What is Sound & How do PA Speakers Work ?

If you drop a stone into a pond it creates waves of water which move out from the impact. When you clap your hands you create waves of air which move out. When these waves hit your ear drum the energy is transferred to the cochlea in your inner ear where it is converted into nerve impulses which you perceive as a sound, “clap”

The pitch of a sound, (how low or high it is) is governed by the frequency of its waves. Low pitch sounds have long drawn out waves whereas high pitch sounds have short bunched up waves.  Frequency is given in Hz which is a measure of cycles or oscillations per second. 1 Hz is exactly one complete wave from peak – trough – peak per second.

The lowest frequency sound Humans can hear is around 20Hz. This means we can just about perceive waves of air vibrating past our ears at anything above 20 oscillations per second. These are the kind of deep rumbling sounds made by earthquakes and distant huge explosions or a space rocket launch for example.

If you attach a loudspeaker to a signal generator and get it to produce a note at around 20Hz you can literally watch the cone driver moving back and forth creating the sound wave. As you increase the pitch it becomes more and more of a blur until you can’t really see the movement (although you can certainly hear the note being produced.)

The cone driver has a metallic coil like a bracelet attached to its back – this fits snugly into a grove within a chunky circular magnet. When a current is applied to the coil it can move back and forth within the magnet. This is essentially all that is needed to create the oscillations that move the air to create sound.

For a sound to be audible it needs to push enough air against your ear drum.  This is known as sound pressure. Because long waves are relatively infrequent they need to be big waves if you are going to hear them. The size or depth of the wave is known as its amplitude; literally the distance from peak to trough. It takes a huge amount of amplifier power and a big cone driver to produce the colossal tsunami sized waves needed to register deep bass notes on your ear drum. This is why you can usually feel the bass more than you can hear it.

Bass loudspeaker drivers are 21, 18 or 15 inches in diameter. They need lots of amplification power in order to shove enough air to produce the long waves of bass that you can hear and feel.

Most bass drivers cover the frequency range from around 35Hz up to around 500 Hz. Producing frequencies of more than 500 Hz is impossible for drivers this size because they are simply too bulky to oscillate much faster than 500 times a second.

Producing very high frequencies is done with compression drivers.  A compression driver is a very small driver with a diameter of 1, 1.4 or 2 inches. These can oscillate at incredible rates, up to about 20,000 times a second. Compression drivers can cater for all the higher frequencies, usually 16 kHz down to about 2 kHz.

A quality loudspeaker should accurately reproduce the signal coming into it by ensuring a comprehensive coverage of all audible frequencies of sound. Average Humans can hear sound waves with frequencies between 35 and 18 kHz.  

Budget speakers comprise a 15” or 12” bass driver with a 1” or 2” compression driver. This is ok but the system completely lacks anything in the mid-range. There is a gap in most budget speakers between about 800 Hz and 2 kHz.

This is ok for most modern dance music because it has very little going on in the mid-range. It’s all bass lines and kick drums with snares and hi-hat. When it comes to classical or vocal rich music and in live sound applications you really miss the coverage of the mid-range frequencies.

In addition to the bass driver and compression driver a professional quality loudspeaker will be fitted with a mid-range driver. These can range in diameter from 4” – 10” but are usually around 6.5”. This fills the gap between 800Hz and 2 kHz and makes a huge difference to the detail and clarity of sound reproduction.      

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