The Morel M3 Loudspeaker

INTRODUCTION AND OBJECTIVES

The M3 loudspeaker was designed to satisfy multiple objectives. It gets its name from the fact that it uses all Morel drivers in a 3-way system. The primary objective was to obtain the best quality in a minimum size 3-way system. The baffle height and width are about as small as possible given the drivers used, and these drivers occupy fairly small baffle area. Indeed, the M3 is smaller than a great many 2-way systems. A related objective was to demonstrate that a 3-way system need not be large.

Another objective was to demonstrate that a 3-way crossover need not be complex. A related objective was to demonstrate that a first-order transient-accurate crossover also need not be complex.

The M3 is only 13.5 inches high by 7 inches wide by 14.75 inches deep. As a result of its sloped baffle, it is only 12 inches deep at the top.

The M3 is not an inexpensive loudspeaker to build, primarily as a result of the use of $217 worth of premium Morel drivers. Total cost of parts per side is just under $300.

THE DRIVERS

The M3 employs the following Morel drivers:

The small 2-1/8-inch square tweeter flange and the 3-7/16-inch square midrange flange make close mounting of the drivers on a small baffle possible. Such close mounting mitigates vertical lobing effects and makes possible a small baffle with good imaging.

THE CROSSOVER

The 3-way crossover is shown in the figure. It is composed of two first-order 2-way crossovers in tandem, the first crossing at 1500 Hz and the second crossing at 4000 Hz. C2 and L2 provide the 1500 Hz crossover, while C3 and L3 provide the 4000 Hz crossover. Both the midrange and woofer drivers have Zobel networks to flatten their impedance curves at high frequencies. An RLC trap circuit consisting of C4, L4 and the inductor's resistance of 1.0 ohm provides a bit of equalization in the vicinity of 750 Hz for the midrange driver to help it integrate better with the woofer.

Time Alignment
Proper operation of first order crossovers for achieving a transient-accurate, minimum phase response requires that the drivers be time aligned. This means that they should all introduce the same time delay to the ear at the listening position. Larger drivers tend to have an effective acoustic center (point of sound origin) that is further recessed from the plane of the front baffle. In the M3 the baffle is sloped back at an angle of 12 degrees in order to accomplish time alignment of the drivers.

First Order Transient Accurate Design
With time-aligned drivers and first-order electrical filters, a transient-accurate, minimum phase system response will be achieved as long as the achieved frequency response is reasonably flat. In some cases, minor tweaks in driver acoustic offset and crossover frequencies are necessary to achieve the desired degree of frequency response flatness. The acoustic responses of the crossed-over drivers in the M3 are not first-order to way beyond the crossover frequency, but this is not necessary to achieve a transient-accurate response. This is why the crossovers can remain fairly simple.

Baffle Step Compensation
Components L1 and R1 at the input to the crossover provide baffle step compensation (BSC). Total amount of BSC is 3.0 dB, with the transition centered at 300 Hz.

HF Lift
Capacitor C1, connected across the BSC network, provides a high-frequency lift above 10 kHz that adds to the "air".

THE WOOFER ALIGNMENT

The box is lightly stuffed with fiberglass and tuned to 44 Hz with a 2-inch flared Precision Port that has a total length of 9.5 inches. The design assumes an effective volume for the woofer of 13 liters (0.46 cubic feet). Response is down 3 dB at 40 Hz.

The box stuffing can also be done with AcoustiStuff. Most of the stuffing is behind the woofer below the bracing shelf. There is also some stuffing surrounding the body of the port tube. Pull the stuffing apart before putting it into the box so that it is of a fairly low density. Stuffing is not very critical, but avoid getting it near the inner mouth of the port.

The box is not lined with anything on its walls.

THE BOX

The box is constructed of ½-inch MDF panels with the exception of the baffle and rear panel, which are ¾ MDF. We can get away with ½-inch panels because the box is quite small and rigid, especially with the added shelf brace. The use of ¾ walls with a box of the same outer dimensions would have seriously reduced interior volume and working width.

Driver locations and offsets are as follows, with respect to the bottom of the baffle:

The baffle extends the full height and width of the cabinet (13.5 X 7.0). The actual length of the baffle is 13.75 inches due to the slope of the baffle. The side panels extend the full height of the cabinet.

The woofer is centered laterally on the baffle. The midrange and tweeter are offset to the right on the right channel speaker such that their centers are 4 inches from the left edge and 3 inches from the right edge. These drivers are similarly offset to the left for the left channel speaker. These offsets help smooth out diffraction effects by causing the driver distances to the cabinet edges to be different.

The drivers are all surface-mounted on the baffle - no routing of flush-mount recesses is needed. The effects of such mounting are mitigated by the use of square-frame midrange and tweeter drivers, since the radius from the center of the driver to its flange edge is not constant.

A horizontal shelf brace made of ½-inch MDF is located 5-1/2 inches above the interior floor of the cabinet. The depth of the shelf is 7.75 inches, and it is positioned 2.5 inches forward of the inner wall of the back panel. This provides a 2.5-inch air-flow space at the rear, and an approximate 3.5-inch air-flow space at the front. The shelf is positioned just above the woofer, and just below the bottom tip of the rear chamber of the midrange (which is slanting down due to the sloped baffle).

The port hole on the rear panel is centered 10.5 inches above the bottom of the cabinet, and is offset in the same direction as the tweeter. The rear chamber of the tweeter is thus centered on the inner flair of the port, but with reasonable clearance.

The crossover board is mounted to the bottom panel of the cabinet, below and to the rear of the woofer. It is mounted with six screws. The screws are retained with plastic or rubber "feet", which also act as spacers to elevate the board about ¼ inch above the floor. Mounting the crossover board in the confined space through the fairly small woofer hole can be tricky and requires some patience.

Double check all of the physical design of the box on paper before building it to make sure that there are no mechanical interferences. Things are tight in this design.

BILL OF MATERIALS

All capacitors used are Dayton Audio polypropylenes. These are very good capacitors, and the use of more expensive capacitors is unlikely to make any sonic difference. Note that the 16 uF cap is composed of a parallel combination of a 15 uF cap and a 1 uF cap. Although the schematic specifies 6.0 uF for C3, the standard 6.2 uF value is plenty close enough. The resistors specified are Mills, but less-expensive Eagle resistors should be fine as well - they are just not as convenient because of their use of color codes rather than printed values.

As you can see, this is not an inexpensive loudspeaker to build, primarily as a result of the use of $217 worth of premium Morel drivers.

DRIVING THE M3

The nature of the first-order electrical crossover design is to produce a load impedance that is fairly flat with frequency, the only exception being the impedance variations due to the vented woofer. This makes for a fairly amplifier-friendly loudspeaker. The input impedance of the M3 is about 6.5 ohms, making it an honest 8-ohm-rated speaker.

Alas, there is a price for everything, and in the M3 the price is efficiency. Given the basic 86 dB efficiency of the MW142 woofer, in combination with the 3 dB of BSC, the efficiency of the M3 is on the order of 83 dB SPL 1-Watt/1 Meter. This means that the M3 would like to be driven by a decent-sized power amplifier so as to minimize clipping when playing at realistic levels. A 200 wpc amplifier would be a good choice.

ACKNOWLEDGMENT