JBL Technical Note - Vol.1, No.3A 电路原理图.pdf
A. Introduction: JBLs low frequency (LF) transducers tall into three major categories, based on their relative efficiencies. Those models in the lowest efficiency category exhibit high linearity and are primarily used as subwoofers and low frequency applications in studio monitors. The mid- efftciency models find their greatest application in general sound contracting work, and the highest effi ciency models excel for LF horn applications as well as use with musical instruments The mid-efficiency models are further available in JBLs standard magnetic configuration as well as in the Vented Gap Cooling (VGC) configuration introduced m 1990 The VGC products provide advantages in heavy duty applications, where their superior power dissipation results in low dynamic compression. It is the purpose of this Technical Note to explain to the professional user how LF transducers are chosen for specific jobs. We will also explain in detail the elements of construction and design of the transducers which optimize them for their particular jobs. B. The Basic Matrix: The primary JBL transducers are grouped into a three-way matrix, shown as follows: Table I. SENSITIVITY MEDIUM HIGH MAXIMUM EFFICIENCY: 0.5% 3% 2% 6% 2%-6% 4% - 10% LINEARITY: mr.n MEDIUM (STANDARD) MEDIUM LIMITED 460 mm (16 in) 2245H 2240H 2241H (E155-8) 380mm(15tn) 2235H fffDf 2226HJ (Haiti 300 mm (12 In) (128H-1) 2202H 2206H (El 20-8) Those models enclosed in parentheses are not in the normal 2200-senes line-up. but they tit structurally into the matrix in the slots indicated. The model 128H-1 is used as the LF driver in the Model 4412 studio monitor system C. Sensitivity vs. Efficiency: it is important to understand the difference be tween sensitivity and efficiency. The reference efficiency of a transducer is a function of its electro-mechanical parameters and is normally expressed as a percentage of the input power which is converted to acoustical power in the so-called piston band range of the trans ducer, over which its response is essentially omnidirec tional. Sensitivity is a measure of sound pressure output at some reference distance on-axis with a reference input signal. In normal industry practice, the reference dis tance is taken as one meter (3.3 ft.). The input signal may be one watt, or perhaps a stated rms voltage (2.83 V is common, in that it is the applied voltage which will produce a power of one watt in an 8 ohm load). It is important to note the frequency band over which the sensitivity measurement is made. Normally, it is stated over a fairly wide range, perhaps as high as 2.5 kHz; In such cases, on-axis directional effects will result in high sensitivity ratings. D. Detailed Description of the Matrix: In this section we will descnbe in detail the internal parts of a loudspeaker. For those who are not familiar with JBL LF transducer construction, we present in Figure 1A a labeled section view Figure IB shows the added detail of the VGC magnetic structure. The cone, voice coil, and frame details cf the VGC products are the same as shown in Figure 1A. 1. MEDIUM-SENSITIVITY TRANSDUCERS: a Uses: Pnmary use is as LF transducers in monitor sys tems as well as subwoofer designed for recording Technical Notes, Volume 1 Number 3A Choosing JBL Low Frequency Transducers studios and and motion picture theaters. For these purposes, long travel at low frequencies is essential, as is low distortion Smooth HF response is also a require ment for proper transition to mid- and high-frequency transducers in the 400 Hz to 1 kHz range b. Sensitivity and Power Handling: Table II. b. Sensitivity and Power Handling Table III. MODEL RATED SENSITIVITY (1 W 1 mi CONTINUOUS PROGRAM POWER REFERENCE EFFICIENCY 2245H 95 dB 600 Warts 2.1% 2235H 93 dB 300 Wans 1.3% 128H-1 91 dB 200 Waits 0.86% c. Internal Design and Construction: VOICE COIL: Flat copper wire (see Table VI). extended over-hanging winding for maximum linearity (see Figure 2A); 100 mm (4 in) voice coil diameter used in larger models; 75 mm (3 in) voice coil diameter used In 128H-1. VOICE COIL FORMERS: Made of aluminum. Kapton polyimide plastic, or a composite for effective heat sinking and mechanical integrity (see Table VII). SPIDER (Inner Suspension): All models use a special design which reduces dynamic offset and instability at high drive levels, resulting in unusually low distortion and tight transient character. CONE Straight-sided and nbbed for stability. Fairly large mass for optimum balance of efficiency, bass output, and low distortion. Aquaplas coating used on 2245 and 128H-1 to optimize stiffness and mass; mass ring on 2235 for desirable efficiency and bass balance. SURROUND (Outer Suspension); Half-roll of polyurethane foam for high compliance and long travel (see Figure 3A). DUST DOME Made ot similar felted material as cone for smoothest HF response. 2. HIGH-SENSITIVITY TRANSDUCERS: a. Uses: Pnmary use is in sound and music reinforcement, mounted in horn and direct radiator reflex-type enclo sures For these purposes. LF cone excursion has been limited and sensitivity increased, relative to the transduc ers of the medium sensitivity class, in order to get greater output in the 50 or 60 Hz range up to 800 Hz. MODEL SENSITIVITY CONTINUOUS REFERENCE W w 1 m; PROGRAM POWER EFFICIENCY 2240H 06 dB 600 W 5% 2225H.J 97 dB 400 W 3.5% El40 8 100 dB 400 W 4.9% 2202H 99 dB 300 W 6% VGC PRODUCTS 2241G.H 96 08 97 dB 95 dB 29% 3-3% 2-5% 2226G.H J 96 08 97 dB 95 dB 29% 3-3% 2-5% 2206H.J 96 08 97 dB 95 dB 29% 3-3% 2-5% c. Internal Design and Construction: VOICE COILS: Flat copper or aluminum wire. 100 mm (4 in) in diameter. Slightly over-hung in the mag netic gap (see Figure 2A) for proper balance of sensitiv ity and linearity. VOICE COIL FORMERS Made of aluminum. Kapton polyimide plastic, or a combination, for effective heat resistance and sinking, and rigidity. SPIDER (Inner Suspension) All models use a special design which reduces dynamic offset and instability at high drive levels, resulting in unusually low distortion and a tight transient character CONE: Straight-sided and ribbed tor stability; however, total moving mass is less than in the case of medium efficiency transducers, without added damping compounds or mass rings. SURROUND (Outer Suspension): Multiple halt-roll (see Figure 3B) provides controlled travel Stiffness is optimized by depth of rolls, weave of cloth, and damping treatment. DUST DOME: Made of similar felted material as cone for smoothest HF response The E140 is similar to the 2225. but with an alumi num dome and less voice coil overhang for slightly more midrange efficiency, peaked high-frequency output, and a punchy sound character The E145 and 2234 are specialized transducers that combine high efficiency with high linearity. The E145 has a short copper coil moving in a very deep magnetic gap (Figure 2B) for maximum voice coil control and excursion linearity. This design also provides maximum low frequency output from a given enclosure size The E145 can move as much air as a high effi ciency 460 mm (18 in) transducer. The 2234 is identical to the 2235 with the deletion of the mass ring This raises the efficiency (mid-band sensitivity) for multiple woofer applications The 2234 is used in the 4435 studio monitor 2 3. MAXIMUM SENSITIVITY TRANSDUCERS: E. Comparative Frequency Response: a. Uses: Primarily in music reinforcement for driving LF horn systems. For these applications, the range of linear travel has been restricted in a trade-off for greater sensitrvrty in the 50 or 60 Hz range up to 1200 Hz b. Sensitivity and Power Handling: Table IV. MODEL RATED SENSITIVITY (t W. 1 m) CONTINUOUS PROGRAM POWER REFERENCE EFFICIENCY E155-8 100 dB 600 Watts 49% 2220H.J 101 dB 200 Watts 8.7% E130-8 105 dB 300 Watts 86% E120-8 103 dB 300 Watts 8.6% c. Internal Design and Construction: VOICE COILS: Flat copper or aluminum wire, depending on transducer sensitivities. Voice coils either slightly underhang (2220) or overhang (El55) the top plate, or are of equal length (E120. E130) (see Figure 2C). Sensitivity is at a premium and is more important than linearity. VOICE COIL FORMERS: Aluminum and Kapton poiyimide plastic composite, for effective heat sinking and rigidity. SPIDERS Stiff, to keep resonance high. CONE: -Curvilinear- on E120. E130. and 2220 and shown in Figure 4C. Curvilinear cones are thin and exhibit controlled high-frequency break-up for extended output One pece cone/compliance construction pro vides increased top-end on El55. while straight sides and deep cone angle give rigidity SURROUND (Outer Suspension): Paper, integral with cone on E155 Multiple half-roll on E120. E130. and 2220. DUST DOMES Thin paper on 2220, aluminum on E120. E130. and E155 for extended HF response and minimum mass. The E130 is similar to the 2220, but with an alumi num dome, and an aluminum voice coil equal in length to the magnetic gap. for maximum high-frequency output. The three families of LF devices are normally mounted in different enclosure types: however, it is instructive to took at the frequency response of the three types mounted under similar conditions Figure 5 shows the 2235. 2225. and 2220 mounted in a 280 liter (10 cubic foot) sealed enclosure The curves were run in halt space that is. the enclosure front was flush with the large baffle surface. At low frequencies, below about 60 Hz, all three drivers exhibit similar response. Above about 80 Hz, the three drivers diverge, and the 2235. the lowest sensitivity model of the three, levels off at a mid-band plateau of 93 dB. The 2225 continues to climb and does not level off until about 400 Hz at 97 dB The 2220 levels off hardly at all. but we can see something of a plateau at about 101 dB m the 400 Hz range. The mid-band levels of the three drivers are a result of their respective efficiencies, as is the reduction of the LF bandwidth of the curves Note the HF response of the three drivers. The 2235 begins to roll off at 1 kHz, and the slight peaks and dips in its roll-off characteristics are due to resonances in the polyurethane surround. Because of its lower mass, the 2225 reaches about 1600 Hz before it begins to roll off It exhibits the smoothest roll-off characteristic of the group, due to its straight, ribbed cone and stiff, multiple half-roll surround. The 2220 exhibits the most extended response of the group, going out to 4 kHz before it rolls off sharply. This is due to break-up modes in its curvilin ear cone. Remember that each of the three transducer types is designed for a particular kind of enclosure and appli cation, and that its sensitivity and bandwidth have been optimized for those applications F. Thiele/Small Parameters While the high-frequency performance of a LF transducer is often unpredictable due to breakup modes and surround resonances, the precise nature of LF response can usually be accurately plotted beforehand through the use of the Thiele/Small parameters. All JBL ported enclosures have been designed using these parameters and. where possible, it is recommended that sound contractors use them. For the convenience of those who have need tor designing special systems, we present here a tabulation of the Thiele/Small parameters for all JBL cone transducers which have been manufac tured during the last two decades (see Table V). Refer ences which will be useful In Thiele/Small simulations are given at the end of this Technical Note (1,2). Work done by D B Keele (3) has extended the usefulness of the Thiele/Small parameters to LF horn systems. Some details of this are shown in Figure 6. where both LF and HF roll-off frequencies have been related to specific Thiele/Small parameters. The high frequency mass roll-off of the driver in horn loaded use. W can 06 easiy computed from the Thiele/Small parameters, and used as a strong indicator of how high m frequency a horn/driver system can be expected to produce power-flat output Field measurements of ThieleSmall parameters often show small variations in VM. fs. and Q. These parameters are all affected by the stiffness of the cone surround and inner compliance, both of which may be influenced by temperature. In general, after a short period of use. the stiffness will settle into its target value. We hasten to point out that the stiffness variation will ncj cause actual response variations, inasmuch as the response, or low frequency alignment, of a loudspeaker system is dominated by enclosure volume, port tuning, and other parameters that are relatively constant. G. Dynamic Compression in LF Transducers An ideal loudspeaker should theoretically respond with the same frequency response with an input of 100 watts as with an input of 1 watt. There should only be 20 dB difference in level. In reality, considerable thermal and mechanical stresses are placed on a loudspeaker with high power inputs. Typically, they reduce its effi ciency and alter its response. The change in perfor mance from low to high power inputs is an important indicator of its behavior in many musical applications. Dynamic compression results from heating of the voice coil. It is the increase in resistance with heat that causes the signal compression. Because of their large voice coils and heavy construction. JBL LF transducers have always exhibited relatively low dynamic compression, compared to competitive models with 2.5 inch diameter voice coils. However. JBL introduced the VGC products in an effort to reduce dynamic compression to even lower degrees and increase general power handling in the process. Figure 7 shows 1 watt and 100 watt superimposed compression curves for the JBL 2226H The curves show compression on the order of 1.5 dB over the range from 100 Hz to about 2 kHz, with virtually no compres sion at lower frequencies. H. Harmonic Distortion in LF Loudspeakers 1. MOTIONAL NON-LINEARITIES: The voice coil length and top plate thickness can only indicate the potential excursion linearity of a loudspeaker (4). Unless the suspension elements are carefully matched to the motor structure, distortion and limited excursion will result. As the ends of the coil move outside the magnetic gap. at each position the motor strength is different from that observed when the voice coil is centered in its rest position. Under transient conditions the coil can actually jump forward, trying to move out of the gap. This is known as magnetic rectifi cation, or DC offset, and it causes a high level of 2nd harmonic distortion to be generated as part of the transient signal. This effect is common to many long coil loudspeakers. By carefully choosing the stiffness characteristics of the outer suspension and spider, this offset can be eliminated and the linearity of the loud speaker improved. JBL loudspeakers have inner sus pensions which eliminate, or substantially reduce. DC offset. In some cases, this offset may be allowed to o
