Directivity

The formal definition of directivity requires a math-ematical discussion in theoretical acoustics. For our purposes, we will discuss directivity by a simple analogy. Most of us have seen a theatrical production, either as an actor or spectator. And as such, we are familiar with the visual effect of spotlights and floodlights (Figure 1).

Spotlight	Floodlight

Figure 1
A spotlight can concentrate the available light into a smaller area.

Floodlights are used for a “wash” of light covering a large area. Spot-lights are used for selective lighting in order to highlight certain characters or set locations. Generally speaking, spotlights have a higher DIRECTIVITY than floodlights. Spotlights concentrate the available light energy into a smaller area.

The sound coming from the mouth radiates in many directions.	Cupping the mouth with your hands will concentrate the sound.

Figure 2

Directivity, as it relates to acoustics and the technology of sound systems, is very similar in concept. A church sound technician at the back of the church will be more successful in communicating to someone on the front platform if he (or she) cups their hands around their mouth (Figure 2). This effectively concentrates the available sound energy into a smaller area. This can be described as a ratio. The directivity ratio is the amount of energy that radiates in a certain direction as compared to the same amount of energy if it were to radiate in ALL directions.

Directivity is a characteristic that is usually associated with sound sources such as loudspeaker systems. In most specification sheets, directivity is listed as the “Q” of the loudspeaker. The sound system designer can use the measured Q or directivity of the loudspeaker in the sound system design. Knowledge of the room's acoustics and the directivity of the loudspeaker will help the designer. They can predict how well the system will work in that particular room. As the size or the reverberation time of the room in-creases, the requirement for a higher directivity system also increases.

Figure 3
A poor loudspeaker will sound quite different as you change your listening position.

One aspect of directivity will separate a well designed loudspeaker system from a poor system. Directivity is frequency-dependent. That is, directivity of a loudspeaker or system will vary from one pitch or frequency to another. A poorly designed system will sound fine if the listener is directly in front of the loudspeaker system. However, it may sound dull or unnatural if the listener is to the side of the loudspeaker system (Figure 3). A well-designed system, often referred to as a CONSTANT DIRECTIVITY SYSTEM, will maintain a fairly uniform directivity regardless of frequency. This is important, especially within the speech frequency range. The loudspeaker designer's goal is to provide a loudspeaker that sounds the same within the coverage pattern of that loudspeaker. Then the system designer can take that loudspeaker and engineer a system that will produce consistent sound no matter where you sit.

by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com

< Back