How a loudspeaker driver works

Rohit Balkishan Dubla

Construction

A typical loudspeaker driver is based on the principle of an electric motor - a coil suspended in a magnetic field (from a magnet). An electric current through the coil causes it to create a magnetic field which interacts with the magnet's field causing the coil to move. This movement is converted to air pressure waves via a diaphragm or cone. These pressure waves are perceived as sound. Thus a loudspeaker converts electrical current into sound. Since it converts energy from one form to another (electrical to mechanical) a loudspeaker is also a transducer.

The above image shows the construction of a typical loudspeaker driver. The voice coil is wound on a light cylindrical former and suspended in the air gap by means of the spider. The spider provides most of the restoring force to the coil - to bring it back to its centred position after every excursion. The diaphragm is also joined, at its apex, to the voice coil former and at the edges to the frame via a surround. The surround allows the diaphragm to freely move to and fro whilst keeping it centred. The magnet provides a strong magnetic field within the gap. When a current flows through the coil, the coil experiences a force perpendicular to its plane (direction depending on the current's direction). As the current changes direction, the voice-coil's movement also changes accordingly. Since the voice coil is attached to the diaphragm, the diaphragm moves too, in turn producing sound. The dust cap prevents dust from entering the air-gap and in many instances may also serve the role of a whizzer to extend the high frequency response of the driver. Full-range drivers commonly employ a whizzer.

Types of loudspeakers

Woofers

A typical loudspeaker driver is incapable of covering the entire audio spectrum. This is a direct result of the physical structure of the driver. Low frequencies need a lot of air to be moved and therefore to reproduce low frequencies (or bass), the driver needs to have a large cone. Large cones are heavy and therefore the entire motor assembly of a low frequency driver tends to be heavy. Being heavy, they cannot move rapidly and thus cannot reproduce high frequencies efficiently. Such drivers are known as woofers. There is a class known as sub-woofers which are basically woofers designed to reproduce very low bass frequencies often down to 20 Hz. Woofers tend to be limited to around 30 or 40 Hz on the lower side of the spectrum and in some applications may need to be augmented with a sub-woofer.

Mid-range drivers

These are optimised to reproduce the mid range of the audio spectrum and as such are neither good for low frequencies nor for high frequencies, though many mid-range drivers can reproduce bass adequately and are known as mid-woofers. These are usually smaller and lighter than woofers.

Tweeters

These drivers are usually small and have very light cones. These are most efficient at reproducing high frequencies. Many tweeters have just a modified dust cap for the diaphragm (and no actual diaphragm and frame/basket) and are known a dome tweeters. Dome tweeters are considered better than the usual cone tweeters where good performance is desired.

Speaker systems

As noted above a single driver is incapable of reproducing the full audio spectrum. So, to get a speaker system that covers the entire audio band, we have to combine two or more of the above. The most common and simplest is a woofer-tweeter combination. This is also known as a 2-way speaker and has the woofer to cover the lower (mid-bass) part of the spectrum and the tweeter to cover the higher frequencies. The upper frequency limit of the woofer and the lower frequency limit of the tweeter are same (as a design parameter) and is known as the crossover frequency. Thus frequencies starting at the crossover frequency and increasing are handled by the tweeter and those lower are handled by the woofer. The crossover frequency is reproduced by both the woofer and tweeter. How well the acoustic outputs of the woofer and tweeter blend at the crossover frequency determines how good or "smooth" the system sounds. It is desirable to have the two drivers crossed over such that the combined audio output of the system is flat across the entire spectrum with no peaks/dips at the crossover frequency. 2-way systems are common for small and/or budget systems where they offer a good balance between cost and performance.

The job of splitting the signals between the woofer and tweeter based on the crossover frequency is handled by a crossover network usually simply referred to as 'crossover'. This is a circuit that splits the lower and higher frequencies and sends them to the respective drivers. The crossover for a 2-way speaker has two filter sections: A low-pass for the woofer and a high-pass for the tweeter.

A speaker system may also be 3-way; employing a woofer, a mid-range and a tweeter. High-end speaker systems tend to be 3-way or higher since there is greater emphasis on performance rather than cost. Needless to say, a 3-way speaker system will need a 3-way crossover consisting of three filter sections: A low-pass section for the woofer, a band-pass section for the mid-range and a high-pass section for the tweeter. 4-way and higher speakers also exist but the cost and complexity involved is rather high and as such most 4-way systems are usually 3-way systems with an additional sub-woofer - this should not be confused with home-theater systems where there are four to five satellite speakers and a sub-woofer.

Thiele/Small parameters

Thiele/Small parameters refer to certain electro-mechanical parameters of a loudspeaker driver that are useful in the process of designing an optimum enclosure for that driver. T/S parameters are usually used to design enclosures for woofers and sub-woofers but may also be occasionally used for mid-range drivers. T/S parameters may be provided by driver manufacturers or may be measured. The significance of T/S parameters stems from the fact that the loudspeaker driver is an electro-mechanical device having both electrical and mechanical characteristics and these have a profound role to play while arriving at a suitable enclosure for the driver.

The driver can be likened to a simple mass (that of the cone, spider, voice-coil, etc - collectively called the motor) suspended by a spring (formed by the suspension and spider's compliance or 'springiness'). When the mass is moved by an external force and released, it will vibrate at its natural frequency. The same applies to the driver - it has a certain frequency at which the 'motor' will vibrate most freely - the resonance frequency (F_s). Since the voice coil is part of the motor and is in a magnetic field, it has some back-emf induced in it as it moves within the field. At the resonance frequency F_s, the coil is moving with maximum amplitude and thus has the greatest back-emf induced into it. As a result the voice coil presents the maximum impedance to the electrical current flowing through it, designated as Z_max. The voice coil also has a fixed DC resistance designated as R_e which is basically the electrical resistance of the voice coil. The following figure shows the impedance curve of a typical woofer and the relation between F_s, Z_max and R_e. These three parameters constitute some of the driver's T/S parameters.

In the impedance curve, we can see that at F_s the driver's impedance is maximum (Z_max) falling rapidly on either side of F_s. Beyond F_s, as the impedance drops, it reaches a minimum value Z_nom which is basically the sum of the coil's DC resistance R_e and the coil's minimum inductive impedance. Beyond the Z_nom frequency, the impedance again gradually rises as a result of the voice coil's inductance.

Around F_s the impedance varies by a very large amount and this presents a very complex load to an amplifier. This frequency, along with the driver's resonance magnification (or Q_ts, another T/S parameter described below) also determines how the driver will behave when put inside an enclosure, or, conversely what enclosure would be suitable for that driver.

The frequencies above and below F_s when the impedance is 0.707 times Z_max allows us to estimate the driver's Q_ts. Thus, Q_ts is defined as,

Q_ts = F_s/(F_h - F_l) where F_h and F_l are the upper and lower frequencies around F_s where impedance = 0.707 * Z_max.

Yet another T/S parameter that is most important from an enclosure design perspective is the driver's V_as - this is the volume of air that has the same compliance as the driver's, when acted upon by a piston of area equal to the driver's diaphragm. It is an indication of the driver's compliance. The more compliant a driver is, the more air it would move for the same amount of electrical input. Together with F_s and Q_ts, V_as lets us determine the optimum enclosure volume and type (i.e. sealed/vented) for that driver.

To summarise, the three most important T/S parameters that are required in order to design a suitable enclosure are the driver's F_s, Q_ts and V_as.