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An octave is a frequency interval between two sounds. It signifies a doubling or halving of a specific frequency.

In sound analysis, Octave Bands are used to divide a frequency spectrum into bands one octave wide. They are identified by their center frequencies (see table below). The octave-band sound pressure levels are helpful when analyzing the entire frequency spectrum. When a frequency spectrum from 31.5Hz to 8kHz is of interest, Octave Bands analysis allows to divide the spectrum into 9 bands. With this analysis, for example, the octave-band pressure levels between 125Hz and 4kHz can be compared and analyzed.

Sometimes, when more information about a frequency spectrum is necessary, a One-Third Octave Band analysis may be employed. ⅓ Octave Bands take Octave Band analysis even further by dividing each Octave Band into three bands (see table below). This time, when a frequency spectrum from 31.5Hz to 8kHz is of interest, a total of 27 octave-band pressure levels can be compared for detailed analysis.

Octave Band Center Frequencies |
One-Third Octave Band Center Frequencies |

16 Hz |
12.5 Hz, 16 Hz , 20 Hz |

31.5 Hz |
25 Hz, 31.5 Hz , 40 Hz |

63 Hz |
50 Hz, 63 Hz , 80 Hz |

125 Hz |
100 Hz, 125 Hz , 160 Hz |

250 Hz |
200 Hz, 250 Hz , 315 Hz |

500 Hz |
400 Hz, 500 Hz , 630 Hz |

1000 Hz |
800 Hz, 1000 Hz , 1250 Hz |

2000 Hz |
1600 Hz, 2000 Hz , 2500Hz |

4000 Hz |
3150 Hz, 4000 Hz , 5000 Hz |

8000 Hz |
6300 Hz, 8000 Hz , 10000 Hz |

16000 Hz |
12500 Hz, 16000 Hz , 20000 Hz |

The FFT, or Fast-Fourier Transform, is a mathematical algorithm to compute the discrete fourier transform. A Fourier Transform converts signals from the time domain into the frequency domain. A FFT provides a finer frequency resolution than simple octave band analysis.

In the US, Type 1 and Type 2 are standard classification that refer the level of uncertainty, how accurate, an acoustical measurement device is. These standards are:

American National Standards Institute (ANSI) S1.4: 1983 “American National Specification for Sound Level Meters”

- American National Standards Institute (ANSI) S1.43: 1997 “Specifications for Integrating-Averaging Sound Level Meters”

Internationally, the uncertainty of sound level meters are classified as Class 1 or Class 2 and are are governed by several International Electrotechnical Commission (IEC):

- IEC 61672-1: 2003 Electroacoustics – Sound level meters. Part 1: Specifications
- IEC 61672-2: 2003 Electroacoustics – Sound level meters. Part 2: Pattern evaluation tests
- IEC 61672-3: 2006 Electroacoustics – Sound level meters. Part 3: Periodic tests

The Leq, or Equivalent-continuous sound pressure level, is the equivalent steady level of sound pressure produced by steady, intermittent, fluctuating, irregular or impulsive sounds over a given averaging time interval. The Leq is the most useful value for describing a noise environment over a given time period. A-weighted frequency is the most common weighting used.

The equivalent-continuous sound level is given by:

where:

T = averaging time interval

= square of the instantaneous A-weighted sound pressure, in pascals, as a function of time t and averaging time Tstarting at t1and ending in t2

= square of the standard reference sound pressure of 20 micropascals

All Type 1 and some Type 2 sound level meters have detachable mics for separate calibration. A detachable mic can be used with an extension cable when the microphone and preamp has to be placed far away from the sound level meter. It can also allow for sound level meters to be used with other types of sensors, like accelerometers and velocity sensors.

IEPE stands for "Integrated Electronics Piezo Electric". It refers to a type of sensor that has a built-in charge amplifier or voltage amplifier. The sensor signal can then be transmitted over ordinary two-wire or coaxial cables cables. An IEPE equipped sound level meter is capable of producing the needed current to support these sensors.

ICP ® is the trade name for a proprietary standard similar to the IEPE standard. ICP ® is an abbreviation of "integrated circuit piezoelectric", and is a registered trademark of PCB Group, Inc

Other proprietary names for the same principle are CCLD, Isotron®, Deltatron®, Piezotron® and others.

Often used in environmental noise monitoring, the statistical (percentile levels) measure the A-weighted sound level that is exceeded for x percent of the time during a measurement period.

Lower percent levels, such as the L 1 or L 10 , tend to be influenced by loud discrete events over a given time period while the L 90 and higher percentiles are useful for quantifying the background noise with very little influence from nearby discrete events.

An input port to allow the recording of a RPM pulse signal

Removable storage card, typically an SD card or a Compact Flash Card

White, Pink noise signal generator used to measure reverberation times or noise reductions. Some meters may have swept-sine signal generators for low signal to noise environments or for measuring very short reverberation times.

Typically bluetooth communication to replace wires to control the sound level meter from a PC

Reverberation time is defined as the time it takes a sound to decay by 60 dB in a room. Not only is reverberation time an important acoustical room criteria, but it is also use in many ASTM standards to measure acoustical absorption, transmission loss and other metrics.

Dosimetry is the measure of noise to which a person is exposed. Noise dose is the metric used for dosimetry that rates the exposure to noise in a workplace. Noise dose is expressed as a percentage of the maximum that is permitted by regulations.

The exchange rate and criterion sound level are the key factors in calculating the noise dose.

The exchange rate expresses how much a sound level should increase or decrease for doubling or halving of the exposure duration.

The Criterion Sound Level is the normalized 8-hour average level, in dB, that represents to the maximum daily exposure to noise.

The Occupational Safety & Health Administration (OSHA) sets the Criterion Sound Level at 90 dB with a 5 dB rate of exchange (see table below). This means that a worker may be exposed to 90 dB noise for a total of 8 hours. When that worker’s exposure to noise is reduced down to 4 hours, the noise limit is raised by the 5 dB exchange rate up to 95 dB.

Duration per day, Hours |
Sound Level, dB |

8 |
90 |

6 |
92 |

4 |
95 |

3 |
97 |

2 |
100 |

1 ½ |
102 |

1 |
105 |

½ |
110 |

¼ or less |
115 |

NC (Noise Criteria) Curves are standard spectrum curves by which a given measured noise may be described by a single NC number (see graph below). The NC curves are the most widely used method for evaluating background sound in buildings.

The most common way of rating noise using the NC Curves is by the Tangency Method. The Tangency Method involves plotting the measured octave band levels against the NC Curve graph as pictured by the example of a solid line in the graph below. NC rating is determined by the highest NC-Curve tangent to the curve formed by the measured octave levels.

For the example below, the highest NC curve tangent to the plotted levels occurs in the 125 Hz Octave Band. As a result, the NC-51 curve number is used to assign a noise rating to the measured spectrum.

Noise monitoring applications require the ability to record the sound pressure level over time. The sound level meters with the Level vs. Time function can automatically record the sound pressure level at various intervals.

The ability to record the sound for either source identification or for later analysis. The recording can be triggered on a level exceedance or at a set time. The recordings are in .wav format at up to 24 bits at 44.1k samples/sec.