If you look at the specifications of speaker boxes, you should see something like Dispersion (H x V): 90 x 50. This is stating the nominal angle of high frequency dispersion in the horizontal and vertical plane. These figures can be used to calculate the nominal spread of the high frequencies in the horizontal and vertical plane (who said all that trigonometry at school was useless?)
Using the calculator below, simply enter the values in the horizontal and vertical angle boxes. Also enter the distance of the measuring point to the speaker (there are actually two calculators below, one for feet and one for metres).
Example 1: The seating area of a small church or hall is 10 meters deep. Use “5” as the throw distance to determine the width of the high frequencies spread half way down the hall.
Example 2: Ceiling speakers (often rated at 110 degrees (circular spread)) are to be mounted on a ceiling 2.4 metres (8ft) high. If the audience is predominately seated you would use a throw distance of 1.2 metres (2.4m – 1.2m) or 4ft (8ft-4ft). This assumes the height of a sitting person is 1.2 metres (or 4ft). Note: for ceiling speakers with a uniform dispersion angle (circular), simply use the same input (110 degrees) for both horizontal and vertical.
Note: These calculations only give an approximation as the specified figures used are only an average of different high frequencies, and are given for when the SPL at these frequencies are 6dB below the level directly in front of the speaker. However they give a good indication of the expected spread of the high frequencies (required for good speech intelligence).
For those who need to know, the formula used for these calculations is: