- Types of drivers
- Interpretation of Catalog Specification
- Enclosure Materials
- Assembly of Enclosures
Interpretation of Catalogue Specifications
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.
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.
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 'ƒ¶' (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.
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..