At first glance, most loudspeakers look about the same yet vary dramatically in price. Most of the reasons lie below the surface, invisible to the naked eye. |
The device commonly referred to as a “speaker” is more correctly referred to as a loudspeaker system. It is comprised of some transducers, a crossover network, an enclosure, and a few additional parts. Loudspeaker system designers must be familiar with the complicated interactions of the components that form the system. An assemblage of good parts does not guarantee a good system. For the remainder of this article I will refer to a loudspeaker system as simply a “loudspeaker.”
If I had to pick one word to describe the loudspeaker design process it would be compromise. Every facet of a loudspeaker’s performance and ultimate cost is a result of trade-offs. This is why it is very difficult to compare loudspeakers from different manufacturers. One may appear to be better by focusing on a single specification (such as its power rating), yet it may be inferior as a system when the performance of the whole is considered.
Let’s look at the parts that make up a loudspeaker in order to get a better understanding of why some are more expensive than others.
The Lowdown on Transducers
The heart of a loudspeaker is its transducers. These are the devices that actually convert electrical energy into acoustical energy. Most sound reinforcement transducers are pistons. Today’s designs have changed little in operating principle from those produced by research done early in the last century. Technological advancements have produced better materials, lighter weight, and lower cost, but loudspeakers still basically produce sound by vibrating the air molecules around them through movement.
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A cast-frame LF transducer (left) compared to a stamped-frame model (right). |
No single transducer can reproduce full-range sound at the levels required for auditorium use. For this reason most loudspeakers are two-way or three-way designs, with dedicated transducers for the low-frequency (LF), mid-frequency (MF), and/or high-frequency (HF) parts of the spectrum. Three-way designs have more transducers, so they typically cost more than two-way designs.
All-Important Frame Design Components
One factor that makes a big difference in the price of low-frequency transducers is the frame design. Stamped frame loudspeakers are lower cost due to their ease of fabrication and lower raw material cost. Cast frame loudspeakers have less of a tendency to flex as the cone moves in and out (Figure 1). They also tend to have higher mass and weight than stamped-frame models, as well as higher cost. Since transducers waste much of the applied electrical power, performance improvements can be realized by improving heat dissipation.
The cone material must compromise mass, strength, and cost. While some esoteric materials have been developed for loudspeaker cones, paper remains the most common material because it represents a good compromise between the required attributes. LF transducers range in cost from tens of dollars to hundreds of dollars. The system designer’s motivation will determine which is used. If they are designing to meet a price point, they will use a cheaper unit. If they are designing for the highest quality performance, a pricier model may be selected.
The MF and HF components in sound reinforcement loudspeakers are usually horn-loaded designs. Horn loading produces high sound-pressure levels as well as control over the sound radiation pattern. Horns are constructed from a variety of materials, including wood, metal, fiberglass, and plastic. ABS plastic horns are very popular due to their low cost, light weight, and ease of fabrication. Fiberglass horns are much stronger but they are also more difficult to fabricate, which increases their cost. Low-frequency horns are often made of wood since they can be very large.
The horn driver (a transducer) plays a major role in the performance of a loudspeaker system. They range from relatively low-cost, piezo-electric devices to compression drivers. Cone-type loudspeakers can also serve as horn drivers. The compression driver is the optimal way to produce a lot of mid- and high-frequency sound. It is preferred over lower cost piezo-electric drivers for its high efficiency and fidelity.
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Compression drivers can vary dramatically in size, weight, and performance. |
Transducer magnet structure is a major cost factor. Large magnets produce higher field density (which generally equates to higher efficiency), accompanied by increased weight. This may necessitate a cast frame for support. Rare earth elements, such as Neodymium, produce stronger magnetic fields than conventional magnet types at a fraction of their weight. Neodymium is also the most expensive magnetic material commonly in use.
Some loudspeaker companies manufacture their transducers. This gives them full control over the supply chain and the ability to optimize a component’s performance for a given loudspeaker system. Most loudspeaker manufacturers use Original Equipment Manufacturer (OEM) suppliers for their transducers. OEMs can be very efficient in the manufacturing process because it is all that they do. Most OEMs build a generic line of transducers that are sold to the public along with proprietary designs that are built to a loudspeaker manufacturer’s specifications.
Crucial Crossover Network
Though it is unheralded in its role, the crossover network is arguably the most difficult part of a loudspeaker to design. The crossover’s job is to band limit the signal to each transducer so that it only delivers frequency content that it can convert into sound. In principle, it is like the transmission of an automobile. A good transmission will allow the wheels to spin faster and faster (increasing frequency) as you steadily increase the vehicle’s speed from zero to 60 mph. The gear changes should be smooth and free of glitches. A good crossover design does the same thing. It provides a smooth transition between the LF, MF, and HF transducers.
Passive crossovers, on the other hand, are filter networks of passive electrical parts — resistors, capacitors, and inductors. Mass-produced generic prefabricated designs are available; but ultimately the crossover network must be optimized for the specific transducers that it is driving. This is a task best left to a design engineer.
Some crossovers are mounted to printed circuit boards (PCB). These have a nice, clean look and are easy to mass produce, thereby reducing cost. This can work well for low-power applications, but the circuit boards can be damaged by heat if driven too hard. They can also warp and crack from the stress caused by large inductors. This is why many manufacturers still hard wire their crossovers. This must be done by hand, and the result isn’t very pretty. But, the end result is more robust (and more expensive) than the PCB crossover.
Some loudspeaker systems have no internal crossover network. They are driven by dedicated electrical processors that perform the necessary signal processing before the signal is applied to the power amplifiers — an active loudspeaker system. While this should reduce the cost of the loudspeaker system itself (less parts), loudspeakers of this type ultimately cost more to implement due to the required external processor and possibly extra amplifi er channels. Active designs can yield higher fidelity and higher sound pressure level (SPL) than passive loudspeakers. Even so, a good passive design can provide excellent performance at a lower overall cost, plus it is usually easier to implement than an active loudspeaker system.
The Cabinet
The cabinet and its construction are a major factor regarding the price of a loudspeaker. Cabinets must be strong and stiff. If the cabinet vibrates it becomes a transducer itself, which will interact with the other transducers and degrade the overall system performance. A stiff cabinet requires a lot of mass, a lot of bracing, or both. The trade-off is the weight of the box. Concrete is an excellent cabinet material, but is impractical for all but custom subwoofer systems. Medium density fiberboard (MDF) is also a good material. It is very dense and low cast, but it is also very heavy and is subject to warping if it gets wet. Particle board cabinets are cheap, but should be avoided. Plywood is lighter and stronger than MDF, but it can have voids between the layers. Baltic birch plywood is best (11+ layers and free of voids), but it is also the most expensive wood for loudspeaker building. Composite materials can be very strong and very light, but are also very expensive. Fiberglass is strong and weather-resistant, yet it is difficult to fabricate and therefore much more expensive than wood or plastic.
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The left horn is fiberglass, the center one is metal, and the right one is ABS plastic. |
Molded-plastic loudspeaker cabinets have become common in recent years. They are lightweight and can be made weather-resistant. The initial high tooling costs incurred by the manufacturer are recovered through mass production over many years. This type of cabinet construction is practical for small-dimensioned enclosures, but more rigid materials are required for large boxes.
Loudspeakers that will be flown over an audience must have special rigging hardware (fly-points). They must also be pulltested to ensure cabinet and hardware integrity. Fly-able loudspeakers don’t look a lot different than ground-stack or polemount loudspeakers, but will generally cost a good deal more due to the required cabinet construction, integral fly-points, and testing.
Self-Powered Loudspeakers
I have shown that the loudspeaker is a system of inter-dependent parts. This can be expanded to include the power ampli- fier. Amplifier selection can be complicated, and the ramifications for choosing the wrong one can be disastrous. This has prompted some manufacturers to incorporate the amplifier and signal processing right into the loudspeaker. These are commonly called self-powered loudspeakers. This was not a practical approach with older amplifier topologies, but modern Class-D amplifiers are relatively small and lightweight, and they are also very efficient. They can be incorporated into the loudspeaker cabinet without undue increases in size or weight.
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Crossover networks can be fabricated on PC boards (left) or hard-wired by hand (right). The telephone handset has been added for scale. |
The result is a plug-and-play loudspeaker that needs only an electrical outlet and a signal from a mixer. Of course, self-powered loudspeakers will be comparatively more expensive than passive models. They can also be more difficult to install since they require AC power. Both passive and self-powered loudspeakers can provide equivalent levels of performance. The pros and cons of each must be considered in light of the application.
Loudspeaker Specifications
Loudspeakers should be tested by the manufacturer and supplied with performance data. Some key specifications include sensitivity, power handling, coverage angles, and impedance. These specs are useful for providing a general idea of a loudspeaker’s performance (and hence its suitability for a project), yet they shouldtson.
not be used for comparison shopping between brands. This is especially true for power ratings, which are seldom understood and usually wildly exaggerated. One big difference between loudspeaker brands is the availability of engineering data for use in computer modeling programs. This data goes far beyond the spec sheet that almost all loudspeakers have. A popular program among sound system designers is the Enhanced Acoustic Simulator for Engineers (EASE). Another is CATT-Acoustic. One data type is proprietary to the EASE program, and others (i.e., the Common Loudspeaker File Format or CLF) are generic. A very quick way to narrow the field when selecting loudspeakers is to only consider models that are accompanied by EASE or CLF data. The testing required to produce this data is extensive, so its availability is usually limited to professional models.
Consistent Quality
Since the loudspeaker is a system of interdependent parts, the selected transducers must be of similar quality and have a similar level of performance. A better MF driver must be accompanied by better LF and HF drivers for the benefits to be realized. The same is true for the enclosure and crossover network. It makes no sense to combine high-quality components with a cheap enclosure. This is like building a house, where it is sensible to be consistent with the quality of materials. You don’t mount solid gold faucets onto a cheap sink. But as with a house, if each part selected is a little better, then the cost of the whole will be much greater.
Conclusion
Remember to compare apples with apples when evaluating loudspeakers. Consider a loudspeaker with the following attributes
- Cast-frame LF transducer
- Fiberglass MF horn with compression driver
- ABS plastic HF horn with compression driver
- Baltic birch cabinet rated for overhead use
- Integral fly-points
- Robust internal crossover network (if applicable)
- EASE and CLF performance and design data
- Technical support department and available parts
The cost of a loudspeaker is affected by all of these factors and others. Even in a competitive marketplace, a loudspeaker with these attributes will not be cheap. If one brand is half the price of all the others, then “buyer beware.” With loudspeakers, you get what you pay for and you can expect to pay for what you get. A loudspeaker represents a set of compromises and trade-offs. A competent sound system designer can weigh these out to select an appropriate make and model for your specific application.
Pat Brown teaches the Syn-Aud-Con seminars and workshops. Synergetic Audio Concepts (Syn-Aud-Con) has been a leader in audio education since 1973. With nearly 15,000 “graduates” worldwide, Syn-Aud-Con is dedicated to teaching the basics of audio and acoustics. For more information visit their website at www.synaudcon.com or call (800) 796-2831.
