AUD202
Audio and Acoustics Theory
The Decibel
Inverse Square Law / SPL Meters
Last Week >
The Human Ear and the Hearing Process
Noise Induced Hearing Loss
Hearing Protection
OH&S Principles
UPCOMING
REPORT!!!
Noise Induced Hearing Loss Report
1000 words
Requires Reference and Bibliography Sections
Lots of things to follow in the JMC Style Guide
Upcoming Events
18 Days - NIHL Report
39 Days - Sound Observations Report
49 Days - Exam
THE DECIBEL
The Decibel is a logarithmic
value that expresses the ratio
between two quantities.
Decibel
White noise -3dB per step
White noise -1dB per step
http://www.phys.unsw.edu.au/jw/dBNoFlash.html
Decibel
White noise -0.3dB per step
http://www.phys.unsw.edu.au/jw/dBNoFlash.html
Logarithms
Logarithms are useful because we can conveniently
represent very large or small numbers, and carry out
multiplication of ratios by simple addition and
subtraction.
Some examples:
1000 watts relative to 1 watt is 30dB
100,000 watts relative to 1 watt is 50dB
100,000,000 watts relative to 1 watt is 80dB
Logarithms
The logarithm of a number is the power which the base
has to be raised to produce that number
The logarithm of 1000 is 3, because 1000 is base 10 to
the power 3
1000 = 10³ = 10 x 10 x 10
Calculating a Difference in dB
The difference in dB between 100 and 1
100 / 1 = 100
Log100 = 2
Log100 = 2 Bells
The decibel is 1 tenth of a Bell, so:
2 Bells x 10 = 20 Decibels
0dB Reference Values
0dBSPL = 0.00002 Pa
0dBV = 1 Volt
0dBu = 0.775 Volts
0dBm = 0.001 Watts
The dB Formulas
Sound Pressure Levels
dBSPL = 20 x log (SPL / SPLref)
Voltage
dBV = 20 x log (V / Vref)
dBu = 20 x log (V / Vref)
Watts
dBm = 10 x log (P / Pref)
Sound Pressure Level (SPL)
Sound Pressure Level is a logarithmic measure of the
sound pressure relative to a reference level
0dBSPL = 0.00002 Pa (20 µPa)
dBSPL = 20log (SPL / SPLref)
SPL is the measured sound pressure (in cm²)
SPLref is the reference sound pressure (0.00002 Pa)
Voltage (V)
Voltage is the potential difference between two
points (e.g. the + and - sides of a battery)
0dBV = 1 volt
dBV = 20log (V / Vref)
V is the measured voltage
Vref is the reference voltage (1 volts)
Voltage (u)
dBu is referenced to 0.775 volts RMS (Root Mean
Square)
0dBu = 0.775 volts
dBu = 20log (V / Vref)
V is the measured voltage
Vref is the reference voltage (0.775 volts)
Power (P)
Power is the rate at which energy is produced or
used
0dBm = 0.001 watts
dBm = 10log (P / Pref)
dBm is the signal level
P is the measured wattage
Pref is the reference wattage (0.001 watt)
Two Line Level Standards
Pro equipment:
+4dBu
Consumer equipment:
-10dBV
+4dBu and -10dBV
Pro equipment signal level is +4dBu
Consumer equipment signal level is -10dBV
0dBu = 0.775 volts
0dBV = 1 volt
SPL and SIL
Sound Pressure Level (SPL) is measured
in Pascals (Pa) and 0dBSPL is 0.00002Pa
Sound Intensity Level (SIL) is measured in
watts per square meter (W/m2). 0dBSIL
is: 10-12W/m2 or 0.000000000001W/m2
SPL and SIL
Sound Intensity (SIL) is difficult to measure
which is why we typically use Sound
Pressure (SPL).
RMS vs Peak
• The peak value is the highest voltage that the
waveform reaches
• The RMS (Root-Mean-Square) value is
the effective value of the total waveform. In audio it
is the continuous or music power that the amplifier
can deliver.
• The effective or rms value of a sine wave of current
is 0.707 times the maximum value of current
RMS vs Peak Voltage
SPL METERS
SPL Meters
In order to measure sound levels we need a
calibrated microphone, preamp and display.
We have various settings to allow us to choose
what we’re measuring (such as transient
response & frequency response).
Digitech SPL Meter
Model: QM 1589
Bruel & Kjaer
2250 Sound Level Meter
SPLnFFT Noise Meter
By Fabien Lefebvre
SPL Graph by Studio Six Digital.
“SPL Graph is an audio level chart recorder for the iPhone… You
can optionally record the audio for the graph, and even email
graph results at the end of a test” (Studio Six Digital 2013).
Weighted Frequency Responses
Fig.1 A-Weighted frequency response
(Au.noisemeters.com, 2014)
Fig.2 A-Weighted frequency response
(Au.noisemeters.com, 2014)
SPL Meters
The options to understand on an SPL meter
are:
• A-weighting versus C-weighting
• Fast or slow response
• High or low (volume range)
Remember, the A-weighting is close to human
perception of loudness
THE INVERSE SQUARE LAW
The Inverse Square Law
In a free field, doubling the distance
from the source results in a level
drop of approximately 6 dB
The Inverse Square Law
Next Week >
The Doppler Effect
Delay Perception
SPL Meters
Links
• Institute of Acoustics > ioa.org.uk
References
Au.noisemeters.com, (2014). Frequency Weightings - A-Weighted, Cweighted or Z-Weighted. [online] Available at:
http://au.noisemeters.com/help/faq/frequency-weighting.asp [Accessed
10 May 2014]