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JBL 4430 and 4435 Studio Monitors
© Harman International, Courtesy Mark Gander and John Eargle


 


The 4430 and 4435 Bi-Radial Studio Monitors were amongst the most successful professional loudspeakers ever produced by JBL. They were in production longer than any other JBL main studio monitor, being introduced in 1981 and not discontinued until 1996 for the 4435 and 1999 for the 4430. David Smith was the engineer responsible for the system design of these monitors. The following is his account of the background and design  of these systems.

2, 3 or 4 way?

A lot of the systems that preceded the 4430/35 were 3 or even 4 way designs. Adding an 8” lower midrange would certainly improve power handling and also clean up the sound at high levels where the woofer’s excursion gets significant. Putting a crossover between the main horn and a small super tweeter type horn is problematic, though. The main horn usually had a lot of depth whereas a super tweeter (such as a 2207) would be considerably shorter. There would be several wavelengths of separation if they are both front mounted on the cabinet, leading to comb filtering in the crossover region. Pushing the supertweeter back to the point where the voice coil planes are aligned can remove the comb filtering although it isn’t always practical and doesn’t work over much of a vertical range of angles. If your main horn has the bandwidth then you are better off equalizing a bit than crossing over to a super tweeter at a high frequency. (This is just my opinion and I do realize that a lot of great JBL speakers were designed contrary to this.)

I came to JBL in September of 1980. It was my second job in the industry. I had started out in an obscure OEM speaker company called Essex Cletron in Cleveland. When Essex announced that they were moving the engineering department to Indiana I thought it was a good time to move on. Interestingly, soon after their move they were bought by Harman and turned into Harman-Motive, a long lasting and highly profitable sister company for JBL.

At the time JBL had a good line of studio monitors including the 4350, the 4343 and the 4315 (a product I much admired). The 4343, and especially the 4350, were very large and were sometimes referred to ironically as "Japanese bookshelf speakers". This is not a slur on their quality, but more reflected their great popularity in the Asian market. As a practical issue their sheer size made them a little over the top. Studios typically build their main monitors into soffets over the control window and the 4350 was just too big. The biggest problem, though, was that UREI was stealing market share with their 811 and 813. These were based on an Altec 604, always a popular unit, with a "time aligned" network and some enhancements to the horn. The sound was good enough and the size was reasonable and it had a good story in the time aligned network.

Don Keele had just gotten his constant directivity horn design software going. You might know that he had really pioneered the first constant directivity horns while at EV. He also patented their design. Altec made thinly veiled copies with their "Manta Ray" horns. Don, now at JBL, was to develop a line of horns and at the same time needed to change the design enough to get around the other patents. The biradial horns were the answer and they were extremely good. One of the first units he developed was a 100 by 100 degree horn. The larger 90 by 40 and 90 by 60 horns were more for theater applications but this smaller, wider angle horn was a natural for a studio monitor. If you look at the polar patterns that are in the AES paper we wrote you can see that the polars are incredibly uniform from 1000Hz to 16,000 Hz.

Why does the 4430’s horn roll off? 

It doesn’t, actually. All compression drivers have an inherent roll off related to their diaphragm mass.  This “mass breakpoint” usually occurs around 3 kHz.  In the lab we would measure a compression driver on a terminated tube.  In practice this is a long pipe (perhaps 4 to 6 ft long) with a thin fiberglass wedge in it.  A small microphone is inserted into the side of the pipe near the compression driver.  This presents the compression driver with a resistive acoustical load so that pressure in the tube represents what the output of a perfect horn would have.  Measured in this way most of the JBL compression drivers would have flat response from several hundred Hz to about 3kHz.  At 3k they would start a gentle rolloff at 6dB per octave until phase plug design and diaphragm modes took over above 10kHz.  You may know that Fanchur Murray did all the compression driver design at JBL in the 80’s.  In my opinion his greatest achievement was the diamond surround and getting the 2” and 4” compression drivers to have smooth response out well beyond 15kHz.

Anyhow, this rolled off response is the raw response that is presented to the horn.  The horn then modifies this response via its acoustic load at the low end its and via its directivity index at the high end.  (Directivity index represents the on axis gain.  It is essentially the “beaminess” of the horn expressed in dB.)  Prior to the constant directivity horns most people assumed that horn/compression driver combinations should have inherently flat response.  Horns were evolved that in effect “equalized” the compression driver.  This ignored the fact that the combinations could only be flat, and only on axis, if the horn exhibited a great amount of high frequency beaming.

With the 100 by 100 horn the polars are so consistent and the directivity index so flat that the on and off axis response looks just like the terminated tube measurement of the compression driver.  It is up to the crossover network to take this rolled off response and makes it flat.

To explain the biradial name, there were a lot of radial horns at the time. Probably the best known examples were the Altec 511 and 811. They took a vertical cross section designed for exponential area growth and rotated this cross section around an apex back in the throat (in effect a radial swing). Slice such a horn from front to back at any angle (through the vertex) and the cross section would be the same. Designing a horn in this way gave good extension to its theoretical cutoff, but poor polar response and a lumpy frequency response that contributed to “horn colorations”. Two independent variables were at play here. The first, the rate of area growth, defines the acoustic load that governs the radiated power curve for the first octaves above cutoff. Independently, the wall contours determine the polar pattern versus frequency. It works out that the side wall angles near the compression driver primarily determine the high frequency directivity. Contours farther out the horn, as the dimensions grow, progressively set the polar curves for lower and lower frequencies. Don developed a sidewall contour that gave a great polar pattern for a wide range of frequencies. These contours (or one contour used twice in the case of a 100 by 100 degree horn) were then used for the horizontal and the vertical shape. With his new designs the special contours were rotated twice around two radial points, hence the name biradial.

One part of the mechanical development of the 4430/35 was of the 100 by 100 horn castings. I remember working with Mark Gander on their construction. First samples were from a low tech molding process (reaction injection molding?) and always a little warped. The back surface that would have to seal to the cabinet tended to bow forward. General quality was much improved by the time production rolled around.

As an aside, a lot of the evolution of horns has been tied to their construction processes. Early horns were usually of a multicell design because a tinsmith could solder them up. More complex horn shapes wouldn’t become practical without molding techniques. Horns that were expected to sell in volume could be cast in aluminum, such as the Altec 511 and 811. Don's big theater horns would have been cost prohibitive (and heavy) if aluminum diecast due to their large size and relatively low projected sales volume. They ended up being made in fiberglass with reinforcement panels molded in. The 4430 horn was a little tricky because it needed side extraction of the tool for the lateral pockets. It also had a separate sand cast throat section that was bolted on from behind and linked the front to the compression driver. Getting the cross section and the juncture between the two just right impacted the response so we played with that variable a fair bit.

Don had just started on a crossover network when I took over the project. I remember that there was a lot of work to get the midrange and tweeter controls to work sensibly. Also a lot of work to get the octave to octave balance right and also to insure the balance of the 4430 and 4435 were identical even though the 4435 was three dB more sensitive. There were a number of listening sessions with Gary Margolis and John Eargle which I, as a young engineer, found very instructional. They had good ears and could identify what octave needed to go up or down a dB to get the balance just right and I wanted to be able to do that!

 How to improve the 4430/35

The only real negative of the biradial horn designs was that the concentration on great polar response sometimes was at odds with the ideal of exponential area growth. This resulted in some ripple to the first couple of octaves of the horn’s response.

The bottom end of a horn/compression driver combo is largely determined by the rate at which the horn’s area grows. Back in the acoustical gramophone days mathematicians had figured out that Exponential area growth gave the most extended acoustical load to the transducer. Exponential growth simply means that the cross sectional area grows a constant percentage for a fixed unit of length along the horn. The slower the area grows the lower the cutoff frequency. For example a 500 cycle cutoff would dictate that horn area should double every 38 millimeters. A 250 cycle horn would grow half as fast with area doubling every 76 mm. For a horn to work well to a certain frequency it will need appropriate exponential growth and also a certain minimum mouth area. You must have both. Having a low flare rate but with inadequate mouth area will lead to choppy response in the horns first octaves.

In the original biradial horns the only truly exponential part was the short section from the exit of the compression driver to the gap that fed the horizontal flare. Making the gap narrow gave great horizontal polars up to a high frequency but pushed the exponential cutoff frequency down to the point that the area didn’t support it. The consequence was a periodic ripple in frequency response. It wasn’t real bad…about 2 dB total ripple.

If you look at the impedance curve you see impedance peaks that correspond to the response peaks. This gave a challenge in the network design. You needed to keep the networks driving impedance low otherwise the voltage at the compression driver would start to follow the ripple of the impedance curve and make the response worse.

With the 4430 network the driving voltage had a ripple of about 1 dB and so the horn’s total ripple was about 3 dB. The consequence of the ripple was a slight coarsening of the sound of the midrange, most noticeable on pink noise. I always thought that biamping would help a little here (directly coupling the amplifiers low output impedance to the compression driver) but what would be really neat would be to drive it with and amplifier with a slightly negative output impedance. This is can be achieved with various amplifier feedback schemes. Then for every frequency where the impedance bumps up the amplifier output voltage would actually drop. A negative output impedance amplifier would act to equalize the first couple of octaves of the horn’s response to something really smooth. Any experimenters out there up to the task??

Equalization of the horn/compression driver was the only out-of-the-ordinary issue for the 4430/35 network.  As mentioned above the high frequency section will have an inherent 6dB per Octave rolloff above 3kHz.  A passive network can equalize that as long as the inherent sensitivity stays high enough to the highest frequency needed.  The sensitivity of the 2425 compression driver on the 100 by 100 horn was about 108 dB from 1000 Hz up until its rolloff above 3K.   By 16K the response had dropped to about 94dB, just enough for the 93dB target of the 4430 and near enough for the 96dB 4435.

The upper section of the network started out as a second order filter followed by an L-Pad for overall treble level and with a first order bypass (a small capacitor around the L-Pad)  to give the highest frequencies a path around the L-Pad.  Basically the L-Pad would pull the lower treble down about 16dB, the bypass would push the highs back up and equalize the horn.

A couple of issues needed to be dealt with:  First, the first order bypass approach still lost a couple of dB at 16kHz, leading to a softer top end than was desired.  Secondly, it would be nice to have sensible response controls for the system.  The L-Pad, because it was bypassed, would only have effect from 1 to 5kHz.  It would become a “lower treble” control.  I wanted separate controls for both the lower treble and upper treble, more in keeping with our other 3 and 4 way systems.

A solution for both was to use a series resonant bypass network.  1 microfarad in series with about .08 milihenries would resonate around 15K and give about 2 dB more output for the highest frequencies.  It also gave a little less around 5k where the 2425/biradial combo was a little hot.  A variable resistor in series the resonant leg gave a nice “upper treble” adjustment so we now had a pair of controls with really useful control centers and range.

After getting the horns working the woofer section was tackled.  The woofer section was a straight second order network with a conjugate to flatten out the woofer’s inductance.  Using a conjugate lets you achieve a more “classic” looking second order rolloff since the network inductor and the woofer’s inductance wouldn’t be interacting.  Most of the work in this network was in getting the best blend between woofer and horn.  When the woofer and network shape looks about right you have to see how the sections add.  Do they add well in phase?  Or in reverse phase?  At what axis do they sum best, and is that where you want them to sum best?  It turns out that with the crossover slopes used and the relative depths of the woofer and horn that the units gave best summing connected in phase (++ we would say) on a measuring axis that was straight out or slight rising.  The 0 degree and 15 degree up curves were both about equally good.  This would work well if the system was floor standing or even if inverted and mounted over a studio window.

At this point we now have a system with all the sections working and giving a reasonable response curve.  We still aren’t done though.  At this point you call over the marketing guys with the golden ears and everyone has a listen.  The curve will be massaged a dB here and a dB there until the group (primarily Gary Margolis and John Eargle for this product) is happy.

I was keen to give a paper on the monitors. Since they were the first to use a constant directivity horn there was some merit to a paper and so I received permission to write it. I did most of the writing, although John Eargle corrected a lot of my grammar and finessed the introduction and my wife captioned the illustrations! If you read between the lines of the paper you can see that we were taking some potshots at the UREI product.

I was pleased that the products were well respected by the Pro community and stayed in the line for a long time. The 4400 series grew after these models. I designed a 4401 and 4411 and after I left JBL Greg designed a baby 4430, the 4425. One thing that was amusing at the time was how people reacted to the shape of the new biradial horns. I remember more than one studio engineer referring to them (affectionately!) as the “Dolly Parton” horns or “Baboon butt monitors”.

© David Smith, February 2005

 
 

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