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Tue 30 May 06 @ 10:22 pm
SUBWOOFERS
History
The first commercial subwoofer was developed during the 1970s by Ken Kreisel, current president of M&K Sound/Miller & Kreisel Corporation in Los Angeles. Kreisel's business partner, Jonas Miller, owned a high-end audio store in Los Angeles, and customers of some of the highest quality electrostatic speakers complained about a reduction of bass response in the electrostatics, compared to conventional loudspeakers; Kreisel's solution was to design a powered loudspeaker that would reproduce only those frequencies that were too low for the electrostatic speakers to convey and thereby fill in the missing sonic information[1]. The first known use of a subwoofer in a recording session was for the mixing of the Steely Dan album Pretzel Logic when recording engineer Roger Nichols arranged for Kreisel to bring a prototype of his subwoofer to Village Recorders. Further design modifications were made by Kreisel over the next ten years (and continuing to the present day), and in the 1970s and 1980s by engineer John P. D'Arcy; record producer Daniel Levitin served as a consultant and "golden ears" for the design of the crossover network (used to partition the frequency spectrum so that the subwoofer would not attempt to reproduce frequencies that were too high for its effective range, and so that the main speakers would not need to process frequencies that were too low for their effective range).
Overview
Subwoofers use drivers (woofer) with cones typically coming in 10" or 12" sizes, but can be as large as 34", and as small as 4".
Nearly all woofers are driven by a voice coil in a magnetic field, connected to an amplifier. The voice coil assembly is basically an electric motor. As voltage is applied to the coil, it generates an electromagnetic field. This field is either repulsed or attracted to the fixed field of the magnet which surrounds the voice coil, 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.
Larger diameters tend to be advantageous because low frequencies involve moving a great deal of air. A recent trend has been for high excursion. Excursion is defined as how far the cone can linearly travel from its resting position. Some newer models can move as much as +/-2.5" , yielding an overall controlled displacement of 5" with the voice coil of the driver staying inside the magnetic field.
Subwoofers are usually powered by a high power amplifier, and often an electronic crossover with a Low-pass filter is used to ensure that higher frequencies will not be directed to the subwoofer.
The need to reproduce these frequencies has increased since older formats, such as vinyl records, have been replaced by digital formats, such as CDs, and particularly 5.1 formats such as Dolby Digital and DTS, in which the ".1" channel is dedicated solely to the subwoofer. The .1 channel is usually dedicated to extended bass frequencies, for example, the low frequencies of a gunshot, double bass, or thunder. This track is often used aggressively by mixing artists.
Professional audio
Subwoofers are found in professional applications such as live concerts, movie theatres, various other sound reinforcement applications (ranging from nightclubs to theme restaurants) and studios. Some of these applications require subwoofers designed for very high sound levels, such as the JBL 4645 or the Electro-Voice TL440 which use a larger than average 18" driver and are certified for use in THX movie theatres. Most movie theatre speakers (situated behind a perforated screen) typically use 15" drivers, so the larger subwoofers are used only to reproduce the lowest frequencies at high sound pressure levels.
Large concert sound systems always use subwoofers (referred to as "subs" by the engineers and crew). The bulk of the sound system is usually "flown" (suspended from the ceiling by chain hoists) and the subs are usually stacked on the stage or the ground in front of the stage to the left and right of the performance space.
An unusual example of the use of sub-woofers came with the release of Earthquake in 1974 where they used a system called Sensurround to create a feeling of an earthquake. This was simply a set of large sub-woofers designed to put out infrasound(felt but not heard). Similar systems are used in theme park rides, such as "Days of Thunder," which uses sound to simulate a physical impact.
Many times the subs are not part of the entire sound mix but are specifically fed just kick drum, bass guitar and other low-frequency content from a separate output on the main mixing console. Popular sub systems in use currently are made by companies such as EAW but usually the subs will be made by the manufacturer of the rest of the PA system.
The 18-inch woofer driver is the primary majority device for pro audio applications. They are usually direct radiating in a ported enclosure built of 13-ply birch. For electronic music events with at least a thousand audience members there are often more than 20 double-18-inch cabinets on either side of the stage. 12-inch drivers in very large folded horns are also becoming popular now. One of the most powerful subs manufactured can play as low as 25 Hz and can cover thousands of feet and uses two 12-inch woofers on a 13-foot (4 m) long folded horn.
Pro Audio subs have to be capable of very high output levels – after all, concert venues may seat 10,000s of individuals outdoors. On average, music applications generally require less capability than movie soundtracks in the very lowest octave, but modern popular music is changing this preconception and this is reflected in the design attention given to the subwoofer section of the PA system nowadays compared to a few decades ago. People who are accustomed to bass in home audio systems and car audio many times think that the subs in a concert PA system aren't putting out that much. As sound pressure is measured in decibels which are a logarithmic scale PA subs can be 10 times more powerful yet only measure a 2 more decibels. Also sound intensity obeys the inverse-square law in relation to distance from the sub and at outdoor events the crowd are many meters away from the PA equipment.
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 an 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.
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 an 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.
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.
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.
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.
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 fill 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.
Frequency response of woofers
For woofers, the frequency is the number of times the cone of the woofer goes in and out per second and is measured in Hertz. So at 20 Hertz, the cone is going in and out 20 times every second. The faster the cone moves, the higher the pitch. The farther in and out the cone moves in each cycle, the louder it sounds.
Enclosures
A loudspeaker enclosure is a cabinet designed for mounting of loudspeaker drive units. The major role of the enclosure is to prevent the out-of-phase sound waves from the rear of the speaker combining with the positive phase sound waves from the front of the speaker. This would result in interference patterns and cancellation causing the efficiency of the speaker to be compromised, particularly in the low frequencies where the wavelengths are large enough that interference will affect the entire listening area.
Most loudspeaker cabinets currently are box-shaped. Although loudspeaker cabinets may at times appear bulky in an environment, they serve a number of necessary purposes.
History of enclosures
Before the 1950's manufacturers did not fully enclose their loudspeaker cabinets. The back of the cabinet was typically left open. Since the rear of the loudspeakers in a cabinet broadcast soundwaves in 180 degrees out of phase from the front where one listens, mixing the out of phase soundwaves into the listening environment causes loss of bass and lower program volumes.
Research from the 1930's by Dr. H.F. Olsen showed experiments that curved loudspeaker cabinets have major benefits by eliminating most soundwave diffraction.
In the 1990's, US Enclosure Company (Ultimate Loudspeaker Enclosure Company) pioneered a method of molding loudspeaker walls into any curved shape using wall materials that were equal to wood loudspeaker walls.
Explanation
The ideal mounting for a loudspeaker would be a flat board of infinite size with infinite space behind it because the rear soundwaves cannot cancel the soundwaves from the front. An 'open baffle' loudspeaker is an approximation to this as the transducer is mounted on a simple board of size comparable to the lowest wavelength to be reproduced. However, for many purposes this is impractical and the enclosures must use other techniques to maximize the output of the loudspeaker.
Loudspeaker cabinets sometimes are "sealed" and sometimes "ported". Ported cabinets allow some of the sound energy inside the cabinet to be released generally to increase bass response. Many other engineering designs exist including acoustic transmission lines.
Enclosures always play a significant role in the sound production, adding resonances, diffraction, and other unwanted effects. Problems with resonance are usually reduced by increasing enclosure rigidity, added internal damping and increasing the mass of the walls of the enclosure. The speaker manufacturer Wharfedale has addressed the problem of cabinet resonance by using two layers of wood with the space between filled with sand. Home experimenters have designed speakers built from concrete sewer pipes for similar reasons.
Diffraction problems are addressed in the shape of the enclosure; avoiding sharp corners on the front of the enclosure for instance. Sometimes the differences in phase response of the different size drivers is addressed by setting the smaller drivers further back, by leaning or stepping the front baffle, so that the resulting wavefront from all drivers is coherent when it reaches the listener. The Acoustic Center of the driver, or physical position of each driver's voice coil, dictates the amount of rearward offset to time-align the drivers.
Ideal loudspeaker cabinets use heavy walls to contain the out of phase sound energy, and the walls are usually made of wood.
'Woofer' and 'subwoofer' enclosures
Enclosures used for woofer and subwoofer are applications that can be adequately modelled in the low frequency range (approximately 100–200 Hz and below) using acoustics and the lumped component model. For the purposes of this type of analysis, each enclosure has a loudspeaker topology. The most common enclosure types are listed below.
Closed-box enclosures
Infinite baffle
Closed box enclosure
A variation on the 'open baffle' is to place the loudspeaker in a very large sealed box. The loudspeaker driver's mass and compliance, i.e. the stiffness of the suspension of the cone, determines the resonant frequency and damping properties of the system, which affect the low-frequency response of the speaker; the response falls off very sharply below the cabinet resonant frequency (Fs), which can be determined by finding the peak impedance. The designer trades off bass response for flatness; the larger the resonant peak in the bass, the lower the speaker will seem to reproduce, but the more over-emphasized the resonant frequency will be. The box must be large enough that the internal pressure caused when the driver cone moves backwards into the cabinet does not rise high enough to affect this. The box is usually filled loosely with foam, pillow stuffing, fiberglass, or other wadding, converting the speaker's thermodynamic properties from adiabatic to isothermal.
Acoustic suspension
The closed-box or 'acoustic suspension' enclosure, rather than using a large box to avoid the effect of the internal air pressure, uses a smaller, tightly sealed box. The box is typically designed with a very small rate of leakage so that internal and external pressures can slowly equalise over time, allowing the speaker to adjust to changes in barometric pressure or altitude. In this case, the true suspension of the driver's cone is the air trapped inside the box which acts as a spring with very close to ideal behavior rather than the mechanical suspension of the speaker driver, which for this application must be very weak, just strong enough to keep the cone centered in the absence of any signal. The drawback of these speakers is their low efficiency, due to the loss of the power absorbed inside the cabinet.
Reflex enclosures
Bass-reflex
Bass reflex enclosure
Other types of enclosures attempt to improve the low frequency response or overall efficiency of the loudspeaker by using various combinations of reflex ports or passive radiating elements to transmit the energy from the rear of the speaker to the listener; these enclosures may also be referred to as vented/ported enclosures, bass reflex, transmission lines (see below). The interiors of such enclosures are also often lined with fiberglass matting for absorption. Reflex ports are tuned by amount of mass within the vent, using appropriate diameter and length to reach this point. This enclosure is the most common as it lends itself to small size and reasonable bass.
Compound or Band-pass
Compound or 4th order band-pass enclosure.
A 4th order bandpass is really just the same as a vented box where the contribution from the driver is trapped in a sealed box which modifies the resonance of the driver. In its simplest form it has two chambers. The dividing wall between the chambers has the driver mounted on it and the panel opposite to it (or the chamber into which the driver faces) has a port. If the enclosure on each side of the woofer has a port in it then the enclosure yields a 6th order band-pass response. This enclosure is considerably harder to design and tends to be driver-specific.
Passive radiator
Passive radiator enclosure
Sometimes a passive radiator (PR) or drone, similar to a speaker driver but without an electrically activated voice coil, is used instead of a reflex port. Passive radiators are used primarily to tune small volumes to low frequencies, where a port would need to be very long. They are also used to eliminate port turbulence and reduce power compression caused by high velocity airflow in ports. Passive radiators are tuned by their mass (Mmp) and the way their compliance interacts with the compliance of the air in the box. Passive radiators add a complication to vented systems which causes a notch in frequency response at the PR's free air resonant frequency and this causes a steeper rolloff below the drone's tuning frequency Fb and poorer transient response than standard vented loudspeakers. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PR's to reflex ports. PR's do add considerable cost to the system, however.
Other enclosure types
Transmission line
The transmission line system is a waveguide system in which the guide shifts the phase of the driver's rear output by at least 90°, thereby reinforcing the frequencies near the driver's Fs. Transmission lines tend to be larger than the other systems, due to the size and length of the line required by the design (1/4th of a wavelength). In 2004 there was a breakthrough in this design category. The coupler transmission line enclosure was designed in part by veteran audio design engineers Brad Judah and Andy Bartha of Nucore Electromagnetics. The new coupler transmission line speaker enclosure design features a non-resonant characteristic and flat frequency response from 14 Hz to 25 kHz combined with high efficiency.
Dipole
Dipole speakers and their radiation pattern.
A dipole enclosure in its simplest form is a driver located on a flat baffle. The baffle may be folded in order to conserve space. A rectangular cross-section is more common than a circular one since it is much easier to fabricate in folded form than a circular cross-section. The baffle dimensions are chosen to get the desired response, with larger dimensions giving a lower frequency before the front and rear waves combine and cancel. A dipole enclosure has a "figure-of-eight" radiation pattern, which means that there is a reduction in sound pressure or loudness at the sides as compared to the front and rear. This is very useful when it is desired to prevent the sound from being heard at places other than the listening room/venue.
