The Low-Down on Dispersion
The fundamentals of natural sound travel uniformly in all directions — a piano, for example, distributes sound throughout a room. Our hearing favors speakers that reproduce sound in the same way. Wide-dispersion speakers sound more real because they too fill the room. Limited-dispersion speakers seem less realistic because they beam, or project sound into only one area.
Wide dispersion throughout a speaker’s bandwidth is difficult to achieve. Most high-frequency and bass/midrange drive units have good dispersion at the lower limits of their frequency range, but they naturally start to beam as they reach their upper-frequency limits. With high-performance high-frequency drivers beaming occurs beyond audibility. Beaming from midrange drivers, however, occurs within the audible range. Speakers with beaming problems will not sound the same in all areas of a room. They may sound balanced in one area, but nasal, dull, or even harsh and shrill in other areas.
Midrange beaming can be reduced by lowering the crossover frequency. The high-frequency driver’s lower range will then provide wider dispersion and the bass/midrange driver’s output can be rolled off before its dispersion narrows.
This is an effective approach but requires the use of a high-frequency drivers that can handle the vast amounts of power it takes to reproduce these frequencies. This driver must be very robust and as a consequence, will be expensive to produce. Many speaker companies are unwilling to incur the cost of building high-power high-frequency drivers, thus not all speakers have uniformly wide dispersion.
A crossover is a network of electrical devices that divides the audio signal into separate frequency bands and directs them to the individual speaker drivers. The frequency at which it does this is called the crossover frequency, or crossover point.
In order to protect a delicate high-frequency driver from damage, many speaker companies will set their crossovers quite high. Unfortunately, this encourages midrange beaming. (Figure 1)
Speakers with more robust high-frequency drivers and lower crossover points do not suffer from beaming problems — they disperse sound uniformly and widely. (Figure 2)