Class Notes Acoustics Demos

Basic Sound Engineering
PP 271 01
http://web.me.com/dtostilane/PP271/PP271_Basic_Sound_Engineering.html
Wednesday, we’re back in the Annex
next door
Classroom Door Code = 16333
Sunday, September 18,
1968: The Beach Boys & 1910
Fruitgum Company.
Concert Technology at it’s peak!
4 Altec Voice of the Theater Speakers
and 12 (!) Mixer Channels!
Sunday, September 18,
October 25, 1968, Virginia Tech Coliseum Crowd of about 3500.
Concert Sound Control Center
Sunday, September 18,
Control center showing two Altec 1567A 5 channel mixer amps and additional gear for mixing to the campus
radio station.
Altec 1567A Tube
Mixer-Amp
Cadac J-Type
Theater Mixer
(No Amp included)
(and no Tubes!)
Sunday, September 18,
The Altec 1567A in pristine glory. All Tube (of course), including sophisticated EQ section and metering!
Note, 1 amp fuse - maximum power would be 120 Watts. (which was not a small amp at that time)
Contrast with Cadac J-Type 66 input 16 output broadway configured console.
Quick Recap- Frequency, Velocity, Wavelength
Velocity
λ=
frequency
Velocity of sound at 72˚ = 1130 ft/sec.
So, for example:
20Hz Wave = 56’ 6”
40Hz Wave = 28’ 3”
80Hz Wave = 14’ 1-1/2”
200Hz Wave = 5’ 8”
1000Hz Wave = 1’ 1-1/2”
10k Hz Wave = 1-1/4”
for quick calculations sound travels 1’ per 1ms
Sunday, September 18,
Recapping - find wavelength from velocity and frequency - or vice versa - if you know wavelength and
temperature you can find the frequency. Note that velocity INCREASES with temperature at 72˚ it’s about 1130 ft/sec. At 85˚ it’s about 1138 ft/sec. At 32˚ it’s about 1086 ft/sec.
SO that 20 Hz wave at 32˚ would be about 2’ shorter, and at 110˚ on a hot Texas day (velocity 1170 ft/sec) it
would be a bit over 2’ longer!
Remember, sound is zones of compression and
rarefaction radiating away from the source
Sunday, September 18,
Recap sound waves = compression and rarefaction zones radiating away in a spherical pattern
Superposition of Waves
Combining two signals yields a new wave
which is the mathematical combination of the
levels of each wave.
Sunday, September 18,
Superposition In-Phase
Equal level Waves add together for twice total level
Sunday, September 18,
Add together the two waves and you get double the level at every point, but the same frequency
Wave shifted 1/2 Wavelength
Superposition With 180˚ Phase Shift
Yields Complete Cancellation
Sunday, September 18,
Shifting the wave 1/2 wavelength lines up the peaks with the valleys - and mathematically cancels out since you
are adding an equal positive and negative pressure at each point.
Stupid Sound Trick # 1
As mic moves away from source, sound takes longer to reach it.
Signals combine showing effect of phase shift due to distance between
microphones
Sunday, September 18,
Notice that cancellation is not perfect due to simultaneous effect of the inverse square law lowering the level of
the distant microphone.
Adding two different waves
Superposition is mathematical sum at each point on wave
Sunday, September 18,
Two different frequencies combine into a more complex shape
Adding same two different waves with phase shift
Superposition is still mathematical sum at each point on wave
But time shift of one wave changes the resulting wave shape
Sunday, September 18,
Shifting the position of one changes the shape slightly, but not a great deal
Adding another wave
Results in a more complex waveform as the addition continues
Superposition is still mathematical sum at each point on wave
Sunday, September 18,
Add another wave and the complexity goes up - note these three have a special relationship - each one is an
octave higher than the preceding - or we could say there is an octave span between 200 and 400 and between
400 and 800.
And yes, we can keep this up all day!
Sunday, September 18,
Here, we’ve broken the equal octave relationships
Sound Pressure Level
Sound pressure can be expressed in Pascals, Newtons/Meter2 or dynes/cm2
but a more convenient measure is the dB SPL, where:
0 dB SPL = 20µPa = the Threshold of Hearing
The range of sound pressure levels typically is considered to be
from 0dB SPL to about 130 dB SPL, which is roughly the
Threshold of Pain
This represents a ratio of over 3,500,000 to 1 from the quietest
sound to the threshold of pain.
Sunday, September 18,
One of many charts showing equivalent levels
What is the background noise sound level in this room?
Sunday, September 18,
Note - the “loudest sound possible” is based on the assumption that it is not possible to create a sound pressure
that would be higher than the atmospheric pressure of roughly 101,325 Pa. While you can see that you certainly
wouldn’t want to be in this sound field, the bald statement that no higher pressure is possible is not strictly
accurate.
Decibel Calculations
Decibels are always an expression of a ratio of one thing to another.
For sound pressure, the reference is always 20µPa.
You almost never actually have to calculate SPL from a pressure reading
since you’re usually working with a meter that reads in SPL directly.
There are many flavors of dB expression, the suffix following the dB tells
you what the reference value is.
Many people use the references incorrectly, many published specifications
do too.
We’ll talk about this a lot.
Sunday, September 18,
Scary Math Screen # 1
!!
!! = 20 ∗ log
!!
Lp = Sound Pressure in dB
P1 = new Pressure
P0 = Reference Pressure
!! = !!"
!!
+ 20 ∗ log!
!!
LPR = Sound Pressure at reference point
Dr = Original Distance from source
Dm = New Distance from source
Sunday, September 18,
We’ll talk more about these formulas in the classroom next time - they’re good to know but you don’t often need
to use them in their full form.
Friendly dB Screen
With Decibels, you can just add and subtract differences. So, a 6
dB increase changes a 20dB level to 26dB
For Pressure, Distance & Voltage, these relationships hold:
+6dB = 2 x Value, -6dB = 1/2 Value
+20dB = 10 x Value, -20dB = .10 x Value
+40dB = 100 x Value, -40dB = .01 x Value
+60dB = 1000 x Value, -60dB = .001 x Value
Human Hearing Response Behaves Similarly
we respond to changes in pressure on a log scale as well
so dB readings “make sense” in the way they relate to our perception
Sunday, September 18,
Friendly dB Screen # 2
Power and Intensity measurements behave slightly differently
For Power & Intensity, these relationships hold:
+3dB = 2 x Value, -3dB = 1/2 Value
+10dB = 10 x Value, -10dB = .10 x Value
+20dB = 100 x Value, -20dB = .01 x Value
Sunday, September 18,
Measurement Signals
Single frequency tones have limited value as test tones
Test signals should excite the environment in all the frequencies we
are interested to test.
Ideally, they should be quantifiable in a way that relates to human
hearing
They should produce readings on our test gear that are easy to
interpret.
The two most common test signals that we use are
White Noise and Pink Noise
Sunday, September 18,
White Noise
Broadband Random Noise containing equal energy per frequency
Why does it sound “hissy” - balanced to the higher frequencies?
As with levels, our perception of the frequency distribution
across the audible spectrum is logarithmic rather than
linear.
We experience the audible spectrum as divided into
equal Octaves - each successive octave contains twice
the number of frequencies as the one before it i.e. there
is an octave between 40Hz and 80Hz, and an octave
between 80Hz and 160Hz.
Since there are twice as many frequencies in each
octave as we go up the scale, and there is equal energy
per frequency in white noise, there is increasing energy
in each octave.
Sunday, September 18,
On a log-scale graph, White Noise looks like this
!
Sunday, September 18,
Pink Noise
Broadband Random Noise containing equal energy per Octave
A -3dB per Octave filter is applied to White Noise
This results in a roll-off which exactly counteracts the
increase due to the doubling of frequencies
!
Sunday, September 18,
Using Pink Noise, Let’s Listen to some Levels
Background Noise Level
80 dB SPL
6dB Difference
3dB Difference
1dB Difference
Inverse Square Law -6 dB per Doubling
Critical Distance
Sunday, September 18,
80 Hz Pattern with no reflections in the room
Sunday, September 18,
80 Hz Pattern with end wall reflections only
Sunday, September 18,
80 Hz Pattern with 4 wall reflections
Sunday, September 18,
500 Hz Pattern with 4 wall reflections
Sunday, September 18,