Audio Fundamentals Part 2
Objectives
Upon completion of Audio Fundamentals Part 2, you will
be able to:


Explain Sound Wave Propagation, and
Explain decibel and decibel in reference to
voltage and power level.
Sound Wave Propagation
Sound waves travel approximately 344 m/sec (1130 ft/sec) in air. The behavior of sound
propagation is generally affected by three things:
1.
2.
3.
A relationship between density and pressure. This relationship, affected by
temperature, determines the speed of sound within the medium.
The propagation is also affected by the motion of the medium itself. For
example, sound moving through wind. Independent of the motion of sound
through the medium, if the medium is moving, the sound is further
transported.
The viscosity of the medium also affects the motion of sound waves. It
determines the rate at which sound is attenuated. For many media, such as
air or water, attenuation due to viscosity is negligible.
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Intensity
When talking about Sound Waves, it's important to remember that they are about the
transmission of energy through a medium. That could include; mechanical energy,
acoustic energy, electrical energy etc.
Looking at energy, we know that a unit of Energy equals a Joule. We also know that
energy cannot be created or destroyed. It can only be turned into another form.
Power, on the other hand, equals energy flowing through a system in a given time,
Intensity is another energy concept to understand when talking about sound waves.
Intensity = energy flowing through a certain area in a given time: unit = Joule / second /
m2 or Watt / m2
In other words, intensity is the amount
of power flowing through a square
meter, perpendicular to the direction
of wave travel.
If you can measure the power of a
source of wave energy and you know
over what area this power spreads
out, you can work out the intensity of
the wave at any point around that
area.
If the source is omnidirectional then
the intensity may be the same where
ever you look, but most sources direct
more intensity in some directions than
others, these are called directional
sources.
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Diffraction of Sound
Spread of Sound Waves
Sound waves can spread in a rather unusual manner when they reach the edge of an
object - this is called diffraction.
The amount of diffraction depends on the wavelength and the size of the object. Click
on the buttons to select an object size. Pay attention to the sound wave as it passes the
object.
In this example the sound wave is larger than the object, represented by the red
rectangle in the center. Diffraction is more pronounced with longer waves. This means
that you can hear low frequencies around obstacles better than you can hear high
frequencies. Another common example of diffraction is the contrast in sound from a
close lightning strike and a distant one. The thunder from a
close bolt of lightning will be experienced as a sharp crack,
indicating the presence of a considerable amount of high
frequency sound. The thunder from a distant strike will be
experienced as a low rumble since it is the long
wavelengths which can bend around obstacles to get to
you. There are other factors such as the higher air
absorption of high frequencies involved, but diffraction
plays a part in the experience.
object
shadow zone
In this example, the obstacle is roughly half the size of the
sound wave. Sound will still bend around the object, but
we begin to see the development of a zone of silence or
Shadow Zone.
object
shadow zone
In this example, the obstacle is larger than the sound wave. We see a large zone of
silence or Shadow Zone behind the object. Because the
object is much larger, there is a lot of energy reflected back
towards the source. There is still some diffraction, but not
much. As we saw with the previous two examples, it's the low
frequencies, the ones with longer wavelengths, which
diffract around the corner.
object
shadow zone
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Diffraction Through a Gap
Diffraction also occurs when a sound wave passes through a gap in an obstacle. When
the gap size is larger than the wavelength, the wave passes through the gap and does
not spread out much on the other side. When the gap size is equal to the wavelength,
maximum diffraction occurs and the waves spread out greatly - the wave fronts are
almost semicircular.
Here we see a small gap, where the wave is able to spread
greatly.
When there is a medium gap we see the wave spreads on either
side of the gap, but not as much as a small gap.
In this image of a larger gap, we see that the wave does not
spread out as much on either side of the gap.
Examples
Noise barriers can be put up alongside major roads - houses behind the barriers are
exposed to less noise if they are in the shadow zone. However, low frequencies are
unaffected by the barriers and can diffract over the top.
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Diffraction also plays a role in allowing us to locate sources of sound.
If you close your eyes, you can generally tell which
direction sound is coming from. When sound reaches
you from straight ahead, the same sound signal is
received at both ears. This is because the head is
more-or-less symmetrical and the sound to both ears
travels an identical path length. Your brain uses
this information to locate the sound in front of
you.
When sound comes from the side directly, or via a reflection, the sound at each ear is
different. Sound to the furthest ear has to diffract (bend) around the head. This means
the sound wave arrives slightly later and is altered in terms of the balance of high and
low frequencies it contains. As we have seen, sounds with short wavelengths don't
diffract as well, so the furthest ear hears less high frequencies. The brain senses this
difference in arrival time and frequency content, and uses it to locate sound.
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decibel
The decibel (dB) is a logarithmic unit used to express the ratio between two values of a
physical quantity, often power or intensity. One of these quantities is often a reference
value, and in this case the decibel can be used to express the absolute level of the
physical quantity.
The decibel is also commonly used as a measure of gain. In audio, gain = amplification
factor. This is a measure of the ability of the amplifier to increase the amplitude of a
signal from the input to the output. It is the amplification usually defined as the mean
ratio of the signal output of a system to the signal input of the same system. It may also
be defined on a logarithmic scale, in terms of the decibel logarithm of the same ratio
("dB gain").
In all phases of audio technology
the decibel is used to express signal
levels and level differences in sound
pressure, power, voltage, and
current. The reason the decibel is
such a useful measure is that it
enables us to use a comparatively
small range of numbers to express
large quantities. The decibel also
makes sense from a
psychoacoustical point of view in
that it relates directly to the stimuli
as perceived by the human
auditory system.
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Power Relationships
Suffixes are commonly attached to the basic dB unit in order to indicate the reference
value against which the decibel measurement is taken. For example, dBm indicates
power measurement relative to 1 milliwatt.
Other suffixes used in acoustics and audio include:







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dBV - volt
dBu - unloaded
dBmV - millivolt
dB SPL - Sound Pressure Level - Probably the most common usage of "decibels" in
reference to sound loudness is dB SPL, referenced to the nominal threshold of
human hearing
dB SIL - Sound Intensity Level
dB SWL - Sound Power Level
dBFS - Full Scale - This is the amplitude of a signal compared with the maximum a
digital device can handle before clipping occurs. Clipping is a form of distortion.
dBTP - True Peak. This is the peak amplitude of a signal compared with the
maximum which a device can handle before clipping occurs.
The bel is defined as the common logarithm of a power ratio: bel=log (power 1/power
0). For convenience we use the decibel, which is one-tenth of a bel: decibel = 10 log
(power 1/ power 0).
This chart illustrate the concept. Notice how a power level of 100,000 is expressed as a
more manageable 50 dB. Psychoacoustically, a ten-times increase in power results in a
level which most people judge to be twice as loud. A 100-watt acoustical signal would
be twice as loud as a 10-watt signal, and a 10-watt signal would be twice as loud as a
1-watt signal.
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Let's expand on the chart above. This chart will allow you to perform quick calculations
arriving at power levels in dB above, or below one watt. For example, 8 watts
corresponds to 9 dB. If you wanted to find what 80 watts corresponds to, note that 80 is
10 times 8, giving another 10 dB. Therefore, 80 watts equals 19dB. Click the zoom picture
to see more.
Every effort has been made to ensure the information in this document is accurate and reliable, however in
some cases changes in the specifications may not be reflected in this document. Christie reserves the right to
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make changes to specifications at any time without notice.
training.christiedigital.com
Audio Fundamentals Part2 v1.0 Sept14