Imagine having to listen to music without hearing much bass response. Wouldn’t be too pleasant would it? Everyone wants to hear the low rumbling of explosions in action movies, and the deep sounding kick drum rhythms in rock music. So, if you want to attain a more immersive listening experience, then ask yourself this – What is a Woofer?
From live sound systems used in concerts, to home theatre systems used by consumers, the woofer has become an essential part in almost every type of sound reproduction system that you can think of. Today, we will learn about its design principles and also look at the various specifications that you should consider, before buying one for yourself!
Woofer? Why Use It?
A loudspeaker driver that is designed to produce low frequencies is called a woofer. These frequencies typically range from 60 Hz to 250 Hz. Most woofers are built based on the electrodynamic driver design, which usually consists of a stiff paper cone, that is driven by a voice coil (attached to the back of the speaker cone) surrounded by a magnetic field.
Both the voice coil and the magnet actually functions as a linear electric motor. According to Fleming’s left hand rule for motors, when current flows through the voice coil, the coil will move in a direction that is perpendicular to the magnetic field (caused by the magnet), causing the coil to push or pull on the cone. Thus, the vibration of the cone will produce sound waves.
Take note that in 2-way loudspeaker systems, the lower frequency drivers (mid-woofers), are designed to also cover frequencies from 2000 to 5000 Hz. Since the 1990s, a type of woofer called subwoofer, which is designed for very low frequencies only, has been commonly integrated into home theatre systems and PA systems, in order to handle frequencies as low as 20 to 120 Hz.
Let us now look at the topics we’ll be discussing for today:
- Design principles
- Power requirements
- Frequency range
A high quality woofer will be able to effectively convert a low frequency amplifier signal to mechanical air movement without sacrificing fidelity. This process can be enhanced and at the same time, be made complicated with the use of a loudspeaker enclosure. If done well, many problems (such as linear excursion requirements) associated with the design of a woofer will be reduced.
Ideally, the woofer and its enclosure must be designed to be complimentary. The enclosure is typically designed to be suitable with the characteristics of the speaker or speakers used. The size of the enclosure largely depends on the wavelengths (of the lowest frequencies) that needs to be reproduced, and it is also bigger than the ones used for mid and high frequency speakers.
The woofer’s cone motion has to be controlled in order to have a faithful reproduction (through sound waves produced by the cone) of the audio electrical signal that is going into the voice coil. A major challenge is to actually damp the cone with no audible distortion, so that it will not continue to vibrate, causing ringing, when the instantaneous input signal falls to zero each cycle.
Another prevalent problem is to manage high excursions, which is usually required in order to reproduce loud sounds with minimal levels of distortion. There are also challenges in presenting an electrical impedance (to the audio power amplifier) which is not too far from constant at all frequencies.
A woofer’s power rating basically states the amount of power the woofer can handle without damage. Take note that many manufacturers cite electrical power “peak” ratings that are attainable only for very brief moments without damage. Power ratings become vital when the woofer is pushed to its limits due to factors such as amplifier failure or overload conditions, and high output applications.
In situations where there is high-volume output, “power compression” may occur. This is a condition where the sound output level decreases after prolonged high power activity. It is caused by the voice coil heating up, and increasing its resistance. Further heating can physically warp the voice coil, causing scuffing or shorting due to electrical or mechanical damage.
In principle, woofers that are designed for PA systems and instrument amplifiers are similar to home audio woofers, except that they have a more heavy-duty build. Their cabinets are typically more durable (for rugged handling) and they have larger cones to handle higher sound pressure levels. Their voice coils are also more robust to withstand higher power, and higher suspension stiffness.
Woofers used in live sound reinforcement systems and PA systems are typically engineered to have high efficiency and high power handling capacity. However, most of the time, the trade-off for having high efficiency is a relatively low excursion capability (inability of the cone to move “in and out” as far as many home woofers can), as they are intended for horn or large reflex enclosures.
Theoretically, most humans are able to hear frequencies as low as 20 Hz (at ordinary sound pressure levels). For an accurate reproduction of the lowest tones, a woofer, or group of woofers, must move a relatively large volume of air, which becomes more arduous at lower frequencies. The larger the room, the more air the woofer’s movement will have to displace in order to produce the required sound power at low frequencies.
We have come to the end of this article. I hope you have learnt a thing or two about woofers and how they actually work. Do you find yourself needing a woofer for your home stereo system?
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