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Os grandes conselheiros on-line Thomann: Choosing a PA System

7. Loudspeakers

No matter how good the rest of the system may be, it can all fall apart at the loudspeakers. There's no digital or analogue debate in this area - it's here that the electrical signals have to be converted to waves of air and pushed out to the waiting audience. Because of this, the speaker cabinet is as important as the ‘drivers’ themselves in the management and diffusion of the sound. You can get deeply immersed in speaker technology if it takes your fancy - our Online Guide to PA Speakers is a good place to start - but here we'll just consider the essential points.

The first issues concern compatibility with the amplifier:

  • The impedance (a kind of electrical resistance, measured in Ohms) of the speakers must match the range of output impedances that the amplifier can handle. Each speaker will usually be rated at 4 Ohms or 8 Ohms. Resistance is a tricky principle, and much depends on exactly how you will connect everything together. The key point is that an imbalance can damage speakers, amplifier, or both, so check the specifications of each element to ensure that they are compatible. Above all, make sure the total impedance of the speakers is within the operating range of the amp. See our Online Guide to PA Amplifiers for more on calculating this.

  • Remember the rule about Watts RMS - match the speaker capacity to the output of the amplifier, allowing about 30% more power from the amp. The idea is that you don't run everything at full volume, but you have enough headroom to cope with the highest peaks.
A typical PA speaker cabinet, with low/mid-range and high frequency speakers

Frequency range

This is an important measure for musicians in particular, because it indicates the most suitable use for a speaker. Is it good for high, mid, or low-frequency sound? Most people can hear sound in the range between 20Hz and 20,000Hz (Hz = cycles per second). The ideal loudspeaker would be able to transmit effectively across the entire frequency range, although in practice this is virtually impossible.

So the key question is whether or not the loudspeaker can transmit consistently and with good quality, within its ideal frequency range. This is known as linear transmission. For the purposes of a general gigging PA, you will need a combination of speakers that offer a broad coverage of the frequency range.

The industry uses frequencies separated by a musical third, and the levels are expressed in decibels (dB). Typically, if a frequency response is shown as 40 - 20,000Hz, +/-3dB, you can assume that the bass frequencies were raised at around the +3dB level, and that the higher frequencies were dampened at around -3dB. The usual warning about manufacturer data always applies - they will tend to use the figures that flatter their equipment, though most reputable companies will show fairly accurate frequency response data and RMS values.

Sound pressure

Sound pressure means volume - the higher the pressure of the sound wave striking your ears, the louder it sounds. It’s measured, perhaps appropriately, in Pa (short for Pascals in this case - strength per unit area).

In fact, humans have a fairly limited audible range, from around 100Pa (the pain threshold for most of us) to about 20uPa. The ear detects these as logarithmic values, measured in deciBels. Zero dB roughly equates to 20uPa, and it usually starts to hurt for us at around 130dB. When considering speakers, the sound pressure level (SPL) is usually shown in decibels. For example a speaker may be shown as 400W RMS, 118dB SPL.

Not all manufacturers use the same measures, but what you are looking for is an indication of how much of the signal received is converted faithfully to moving air. The higher the number, the greater the efficiency and/or sound pressure produced by the speaker. If, for example, you compare two speakers that have a difference of 3dB in their sound pressure ratings, then you can assume that you will need roughly twice the power to achieve the same volume from the unit that is lower rated, and therefore less efficient.

Active loudspeakers

For smaller venues, active loudspeakers are often a cost-effective option. These have amplifiers built-in – you simply connect them directly to a mixer. Typically, the active loudspeaker cabinet will contain mid-range and high frequency speakers. If you are connecting keyboards, or amplifying the bass drum, it's worth adding at least one sub-woofer cabinet to handle the bass frequencies. In these configurations, the bass cabinet will usually contain a crossover switch to distribute the frequencies between the appropriate speakers.

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