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Loudspeakers Buying Guide

by October 19, 2002
Loudspeakers Buying Guide

Loudspeakers Buying Guide

A loudspeaker is an electromechanical transducer that converts electrical energy into mechanical vibrations. One would think this process is trivial enough to produce similar results in all loudspeakers. In an ideal world this may be possible. However, we do not live in an ideal world. Our ear is not an ideal instrument of sound perception and sound waves do not propagate through space ideally. There are countless factors and attributes in determining how well a loudspeaker system will faithfully reproduce a given source. It is the purpose of this document to highlight some of the important fundamental principals and attributes of quality loudspeakers to assist the consumer in choosing the right model and type based on their needs, budget, and personal preferences.

 

Driver Topology

Loudspeakers come in various flavors. Choosing one type is a function of your personal tastes, budgets, and needs.

Two-way systems
2-way systems consist of a woofer(s) and a tweeter to reproduce the audible frequency spectrum. In 2 way systems it is usually not recommend to buy a system with a woofer larger than 7". Anything larger, in most cases, may not faithfully reproduce the midrange frequencies with realism, clarity and detail since a large driver (8" or greater) is designed primarily to produce bass frequencies below 200 Hz and can suffer from excessive cone break up and beaming which will have deleterous affects on its ability to accurately reproduce frequencies in the midrange band (200Hz - 4KHz). There are many high quality 2 way systems employing 6 1/2" woofers or smaller that would sound outstanding throughout the entire frequency spectrum when properly mated with a quality powered subwoofer. It's usually a good rule of thumb not to operate a driver at frequencies above the wavelength of the diameter of the cone to avoid beaming and/or doppler effects.

Two & half way systems
2 1/2 way systems have recently entered the hi-fi market of loudspeaker design. This design employs 2 bass/mid-bass drivers usually identical in design and size and a tweeter. Usually the bottom woofer is crossed over to produce bass frequencies below 200Hz and the top woofer is crossed over to produce midrange frequencies between 200Hz - 4KHz. The interesting thing about this design is that although the two woofers are crossed over at different frequencies, they acoustically couple to synergistically enhance overall system bass response.

Three way systems
3 way systems employ a woofer(s) crossed over to produce bass frequencies, a midrange driver crossed over to produce mid frequencies and a tweeter crossed over to produce high frequencies. This type of system, if implemented correctly, can achieve full frequency response at very high fidelity with little or no need for an added powered subwoofer. However, it is recommended to purchase a quality 2 way bookshelf system over a cheaply designed 3 way system in the same price range as it will almost always offer better performance with only a possible sacrifice in bass response. If bass response becomes an issue with the bookshelf speakers, one can always add on a powered subwoofer when finances permit.

Cubed Systems
While these systems may be very popular, they are by far some of the least accurate and worst sounding so called "speaker systems" devised. Although, most consumers do not realize this because these speakers are well marketed and their main emphasis is big sound in a very small footprint. I deliberately kept this topic separate to stress its importance. Avoid these types of speakers at all costs !!!! Most of them are basically two way system with an unpowered bandpass so called "subwoofer" which produces bass frequencies between 45Hz and 200Hz while the tiny cubed speakers produce frequencies between 280Hz and 13KHz. The problems with this system are as follows:

  1. This so called "subwoofer" produces bass frequencies up to 200Hz, which are very directional and localizable and also contain some midrange frequencies critical to vocal reproduction. This sub does not produce bass frequencies that quality subwoofers are designed for (90Hz and below). Instead it over emphasizes midbass frequencies (50-80Hz) to give the illusion of bass to the unknowledgeable consumer in the demo room.
  2. There is a gap of at least 80Hz in the crucial midbass / voice frequencies which is blatantly obvious while listening to vocal music.
  3. The cubes cannot properly handle the midrange and high frequencies since they are too small to be adequate midranges and too large to be good tweeters. They roll off sharply at 13KHz, which is considerably below human hearing capabilities (20KHz) and have a deliberate bump between 7kHz to 9kHz to give the illusion of treble in a showroom floor. There are many alternative loudspeakers in this price range, at half the price for that matter, which will clearly outperform them in every respect. If a small footprint is critical to your application, there are many quality satellite speaker systems with self-powered subs that will sound superior to these, cost the same or less, and allow for better bass management and upgradability. Let your ears be the judge in this matter rather than brand or employed marketing techniques. You will soon come to the realization and ask yourself how you every considered these cubed speakers from the beginning.

Loudspeaker Drivers

Cone Materials
Loudspeaker cones can be manufactured from various materials depending on driver implementation, desired frequency response and budgetary considerations. High frequency drivers (dome tweeters) are usually made from metallic or fabric materials. Usually metallic drivers (IE. Aluminum, Titanium) tend to be very revealing and detailed but may tend to be bright sounding with certain program materials or electronics, especially if mated with a poorly designed crossover that doesn't fully account for high frequency resonant modes inherent in many metal dome designs. Fabric domes ( Silk, Mylar's) tend to have a more reserved sound with less emphasis on brightness and more on smoothness. Of course, this doesn't apply one is better than the other, just different, usually requiring different design trade offs and considerations. Paper drivers for tweeters should usually be avoided as paper is not the proper material for accurate high frequency reproduction. In addition, most paper tweeters are 2" or more in diameter. A driver this size is usually not suited to produce high frequencies with the accuracy, finesse and detail, and can also suffer from larger break up modes leading to higher distortion. Quality midrange and bass drivers are usually constructed from Paper Composites, Kevlar, Aerogel, Aluminum, or Polypropylene materials, which have very high strength/weight ratios and tend to be immune from flexing during large excursions. This allows the driver to react quickly during transitions in music and minimizes driver distortion. Many high end woofers achieve excellent sonic performance when constructed from high quality paper laminates. If done right, paper woofers can outperform many exotic drivers made from fancier materials.

 

Driver Types

There is a wide diversity of driver types depending on desired frequency response, size constraints and budgetary applications.

Woofers : Woofers are usually between 6 1/2" and 15" and are designed to produce frequencies from 20 - 500 Hz.

Midranges : Midrange drivers are usually between 2.5" - 6 1/2" in diameter and designed to reproduce frequencies between 200Hz and 4KHz.

Tweeters : Tweeters are usually between 1/2" to 1" and are designed to reproduce frequencies between 2KHz to above 20KHz.

Horn loaded Tweeters : These drivers are usually designed as midranges or tweeters or both, have extremely high efficiency and narrow dispersal patterns best suited for loud concerts, sound reinforcement, and movie theater applications. In most cases, these type of drivers are not always well suited for quality two channel or multi channel surround systems in the home due to reactive rooms and generally close proximity of the listener to the speakers. Higher fidelity for home audio applications may be realized with conventional cone midranges and domed shaped tweeters, or ribbon / ESL equivalents. However, there are instances when properly designed horn-loaded speakers are mated with warm-sounding electronics to yield excellent, dynamic and non-abrasive fidelity with reasonable applied power.

Electrostatic Speakers
Electrostatic speakers are designed as tall thin planar metallic membranes that operate in with regards to "like" electrical charges repel one-another and "unlike" electrical charges attract one-another. Electrostatic speakers are basically a stretched thin plastic membrane on a rigid frame, coated with a low-mass electrically conductive substance. They are fabricated into two stiff, flat electrodes, called the stators, and are insulated to prevent electrical discharge. Each stator has the same area as the membrane and are placed one on either side of it. The membrane is placed exactly equidistant between the two stators. The stators must have holes in them to permit sound to pass through. In most cases these speaker systems are not very efficient, require large amounts of power to drive them correctly. The generally have poor bass response in most cases and require a powered subwoofer to accurately reproduce the entire audible frequency spectrum. They have many advantages in that they may image better and create a more open and expanded soundstage than conventional cone speakers.

Passive Radiators
A passive radiator is a device that looks just like an ordinary driver, except it has no magnet or voice coil. A radiator is usually a highly compliant device, with a similar cone material and surround found on regular active drivers. A clear advantage of the radiator is the absence of port noise, and some audiophiles claim the radiator provides a better sounding bass than a ported enclosure. Disadvantages include difficulty in tuning, and the extra required baffle area for the radiator. One must take caution in buying loudspeakers with this type of driver implementation. Many manufacturers simply implement one or several of these drivers disguised as normal woofers to give the illusion of more drivers in the cabinet. In fact some companies take it one step further and utilize passive radiators and porting in the same enclosure. Incorporating both is redundant and unwarranted as it will most likely result in loose, sloppy and undefined bass response.

 

Driver Baskets

Driver baskets may be constructed from various types of metals depending on budget, and design goals. Steel baskets tend to be cheaper in design and have some disadvantages if not properly implemented. They must be designed with special care so that the driver magnetic strength will not be compromised by the baskets magnetic properties resulting in a less powerful motor structure which will weaken response time and increase overhang distortion. The most critical disadvantage of steel baskets is their low resonate properties (80Hz or so) which if not properly damped may couple to the cabinet causing unwanted coloration in sound. Cast Baskets are always better in design as they do not suffer from the previously mentioned disadvantages of Steel Baskets. In most cases, this should not be an issue providing that the drivers are properly designed and implemented. However it is important to note the potential of this problem.

 

Crossovers

In a sense, crossovers are the brains of the loudspeaker system. They are designed to optimize driver performance and assure proper blending of the drivers enclosed in the speaker system. The quality of the components used in the filter system have a direct impact on the fidelity of sound which can be achieved. Polypropylene capacitors are preferred over Mylar's due to their low ESR (effective series resistance) and tighter tolerances. In addition ceramic resistors are preferred since they have high power handling and low DCR (DC series resistance). Air core inductors are preferred over iron core for the following reasons:

  • Eddy Currents : When inductance is applied to an iron core, voltage is induced in the core. The eddy current flows in opposite direction to the coil flux, which results in power loss and increased distortion. Eddy currents are negligible in air cores because voltage isn't induced in a non-magnetic core.
  • Hystereses Losses : Magnetism is induced in an iron core when voltage is applied to it, magnetizing the core in one direction. Additional power is needed to reverse the magnetic field, and the reversal may not be instantaneous resulting in unwanted distortion. Therefore, air cores have substantially reduced Hystereses distortion due to the fact that they have no magnetic core. It should be noted that Hystereses becomes greater at higher frequencies since the frequency changes much more rapidly , in excess of 20,000 times per second (20KHz).
  • Saturation : Induction decreases when more power is applied. The reason for this inverse relation is when all the magnetic lines of induction become full, or saturated, only a small additional amount of induction can be produced which leads to distortion. Iron cores often become saturated 2 or 3 times faster than air core inductors.

In general, air core inductors with the largest practical diameter wire are always the preferred parts to use in cross-over design providing that they are within size constraints (larger values result in larger parts) of the cabinet and meet the budget constraints of the speaker system.

Filter slopes and topologies are a function of the drivers employed, positioning of the front baffle in relation to the drivers and budgetary constraints of the system. The main importance of the filter network is to ensure as close to flat frequency response over the entire audible spectrum as possible, to present the amplifier with a controlled input impedance, and to optimize driver performance by electrically limiting each driver to produce the frequency band it is designed for.

Check out our Crossover Topologies article for more information on this topic.

Enclosure Types

Acoustic Suspended
Acoustic suspended speakers (aka. Infinite Baffle) are sealed enclosures which rely on back wave energies of the woofers to radiate within the cabinet to produce the bass frequencies. While this type of enclose has several advantages (IE. tight and quick bass response, easy to design, frees from port noise) it has some tradeoffs, namely a decrease in efficiency, linearity, and bass output.

Note About Acoustic Suspension & Infinite Baffles
Acoustic suspension and infinite baffle are two different types of enclosures which are often confused as one in the same.
An acoustic suspension speaker has a compliance of the air volume inside the box which is less than the compliance of the woofer by a factor of three or more. In other words, the box is acting like a spring and assisting the driver.
An infinite baffle speaker is an enclosure made large enough so that the compliance of air within the enclosure is greater than the compliance of the woofers suspension. So in other words the woofer is using it's own suspension to control the cone. According to Clayton Oxedine (Audio hobbyist and Speaker Designer) There are not many Infinite Baffle (IB) drivers around today because it seems manufacturers have taken the approach that most people want smaller boxes. Also IB type drivers tend to be higher Q drivers which will not give as good of transit response.

Bass Reflex
Bass reflex enclosures usually have a port(s) or passive radiator to tune a cabinet to a specific frequency determined by the bass driver implemented and volume of enclosure. The obvious advantages of bass reflex systems over acoustic suspended designs are an increase in efficiency and bass output. However, unless properly designed, bass reflex systems may sacrifice some accuracy in bass response. In addition, if the port is not designed properly the system may suffer from port velocity problems resulting in unwanted port noise or loose and sloppy bass attributed to improper box tuning ratios with respect to port size and driver compliance.

Transmission Line
Transmission Line enclosures are a design in which the driver is at one end of the enclosure, with an internal path which consists of a series of bends or curves that lead to a port at the other end of the enclosure. The path length is a fraction of the wavelength at low frequencies. Stuffing the box with either long fiber wool or polyester batting may increase the length of the path. This produces a phase shift in the back waves that reinforces bass at low frequencies. Enclosures must be very large, but low-end response of these systems is legendary among audiophiles. These designs, are tricky since no standardized method for configuring these enclosures exists. One trade off to this design may be a decrease in power handling when compared with other designs, however drivers may be capable of responding their lowest frequency capabilities.

 

Cabinet Construction

One of the most important attributes in determining sound quality of a loudspeaker system is the cabinet that encloses the drivers and magnetics. Quality of materials used determines how well the cabinet will remain free from unwanted resonance. Usually MDF is preferred over particle board because it is easier to work with and is made of higher density wood fibers. As a rule of thumb, the speaker cabinet should be at least 1/2" thick with emphasis on the thickness of the front baffle when possible. Bracing internally also plays a vital role in controlling cabinet resonance. Either cross bars or flat baffled boards should be used in key points of the cabinet to keep the it in static equilibrium during dynamic stresses caused by driver resonance. Dampening material should be used to help deaden the sound in the cabinet. This material also helps to trick the woofer into playing lower in frequency by giving the illusion of a larger internal volume. The midrange driver is usually isolated in its own chamber inside the cabinet, stuffed with insulation to help smooth out the midrange response and minimize coloration from back waves of other drivers.

 

Cabinet Finish and Appearance

Finish of the cabinet plays no role in the overall contribution of sound quality of a loudspeaker system. It is merely a statement of fashion and prestige. While it is desirable to have a speaker system with a nice wood veneer finish, it should not be a primary concern as these types of finishes add cost to the system without an increase of performance. In addition, speaker companies that offer their products in multiple finishes add further cost as they have to pay more for manufacturing each model in lower numbers while stocking larger variety of finishes. There are many examples in the audio industry of speakers with black vinyl finishes that sonically outperform costlier speakers with fancy wood finishes. In other words, don't be fooled by a speakers appearance. It is what's inside that matters when sound quality is the primary concern. Choosing a speaker system merely on appearance or name brand recognition may be the worst mistake a consumer can make when they are evaluating new speakers.

 

Recommendations

Now that we have defined the basics of loudspeaker, we are ready to evaluate systems. Before going to the hifi stores, consider what your needs (ie. music preferences, room size, listening habits, etc) and budget constraints are.

 

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About the author:

Gene manages this organization, establishes relations with manufacturers and keeps Audioholics a well oiled machine. His goal is to educate about home theater and develop more standards in the industry to eliminate consumer confusion clouded by industry snake oil.

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