Instruction Manual

Sound Level Meter Types 2245 – Instruction Manual Page 48 of 110
4.7 Directional Responses
This section gives directional responses for plane progressive sinusoidal sound waves
normalised to the response in the reference direction. Influence of the windscreen UA-
1650 is also given as tables in Appendix A. Only the resulting directional responses are
given here as graphs.
The sensitivity variation graphs show the absolute maximum difference between the
sensitivities at any two sound incidence angles within the specified interval of angles.
For example, at each frequency the value for ±30° is the difference between the highest
and the lowest sensitivity found in a circular cone with an opening angle of 60°, the top
at the position of the microphone and with the reference direction of incidence as the
axis. Because the angle interval defines a three-dimensional geometric shape there is
only one set of graphs with sensitivity variations, also where the directional response is
shown for two planes. The graphs show the maximum variation for the two
measurement planes combined. The sensitivity variations are also given as tables in
Appendix A.
Limit curves are drawn on the sensitivity variation graphs in the following sections.
These curves represent the IEC 61672–1:2013 standard’s acceptance limits.
IEC 61672–1:2013 defines relative directional response in clause 3.17.
The relative directional response for a reference angle can be derived from the specified
directional response by normalization with the values (in decibels) in the reference
direction; that is, for each frequency the value at the reference direction is subtracted
from the values at all angles so that the resulting value in the reference direction is 0
dB.
IEC 61672–1:2013 requires information on the directivity index.
The directivity index definition in the standard is not very precise; therefore, we have
chosen to use the definition in IEC 61183:1994 chapter 4 so that the directivity index in
decibels equals the difference between the free-field response in the reference
direction and the diffuse-field response without the sound level meter electrical
responses.
The directivity index for a specific microphone configuration can therefore be found by
subtracting the Acoustical Response column minus the Electrical Response column in
the relevant diffuse-field table Table A. 4 to Table A. 5), from the Acoustical Response
column minus the Electrical Response column in the relevant free-field table (Table A.
2 to Table A. 3).