JBL - K2 S9800 - White Paper (english) (2001).

JBL White Paper 

The New K2.S9800 Loudspeaker System 

By JBL Technical Staff 

TIMOTHY PRENTA 

Director, JBL Consumer Engineering 

JBL Consumer Products 

GREG TIMBERS 

Chief System Engineer 

JBL Consumer Products 

JERRY MORO 

Senior Transducer engineer 

JBL Consumer 

DOUGLAS BUTTON 

Vice President, Research & Development 

JBL Professional 

Editor 

John M. Eargle 

Advisor 

JBL Consumer & Professional

FIFTH DRAFT (15 May 2001) 

JBL White Paper: The New K2.S9800 Loudspeaker System 

A. Introduction The Design Brief: 

By JBL Technical Staff 

The advent of high-density recorded media for the consumer has challenged manufacturers of 

loudspeakers and microphones to push the frequency response envelope of transducers and 

systems up to 50 kHz in order take advantage of the program signals that can now be recorded in 

the DVD-Audio and Sony-Philips SACD software formats. The issue is not so much whether 

listeners can hear this uppermost octave as such, but rather that the bandwidth extension ensures 

smoother and less aberrated frequency response in the upper portion of the normal audio 

bandwidth than we have known before. 

Another major consideration is uniformity of acoustical power radiation from the loudspeaker 

into the listening space. There are two aspects here: smooth on-axis response and controlled 

vertical and lateral radiation angles. Studies carried out by various technical groups within the 

Harman organization have shown a universal preference on the part of professionals and critical 

consumers for loudspeakers that exhibit such response uniformity. 

At the same time, there is an ever stronger demand from critical listeners for lower distortion in 

loudspeaker systems, effectively matching the low distortion normally found in the best 

microphones, recording systems and playback electronics. 

In addition to the foregoing requirements, critical listeners demand realistic playback levels with 

minimal compression of dynamics. While there may be several approaches to achieving 

bandwidth extension and uniform power response, there remains only one way to ensure high 

playback levels with high dynamic linearity — and that is through the use of over-designed 

transducers that are capable of output levels many times greater than will normally be required of 

them. 

In critical loudspeaker auditioning sessions, listeners become very aware of subtle timbral 

aspects which are often difficult to measure. Such matters as cone materials, high frequency 

diaphragm materials and dividing network component integrity become important design 

considerations in defining a modern state of the art loudspeaker system. 

Finally, the appearance and shape of a fine loudspeaker system must reflect natural and pleasing 

proportions, show use of the finest materials, and be acoustically correct insofar as physical 

boundary conditions for transducer performance is concerned. 

In this White Paper we will examine transducer, horn and system designs in detail, discussing all 

aspects of their engineering and development. We will then move on to the evolution of the 

1

system concept and the integration of the elements into the whole. We will conclude with a 

discussion of system setup and optimization in typical listening spaces. 

B. The Transducers: 

Low-Frequency Driver: 

The Model 1500AL LF driver was designed by Jerry Moro, Senior Development Engineer for 

JBL Consumer Products. It is a 380 mm diameter device with a 100 mm voice coil completely 

immersed in a radial field generated by an Alnico 5DG magnet. A section view of the driver is 

shown in Figure 1. Its salient features are: 

1. Large outer suspension, made of EPDM foam rubber material, which offers low mass, low 

losses and much improved longevity. 

2. Double Nomex inner suspensions, inverted with respect to each other for cancellation of evenorder 

distortion components. All suspension elements are tailored for maximum mechanical 

displacement linearity. 

3. A very large aluminum shorting ring positioned below the gap to reduce global flux 

modulation of the static field. This eliminates the demagnetization problems that have plagued 

Alnico motor designs in the past. 

4. Use of alternating copper-steel laminations in the top plate. The presence of the copper rings 

provides the familiar JBL shorting ring adjacent to the voice coil to reduce its inductance and to 

further stabilize the local magnetic field throughout the steel in the long gap length. The presence 

of the steel laminations produces an effectively narrow magnetic gap through the "sharing" of 

space with the copper. 

5. The use of Alnico 5DG magnet material provides a stable magnetic operating point under high 

drive conditions, further reducing distortion due to flux modulation. The magnetic circuit has 

been optimized for flux consistency throughout the normal range of voice coil movement, and 

the moving system has a linear operating range of ±12.7 millimeters. 

6. Choice of cone material. A traditional felted material of thick cross-section was chosen for its 

combination of high internal damping, mass, mechanical stiffness and freedom from acoustical 

coloration. 

7. A fully vented frame and motor structure is used for minimizing acoustic losses and for 

cooling of internal parts through optimum transfer of air into and out of the magnetic structure. 

Altogether, these factors provide: lower losses at high frequencies, reduced harmonic distortion, 

and improved power handling. 

Pertinent mechanical, magnetic and acoustical parameters of the 1500 Al LF driver are: 

2

Flux density: 0.52 tesla throughout the 40.64 mm long gap 

Bl product: 21 T»m 

Voice coil resistance: 5.3 ohms 

Voice coil winding length: 20.32 mm 

Free-air resonance frequency: 26 Hz 

Peak-to-peak: linear excursion capability: 25.4 mm 

Weight of magnetic structure: 13.6 kg (without cover) 

Driver sensitivity (1 watt at one meter): 94 dB SPL 

[Insert Figure 1 about here.] 

Mid-Frequency Compression Driver and Horn: 

The model 435 Be compression midrange driver was designed by Douglas Button, Vice 

President of Engineering for JBL Professional. It makes use of a 75 mm diameter diaphragm 

operating into a traditional Western Electric type annular slit phasing plug. The use of 

neodymium magnetic material keeps the size of the driver to a minimum. Figure 2 shows a 

section view of the midrange compression driver. 

Its salient features are: 

1. As an intermediate size between JBL's large and small format compression drivers, 

derivatives of the 435 have found great use in JBL's recent professional line arrays, where the 

small size of the driver and its magnetic structure allow them to be used in multiples for driving 

high frequency line arrays without the design and fit problems that would be unavoidable with 

large format drivers. 

2. The phasing plug and initial flare development are both of the newer rapid flare type, which 

reduces second harmonic distortion by up to 6 dB, relative to JBL's earlier driver technology. 

3. With efficiency in the range of 25 - 30%, the driver can deliver high amounts of acoustical 

power with relatively little electrical input. The diaphragm is made of beryllium, whose pistonic 

behavior is maintained to well beyond 15 kHz. The diaphragm is coated with a fine layer of 

Aquaplas for damping of spurious resonances. 


4. Pertinent magnetic and acoustical details: 

Mass of moving system: 1.25 g 

Flux density in gap: 20 tesla (20 kGs) 

. Magnetic assembly weight: 1 kg 

Figure 5 shows a plane wave tube response curve of the 435 Be driver. 

3


The 435 Be driver is attached to a Bi-Radial horn designed by Greg Timbers, Chief Engineer 

of JBL Consumer. It is executed in a high impact plastic that has high dimensional stability. 

Horn section views are shown in Figure 3. The -6 dB beamwidth plots and directivity data are 

shown in Figure 4. Note that the pattern control is uniform (±10°) from 1 kHz to 10 kHz. The DI 

is within ±1 dB over the same frequency range. 

[Insert Figures 3 and 4 about here.] 

Ultra-High-Frequency (UHF) Compression Driver and Horn: 

The 045 Be UHF compression driver was designed by Tim Prenta, Director of Engineering for 

JBL Consumer products. With its 25-mm beryllium diaphragm and 50-mm neodymium magnetic 

structure, this is the smallest compression driver that JBL has ever designed. The pure beryllium 

diaphragm is less than 0.04 mm thick and has a mass of only 0.1 gram. The single layer 

aluminum ribbon voice coil is wound without a former and attached directly to the diaphragm. 

The driver employs the smallest annular slit phasing plug that JBL has ever designed. Each 

phasing plug assembly is manufactured by modern stereo lithography techniques for absolute 

dimensional integrity. A section view of the 045 BE driver is shown in Figure 5. 

The extremely low mass of the moving system, high magnetic flux density and the high rigidity 

of beryllium produce response that is essentially flat from 10 kHz to 48 kHz, as seen in Figure 6. 


The UHF horn was designed by Greg Timbers. It is properly scaled to maintain a coverage angle 

of 60 degrees in the horizontal plane and 30 degrees in the vertical plane over the frequency 

interval from 10 kHz to 50 kHz. 

Pertinent magnetic and acoustical details: 

Mass of moving system: 0.3 g 

Flux density in gap: 20 tesla (20 kGs) 

Magnetic assembly weight: 0.21 kg 

Figure 7 shows various views of the UHF horn. Figure 8 shows pattern control and directivity 

for the horn. 


C. System design and component integration: 

4

Figure 9 shows front, top, side and perspective line drawings of the systems, illustrating the 

various curved transitions that define the smooth acoustical boundary conditions for the LF 

driver and the MF and UHF horns. 

The dividing network topology combined with the acoustic behavior of the individual 

transducers provides 24-dB-per-octave transitions between adjacent system elements. The 

response of each element is -6 dB at the transition frequency. These rapid transitions provide 

more uniform system response over a larger vertical listening angle than that provided by lowerorder 

slopes, and this translates into an increased solid listening angle for the system. The 

contribution of each element, along with the overall on-axis response of the system is shown in 

Figure 10. 

Another important factor in the design of the dividing network is the use of DC biasing of the 

capacitors in this system. The biasing ensures that normal zero-crossings of the audio waveform 

from plus to minus are displaced from actual zero potential and thus do not force the capacitors 

to undergo a zero charge condition, even momentarily. This is analogous to "Class A" behavior 

in amplifiers. 


Details of the system's directional response are shown in Figure 11. This family of curves is 

defined as follows: 

Curve 1. On-axis response. 

Curve 2. Average of response over 30° horizontal and 15° vertical angles. 

Curve 3. First reflected sound power sum. 

Curve 4. Total reflected sound power sum. 

Curve 5. On-axis overall directivity index (dB). 

Curve 6. On-axis directivity index for first reflections (dB). 


The overall trend is for the system to be fairly nondirectional at low frequencies, reaching a 

plateau of uniformity at about 1 kHz, where the midrange Bi-Radial horn comes into play, and 

extending beyond 20 kHz as the UHF takes over. 

Under these conditions the system will integrate well with a wide variety of acoustical 

conditions. In a room where the boundary absorption is uniform over the midrange, the system 

will provide ideal spectral coverage over a broad listening angle. 

The impulse response of the system is shown in Figure 12. The response shows system time 

alignment within an interval of about 350 microseconds, along with highly damped low 

frequency response 


5

From an electronic point of view, we want to ensure that the system does not present a 

troublesome electrical load to any amplifier that is chosen to drive the system. Figure 13 shows 

the modulus of impedance as not dropping below 6 ohms at any point over the range below 10 

kHz. The drop to about 3 ohms at 50 kHz is considered negligible. 


Overall system distortion at reference level of 102 dB at a measuring distance of one meter is 

shown in Figure 14. Because the fundamental response is so uniform we can add at the right edge 

of the graph an approximate scale indicating distortion percentage from the range of about 60 Hz 

to 20 kHz. The second and third harmonic distortion components at this extremely high playback 

level are amazingly low. Only in the range above about 5 kHz does the value of second harmonic 

(dashed curve) rise slightly above one percent. 

[Insert Figure 14 about here] 

D. System Integration into the Listening Room: 

The target consumer for the K2.S9800 system will probably already know enough about 

loudspeaker positioning in the home listening environment to ensure good results with a pair of 

these new loudspeakers in normal stereo configuration. It is for the listener who wants to use 

these loudspeakers in a surround sound configuration that we offer the following 

recommendations. 

The ITU (International Telecommunications Union) standard for surround loudspeaker 

placement is shown in Figure 15. When possible, the circular outline of loudspeaker location 

should be maintained. If it is necessary to place the three frontal loudspeakers against a wall, then 

the center loudspeaker should be time-delayed so that its effective distance from the primary 

listening position will be equal to that of the left and right front channels. In calibrating the 

system levels at the primary listening position, any differences in distance to the primary 

listening position should be taken into account so that both level and relative timing from all five 

channels will be the same at the center listening position. 

Since the ITU recommendation is basically a broad guideline, individual users may modify it in 

minor ways to fit the requirements of a given listening space. Specifically, the placement of rear 

channels may be changed slightly to allow for difficulties in room setup. Some listeners may 

prefer the rear channels to be slightly greater than the ±110° limit shown here. In any event, both 

rear channels should be at the same off-axis angle, relative to the center channel, to maintain 

system left-right balance. 

Figure captions: 

1. Section view of 1500AL low frequency transducer. 

2. Section view of 435 Be mid-frequency compression driver. 

6

3. Vertical and horizontal profiles of the mid-frequency Bi-Radial horn. 

4. -6-dB beamwidth and directivity data for the mid-frequency horn over its nominal operating 

range. 

5. Section view of the 045 Be UHF compression driver. 

6. Unequalized on-axis response of the UHF system. 

7. Vertical and horizontal profiles and views of the UHF horn. 

8. -6-dB beamwidth and directivity data form the UHF horn over its nominal operting range. 

9. Front, side, top and perspective line drawings of the complete system. 

10. On-axis contributions of LF, MF and UHF sections of the system. 

11. Overall system directional and room response. 

12. System impulse response. 

13. System modulus of impedance. 

14. Second and third harmonic distortion at 102 dB SPL at a measuring distance of one meter. 

15. Target loudspeaker locations for surround sound playback. 

7

JBL - K2 S9800 - White Paper (english) (2001).

Create successful ePaper yourself

Delete template?

Save as template?