Woofer
From Wikipedia, the free encyclopedia
Image:Modernwoofer.jpg Woofer is the term for a loudspeaker driver that is designed to produce low-frequency sounds, typically from around 40 Hertz up to a few thousand Hertz. The name, derived from one of the words for a dog's bark, contrasts with the name used for speakers designed to reproduce high-frequency sounds, tweeters.
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[edit] Description
Nearly all woofers are driven by a voice coil in a magnetic field, connected to an amplifier. The voice coil assembly is an electric motor. When current flows through the voice coil wire, the coil moves according to Fleming's left hand rule, causing the coil to push or pull like a piston. The voice coil is cemented to the back of the speaker cone, which creates sound waves as it's pushed back and forth.
[edit] Woofer design
There are many challenges in woofer manufacture such as stopping the cone cleanly at each end of the in/out cycle, loudness which requires the cone to move farther in and out, and ringing when the cone is underdamped. There are challenges with maintaining a stable impedance. Woofer design is about effectively converting a low frequency amplifier signal to mechanical air movement with high efficiency.
Resonant frequency is one of a woofer's parameters and is determined by the compliance (flexibility) of the cone suspension, the mass of the cone, the magnetic field strength and the air resistance behind it. The lower the resonant frequency, the lower the frequency of sound that may be produced without distortion. Resonant frequency is listed in the Thiele/Small parameters as Fs.
All woofers have electrical/mechanical properties that dictate the correct box size and crossover components for a given finished loudspeaker. A given woofer may work well in one application and not in another. It is important to know and understand the Thiele/Small parameters in order to build a satisfactory loudspeaker in an enclosure.
[edit] Cone materials
All cone materials have advantages and disadvantages. The three properties designers look for in cones are light weight, stiffness and lack of coloration/ringing. Exotic materials like Kevlar and magnesium are light and stiff but can have ringing problems. Materials like paper, coated paper and polymer will ring less but can be heavier and not as stiff.
There are good and bad woofers made with all types of cone materials. However, there is a lot more to driver construction than just cone material.
[edit] Frame design
The frame, or basket, is the metal structure that holds the cone, voice coil and magnet in the proper alignment. There are two types of frames, stamped steel and cast. Stamped steel, as the name implies, is a lower-cost alternative, stamped from sheet metal. The disadvantage of this type of frame is the woofer basket may flex or even resonate if the speaker is driven at high volumes. Cast baskets are more expensive, but are usually much more rigid, and therefore the preferred design.
[edit] Power handling
A popular woofer measurement is power handling, an average amount of power the woofer can take. This rating is not well regulated and many woofer manufacturers advertise exaggerated numbers. The only time wattage ratings become important are in very high volume (loudness) situations and very low amplifier power situations. In high volume situations, a woofer's voice coil may overheat and damage the woofer. In low power situations, the amplifier will clip and send a distorted signal to the woofer, damaging the voice coil. For normal listening level applications, this number can be ignored.
There are two types of power handling. Thermal (heat) and mechanical. Mechanical power handling may be reached in situations such as a ported (also known as bass reflex, or vented) enclosure, where the box no longer retains control of the woofer. Here the woofer will physically travel too far and will be physically damaged. On the flip side, thermal power handling may be reached (particularly in small sealed box designs) when too much power gets fed to the woofer but it does not exceed its stroke capability.
[edit] Public address (PA) and instrument applications
Woofers designed for public address (PA) and instrument applications are similar in makeup to home audio woofers. Key design variances are: Cabinets are built for regular shipping and handling, woofer cones are usually larger to allow for higher sound levels, voice coils are more robust to withstand higher voltages. Generally, a home woofer used in a PA/instrument application will fail in short order. A PA/instrument woofer used in a home application will not have as much low volume detail.
Pro audio woofers have high efficiency, and high power handling. The trade off for high efficiency is a relatively low excursion capability (they cannot move in and out as far as a home woofer does) and limited extreme low bass response. Because of this, most pro audio woofers are not designed to play extremely low in the musical spectrum. 30-40 Hz is usually the lower limit of such components.
[edit] Frequency ranges
Humans can hear down to around 20 Hertz. A loudspeaker that can produce bass down to 45 Hertz will sound full range to most people. Many small loudspeakers are designed to produce bass down to around 80-100 Hertz because it is assumed the end user will be using a subwoofer to cover the bottom 2 octaves. But to accurately produce the bottom octaves, a woofer must be large enough to move an appropriate volume of air for a given room. The larger the room, the larger the woofer will have to be to produce the required SPL in the room.
The chart below defines the general operating ranges of different sized woofers. The green area represents the optimal woofer range while the yellow represents the extended range. The purple area represents the music range of almost all instruments. The lighter purple areas extend the instrument range to include rarely played notes, say the first and last 10 keys on the piano. Comparing the instrument versus driver ranges, one can get an idea of the speaker building problem: no woofer does everything well.
