Measuring the noise in passive mode

Low noise is a key factor for high quality and reliable measurements. The performance of the ST90 system will always be limited by different noise sources. A high noise level will mask the echoes and make them difficult to read and interpret. The noise is measured while the ST90 system operates in Passive mode with the transmit pulses disabled.

The ST90 system is turned on and operates normally.

  • The ST90 system is in Normal mode, but TX Power is set to Off to prevent transmissions.

  • The transducer is retracted to its upper position.

  • Neither tools nor instruments are required.

In order to do this test, the ship must be "silent".

  • The water must be as deep as possible. Recommended minimum depth is 100 metres.
  • There must be no other vessels in the vicinity.
  • The vessel must lie still in the water.
  • As much machinery as possible must be turned off. It is particularly important to turn off electrical motors, as well as cooling systems and hydraulic pumps that may cause electric noise.
  • To prevent interference, all other hydroacoustic instruments must be turned off.
Note:

With the vessel in port, the environmental conditions are not satisfactory. In the shallow waters of the port, noise from other vessels, dockyard workers or machinery will cause unreliable test results. If you do this tests in a busy harbour, or with noise sources present, the sensitive receivers will detect all the noise in the nearby waters.

This task allows you to check the noise conditions. To measure the noise level, observe the value presented on the Noise page in the Diagnostics dialog box.

Note:

Without external noise sources the level should not exceed xxx dB. If this level is exceeded in any direction you must locate the noise source(s). If the noise originates on your own vessel we strongly suggest that relevant actions are taken to reduce the noise and thus increase the performance of the ST90 system.

Procedure

  1. If applicable: Make sure that you have sufficient water depth below the keel before you lower the transducer.
  2. If applicable: Lower the transducer to its bottom position.
  3. Select the default settings.
    1. On the Main menu, select User Settings.
    2. Observe the Factory Settings list.
    3. Select the setting you want to use.
    4. Select Activate Selected Setting.
    5. Select OK to apply your changes and close the User Settings dialog box.
  4. At the bottom of the display presentation, select a suitable presentation mode, and activate the Horizontal view.
  5. On the Main menu:
    1. Set Tilt to the correct compensated value.
    2. Set the bearing to the correct compensated value.
  6. Make the following preparations.
    1. On the Main menu: Set Range to 1500 m.
    2. On the Operation menu: Set Tx Power to Off.
    3. On the Active menu:
      1. Set Pulse Type to CW Medium.
      2. Set Frequency to the start frequency: 14 kHz
      3. Set Vertical TX Sector to Narrow.
      4. Set Noise Filter to Off.
      5. Set the Bottom Filter Threshold to Off.
    4. On the Setup menu: Select Diagnostics to open the dialog box.
  7. Select Noise to open the page.
  8. Select Start Noise Measurements.
    1. Observe the noise curve.
    2. Read the maximum and average noise values.
    3. Repeat for selected frequencies.
    4. Record the frequencies and the measured values in the result table.
  9. Finish the test.
    1. Close the Diagnostics dialog box.
    2. Hoist the transducer back up to its upper position.
For each transmit frequency (in kHz), write down the measured noise levels (in dB).
Frequency (kHz)MaximumAverage
14
19
24
RequirementsResults
Maximum noise level (in dB) shown on the Noise page is: xxx If this level is exceeded in any direction you must locate the noise source(s). If the noise originates on your own vessel we strongly suggest that relevant actions are taken to reduce the noise and thus increase the performance of the ST90 system.
Maximum noise level (in dB) measured on the Noise page:  
Date/Signature: