Sound Field Measurement and Calibration

What needs to be done: 

Routine calibration of the test booth sound field is necessary for accurate threshold measurements during sound field testing.     

Things to be aware of:  

There are currently no standards for sound field calibration.  For such a standard to exist, sound booths would have to be standardized essentially as a coupler.  During testing it is important to ensure that the sound field is uniform for the position in which a subject will be tested.

Sound field calibration is carried out for two reasons.  First, it establishes the relationship between sound pressure level (SPL), measured at a specified point in the sound field and the corresponding attenuator setting on the audiometer (assuming that the audiometer is calibrated).  Second, it establishes the SPL corresponding to normal thresholds (audiometric zero) so that individual patient’s hearing levels may be compared to this standard. 

The following are factors that need to be considered when conducting measurements or calibrating a sound field:

1. Sound Delivery System
1. Type of signal to be delivered
2. FM (frequency modulated) or NB (narrow band) noise – provide frequency specificity and stable SPL’s

More detailed information on this topic can be found here

2. Ambient Noise
1. Conduct measures with a sound level meter with all equipment on (internal noise that would normally be present)
2. Calculate lowest level at which a threshold can be accurately obtained (ensuring that thresholds are not being masked)
3. Check that these are within standards (refer to ANSI S3.1-1977 for ears uncovered)
4. Ambient noise measures should be taken twice per years, or minimally annually
5. Ambient noise measures should also be taken when there is a new piece of equipment in the sound booth, or a new noise source in the testing vicinity

More detailed information on this topic can be found here

3.  Constant position for subjects
1. SPL changes with position changes (this especially applies for children)
2. Body baffle effects are also a factor

More detailed information on this topic can be found here

4. Speaker Location
1. Setting at an azimuth to the listener (0, 45, or 90 degrees)
2. High frequencies should come out of the speaker at ear level (on azimuth with the speaker) for the subject with a two or three-way speaker system (shorter wavelengths are more directional).

More detailed information on this topic can be found here

5. Room construction and Characteristics
1. Reflective materials, constant object position, constant subject position, single or double walled sound booths

More detailed information on this topic can be found here

6. Calibration methods
1. For selecting a test position when coming into a new situation or equipment change occurs

More detailed information on this topic can be found here

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