BCST1101 Audio Measurement
Human Hearing Sensitivity
humans can discriminate very small changes in the intensity (amplitude) of quiet sounds, but are much less sensitive to changes in louder noises
Dynamic Range
defines as the difference between the smallest and the largest values
in audio/ sound we are discussing the range of amplitude or loudness of the sounds
Air Pressure and Sound
Air pressure at sea level is about 101,325 pascals (Pa)
or 1 Atmosphere
About 10332 kg per square meter
there’s a constant pressure
Ambient Air Pressure
that constant pressure is also known as the ambient air pressure
ambient:
- existing or present on all sides
sound is result of changing air pressure
sound is not constant pressure
atmospheric pressure is relatively constant
sound is pressure changes relative to ambient air pressure
No sound doesn’t mean no pressure
silence ~101,325 Pa
Sound is made by pressure going higher and lower than ambient pressure
Air Pressure and Hearing
the faintest sound humans can hear are changes in pressure measuring about 0.00002 Pascals
The loudest sound we can tolerate is somewhere around 200 Pascals
Lowest threshold
Or put another way:
the faintest sound we can hear is 20 micro pascals
Human Hearing and Loudness
loudness is the perception of intensity
how strong does a certain sound feel
human perception of loudness is not linear
doubling the power of the sound wave does not double the loudness
Power makes it heard
the power intensity ration between the faintest audible sound and the loudest sound we humans can tolerate is one to one trillion
One trillion to One
one million millions
1 followed by 12 zeros: 1,000,000,000,000
So the ratio of human hearing sensitivity to the power or sound. audio is 1:1,000,000,000,000
this is our “dynamic range”
Loudness vs Volume
loudness is perceived
its how intense we think the sound is
so loudness is what we sense, what gets interpreted by our brain
meaning loudness is a subjective experience
what’s Lous to one person may not be as loud to another
volume is measured
volume is the measurement of the power of the sound and/or audio
volume is therefore an objective experience
what’s high volume on one device is high volume on other devices
Logarithms and measurement
because of the non linear response to loudness we need a scale that is also non linear
we use a logarithmic scale
Logarithms
one that goes up in steps of ten times per step to measure the power of sound and audio
why logarithms?
compare to a linear scale loudness scale
with a logarithmic scale
Linear Measurement
this scale has one Pascal pressure unit for each unit of loudness
this is called a linear scale, one step up for each step across
problems with linear audio scales
Soft sound levels cluster at the bottom and are hard to differentiate from zero
logarithmic measurement
a logarithmic scales make the different loudness levels clearer and easier to separate from each other
each step up the scale of sound intensity is 10 times the energy or power of the previous step
Logarithms and Decibels
decibels are a logarithmic measurement
Measuring sound using logs
for example, we have an amplifier with unlimited power output and a knob where each number on the knob sounds twice as loud as the previous number
3 is twice as loud as 2
4 is twice as loud as 3
they ay 10 watts is coming from our amplifier with the volume knob set to 1
our sound intensity is at 1 on a scale of loudness
that takes 10w of power
now we want to increase the intensity of the sound
Logarithmic scales
so we have a scale that can make big jumps in power measurement in a small space
no amplifier on earth has the ability to deliver 10 times with each step up on the knob
each one step of the scale increases the amount by multiplying by some amount
whereas on a linear scale, each ones step of the scale increases the amount of adding 1
f-stops are logarithmic too!
example the f-stops in a camera are logarithmic since each full stop up increases light coming in by a factor of 2
f5.6 is four times the light of f2.8
f5.6 is 2 stops up from 2.8
Why use a logarithmic scale?
because of the high dynamic range of human hearing
if we used a linear scale we’d need a trillion units to measure sound levels
with a logarithmic scale we end up with about 10 units or so
Logarithmic scale in audio
the logarithmic scale in audio uses powers of 10
Logarithms and Audio
we use a logarithmic scale
decibels
that goes by be powers of ten, to measure the power of sound
each step up the scale is 10 times the energy of the previous step
but not 10 times as loud
Logs,Bels and Decibels
bels?
the original audio scale was the Bel
a decibels isn 1/10 of a bel
FYI bels are too big of a unit to be useful
each step covered way too much power
so that means to get a useful measurement in decibels we need to do one more bit of math
Cutting Logs down to size
up to now we’ve ben talking about a scale that goes up 10 times per single step
that’s too big to be useful
so we’ve divided up the steps into 10 smaller steps
the decibel scale
all human dynamic range is from 0-120 on the dB scale
all human dynamic range would be from 0 to 12 on this scale
what is a decibel?
a decibel (dB) is a unit for comparing the intensity of two different sounds
it is not a unit of absolute measurement
it’s a relative scale
because decibels are a comparison, we need a reference point to use them
for example, sound in air is measured against air pressure at sea level
so the reference point is air pressure
How the relationship works
the scale for measure is decibels
the reference point is air pressure
aka sound pressure level
so the unit of measurement is the dB split express as 28 dispel
some weirdeness about spl
0 dBspl should be no change in air pressure from the ambient 101,325 pascals, right?
but since humans can't detect changes in pressure less than 0.00002 pascals
OdB split is actually 0.0002 Pascals
why?
Because dispel is measurement of sound, 0 dB only needs to be the lowest pressure that humans can detect
not absolutely zero air pressure
FYI
FYI 120Dbspl is the “threshold of pain” where the loudness of a sound can actually hurt
since the threshold of pain is based on loudness not volume it can vary depending on the listener
Decibel scales DbFS
the terms dB FS means decibels relative to fulls Cale. it is used for amplitude levels in digital systems with a maximum avaible peak level where 0 dB FS is assigned to the maximum level
a signal that reaches 50 percent of the maximum level would, should have a value of -6 dB FS
all peak measurements will be negative numbers
why dBFS?
digital audio compares sound to numbers (bits). more bits = higher but depth = more sounds
digital systems are limited by the bit depth. 0 dbfs is chosen as the absolute maximum measurement
any single that exceeds the maximum ceases to exist since there are 0 bits left to count the data
dBu and dBv analog scales
what is dBu?
a logarithmic voltage ratio with a reference voltage of 0.7746 volt = 0dBu
what is dBv?
a logarithmic voltage ratio with a reference voltage of 1.00000 volt = 0dBv
analog audio compared sound to voltage and current
You cannot convert digital to analog
you can never match dBFs and dBu/dBv
dBu/dBv is volts - you measure it with a volt meter
analog audio: positive and negative voltage
dbfs is in contrast a binary number
digital audio: seroes and ones
the reference point of analog and digital are so different that they can’t be converted
instead you measure each as you do the conversion
example - send tone at 0dbu from the analog source. set the dbfs scale to -20dbfs to match the tone and start your transfer
Remember this power ratio
the original scale was Bels - 1 bel = 10 times the power, 2 bels = 100 times the power
divided bels by 10 to make a more useful scale, decibels
for decibels = 10 decibels is 10 times the power, 20 dB is 100 times the power, 30 dB is 1000 times more power
Sound and Audio
Amplitude
the distance above and below the centreline of a waveform
Frequency
the rate at which a sound wave repeats any number of cycles within one second
Wavelength
the distance between peaks of a wave in one cycle
Phase
the time relationship between multiple sound waves
measured in degrees
range 0-359
Frequency Range
the lowest and highest frequencies of a signal
Hertz
a frequency measurement unit (cycles per second)
Transient
a short duration, high level sound. Such as hand clap or snare drum hit
Axis - Microphones
on axis - if you envision a polar xis around a microphone, on axis would be at zero degrees
at the front of a mic
this is the area where the microphone’s frequency response is best
off-axis is at the side or back of the microphone
sound is distant, “thinner”, more “roomy”
Dynamic Range
the lowest and highest possible volumes of a signal
often used to describe limitation of equipment
Headroom
maximum amount of level equipment can take before you cause distortion by “clipping”
Noise Floor
the level of noise below which the signal is weaker than the noise
Signal to Noise ratio
this is the relationship between the signal and the inherent noise of the environment or electrical noise from equipment
measured between recorded sound and the top of the noise floor
Gain
gain - increasing the signal level
usually by amplification - adding electrons (increasing current)
unity gain
not adding or subtracting to the overall signal level
audio going in comes out the same level
Audio Compression
different from data compression, data compression is to make files smaller
audio compression is to increase the headroom without reducing the apparent volume
also to prevent slipping distortion
Treshold
the threshold is the level at which the compression effect is engaged
for example, if the threshold level is set at -10dB, only signal peaks that extend above that level will be compressed
Knee
the knee referee to how the compressor transitions between non-compressed states. Most compressors allow you to choose either a “soft” or a “hard” knee
Ratio
ratio specifics the amount of compression applied to the signal. this setting is expressed in decibels.
a ratio of around 3:1 is considered moderate compression
5:1 would be considered medium compression
8:1 starts getting into strong compression
Audio Distortion
a distortion is the alteration of the original shape of an object, image, sound, waveform other form of information or representation
in audio, distortion is the alteration of the shape of the sound or audio wave. this changes the sound of the wave
often but not always distortion is caused by having the signal at too high a level
this is also called overdriving, or over modulation or clipping
exceeding the headroom of your equipment is the most common cause of distotion
other causes exist
Distortion caused by clipping
abrupt limit to level, ie digital = hard clipping
flexible limit to level, ie analog = soft clipping
noise isn’t distortion, but distortion is noise
the addition of noise or other extraneous signals is not considered to be distortion
however, the effects of distortion are sometimes considered noise
any time an audio signal is “processed” some amount of distortion is introduced
Normal recording - no distortion
amplitude of the signal falls comfortable within the range
this is a well recorded signal
boosted signal - some distortion
here, the signal is amplified by 250%
recording components can no longer accommodate the dynamic range
strongest portions of the signal are cut off
this is where distortion occurs