Types of drivers

Drivers can be classified into several types depending on the configuration of their diaphragms. Drivers have progressed with the strength intrinsic to each type best suited for respective user's purposes, rather than any particular types of drivers having been made do for all purposes. Let's spend some time going over typical types of drivers.

● CONE TYPE DRIVERS With conically shaped diaphragms, this is the most often used type of driver. As diaphragms are called cone "paper", paper pulp has been the major source of diaphragm materials. However, a variety of materials is now available. Typically used materials, other than paper pulp, are PP (polypropylene) and carbon fiber. Application of this structure is extensive enough to cover full range drivers which can reproduce the entire audio spectrum, woofers for the purpose of bass sound reproduction, mid-range drivers intended for reproduction of medium frequency ranges, and tweeters for high pitch. ● DOME TYPE DRIVERS Diaphragms can also be dome shaped. Sound is radiated directly from domes. The advantage of this type of driver is that it has very good directionality. Its application is common for mid and high frequency reproduction. A lower-extending coverage available from many types of dome tweeters, compared to horn type tweeters, makes them reasonable and useful elements to form 2-way systems. Dome tweeters can be classified into 2 categories depending on dome materials, soft dome types where soft materials such as silk, cotton or polyester film is used, and hard dome types where hard materials such as aluminum or titanium is used. ● HORN TYPE DRIVERS This type of driver is characterized by a horn attached in front of the diaphragms. The typical application is reproduction of mid and high frequency ranges. Acoustically loaded horns provide this type of drivers with a high level of efficiency and good transiency. The above 3 types of drivers represent most typically used sound radiation systems. Another key component of drivers is their magnetic circuits. Magnetic circuits can be classified into 2 categories if we leave out the special types. ● OPEN MAGNETIC CIRCUITS Magnetic circuits for most drivers are built on the basis of Fleming’s left hand rule. Drivers of this principle are called electrodynamic types. Of these, open magnetic circuits, using ferrite magnets in many cases, aremost often used. This structure, as illustrated in Figure 1, inevitably permits magnetic flux leakage, causing picture distortion on video monitors when they are brought close to each other. Recent technology has made it commercially possible to produce drivers where magnetic flux leakage can be cancelled for use in audio/visual applications. ● CLOSED MAGNETIC CIRCUITS This type of magnetic circuits is constructed so a permanent magnet can be inserted in a pot shaped yoke, which is typically called a pot type yoke. Alnico magnets are the type of magnet used. Magnetic fields produced by this type of magnet are stronger than those of ferrite magnets, if their volume is equal. This structure permits no magnetic flux to leak out, making drivers acceptable for use with audio/visual systems. Despite many advantages provided by alnico magnets, the lack of economy due to the fact that alnico is an alloy containing cobalt, which is one of the rare metals, may be the only discouraging factor.

Interpretation of catalog specifications

Catalogs carry many figures as "specifications". Let's try to grasp the meaning of these figures.

● LOWEST RESONANCE FREQUENCY (fo)

This frequency is usually referred to as 'f zero' and expressed in units of Hertz. This can be regarded as an element which determines the lowest reproducible frequency. We will now discuss this a little further. Gently hitting a tuning fork generates a consistent frequency. This is because the fork vibrates and produces sound at its resonance frequency. Forks are not the only objects which have resonance frequency, but so does everything else. Designation of the lowest resonance frequencies as 'fo (f zero)' easily reminds us that it is the lowest resonating frequency. The ‘fo’ of drivers is, unlike that of the diaphragms themselves, the frequency at which the vibration system comprising elements such as the equivalent mass (Mo) of the vibrating portion, and the edge and damper sustaining the system, can move back and forth freely. The point we must keep in mind is that ‘fo’ refers to that of unenclosed drivers, not of drivers in an enclosure. The lowest resonance frequency of drivers mounted in an enclosure will be higher than 'fo' due to the air in the enclosure acting like a pneumatic pring. The success in designing a good enclosure depends on what frequency to allow the system 'fo' to rise to.

● Qo

The figure quoted as 'Qo' is another element of importance for successful driver enclosure design. This value represents the sharpness of the resonance frequency (resonance sharpness). The larger this figure, the sharper the resonance. Drivers of 'Qo' of up to 1 or so are considered to be good performers.

● EQUIVALENT MASS (mo)

This is the sum of the mass of the vibratory system and the pneumatic resistance of air present in front of and in back of the diaphragm (additional mass). In short, it is the actual mass when a driver is working with its diaphragm moving back and forth to generate sound. Most of the mass is that of the vibratory system while it includes the mass of the air involved. The unit of measure is the gram, and is called 'mo'. 

● EFFECTIVE VIBRATION RADIUS (a)

The effective vibration radius is the radius of the actual moving portion (e.g. cone papers) contributing to produce the sound, not normally quoted driver diameters. On some occasions, catalog values may include the edge dimensions.

● IMPEDANCE

This figure represents the input impedance measured at the driver terminals. It needs to be noted that impedance is input frequency-dependent, except for some technologies such as RP system tweeters. Nominal impedance shown in catalogs is the impedance of the point where the frequency response hits the lowest point above the 'fo' frequency. The unit is 'Ω' (ohms).

● SOUND PRESSURE LEVEL (S.P.L.)

This is the indicator of driver efficiency. The sound pressure level (loudness) with 1W input applied into the driver is indicated. The greater this figure, the greater is the conversion efficiency of input signal into sound when the same input power is applied. For example, assuming 90dB and 93dB drivers, the sound pressure level output from a 90dB driver with a 10W input will be equally loud as a 93dB driver into which a 5W input is fed. When using a tweeter, supplementary to a full-range driver, or designing 2- or 3-way driver systems, tweeters should have greater figures (higher efficiency) than the woofer being considered for use to form the system. This consideration is unnecessary for systems where a single full range driver is used. Figures are shown in dB.

● CROSS-OVER FREQUENCY RECOMMENDATIONS

Regarding cross-over recommendations given in catalogs for drivers designed for use in multi-way (multiple driver) systems, points of attention are different between mid- or high-frequency units and bass units. Tweeters need to be given very special attention. Mid- or high-frequency units are liable to be damaged (voice coils being burned out) when bass signals are input in excess of their reproduction capability. In order to avoid damage from such cause, as well, observation of the recommended cross-over frequencies is of cardinal importance. With mid- or high-frequency units, cross-over frequency recommendations imply that 'apply frequencies higher than the recommended cross-over frequency’, while, with bass reproduction units, 'use this driver at frequencies lower than the cross-over frequency' is implied. Unlike mid- or high-frequency units, low frequency units such as woofers will stand undamaged if higher frequencies than the recommendation are fed into them. Without regard to whether bass, mid- or high-frequency units, use of drivers above or below the recommended cross-over frequencies, as appropriate, is highly advisable in consideration of the reproduced sound.

● INPUT POWER

Indication of input power may be for both 'maximum allowable input power' and 'rated input power', or for either one of them. The fact that they are given different definitions makes it necessary to confirm that such figures are adequate for your intended application.

(1) MAXIMUM ALLOWABLE INPUT POWER

This term defines the maximum input power which can be momentarily applied to the driver. However, note must be taken that such power is a measurement of certain individual frequencies, and thus, not allowed for every frequency point. To be safe, input power defined using the term 'Music' should be considered, in that it indicates peak input power encountered during reproduction of a general music source. In no sense do input values mean that no sound will be produced unless such power is input, nor may drivers be damaged unless the output power of an amplifier is lower than the input value. Under ordinary listening conditions in homes, the possibility of excessive power being input will be rare even with an amplifier having output capability of up to several hundred watts, except under extraordinary conditions. Apart from occasions for PA purposes or where drivers are driven for experimental purposes, input values may be regarded as certain guidelines for enjoyable reproduction of Hi-Fi sound in homes.

(2) RATED INPUT POWER

This value represents the upper limit of the power which may be continuously input into drivers. It must be noted, however, that, as with the maximum allowable input power, such values are effective only for certain selected frequencies, not for the entire audio spectrum. Nor, does it mean that such value can be input if the frequency of interest is constant, as for some special purposes such as measurement or experiment purposes. Continuous input of a single frequency should be interpreted as if the driver load is substantial and very burdensome.

Enclosures

Drivers produce sound by their diaphragms moving back and forth to create compressional waves. The sound radiated from the front and back of diaphragms is identical except that their phases are opposite. What will happen if drivers are driven unbaffled, or mounted to nothing? As illustrated, sounds radiated from the front and back of diaphragms cancel out each other, silencing bass sound. The purpose of a baffle is to isolate the front sound from the back sound, which allows for bass sound to be audible. There are many types of enclosures available where a variety of baffles are incorporated. Next, we will discuss representative enclosures.

● FLAT BAFFLE TYPE A baffle is the only means to isolate the front sound from the back sounds. Drivers are mounted on baffles of a certain size. The larger the baffle, the more advantageous is reproduction of bass sound because the back sound must go a longer roundabout distance before it can get to the front of the enclosure. In applications of this method, drivers must be located off center of the baffle. Some disadvantages that bass reproduction capability is not as good as other enclosure systems, which is inevitable with flat baffles, may be justified by the ease in building enclosures, and sound with openness. This type of enclosure should be located rather distant from and not parallel to the walls. This is to avoid disturbance of back sound by the walls. ● OPEN-BACK TYPE An open-back type enclosure can be attained if the edges of the flat type baffles are bent. The idea is to make flat enclosures smaller to overcome the necessity of a larger size baffle for richer bass sound. Note must be taken, however, that backward bends sometimes function as a thick pipe. Pipes present at the back of drivers will be a source of pipe size dependent resonance. This fact suggests the possibility of appearance of strong peaks in reproduced sound if enclosures are excessively deep. It therefore is recommended the depth of back opening be at a reasonable extent. ● SEALED TYPE Enclosures of this type have the back of the baffles completely housed in a box of an appropriate size to prevent sounds from being radiated from the back of drivers, confining the back sound within the box. Thus, only sound radiated from the frontal side of drivers will reach the listeners.The advantage of sealed type enclosures is peculiarity-free bass sounds and good transient characteristics of bass sound. One factor in building successfulsealed type enclosures is careful bonding using adhesive to ensure satisfactory sealing. ● PHASE-INVERTING TYPE (Bass-reflex type) A phase-inverting type enclosure is called a bass-reflex type enclosure. This type of enclosure, as well as the sealed type, is one of the standard enclosure systems. Confining the sound exiting from the back of a baffle within an enclosure is the theory of the sealed enclosure system. With phase inverting enclosures, a port (duct) is provided to enhance bass sound of a selected frequency range. This can be carried out by making back sound present only within the enclosure of a selected range of frequencies resonated and inverted, and by adding such processed sound to the sound originally present at the frontal side of the baffle through the port. With phase inverting enclosures, over sealed enclosures, reproduction of an extended bass frequency range can be expected if the enclosure volume is the same. ● DOUBLE BASS-REFLEX TYPE The double bass-reflex system is intended for further enhancement of bass sound, and can be achieved if an additional bass-reflex process is given to bass-reflex system enclosures. Calculation of parameters of double bass-reflex system enclosures, which function in a very complex mode, often requires experience, intuition and the sixth sense. It is, therefore, strongly recommended as many and various calculationexamples as practical be reviewed and knowledge be built up before designing your own version. ● BACK-LOADED HORN TYPE Being one of the systems where sound radiated from the back of drivers is utilized to enhance bass, including bass-reflex and double bass-reflex types, back loaded horn type enclosures make the most efficient use of such back sound. Back-loaded horn system enclosures work in such a manner that, as bass comes out of a horn mounted on the back of a driver, the rest of sound spectrum, e.g. mid- and high-frequency ranges, is radiated directly from the driver. Enclosures of this system are highly efficient by actively utilizing sound output from drivers, and respond well to the subtlest componentsof music signals.

Enclosure materials

Among the many types of materials available even for general consumers, plywood will be the best choice in terms of price, processability and acoustic characteristics. Typical easy-to-obtain plywood types are presented below while many more are available.

(1) LAUAN PLYWOOD

This plywood is one of the most widely available plywood. Plywood of lauan, which is a tropical wood, is commercially available in various thicknesses. Its appropriate level of rigidity makes this material one of acoustically excellent candidates. Only solid plywood should be chosen for enclosure production purposes.

(2) LINDEN PLYWOOD

Linden plywood is generally lauan plywood laminated with a linden veneer. With a surface smoother than that of lauan, and with very acceptable appearance, this type of plywood is a good choice for aesthetic acoustic applications. Plywood made only of linden is too soft for acoustic applications. 

(3) OREGON PINE PLYWOOD

This soft plywood made from North American trees is characterized by reproduction capability of high quality sound. No materials other than this may be appropriate for enclosures where advantages of enclosure vibration is positively intended to be taken.

(4) LUMBER CORE MATERIALS

This material is composed of glued-up square-cut wood, and used as the base material, with a surface material sandwiching the base material. Often laminated with linden material, this plywood is rather soft and, occasionally, has a hollow structure. This structure makes these materials acoustically unsuitable. If its use is still chosen, candidate materials should be closely inspected whether to have a suitably high density structure. 

(5) PARTICLEBOARD

This product is made of glued and pressed wood chips to form sheets. Bonded chips using adhesive are extremely hard and heavy. The use of large sized chips makes these boards unfit for screwing or nailing. Also, an attempt to glue these boards together will result in difficulties. In this respect, it is not very adequate for the general public to process, compared to most other materials.

(6) MDF (Medium density fiberboard)

Like particleboard, these products are also glued and compressed under heat. What makes this material differ from particleboard is that this MDF is made from very fine wood powder. This construction permits use of wood screws for ease of processing.

(7) LAMINATED WOOD BOARD

These are boards made up of glued square-cut pieces of wood. Nowadays, use of many types of choice woods is possible, and subsequently, excellent sound, thanks to the availability of such choice natural woods, can be enjoyed. However, the physical property inherent in such natural woods, such as its likelihood to warp, remains present even after they have been made to form enclosure components, resulting in occasional distortion or cracks in enclosures.

Assembly of enclosures

No more than mere cutting materials to predetermined dimensions is necessary before assembly of enclosures. However, bonding options are abundant. The most often used bonding means is the use of the ‘plain joint’ method. In addition to means using ‘tenons or grooves’ prepared prior to assembly work, this ‘plain joint’ is another very effective method. For assembly, wood work adhesive is used. Unlike bookshelves or chairs where nails or wood screws are a main bonding means, wood work adhesive is the key component in assembling enclosures. Wood work adhesive is an effective means for maintaining tight seals between bonded surfaces. Application of a more than sufficient amount of adhesive, followed by immediate wiping of extra adhesive, squeezed out from member meeting surfaces, will permit neat and tidy finish. The temporary use of nails, wood screws or sash clamps for glued portions before the adhesive sets will ensure formation of strong and solid bonding. Should the structure provide removability of the back board, or some similar ideas are incorporated, used of packing (e.g. gap filing tape) is highly recommended.