decibel, a tenth of a Bel. dB is a
logarithm unit of measurement that expresses the magnitude of
physical quantity relative to a specific reference level. Not like other
physical units, dB does not mean anything by itself. It only
mathematically reflects the magnitude of a ratio in logarithm. Decibel
is used for many measurements in acoustics such as spectral
magnitude, sound pressure level and so on. The reason that decibel is
widely used is that human ears perceive sounds in logarithmic scale
instead of linear scale in both frequency domain and amplitude
domain. The decibel is widely known as a measure of sound pressure
level, but is also used for a wide variety of other measurements in
science and engineering, most prominently in acoustics, electronics,
and control theory. In electronics, the gain of amplifiers, attenuation
of signals, and signal to noise ratios are often expressed in decibels. It
confers a number of advantages, such as the ability to conveniently
represent very large or small numbers, a logarithmic scaling that
roughly corresponds to the human perception of sound and light, and
the ability to carry out multiplication of ratios by simple addition and
subtraction.
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6. So now we are gong to talk about the different decibel
measurements that we use in audiology. But before we do that, let me
talk about temperature. You might wonder why. The first bullet says
we need to develop or define a beginning point. 0 DB doesn't
necessarily mean absence of sound. We can draw some comparisons
to temperature. Hopefully that will help you understand zero sound.
If I told you it was seventy degrees outside, you'd know what you
would experience outside. If I told you it was zero degrees outside,
we'd both understand that does not mean there are no degrees
outside. That's comical to consider. Zero degrees is still a level of
temperature. Zero degrees is ten degrees below ten degrees of
temperature and ten degrees above minus ten. I could say you better
put on more clothing because it's minus ten degrees out, but if you’re
my son, you’re still wearing shorts. So his perception is different. But
regardless, we know that zero degrees doesn't mean the absence of
temperature. In the same way, we often talk of zero DB. We're not
referring to an absence of sound there are levels of sound at minus
five DB, minus 10 DB, and minus 20 DB. Our task is to define the
zero point. What is zero degrees? What is zero decibels? Let's go
back to temperature and to use a common source. Something has
happened physically. We measure our temperature in comparison to
that physical occurrence. When that happens, we say there you go.
That is zero degrees. That's our starting point. That physical
occurrence with temperature is the freezing of distilled water. We put
water out and lower the temperature down until the water freezes.
We designate that temperature as zero degrees. Then we will make
demarcations and increase the temperature in equal increments and
have plus ten, twenty, thirty and on. We will also have minus five,
ten, fifteen, and minus twenty. Below that level of zero where water
froze. So with temperature, we've defined or developed a beginning
point. That point is freezing water. We've taken everything from
that.
7. We need to do the same with sound. We need to find a beginning
point of zero and a threshold. Again, remember that threshold in
audiology designates the lowest point in intensity that a person can
perceive the stimulus. If the stimulus is made less intense, we're
below the individual's threshold. If you deliver the sound above the
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intensity of that threshold, they begin to hear that. In audiology this
is measured in various ways depending on the testing you are
conducting. We are going to discuss: Intensity Level (IL), Sound
Pressure Level (SPL), Hearing Level (HL) and Sensation Level (SL).
Now before we get started, I don’t want you to get overwhelmed with
understanding the math. I just want you to understand the difference
between the types of dB measurements we use in audiology, how it is
measured and when we would use each type of measurement.
Understanding this chapter requires no special knowledge of
mathematics or physics, although a background in either or both of
these disciplines is surely helpful. Fortunately we do not complete the
mathematical calculations we will be discussing, audiometers and
hearing instrument specifications do the job for us.
8. Under some circumstances it is useful to express the dB with an
intensity reference. Decibels measure power in intensity levels,
expressed in watts. The term "intensity" is used exclusively for the
measurement of sound in watts per unit area. More specifically, the
unit of measure for intensity is the watt per square meter (watt/m2)
To describe the strength of sound in terms other than strict intensity,
one can use "magnitude" "strength", "amplitude", or "level" instead.
Sound intensity is not the same physical quantity as sound pressure.
Hearing is directly sensitive to sound pressure, which is related to
sound intensity. In consumer audio electronics the level differences
have been called "intensity" differences, but sound intensity is a
specifically defined quantity and cannot be sensed by a simple
microphone, nor would it be valuable in music recording if it could.
9. The intensity reference in a given system may be expressed as IR,
which is the number of watts of the reference intensity. The output,
for example of a loudspeaker, of the system may be expressed as Io,
so that a ratio may be set up between the intensity reference and the
intensity output. The most common approach to sound intensity
measurement is to use the decibel scale, which uses logarithms.
Logarithms are exponents- For example 102 the 2 is an exponent and
tells us how many times to use the 10 in multiplication. 102 is type of
scientific notation. Another way to write this is 100; still yet another
way to write this same value is 10 x 10 =100. This is basically a short
hand to save space and simplify calculations of immense proportion.
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So, in solving for the number of decibels using an intensity reference,
the formula is: dB = 10 x log of I0 (intensity output) over IR (intensity
reference). So, if the intensity reference of a sound system IR is
known then the preceding equation may be used to determine the
number of decibels of the output either above or below the reference.
If the intensity output (I0) and the intensity reference (IR) are exactly
the same (I0=IR) then the ratio is 1:1. Because the log of 1 is 0, use of
the formula shows the number of decibels to be 0. So, 0dB does not
mean that the sound is absent, but rather that the intensity output is
the same as the intensity reference. If the intensity output were
changed to 10 to the 1st power watt/meter squared, then the number
of decibels would be 130. Please refer to table 3.1 in your book on
page 45. It shows that as the intensity output increases, the ratio
increases, raising the power of the log and increasing the number of
decibels.
10. Here is another chart showing intensity in reference to decibel
level. Remember, the decibel scale is a logarithmic scale. Each ten
decibel jump on the scale is ten times higher then the previous
number. A difference of 10 decibels means the sound is twice as loud.
If the intensity of sound is increased by a factor of 100, the new sound
level will be 20 units greater. This chart also gives good examples of
the loudness of the decibels levels.
11. In audiology or science in general, we can define also define sound
intensity in terms of pressure or power. People in the audiology
profession gravitate to pressure measurements. Probably the most
common usage of "decibels" in reference to sound loudness is dB SPL,
referenced to the nominal threshold of human hearing.
So to understand how this is measured, we must first now that
pressure is defined as a force per unit area. A unit of force is a
Newton (N). So we have to define the force and the unit area. And in
our string of logic, we have to define a newton. Force per unit area is
a Newton. Basically there are three components to defining this. One
Newton is a force, and we have to define a force. One Newton is a
force that will accelerate. As we define a Newton, do a visualization
I’ve seen used in the classroom. Pretend I take an eraser off the
blackboard and hold it up and say pretend this is one kilogram mass.
I put the eraser on the edge of the desk. Going back to one newton,
an eraser is not anywhere near one kilogram mass but we pretend it
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is. We need to accelerate that one kilogram mass the distance of one
meter. The desk I use is probably longer than one meter. Pretend the
width of the desk is one meter. The one kilogram eraser is at one end.
I put my finger on the end of the eraser and push on it. I indicate to
the students I need to move the eraser from one end of the desk to the
other end or the distance of one meter. And I need to accomplish this
task in one second. If I do that, I have moved one kilogram one meter
in one second. The thing we need to focus on is the force that I
applied with my finger to the end of that eraser or that one kilogram
mass. That is the key point. I applied just enough force, or the right
amount, that will satisfy all three bullets. If I apply more or less force,
we will not have these three perimeters relating to each other in the
proper manner. The mass, the distance, and the time. So the focal
point is that I have applied the right amount of force to accomplish
those three parameters correctly and that force is one Newton.
12. Let's move on to talk about units of sound pressure. As you may be
aware, many units of measurements are named after famous people.
Like we called the force that defined pressure Newtons. They were
named after a famous person, Sir Isaac Newton. By convention, units
of sound pressure are called Pascals, abbreviated Pa. This is named
after Blaise Pascal. So if we say twenty micro Pascals or ten Pascals or
whatever, that would sound familiar to someone working in the
hearing field. So going back to the Newton, a Newton is a force that
will move one kilogram one meter in one second. What is a Pascal?
One Pascal equals one Newton. That force we talked about at the end
of my finger. Then you spread out the force over a defined area. You
realize if you take the same force and apply it to a large area, you'll
have a smaller force per unit of area than if you apply the same force
to a smaller area. One Pascal is equal to one Newton per square
meter. The force of one Pascal is one Newton, but we'll apply that to a
square meter. You can picture a square meter. One Pascal equals a
Newton per meter squared. So, if you take one portion of the square
meter, try to imagine how miniscule or small the pressure is that
we're talking about here. Now it should be no mystery to you that we
have many levels of sound pressure. We can make the sound
pressure very intense or weak. So the smallest sound variation
required to produce a just audible sound to healthy young ears is
approximately ... before we go on, we'll review we're looking for the
softest sound that young healthy ears can just detect, some say this is
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roughly the sound of a mosquito flying 3 meters away. That sound is
approximately 0.00002 Pa or twenty micro Pa. Micro means onemillionth part of. So to give you an appreciation of the small units in
pressure we're talking about when we get into the area of sound of
one Pascal-It's the force of one Newton per unit area. That's small in
it self. Take one of those and slice it into one million parts and take
twenty out of the one million parts. That would be 20 micro Pascals.
That's this small measurement. By convention, it was decided by a
body of scientists that 0 dB SPL (sound pressure level) would equal
20 micro Pascals. The 20 micro Pascal is the freezing water
phenomenon. That's our zero point. Therefore, freezing water equals
0 degrees Celsius, and 20 micro Pascals equals zero dB sound
pressure level. In our previous discussion we established our sound
reference point as 0 dB SPL. We discussed the fact the reference
point was 20 micro Pascals. We want to develop a chart where we can
record the results of our testing of patients and have a permanent
record of their ability to hear.
13. What you're looking at here is the beginnings of such a chart.
These charts are called audiograms. So if you look at this chart in this
slide, you'll see across the top the frequencies from 125 up to 8000 in
octaves. Down the ordinate on the left side, you can see the dB scale.
This chart is arranged to start at zero. We know that loudness is
measured in decibels (dB). Decibels on an audiogram are called
'Hearing Level', which we will get to in a minute. I just want you to
understand how the audiogram is designed before we go any further.
So, 10dB is quiet and 100 dB is loud. The numbers called 'Frequency'
are the pitch. These are defined using Hertz or Hz. A small number is
a low sound and a big number is a high sound. Although we usually
only test from 125 Hz to 8000 Hz. On the interactive website included
with your textbook under learning supplements, you will find
information on the audiogram. You can listen to the pitch of the
frequencies from 250 Hz to 8000 Hz. Humans can hear from 20 to
20,000 Hz. Dogs can hear 40 to 40,000 Hz. As a reference, a piano
lowest note is 27 Hz and the highest note is 4186 Hz. We define the
amount of hearing loss that a person has based on their threshold at a
given frequency. You can see the different levels of hearing loss listed.
Again, we will cover this in detail later. I just want to give you a
reference since we will start using audiograms at this point. The
audiometer was designed as an instrument to test hearing sensitivity
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at different frequencies.
14. So we will now go back to Sound Pressure Level. On this
audiogram, note the zero in the upper left hand corner. On this
audiogram it refers to 0 dB SPL. That zero listed above 125
represents 0 dB sound pressure level. The horizontal line to the right
of zero across this chart over to the zero on the right, that represents
0 dB sound pressure level. We said 0 dB sound pressure level was
established as our reference by convention. It was done at that level
because this was a level of sound intensity that could be just
perceived, barely audible, by the best ear. Originally each audiometer
manufacturer determined the Sound Pressure Level required to
barely stimulate the hearing of an average normal-hearing individual.
Needless to say, there were some differences from manufacturer to
manufacturer. At this point, think about a few things. When we
evaluate hearing, and we make judgments as to how well people hear,
we don't want to judge people at a standard against the level that
represents the best ear. We probably would rather make judgments
as to a normal kind of reference. In other words, what if a number of
people were tested that we knew had normal or good hearing, where
would that reference be rather than 0 dB SPL? Second, we know that
the human ear is not equally sensitive to all frequencies, those octaves
on the top there, 125, 250 etc. The human ear doesn't hear those
equally well. So we ought to test how the normal ear reacts to a
stimulus at each frequency. Well, this is what happened in 1938. A
person named Beasley went to the Wisconsin state fair and set up a
booth or station. He took his audiometer and ran a lot of young
adults through his station to test their hearing. He wanted to see how
sensitive their ears were at those frequencies or octaves were from
125-8000. He then averaged their threshold. Their results were
averaged. He found that the normal human ear is not equally
sensitive to the different frequencies. This began additional studies
testing different people under better conditions, which led to updated
standards. The American National Standards Institute (ANSI)
published the most recent U.S. standard in 2004 . That standard is
the ANSI 2004 standards for audiometric testing or Audiometric 0.
This audiogram charts the current standards. This is an audiogram
charted out in dB sound pressure level. The x marks on the
audiogram represents the amount of sound pressure it took for the
normal hearing subjects to just perceive the sound. Also I've put on
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there they've specified that this be done under TDH 49 earphones or
that these results were obtained while using those earphones. What
the ANSI studies found as well was what that the human ear is not
equally sensitive at each frequency. If it were, then when they tested
the subjects to determine their threshold the dots would be in a
straight horizontal line. But that is not the case, we have a curved line.
Not only that, but the curved line is not up at zero dB SPL. So, in the
normal average ear, the hearing is less sensitive than the standard we
have of 0 dB SPL. In the ANSI 2004 standard, it takes 7.5 dB to reach
0 dB hearing level at 1000Hz. If you take the time to look at the ANSI
standard at each level, you will see that each of those are different at
each frequency. Each x represents 0 dB Hearing Level, that’s different
than dB SPL. So at 250 Hz, the ANSI standard indicates you need
26.5 dB SPL to reach 0 dB hearing level and at 8000 Hz it takes 13.0
dB SPL to reach 0 dB HL. Or that 13.0 SPL is the sound pressure that
it takes for the normal ear to just detect the sound stimulus.
15. So to create a summary of what we've discussed you see the
different columns. The first column you see 0 dB hearing level,
they're all identical. The second column is the frequency in Hz Equals
then you have the varying SPL levels. Focus on 1000 Hz. At 1000 Hz,
0 dB hearing level is 7.5 sound pressure level. If you go back to the
previous slides, you'll find that to be true with the ANSI 2004
standard. If we change frequency and go to 500 Hz, 0 dB HL
(hearing level) is actually 13.5 dB SPL. It takes more dB SPL for the
normal ear to just detect a stimulus at 500 Hz. If we go to 250 Hz, it
takes 26.5 dB SPL to reach 0 dB hearing level for the normal ear. You
can go through and look at those values for the different frequencies.
You can see that If you're wondering if you'll have to memorize all
these values per frequency, the answer is no. I have to consult proper
reference myself. You're just required to remember the difference in
SPL and HL at 1000 Hz. Under the ANSI 2004 standard, the
difference at 1000 Hz, the difference between SPL and HL is 7.5 dB.
So, in summary as you change frequency the SPL changes. The
constant is the first, zero dB HL. We'll make a conversion in the next
slide. I hope you understand it. It's necessary you do understand it
because we'll work from this new converted audiogram for the rest of
the course. We have all these zero dB hearing levels and go to the
next slide to discuss this further.
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16. As you look at this chart, you might think this is the same thing we
were looking at when we were discussing SPL, but it's not. It's the
same frequencies, 125 up to 8000 Hz this time it’s across the top. But
look at the side now. Now these numbers, 0-110, are dB hearing level.
To emphasize this, I pointed out that it is referenced as being decibels
measured in hearing level. Across the top is 0 dB HL. Now we have a
chart that has converted dB SPL into dB HL, hearing level. In one of
our previous slides, at each frequency, the difference between SPL
and HL was actually 0 dB hearing level. In other words 7.5 dB at
1000 Hz SPL is 0 dB hearing level. Now we have converted from SPL
to HL; this chart we're looking at right now represents what will
become our audiogram, hearing chart. When we test people's hearing
we will put markings on this chart that will represent their individual
threshold. They may have an impaired ear. So the markings for a
given ear might be at 30, 40, to 70 at the different junctures. We can
look at their impaired threshold and compare it to the zero dB
hearing level, which is a representation of normal hearing at the
different frequencies. What we've doing here is we've taken the
crooked dB HL line off of the SPL audiogram and straightened it out
and pulled it across so now they're all 0 dB HL. Instead of a crooked
line, we have a nice straight line at 0dB HL. But it is important for
you to know that each of them represent a different sound pressure
level. If you need to, you might have to go over that a number of
times to get that done and clarified in your mind.
17. To this point, we have discussed dB SPL and dB HL (hearing
level). Now we want to talk about dB SL or sensation level. In my
mind, this is the simplest of the three to conceptualize. No hard math
or logarithms-yeah! dB sensation level is used to designate an
intensity level above threshold. Try to think about that. What we
have to do is know the patient's threshold at a given frequency before
you identify the SL level. For instance if you had tested a patients
hearing and found that their threshold (the softest sound that they
could hear) at 100Hz was 25 dB. Then I said to present a stimulus to a
patient at 20 dB SL at 1000 Hz and the patient's threshold at 1000 Hz
was actually 25 dB HL, you would deliver a stimulus to the patient 20
dB above their threshold, which is 25. 20 dB represents sensation.
You would take their threshold of 25 and go 20 dB above it. That
would be 20 dB SL or 45 dB HL. So you would add 20 to it and
present a tone at that level. That would actually be 45 dB on the
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chart. You're presenting the tone at 45 dB HL. But the person has a
25 dB hearing loss. So you're presenting the stimulus at a sensation
level of 20 dB. There are many times in testing we want to find a
person's threshold but there are some times we want to deliver
stimuli at a constant level above their threshold. If you had three
different patients, and each had a different audiometric threshold at
1000 Hz. Patient one has a threshold of 20 dB. Patient two has a
threshold of 30 dB. Patient four has a threshold of 40 dB at 1000 Hz.
You can present a stimulus so it would be equally intense or loud for
each of them. You could say let's present a 20 dB SL stimulus to all
three patients. Patient one has a threshold of 20. You add twenty to
that so you would present the tone to them at 40 dB HL. Patient 2
threshold is 30. You go twenty above his threshold, and you'll present
the stimulus at 50 dB HL. You take a constant 20 SL and add to the
threshold of 40 with the third patient. So forty and twenty is sixty dB
HL. That means each patient received the stimulus at a constant
value above his or her thresholds.
18. So in summary what we've tried to do is to discuss the nature of
dB IL, dB SPL, dB HL, and dB SL. It's essential you learn the
relationship between those four designated levels of sound intensity.
It is very important to use the letters SPL, IL, HL or SL to identify
what kind of dB you are using. Remember, The HL (hearing level)
reference is used only for audiometric purposes and has as its
reference the average normal-hearing person’s thresholds at each
frequency. SPL (sound-pressure level) is used for most other
measurements and has as its reference 20 micropascals or 0.0002
dyne/cm2.
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