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System Calibration

An IRIS System Calibration is a calibration of the entire hardware system (including source, receiver and everything in between) to enable measurement of sound strength related parameters.

The sound strength parameters compare the energy of the measured room impulse response to the energy of the impulse response measured in a free field at 10 m from the source 1.

The sound strength calibration process can be conducted in either a diffuse field or free field. The diffuse field method is preferable because it includes the directional responses of both the source and receiver in one measurement, and requires fewer measurements than the free field technique.

Note

A system calibration document is also used to store the automatic source-receiver distance measurement calibration value. Please see Source-Receiver Distance for details.

Hardware Requirements

  • A source amplifier with a fixed or recallable gain control. The gain setting use for system calibration measurements must be recalled precisely for subsequent room measurements.

  • The types of cables and adapters used to connect the source to the audio interface during calibration measurements must also be used for real room measurements.

    For example, please do not use an unbalanced cable to connect the amplifier to the audio interface if a balanced cable was used during calibration.

Diffuse Field Method

The diffuse field method requires a reverberation chamber. A volume of about 200 m3 is ideal.

Measurement Positions

Since the chamber will not likely be completely diffuse at lower frequencies (standing waves), you should conduct several measurements within the chamber with different source and receiver positions.

We recommend at least 2 source positions and 3 receiver positions (at least 6 measurements in total). The source and receiver should ideally positioned at least 2 m from any boundaries and each other. Their specific orientation is not important.

Steps

  1. Set up your IRIS kit as normal (see Make a Measurement).

  2. Set the source power amplifier to a sufficiently high volume and take note of this volume setting. It will need to be recalled precisely for any future measurements on site.

    Tip

    The level required to excite a reverberation chamber may not be loud enough to excite a large dead space. Please set the power amplifier gain sufficiently high and attenuate the signal going to the amplifier using the output gain slider in the Record Measurement window. IRIS takes the output gain slider into account when processing measurements.

  3. Place the source and receiver in the initial positions.

  4. Create a new IRIS project which will only contain these calibration measurements.

  5. Configure the STIMULUS SETTINGS with a 30 second sweep time and a suitably long impulse response time (at least 10 seconds for most reverberation chambers).

    Note

    You should achieve a decay range of at least 45 dB in each band.

  6. Record all of the measurements.

  7. Your IRIS project should now contain a collection of calibration measurements. Please delete any duplicate measurements before proceeding.

  8. Select Project menu, then Create System Calibration Document.

    Create System Calibration Document

  9. Select the diffuse field option.

  10. Enter the air volume of the reverberation chamber in cubic metres.

  11. Click CALCULATE. The will determine the relative sound pressure level at 10 m from the source in a free field. It may take a few seconds.

  12. The Distance correction is a value in metres added to the automatically calculated source-receiver distance, which may be necessary if there is additional time-delay in the output system such as from digital equalisation in a power amplifier. Please refer to Source-Receiver Distance for details.

    Note that this setting has no affect on the strength calculation.

  13. Enter a description for the system calibration document.

  14. Click SAVE. IRIS will create the system calibration document on disk.

    Note

    IRIS will save additional information about the hardware used for making the calibration measurements, including the Receiver and Audio Device. IRIS will give a warning if you try to use a system calibration document with hardware of a different type. However, it's still up to you to check your hardware setup is the same as during calibration measurements.

Theory

The relative sound pressure level 10 m from the source in a free field (LpE,10) is calculated from spatially averaged diffuse field measurements using the following:

L_{pE\mathrm{,10}} = L_{pE} + 10\log\left(\frac{0.16V}{S_{\mathrm{0}}T}\right) - 37\ \mathrm{dB}

where

  • LpE is the spatially averaged sound pressure exposure level measured in the reverberation chamber.

  • V is the volume of the reverberation chamber in cubic metres.

  • S0 is equal to 1 m2.

  • T is the reverberation time in seconds.

Free Field Method

The free field method requires an anechoic chamber, but a sufficiently large reverberant space may be an acceptable approximation.

ISO 3382-1:2009 1 specifies positioning the receiver at least 3 m from the source.

The ISO standard suggests making measurements at 12.5° around the source to smooth the directional response of the source (horizontal plane assumed).

Tip

It may be more practical to leave source and receiver stationary and to rotate the source about its centre.

Large Reverberant Space

Using a large reverberant space will probably not comply with the standard, since it will be difficult to place the receiver more than 3 m from the source without being affected by the room's reverberance.

Ideally, the 3 m condition should be met as well as the source and receiver being positioned well within the room's reverberation radius. They should be placed far away from any room boundaries.

If possible, place a large piece of highly absorptive material on the floor between the source and receiver to eliminate the floor reflection.

Steps

  1. Set up your IRIS kit as normal (see Make a Measurement).

  2. Set the source power amplifier to a sufficiently high volume and take note of this volume setting. It will need to be recalled precisely for any future measurements on site.

    Tip

    Set the power amplifier gain sufficiently high, and if necessary, attenuate the signal going to the amplifier using the output gain slider in the Record Measurement window.

  3. Set up the source and receiver.

  4. Accurately measure the source-receiver distance (or use the automated source-receiver distance measurement feature of IRIS).

  5. Create a new IRIS project which will only contain these calibration measurements.

  6. Configure the STIMULUS SETTINGS with a 30 second sweep time and a suitably long impulse response time.

    Note

    You should achieve a decay range of at least 45 dB in each band.

    Tip

    If you are measuring in a large reverberant space, capture the whole decay as you can window the response later.

  7. Record all of the measurements.

  8. Your IRIS project should now contain a collection of calibration measurements. Please delete any duplicate measurements before proceeding.

  9. Step through each measurement and inspect the waveform to identify a suitable window size to isolate the direct sound.

    Note

    Windowing may limit low frequency information. The window should be as long as possible.

    Tip

    If you measured in a reverberant space, locate the latest possible time after the direct sound before the first reflection.

  10. Select Project menu, then Create System Calibration Document.

    Create System Calibration Document

  11. Select the free field option.

  12. Enter the source-receiver distance in metres.

  13. Enter the direct sound window duration in milliseconds.

  14. Click CALCULATE. The will determine the relative sound pressure level at 10 m from the source in a free field. It may take a few seconds.

  15. The Distance correction is a value in metres added to the automatically calculated source-receiver distance, which may be necessary if there is additional time-delay in the output system such as from digital equalisation in a power amplifier. Please refer to Source Receiver Distance) for details.

    Note that this setting has no affect on the strength calculation.

  16. Enter a description for the system calibration document.

  17. Click SAVE. IRIS will create the system calibration document on disk.

    Note

    IRIS will save additional information about the hardware used for making the calibration measurements, including the Receiver and Audio Device. IRIS will give a warning if you try to use a system calibration document with hardware of a different type. However, it's still up to you to check your hardware setup is the same as during calibration measurements.

Theory

The relative sound pressure level 10 m from the source in a free field (LpE,10) is calculated from directionally averaged free field measurements using the following:

L_{pE\mathrm{,10}} = L_{pE,d} + 20\log\left(\frac{d}{10}\right)\ \mathrm{dB}

where LpE,d is the directionally averaged sound pressure exposure level measured at a distance d metres from the source in a free field.

Apply System Calibration

The system calibration document can be specified before or after a measurement has been recorded.

Before

In AUDIO SETTINGS, specify the desired System Calibration document, and future measurements will use this calibration document.

After

You can set or change a system calibration document after a measurement has been recorded.

  1. Select Change System Calibration from the Measurement menu

    Change System Calibration

  2. Select the system calibration document from the list.

  3. If you want to apply this document to all measurements in the project, check Apply to all...

  4. Press OK.

Note

If you attempt to apply a system calibration document which was conducted with different equipment (different Receiver and Audio Device, IRIS will let you proceed but will give you a warning.

In-Situ Calibration Check

Before conducting any sound strength measurements, we highly recommend making an initial verification measurement.

Soon after calibrating your system, set up the source and receiver very close to each other. The idea is to roughly simulate free field conditions by placing the source and receiver well within the reverberation radius of any space. For example, the photo below shows a TetraMic placed 15 cm from a loudspeaker grille on our dodecahedron unit.

In-Situ Calibration Check

Conduct a measurement and make a note of the mid-frequency G.

Each time you go on site, repeat the above setup and conduct a measurement. The mid-frequency G value should be very similar (within 1 dB). If different by much more than 1 dB, there is probably an issue with your hardware and you should carefully check your hardware setup.

It is possible to approximately predict what the mid-frequency G value should be for this verification measurement:

G_{d} = 20\log(10 / d)

where d is the short distance in m between the acoustic centres of the source and receiver.

In the photo above, the TetraMic is 15 cm from a grille, which happens to be 30 cm to the acoustic centre of the dodecahedron. The predicted G value is therefore 30.5 dB, and in practice, values of 30.5 ±0.5 dB are measured on site when the system is working correctly. Highly reverberant spaces may increase this value slightly.


  1. International Standard ISO 3382-1 Ed. 1, Acoustics – Measurement of room acoustic parameters – Part 1: Performance spaces (2009).