Rule 6 - components

Part 4: loudspeakers

Select and integrate audiophile-quality components


Loudspeakers are the most prominent component in an audio system, and they are sensitive to placement and their companion amplifier. Room integration is a critical factor if the potential of the loudspeaker is going to be realized.

The final link in the audio chain

 A loudspeaker is a transducer that may utilize several component drivers (e.g., tweeter, midrange, and woofer) and a passive crossover mounted in an acoustically tuned cabinet. The crossover contains electronic parts that filter the overall signal into discrete frequency ranges and send them to the respective drivers. Loudspeakers function by converting electrical energy into mechanical energy creating sound pressure waves that emanate as a coherent and balanced waveform we hear as sound. A shout out to Edward Kellogg from General Electric in 1928 who patent the first dynamic woofer design and to James B. Lansing from Altec Lansing who improved on the design for the movie theatre and later became JBL commercializing loudspeakers. Fast forward to Dr. Floyd O’Toole of Harman (AES papers 1982 to 85) who provided a criterion for making good loudspeakers which many loudspeaker designers have come to appreciate.

 

Designing process

The cabinet, driver components selection, and crossover are usually applied by computer-aided design (CAD). Thereafter, the build prototyped is optimized through an anechoic chamber or hung high in an open area where it is measured without surface interferences for its frequency response. The next phase would be measurement and listening in a real-world environment to try and correlate subjective evaluation with measurements. The loudspeakers are further tweaked to achieve the desired performance outcome at a given price point.

 

Loudspeaker’s sensitivity

This indicates the sound pressure level (SPL) at a given power level normally at 1 watt and a specific distance normally 1 meter. As an example, a loudspeaker’s sensitivity will usually be expressed as Sensitivity: 98 dB SPL, 1W (or 2.83V) @ 1m into 8 ohms. We use this measurement to calculate the SPL rating in conjunction with the continuous power rating of the amplifier to determine its efficiency when listening from a distance. The higher the sensitivity rating, the more efficient the loudspeaker is, enabling flexibility in choices for lower (also cheaper) power amplifier ratings.

Therefore, anything below 89dB sensitivity requires higher-powered amplifiers (watts) to comfortably drive the loudspeakers.

Loudspeaker’s frequency response

This describes the ability of a loudspeaker to reproduce the full audible spectrum which is normally 20 Hz to 20 kHz. Each driver in a loudspeaker cabinet will have its unique frequency response and together with the aid of a crossover network and cabinet design will combine to create the desired frequency response. A loudspeaker’s frequency response will usually be expressed with a tolerance, such as Frequency Range: 20 Hz – 20 kHz (+/- 3 dB). A given frequency range from a loudspeaker without a tolerance qualifier can be misleading. A full-range loudspeaker would read 20 Hz to 20 kHz, normally floor standers. For bookshelves and monitors, loudspeakers will usually not go below 40 Hz requiring a companion subwoofer to achieve the full range and fidelity for most recordings.

 

Loudspeaker’s dispersion pattern

 This informs the dispersion pattern on the horizontal and vertical planes which are often based on the points at which the loudspeaker’s output is -6 dB at a particular frequency, such as 1 kHz. You might see it expressed as such: Dispersion: 110 degrees horizontal, 60 degrees vertical (-6 dB @ 1 kHz). However, the frequency response of a loudspeaker is different when measured from 90 degrees off-axis versus perfectly on-axis. Usually, the high frequencies will begin to drop off in level the further off-axis you get. The dispersion patterns depend on the tweeter design, cabinet structure, and toe-in position of the loudspeakers. Suffice it to say, a wide dispersion pattern loudspeaker is desirable as the reflections at certain frequencies remain constant mitigating comb filtering effects that are detrimental to good sound.

 

Types of purposed build loudspeakers

 

Common types of design configurations for loudspeakers

 Two-way – uses a woofer and a tweeter, a more seamless crossover advantage over a 3-way design

 Three-way – uses a woofer, midrange driver, and tweeter, coherency between drivers may be suspect unless a complex crossover is used with strategic driver positioning on the baffle for absolute phase alignment

 Composite – use 4 ways or more with many drivers arrayed on the baffle (not recommended for high-end audio unless it’s a vertical array column design)

Coaxial aka dual concentric – uses a woofer with a tweeter built in the center of the woofer where the dust cap usually resides. This design is used to enable sound from two driver types to produce a conical dispersion pattern emanating from a single axis (point source). This design offers superior imaging either far or near-field listening because it allows a wider field of listening to a synchronized summation where the pattern of response is symmetric around the axis of the loudspeaker. The challenge is perhaps to time-align both drivers to be coherent at all angles and prevent low-frequency intermodulation distortion due to excessive cone excursion which is known as the Doppler effect.

Active loudspeakers

Active loudspeakers usually refer to Subwoofers and Soundbars. However, they have found their use in home audio, music studios, and for home computer desktop systems. Active loudspeakers have their amplifier built into the cabinet and the respective controls are normally outside the back of the cabinet. There are but a few advantages such as reducing the number of components (amplifier, connecting cables, preamplifier in some instances) needed in the audio chain. Therefore, they are said to be cost-effective. 

Present-day active loudspeakers have electronic crossover networks instead of passive, matching amplifiers, EQ room correction, and even an internet streamer built-in. They are produced and marketed for their versatility, convenience, and even cost-benefit which is appealing to the everyday consumer. However, they have several disadvantages, such as a failure of one feature making the entire loudspeaker useless and therefore music unavailable due to its incorporation of several audio components. Audiophiles may have issues with this design concept. Because of the small space available inside the loudspeaker cabinet, the manufacturer would normally use a class D amplifier because of its smaller form factor. Class D amplifiers may not sound as good if all the other things remain equal in terms of distortion and linearity. Although it has relatively more power for its size, it does not boast discreet design circuitry.

The room correction EQ built into the design is meant for home theatre and is, therefore, suspect for audiophile use because when correcting peaks and dips, the corrections may introduce other problems like phase anomaly and amplifier clipping during peaks. The other disadvantage is that you don’t have the luxury to change the voice of the loudspeakers by experimenting with different connecting cables or amplifiers because they are already built-in except for the front end. You would have to live with the loudspeaker’s voice. The other problem is the existence of a hostile operating environment found inside the cabinet such as heat and vibrations. Cabinet vibration from the drivers affects all the electronics (microphonic) within the cabinet. If the cabinet is not suitably vented, heat generated within will accumulate and without sufficient heat sinks and venting will affect the life expectancy of the electronics. If the drivers’ magnets are not amply shielded and isolated will create a magnetic field inside the cabinet and affect the other electronics. A carefully thought-out design to address these problems may not be cost-efficient from a manufacturing standpoint. 

Studio Monitor loudspeakers

 As the name implies, they are designed and used in a recording studio setting to translate music for the mixing engineer who will set up the stereo soundstage and add special effects to a recording if required. Therefore, studio monitors are designed to generally have a flat frequency response where neutral tonal features are crucial to help recording engineers assess the quality of music. This is necessary to effectively translate a recording anticipating a studio that is fully treated with acoustic materials and the loudspeakers optimally positioned. Studio Monitors are usually active loudspeakers. On the other hand, consumer loudspeakers and high-end types are generally ‘voice’ as the designer intended based on their topology to sound good in a typical home listening environment. Therefore, they don’t have a flat frequency response so the peaks and the bumps they anticipate in a typical home environment will complement the ‘voice’ of the loudspeaker to sound good. They usually produced the desired bass response for the consumer’s listening benefit. Therefore unless you have a well-controlled listening environment (neutral) it is not advisable to use a studio monitor quality type loudspeaker.

Typical loudspeaker design concepts

Infinite baffle aka sealed box – A closed enclosure is perhaps the most popular, it primarily designed is to absorb the rearward sound wave avoiding cancellation effects. Because the enclosed air acts as a spring which reduces the woofer compliance and as a result it raises the resonant frequency of the cone, therefore a larger cabinet is desirable to increase the compliance for a lower resonant frequency. A smaller enclosure would need a compliant suspension (higher Q) of the woofer to keep the resonant frequency low. The cabinet material inside is critical for sound absorption to minimize panel vibrations in an infinite baffle design. 

Open baffle – A rare design commercially. It has an open sound and is more transparent with fewer room modes because of the figure of 8 dispersion pattern, special woofers (high Q drivers) are used to control the drivers for good bass response. A dipole peak at some frequencies usually at 1khz is common but may be cured by a wider baffle to shift the peak to a lower frequency. This would lose some of the advantages of an open baffle design, as the baffle gets wider, baffle reflection sets in and affects imaging. However, it is relatively more transparent because of the absence of a cabinet (box resonance). The frame design (shape) is critical to handle low frequencies. They cannot be placed next to a wall because of time smearing issues (need at least 6ms delay – i.e., about 3 feet or more away from the rear wall).  

Bass reflex aka ported – This design allows the back wave of a loudspeaker cone to be routed through an open port (sometimes called a vent or tube) in the enclosure. This port will reinforce the overall bass output although there would be frictional losses. These ports may be located on the front, sides, or rear of the loudspeaker cabinet and for frequency tunning purposes can vary in depth and diameter. The air inside the cabinet is pressurized by the woofer movement which will be reduced by the port at some frequencies. Some frequencies may be out of phase that may cancel off each other. This design is popular for bookshelf loudspeakers to make the cabinet sound “larger”. This design is more economical and easier to implement. 

Transmission line aka waveguides – an enclosure design (topology) that uses an acoustic transmission line within the cabinet that is used to extend the low frequencies in-phase with the woofer. However, the airflow exiting may cause port noise (chaffing). Directing the loudspeaker cone's rear sound wave through such a port can often be an effective way to increase output volume, reduce distortion, and improve bass definition and extension. The ports may be situated in the front or at the rear of the cabinet. The cones are usually co-axial designs. This design provides the best low frequency and imaging with all things being equal. 

Passive radiators aka ‘ABR’- acoustic bass radiators – loudspeaker drivers are housed in a sealed enclosure with a passive radiator, i.e., a driver cone that is not electrically connected (inert) to the crossover and usually without a voice coil. The passive cone would be moved by the air pressure inside the enclosure augmenting the low frequencies. There are no port noises to contend with. The surround and spider of the active cone in these designs should be stiffer (high Q) for better transient response (cone excursion compliance). Ideally, a passive radiator driver should be similar in size to the active woofer (spider and surround) but without the magnet and electrical connection so that it has the same moving mass, keeping both cones moving together. The passive woofer may be situated in the front, rear, or below the loudspeaker cabinet. A passive radiator design is the best way to get extended bass from a smaller form loudspeaker system without port distortion.

Vertical line array aka column – these designs consist of multiple drivers, arranged vertically that provide a normally wide horizontal dispersion pattern to direct the sound energy more at the listener. The taller the line array, the better the control of the vertical coverage of the low frequencies. This reduces reflections toward the ceiling, and back into the listening space. Multiple drivers moreover have the capability of producing higher SPLs enabling the drivers to operate way below their stress point i.e., achieving a more linear performance.

MTM aka D'Appolito – uses three drivers; a mid-woofer, tweeter, and mid-woofer configuration where it gets its acronym from. The drivers are placed in a vertical array pattern to achieve sound emanating from a single-point source much like a coaxial design which is its unique design characteristic. Joseph D'Appolito perfected this design by correcting a lobe tilt issue by using a 4th order crossover topology hence his name is attributed to this design. The disadvantage may be restrained dynamics unless augmented by a subwoofer or using larger (more powerful) drivers.  When vertically oriented, they reduce horizontal ‘lobing’ between drivers producing a smoother response.

Horn-loaded – uses an acoustic flange shape horn with a long throat and a flaring out duct to increase the overall efficiency of the voice coil motor. It may incorporate a woofer driver mounted in a loudspeaker enclosure which is divided by internal partitions called a folded horn loudspeaker. The horn serves to improve the coupling efficiency between the loudspeaker driver and the air. The result is greater acoustic output power from a given driver. They are usually a more efficient loudspeaker and are normally used with tube amplification for a better sound. Horns would normally sound harsh when paired with solid-state amplifiers.

Ribbon Planar Loudspeakers - an elegant and simple design that uses a very thin ultra-light film or aluminum ribbon diaphragm. The ribbon is conductive and is positioned in a transverse magnetic field. The ribbon is either straight, pleated, or folded. The ribbon is held tight on its top and bottom surrounded by powerful magnets that oppose and attract, pressurizing the air that we hear as music. They produce a figure of 8 dispersion pattern and are therefore less susceptible to sidewall reflections and must therefore be placed away from the rear wall. There are usually two sets of different-sized ribbons for the highs and the lows. Though they don’t often have deep low frequencies they excel in imaging and an expansive soundstage.

Electrostatic Loudspeaker – Uses a high-voltage electric field to vibrate a thin membrane (mylar) between two perforated conductive plates called stators. The mylar handles the full range of frequencies which excludes the need for a crossover. It has good imaging and transient response without phasing and discontinuity issues that are inherent in most dynamic loudspeaker designs. Must be handled with TLC. It May also pose an electrical safety hazard when there are young children at play.

Subwoofers – A subwoofer's purpose is not to provide more or louder bass but to extend the lower frequency range of your main loudspeakers

They use low-frequency woofers in a cabinet usually with a built-in amplifier (active) with controls for phase, volume, and a selection of low-frequency crossover roll-off points for proper integration with the main loudspeakers. The primary goal is to extend low-frequency response to an ideal point of 20Hz which the large majority of loudspeakers out there are not designed to reach even with most subwoofer models. The secondary goal is to reinforce low-frequency energy (SPL) providing the much-needed kick and slam required for some music genres such as rock, full symphonic orchestra, and techno-pop. They would more importantly maintain with relevant musical instruments the timbre integrity such as organ, drums, Grand Piano, Double bass, electric bass guitar, etc.

Subwoofers are commonly used in pro-audio for sound reinforcement duties such as concerts, auditoriums, discos, and even in vehicles. They are also used for Home Theatre Systems for the ultimate experience of movie effects. This may perhaps be a reason why most audiophiles would shun their utility notwithstanding the added expense, space, and their daunting integration. Home audio loudspeakers such as bookshelves, mini/studio monitors, and some floor-standing loudspeakers will be well served with subwoofers. It is pertinent to note that it is challenging for any loudspeaker manufacturer to design a one-box full-range loudspeaker because it would also compromise their other design parameters, predominantly electrostatic and planar designs. Full-range loudspeakers require complex crossovers for time alignment and sturdy reinforced cabinets or frames in the case of planar designs which would simply be a design nightmare and cost prohibitive. Moreover, the placement of the full-range loudspeaker is difficult because bass frequencies and the middle to high frequencies react to the room differently. This is why state-of-the-art loudspeakers are two cabinet designs with the bass duties kept separate which is a logical approach to loudspeaker design and ease of setup.

There are 3 primary kinds of subwoofer designs; Sealed (infinite baffle), Ported (vented), and Passive Radiator with their corresponding pros and cons. Like the previously mentioned loudspeaker design concepts, subwoofers are subjected to the same predicament. Suffice it to say, the Ported design utilizes a port (cylinder) or vent (slot) to achieve better bass extension for a given size cabinet, however, they lose some volume (SPL) and are also susceptible to port noise (aka chaffing). The Sealed cabinets are superior in maintaining volume (SPL) with less distortion and are faster in their attack, however, they are less efficient and need a relatively larger cabinet to compensate for their lack of bass extension. Finally, the Passive Radiator, which has an active woofer and another supporting passive woofer normally down-firing with no voice coil but will radiate in tandem with the active woofer to provide that added bass extension while maintaining the cabinet size of a ported design to achieve similar efficiency. 

In all active subwoofers, you will find control settings on the cabinet for you to integrate the subwoofer into the room and the main loudspeakers. There are three basic areas of control; volume, frequency range, and phase. You need subwoofers with phase control to ensure they don’t cancel off crucial frequencies and do not lag (slow). A volume control knob is an important feature to level match with other subwoofers used and the main loudspeakers.  The larger the woofer size the greater the low-frequency extension and therefore the greater the amplification power to adequately reach those frequencies at reasonable SPLs. An ideal subwoofer should have the potential to reach 20Hz with at least 300 watts of amplification power. The woofer size should be at least 12" and ideally 15" or 2 x 12" twin woofers. Speed and control is the challenge to any subwoofer design. Speed is achieved by the size of the woofer, the smaller the diameter the faster the excursion and recovery. For control, the higher the amplification the better the control notwithstanding the headroom that is much needed to handle aggressive transients. To achieve the speed, manufacturers would use smaller woofers or twin woofers to reach the desired frequency response or employ a passive radiator or port whilst maintaining the use of the smaller size woofer. However, the cabinet size will correspondingly increase to facilitate lower frequencies and magnet size for speed when using larger woofers including greater power amplification. Therefore, size matters when it comes to subwoofers, and with size, the cost of build would correspondingly increase. Read also Scaffold: Loudspeaker Placement to understand the placement methods that will best integrate a subwoofer into the main loudspeaker system.

Loudspeaker burn-in

Burning in your loudspeakers is usually recommended by playing audio preferably pink noise (full frequency) through them for at least 40 hours of continuous play for most loudspeakers. You may want to use a burn-in playlist of music and noise tracks in the various frequency range from highest to lowest or a downloadable digital track that loops different noises and frequencies. Many organizations on the internet offer downloadable burn-in files. Burning in your loudspeakers should always be done at a moderate volume - no need to be loud. The enclosure, mechanical parts as well as electronic components in the crossover will settle and the sound will eventually be more refined much like a new acoustic guitar or violin.

 

Exotic materials used in loudspeaker designs

A wide selection of exotic materials are used for dynamic drivers today to achieve smooth frequency response, low harmonic distortion characteristics, and a wide dynamic range. Materials used for the woofer cones (diaphragms) instead of the traditional paper cone are Kevlar, Ceramic, Polypropylene, Carbon Fiber, Polymer-Graphite Composite, Aluminum, and Beryllium for their rigidity, and lower mass to move quicker and respond well i.e., provide a better damping factor. For tweeters, titanium and beryllium are much sought after. Today’s tweeter and midrange designs are flange (waveguide) promising better dispersion patterns. For enclosures, the venerable MDF (medium-density fiberboard) is the material of choice because it is sonically dead and better at absorbing vibrations. The material is flat, stable, and dense making it an ideal material for loudspeaker enclosures. MDF is homogeneous having the same density throughout with no core voids. Although it is not as stiff as other types of wood, this flexibility is an advantage in a hostile vibration environment to keep the entire cabinet from physically moving, which is also why adequate bracing is a must to keep the cabinet together.


A dozen recommended tips and tweaks


 

Conclusion

Loudspeakers will make the most difference in a high-resolution audio system. Their distortion (THD and frequency response) between them is more pronounced i.e., higher in magnitude. Loudspeakers have the hardest job of all other audio components to convert electrical energy into mechanical energy. The design and materials used will determine the ‘voice’ of the loudspeakers in terms of tone and frequency response. That means its job is simply to produce accurate tones across the entire audible spectrum. Therefore, allocate the highest budget possible to enable the integrity of this critical conversion process. Be mindful that its efficiency (SPL-dB to impedance-Ohm) is reasonable, otherwise, you would have to spend quite a bit on the amplification to get the job done. Your room size and its configuration will determine the size of your loudspeakers and to some extent its design philosophy. Moreover, as it will be intruding into the listening space, it should look pleasant to you and your better half, so choose wisely unless you want sleepless and lonely nights.

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