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The DecibelInverse Square Law / SPL Meters
AUD202Audio and Acoustics Theory
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Last Week >
The Human Ear and the Hearing ProcessNoise Induced Hearing Loss
Hearing ProtectionOH&S Principles
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UPCOMING REPORT!!!
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1000 wordsRequires Reference and Bibliography SectionsLots of things to follow in the JMC Style Guide
Noise Induced Hearing Loss Report
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Upcoming Events
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THE DECIBEL
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The Decibel is a logarithmic value that expresses the ratio
between two quantities.
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Decibel
White noise -3dB per step
White noise -1dB per step
http://www.phys.unsw.edu.au/jw/dBNoFlash.html
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Decibel
White noise -0.3dB per step
http://www.phys.unsw.edu.au/jw/dBNoFlash.html
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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 30dB100,000 watts relative to 1 watt is 50dB100,000,000 watts relative to 1 watt is 80dB
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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
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Calculating a Difference in dB
The difference in dB between 100 and 1
100 / 1 = 100Log100 = 2 Log100 = 2 Bells
The decibel is 1 tenth of a Bell, so:2 Bells x 10 = 20 Decibels
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0dB Reference Values
0dBSPL = 0.00002 Pa
0dBV = 1 Volt
0dBu = 0.775 Volts
0dBm = 0.001 Watts
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The dB Formulas
Sound Pressure LevelsdBSPL = 20 x log (SPL / SPLref)
VoltagedBV = 20 x log (V / Vref) dBu = 20 x log (V / Vref)
WattsdBm = 10 x log (P / Pref)
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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)
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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 voltageVref is the reference voltage (1 volts)
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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 voltageVref is the reference voltage (0.775 volts)
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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 levelP is the measured wattagePref is the reference wattage (0.001 watt)
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Two Line Level Standards
Pro equipment: +4dBu
Consumer equipment: -10dBV
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+4dBu and -10dBV
Pro equipment signal level is +4dBuConsumer equipment signal level is -10dBV
0dBu = 0.775 volts0dBV = 1 volt
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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
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SPL and SIL
Sound Intensity (SIL) is difficult to measure which is why we typically use Sound Pressure (SPL).
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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
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RMS vs Peak Voltage
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SPL METERS
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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).
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Digitech SPL MeterModel: QM 1589
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Bruel & Kjaer 2250 Sound Level Meter
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SPLnFFT Noise Meter
By Fabien Lefebvre
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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).
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Fig.1 A-Weighted frequency response (Au.noisemeters.com, 2014)
Fig.2 A-Weighted frequency response (Au.noisemeters.com, 2014)
Weighted Frequency Responses
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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
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THE INVERSE SQUARE LAW
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The Inverse Square Law
In a free field, doubling the distance from the source results in a level
drop of approximately 6 dB
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The Inverse Square Law
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Next Week >
The Doppler EffectDelay Perception
SPL Meters
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Links
• Institute of Acoustics > ioa.org.uk
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ReferencesAu.noisemeters.com, (2014). Frequency Weightings - A-Weighted, C-weighted or Z-Weighted. [online] Available at: http://au.noisemeters.com/help/faq/frequency-weighting.asp [Accessed 10 May 2014]