Loudspeaker Facts:
Directivity as a Design Issue
Page 2
First page - "Throw" Away Lines
The Light Analogy
Let's go back to the light analogy, and for the sake of demonstration, lets say the bass frequencies of 20Hz to 100Hz are red, the frequencies of 250Hz to 1000Hz are blue, and the high frequencies above 1000Hz are green.
In this image we have the impossible ideal represented, this shows a single loudspeaker device with the same directivity at all frequencies, giving us the same frequency balance (white color) at all listening positions. This only exists in advertising literature and in the minds of salesmen.
In this image we can see a room lit by a red bass lamp, note that it is higher in level near the front, and gets dimmer towards the back, just like we found outdoors. There is a lot of red light below and behind the bass device as well.
In this image you can see the balcony is slightly more brightly lit by our blue mid light, indicating our speaker system is pointed at the balcony from the mounting position above the stage at the front. This "blue" frequency range is more directional than the red low frequencies, but less directional than the green high frequencies.
In this image you can see the more directional green high frequency light has been able to provide better control of the coverage and really get all the way to the balcony. This is typically where the rated coverage of a loudspeaker system is derived from, but that has little or no relationship to the coverage through the entire frequency response of the loudspeaker system.
In this image, we've got the bass, mid and highs on and you'll notice that there is variation in color in the balcony coverage, it is a bit greenish yellow, it isn't getting enough bass in the balcony to make the coverage balanced (white light), so we'll have to try to fix that.
In this image, we've turned up the bass to get the frequency balance in the balcony, but look at how red the floor has become in the process. The bass levels on the floor and especially on the stage or altar are much higher. This system will feedback in the low frequencies long before you can make the voice microphone sound natural, but the balcony will sound OK for pre-recorded music. This is what typically happens with a full range "long throw" box when it is used to cover a distance beyond what its directivity would support.
This loudspeaker cluster design is tricky business
Designing a loudspeaker cluster involves more than just checking the coverage at 2,000Hz in a computer modelling program. It is important to understand that loudspeakers have variation in directivity, and that just because it looks good in the computer model at 2kHz, does not mean that it will sound the same in the balcony, or the back of the room. In the light analogy example above we could add all sorts of green spotlights to cover the balcony, but we can't get beat the inverse square law and it's effect on the red bass lamp. Because the low frequencies are near omnidirectional like a standard light bulb, you can't "point" them at the balcony. The low directivity of the low and low-mid frequencies will limit the permissible "throw" distance of any loudspeaker system in any venue. You may get intelligibility in the balcony, but you won't get a nice balanced frequency response. This may require a balcony fill loudspeaker to achieve the musical response and natural voice quality that is required for a church or theatre.
First page - Throw Away Lines
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