Detailed explanation of dynamic speaker structure design

The most widely known and used form of loudspeaker is the dynamic speaker. It can be found in many electronic products, from radios to Bluetooth speakers and public address systems - in fact, anywhere an electrical waveform needs to be converted into audible sound.

Dynamic speakers perform well and can be manufactured relatively easily. However, like all transducers that convert electrical waveforms into sound, their operation can be complex because dynamic speakers connect the electrical and mechanical domains.

Dynamic speakers use the magnetic effect created by flowing electric current as the basis of their operation.

When electric current flows in a wire, a magnetic field appears around it. The effect is increased when the wire is wound into a coil.

Dynamic Speaker Operation

If a coil of wire is placed in a steady magnetic field created by a stationary magnet, the two magnetic fields will interact. Opposite poles attract, like poles repel. This means that current flowing in the coil will cause the coil to be attracted or repelled by the stationary magnetic field - the magnitude of the force is proportional to the current flowing.

If the coil is attached to a large diaphragm, then the sound waves will be transferred into the air more efficiently.

This is the basic concept of dynamic speakers.

Dynamic speaker structure

The construction of dynamic loudspeakers follows a fairly standard format, although there are major differences in electrical and mechanical dimensions and parameters, as well as the materials used.

Cross section through a typical dynamic loudspeaker

(1) Magnet: The magnet provides a fixed magnetic field that the coil's magnetic field opposes. Magnets are usually made of ferrite or strong neodymium.

(2) Chassis/Frame: Dynamic speakers are usually built on a circular frame - sometimes it can be oval. The frame forms the basis of the speaker and provides its structure, although it does not perform any active part in operation.

(3) Diaphragm or Cone: There is a cone or diaphragm at the front of a dynamic speaker. This transfers the vibrations of the dynamic coil into the air, presenting a large surface area. Speaker diaphragms can be made of fabric, plastic, paper or lightweight metal. The outside of the diaphragm is fixed to the edge of the frame - there is usually some undulation at the end of the diaphragm to make it easy for the body to vibrate.

(4) Diaphragm undulation: Diaphragm undulation allows the main part of the diaphragm to move freely in a linear manner.

(5) Dust cap (dome): This part of the dynamic speaker protects the voice coil from dust and dirt.

(6) Voice coil and front voice coil: The dynamic coil is a key element of the loudspeaker. It receives current from the audio amplifier. The current flow generates a magnetic field, which interacts with the permanent magnet, which generates a force that moves the coil, thereby moving the diaphragm to which it is mechanically connected, thereby generating sound waves.

(7) Spider (suspension): This is a flexible corrugated support that holds the voice coil in place while allowing it to move freely. It is not present on some low-end speakers, but provides a very useful support, ensuring that the coil is properly centered. Normally, the space around the coil is limited to maintain maximum efficiency, and the coil needs to not touch adjacent magnets. If it does touch, a harsh sound will be heard as the speaker coil moves in and out.

Speaker unit showing the suspension

Dynamic Speaker Notes

When designing and using dynamic loudspeakers, it is useful to understand some of the design limitations, trade-offs, and issues.

Doppler distortion: Research has found that when a speaker reproduces both high- and low-frequency notes simultaneously, the motion caused by the low-frequency tone causes a Doppler effect on the high-frequency tone.

Cone Construction: The design of the diaphragm cone is a difficult compromise. The cone needs to be rigid, yet have low mass. Cone flexing can cause signal distortion, so a rigid structure is needed. Low mass is also needed to minimize inertia so that the speaker can respond accurately to changes in sound. Fortunately, in recent years, new materials have helped improve performance.

Cone Suspension: The way a dynamic speaker's cone, or diaphragm, is suspended has a major impact on its operation. The suspension needs to keep the cone aligned while also allowing for free movement in and out. Many speakers also ensure centering of the coil by using additional suspension near the coil. Older speakers used what is called a spider at the front of the coil, a helical coil that was attached to the pole piece with screws. This allowed the speaker to be manually centered if it got out of alignment.

Cone resonance: The cone resonance frequency is usually quoted in speaker specifications. Below this value, the output falls off at a rate of 12 dB per octave, so the lower the cone resonance, the better the bass response.

Dynamic speakers are delicate pieces of mechanical and electrical engineering. There is a balance between many conflicting requirements, but the result for a good speaker is that it sounds outstanding.