Lesson 5: Speakers

Speaker Parts

Speakers and microphones operate on the same principles, and can theoretically do each other's jobs.Just the same as there are Dynamic microphones that work on a moving coil in a magnetic field, there are also Dynamic Speakers, which use a moving coil in a magnetic field. The obvious difference is that the microphone generates electricity as sound waves move the diaphram which is physically connected to the coil, whereas the speaker coil moves in the magnetic field creating sound waves by in turn moving the diaphram (cone) connected to the voice coil.

In the diagram can be seen the magnet with the soft iron casing extending the south pole up beside the northpole, thereby concentrating the magnetic field in the airgap between the two pole faces.It is in that airgap is located the voice coil, which is wrapped around the aluminium tubular coil former. The magnetic field generated by electrical current flowing through the voice coil causes the voice coil and attached cone to move in or out depending on the direction of the current, and because the audio signal is AC the cone will then vibrate back and forward at the frequency of the audio signal generating sound waves.

Piezo Speaker

The second most common type of speaker works on the piezo electric principle we studied in the sources section, except in reverse. A piezo crystal vibrates when an alternating current such as an audio signal is applied to it. By attaching a speaker diaphram to the crystal the sound waves generated become audible.

These types of speakers are much more comfortable with high frequencies, and in fact usually don't even respond to frequencies below about 1.5kHz. They are so good at rejecting low frequencies that they don't even require a crossover network. To increase efficiency and directionality they are usually equipped with an exponential horn casing.

This is not the end for the speaker lesson, because the most important thing about a speaker is not it's principle of operation, but it is the box, or enclosure that it is mounted in. Speakers are about creating sound waves made up of compressions and rarefactions by pushing and pulling the air. If a speaker is not mounted in a box then what happens is that the air that is pushed in front of the cone rushes around and fills the rarefaction created on the other side of the cone. You will feel and sometimes see the vibrations of the cone but you won't hear very much.

Sealed enclosure

The solution is to create a baffle to stop the air rushing around between the front and back of the cone. These baffles come in many different forms, and each one has it's own characteristics.

The most efficient baffle is if we cut a hole in the wall or ceiling of the room and mount the speaker in the hole. This is called an "Infinite baffle". The problem here is, of course, not everyone likes you cutting holes in their walls just so you can have the most efficient speaker system.

A more practical way of creating an infinite baffle is to mount the speaker in a "Sealed Enclosure" which is just a box with no holes in it. This creates its own problem in that as the speaker cone moves out it creates a low pressure zone inside the box, and when it moves in it creates a high pressure zone inside the box. Both of those conditions cause a resistance to the movement of the cone, which thereby causes a distortion to the wave it is producing.

This is not always bad, for example; whereas hifi speakers are designed to be soft and light in order to follow every tiny movement of the studio processed recording, musical instrument and PA speakers have heavy, stiff cones to protect them from all the harsh transients (short, loud bursts like thumps and pops) of the raw sound. Having an enclosure like this adds extra protection to the speaker. This type of enclosure was used in the popular Roland "Cube" series of instrument amplifiers.

A bass reflex enclosure

Unfortunately, putting speakers in boxes has other side effects. The biggest being resonances. Every enclosed area (room, box) has resonances caused by the container's dimensions. If you want to know more do some research into Helmholtz resonators. A simple explanation of this process is that you can change the resonance of a container (eg bottle) by altering the shape of the neck. In speaker terms, this means if we put a "port" (hole) in the box we can affect the resonance of the speaker box by altering the shape of the port. The most common of these "Ported Enclosures" is called the "Bass Reflex" enclosure. By changing the length of the tube behind the port we can reduce the resonance peak of the enclosure giving a more consistent frequency response to the enclosure. Another way to reduce resonances is to stop the waves bouncing around inside the box by lining it with acoustic absorbing material (wadding).

Musicians have long known about the efficiency gained by putting an exponential horn shape at the noisy end of their instrument. This is no different when it comes to speaker enclosures. By placing a horn shape in front of the speaker it increases the efficiency and directionality of the speaker. Horn shapes are however related to wavelength (ie, frequency) and, as the frequency goes down the horn length needs to be increased to maintain its effectiveness. This is ok for High frequencies where the wavelength is measured in millimetres and centimetres like the piezo one in the picture above, but when wavelengths start to measure more than half a metre its not practical to build a straight horn in front of the speaker.

Folded horns

Enter the "Folded Horn" enclosure. This type of structure allows increased wavelengths (ie, lower frequencies) without having to use long, straight horn structures in front of the box, somewhat similar to how a french horn is able to reach the same low notes as a trombone.

J Bin

There are 2 main types of folded horn configurations: the "J bin" and the "W bin". The J bin has a single horn and the W bin has a double horn. These are very popular with very big sound systems because of their efficiency, but unfortunately there are 2 drawbacks. One is the size of the box, and the other is the fact that the speakers are facing backwards, and this means can not be used in conjunction with conventional front facing speaker enclosures without a phase shifter, otherwise the wave coming from the folded horn will be 180 degrees out of phase with the other and the two waves will cancel each other out.

As it happens, big sound systems based on using different boxes for low, mid, and high frequencies are gradually being superceded by Arrays of identical, "fullrange" boxes (many of which carrying their own inbuilt amplifiers).

I remember going to a concert in a sports stadium back in the early 90's which was sponsored by Bose. At the time their main stage speaker was the 802, which was built around 8 four inch speakers in each box. For their front house they had 2 stacks of 802s numbering about 40 on each side of the stage. Even back then the sound was pretty huge. The concept being that many small cones can move as much air as a couple of large ones. These days, of course, the speakers in the arrays are far more efficient, and most are built using 3 different speaker sizes in each box.

We're now ready to move on to Effects Processors 1. Click here to go to the lesson about Time and Amplitude Processors, or click here to return to the Stage & Studio Equipment index page.