What are PA speakers

7. Loudspeaker boxes

At the end of the signal chain, the signal that we have amplified is converted back into an acoustic sound event. This is done by loudspeakers that work according to the electrodynamic principle.

In addition to the loudspeaker, which of course should be of good quality, the housing also plays a decisive role in the sound. Because of this very complex sound shaping, loudspeaker boxes are considered to be the weakest link in the signal transmission chain (see also online guide on the subject of "loudspeakers").

Online guide speakers

The following is important:

  • The impedance of a box (resistance specification) must match the output impedance of the output stage. Common values ​​here are 4 or 8 ohms.
  • The higher the RMS value, the more resilient the loudspeaker is. The specification is made in watts, e.g. B. 100 W RMS. The output power of the amplifier must always be higher than the RMS nominal load capacity of a loudspeaker, based on the impedance of the amplifier (power / impedance). This avoids the dreaded clipping (overdriving) of the power amplifier, which in the worst case can destroy the loudspeakers.

Sound pressure

When people talk over loudspeakers, most of them care about how loud it is. More or less volume is perceived as a change in sound pressure, i.e. the air pressure that is generated by the loudspeaker membrane. It should be noted that the sound pressure difference audible by humans is rather modest. The hearing threshold is 0 dB, the pain threshold (about a jet engine) is 130 dB. A high dB value therefore indicates high sound pressure or high efficiency.

If you compare two loudspeakers, one of which has 3 dB less sound pressure (measured at 1 W / l m), then that practically means that the weaker loudspeaker needs twice the amplifier power to be just as loud as the one with the higher sound pressure rating. If I want to have 3 dB more sound pressure with the weaker loudspeaker, I have to double the amplifier output. A loudspeaker with a high sound pressure level shows its efficiency by being significantly louder than a loudspeaker with a lower sound pressure level with the same amplifier power.

Transmission range

The term "frequency response" is often found in data sheets for devices and loudspeakers. This is to indicate how the transmission quality of a sound signal is by the device. Transmission of all frequencies would be optimal - unfortunately this is technically impossible. By and large, we are interested in the range between 20 and 20,000 Hz (although most people can only perceive frequencies up to approx. 16,000 Hz). The human voice is in the range between 80 - 16000 Hz, which can be reproduced well with a full-range loudspeaker (over the entire frequency range). Full-range loudspeaker boxes usually contain two or more individual loudspeakers, which are optimally matched to one another thanks to an internal crossover. Full-range loudspeakers are the ideal solution for simple sound reinforcement tasks, are easy to transport, easy to set up and are easy to expand (more on this in a moment).

If your music also places emphasis on the lower frequency range (bass guitar, bass drum, piano and synths, etc.), the full-range loudspeaker must be given a helping hand at this point. Additional bass speakers must be connected. Many bass boxes have an integrated crossover network that provides the signal that has been freed from the bass component (and reproduced by the bass speaker) at an output socket. The full range box can then be connected there. A solution with an external active crossover is more powerful but much more complex. Here the signal in front of the power amplifier is divided into precisely defined frequency ranges (corresponding to the transmission range of the speakers used), then the divided frequency ranges are each amplified and routed separately to the appropriate speakers. A separate output stage is required for each frequency range.

Connections Fig. Left: passive bass box; Fig. Right act. Crossover

Active loudspeaker boxes

Active satellite systems are very practical, especially for smaller sound reinforcement tasks. Two small tweeter / mid-range speakers are controlled by an active subwoofer, inside of which all crossovers, amplifiers, filters, etc. are permanently installed. Not only is it easy to transport and the short set-up time is an advantage, such compact systems can also boast sound technology. The full-blown subwoofer with active crossovers in combination with the two satellite speakers usually sounds much better than two full-range boxes. In addition, with active loudspeaker boxes there is no amplification by an output stage - the output stage is already permanently integrated in the box. Active boxes can be connected directly to the mixer.