Following is a step by step guide for making quarter-wave loaded enclosure design using Fredo 6.5 inch Jumbo subwoofer. The article and the enclosure is made by one of our respectable community member, Mr. Ravi Kumar Rao.

Readers are assumed to have basic knowledge about Enclosure design parameters. Though we wouldn't be able to help each and everyone with their doubts (which may be self explanatory) but will definitely answer queries regarding the design of QWL.

QUARTER-WAVE LOADED LOUDSPEAKER – AN INSTRUCTABLE By Ravi Kumar Rao

Table of Contents

Preamble..........................................................................................................................................3

INTRODUCTION.............................................................................................................................3

SYSTEM OPTIONS.........................................................................................................................4

IN RESPONSE..................................................................................................................................4

DESIGN CONSIDERATIONS........................................................................................................5

QUARTER-WAVE LOADING........................................................................................................5

HIGH FREQUENCIES....................................................................................................................6

Links...................................................................................................................................................7

CONSTRUCTION...........................................................................................................................7

LOUDSPEAKER MOUNTING.......................................................................................................8

INSTRUCTABLE..............................................................................................................................10

Materials Needed...........................................................................................................................10

STEP 0...............................................................................................................................................11

CUTTING DIAGRAM......................................................................................................................12

STEP 1................................................................................................................................................13

STEP 2..............................................................................................................................................14

STEP 3..............................................................................................................................................15

STEP 4..............................................................................................................................................16

STEP 5..............................................................................................................................................17

STEP 6..............................................................................................................................................18

Preamble

An updated design that should appeal to all audiophiles!(QWL left and Right Speakers, Centre SubWoofer)

The QWL design originally appeared in 1988 (ETI, Aug ’88), and a modified version appeared in EPE Oct 1999 This is my take on the design incorporating , IMHO improved drivers (using the FREDO 6.5” woofer/Midrange,and the DS18 PRO-TW120B Aluminium Super Bullet Tweeter 1- inch with a suitable 12 dB crossover. I had built this speaker more than a few decades back and have found it very good and smooth and, it is hoped that the design will appeal to existing and NEW owners of the QWL both in terms of an upgrade, as well as to new constructors looking for a wide frequency range loudspeaker system occupying a reasonable floor space. I must admit that I have borrowed quite heavily form the the updated article and make no claim of originality etc

INTRODUCTION

The factors governing the size of a loudspeaker invariably involve considerations of overall performance, cost, and domestic acceptability. Referring specifically to moving coil drive units and their enclosures, the larger the size of the enclosure the more extended is the bass response, which in turn adds to the scale of the sound by providing a certain low frequency ambience. The full benefit of this improved ambience can only be achieved in a large room where, of course, the larger size of enclosure stands a better chance of being domestically acceptable. Unfortunately, the larger size loudspeaker system also tends to be more complex, because the use of a larger diameter bass unit requires the addition of a smaller unit, to cover the critical mid-frequency range adequately smoothly, before handing over finally to a high frequency unit. Overall then, the design becomes more expensive arising from difficulties in integrating the responses of the individual drive units over the complete frequency range and constructing the cabinet to keep wall vibration to a minimum. Please note that the original article has suggested 6mm plywood side panels, but I recommend 12mm along with the rest of the panels for overall rigidity.

SYSTEM OPTIONS

From what has been outlined above, it is not surprising to find that generally ,the loudspeaker system manufacturer has concentrated on the smaller enclosure designs to achieve a cost reduction without an appreciable sacrifice in performance. Although it is more difficult to maintain the low frequency response of the small enclosure speaker, there are also a number of important advantages associated with a reduction in the dimensions of the enclosure. Thus, it is not unusual to discover a so called budget design that represents really excellent value for money with a performance that can only be surpassed by designs costing a great deal more; sometimes by a factor of three or more. The advantages arising from the smaller enclosure stem from the dimensions of both the cabinet and the loudspeaker drive units. The smaller cabinet dimensions provide not only a reduction in material costs but also a significant increase in structural stiffness, reducing unwanted radiation from the cabinet walls. The narrower frontal area also improves the sound distribution.

By using a smaller bass unit the complete frequency range can be covered simply by adding a high frequency unit or tweeter. The “seamless” coverage provided by the small bass/mid-range unit can then be made to extend smoothly just beyond the critical mid-frequency range easing the crossover design and producing a radiation pattern conducive to a natural spread of sound and a usefully wide stereo sound stage.

IN RESPONSE

As far as a reduced power handling at low frequencies is concerned, it does not represent too serious a disadvantage. This is because in most types of music, including the organ, the power is distributed in such a way that, below about 200Hz, spectral levels fall off naturally at a rate of about 3dB/octave. However, if the small enclosure results in too abrupt a roll-off in bass level below 100Hz then the bass lightness becomes readily apparent, reducing the enjoyment of certain types of music. It follows that there is a limit to the economy feasible with the small diameter bass unit if it is to provide reasonably linear long-throw cone excursions necessary for adequately low frequency radiation.

When it comes to domestic acceptability, the small enclosure has an obvious advantage particularly if it could be tucked away inconspicuously on, say, a shelf. However, close proximity to a wall can give rise to interfering standing wave patterns. These can cause some deterioration in the stereo image and, in the author’s experience, an obvious audible improvement, in terms of depth and precision of stereo image, is obtained by spacing the loudspeaker units out from the wall by anything up to about one meter.

Continuing to follow the small enclosure approach, the additional requirement is for a pair of loudspeaker stands to locate the enclosures in space such that they are (a) stable and (b) at a height such that the high frequency information reaches the ears of a seated listener without undue absorption by soft furnishings. Possibly the next logical step is to consider whether the space within and under the stands could be usefully utilized perhaps to enhance the low frequency performance.

Acoustic engineering point of view, is to deliberately use and exploit the characteristics of a resonant pipe in such a way that the loudspeaker unit is correctly loaded and terminated at the low frequencies, whilst adequately suppressing unwanted pipe resonant modes. The low frequency efficiency of such an arrangement is somewhere between that of a horn and a bass reflex enclosure and therefore reduces the demands made on the low frequency excursions of the small diaphragm bass speaker unit.

Bearing all these points in mind, the author embarked on a design aimed at satisfying the requirements for low cost, free-standing configuration, and the minimum of dimensions consistent with an adequate low frequency performance. The principle involved utilizes the properties of a closed-at-one-end quarter wavelength pipe as originally proposed by Voigt in his patent and subsequently adapted and described by R.West and R. Baldock in their designs. The design produced by R. West was intended for a corner position with the speaker unit firing into the corner to spread the high frequency sound by reflection from the walls, and R. Baldock’s designs were intended for either a semi-omnidirectional sound distribution or a wall reflected distribution.

DESIGN CONSIDERATIONS

A free-standing loudspeaker cabinet with similar dimensions to that of a small enclosure on a stand, if of conventional design and construction, can present a difficult acoustic problem to the designer because of the long narrow parallel cabinet walls. The walls will tend to vibrate and resonate at similar frequencies giving rise to a resonant pipe-like colouration to the low frequency sound, which is difficult to control and eliminate.

Present day practice favors loudspeaker operation away from corners and walls, firing directly at the listeners. It is the author’s aim to revise the previous design to cater for this latest approach, which is more closely matched to the improved stereo depth and imaging available from some of the current stereo sources.

QUARTER-WAVE LOADING

Having finally decided to keep to the original Quarter-Wave Loading (QWL) enclosure design, an overall view and cabinet dimensions (two required) can be gained by looking at Fig.10 (Cutting details and dimensions for a single enclosure - two for stereo) and Fig.11.(Enclosure dimensions and construction.) The finished appearance of the completed two-speaker “column” can be seen from the accompanying photograph at the beginning of this article. The bass enclosure consists of a quarter wavelength rectangular section pipe with a linear taper, resonant at about 50Hz. The bass loudspeaker unit is situated at approximately half way along the acoustic axis in the best position to suppress higher order resonant modes. At resonance the acoustic pressure is high at the tapered end and is still reasonably high at the loudspeaker unit. This ensures that effective acoustic loading is presented to the loudspeaker cone and small excursions of the cone at high pressure are manifested as much larger low pressure movements of air out of the port at the bottom of the enclosure. Such a process, similar to horn loading, contributes to efficient bass frequency operation with low distortion, up to a frequency of 200Hz, where direct radiation from the cone takes over.

The enhanced bass response produced by this method of loading compared with that from the same unit in a 10 litre sealed enclosure is shown in Fig.1, where the curves were obtained under identical measurement conditions. Bearing in mind this result, it can be seen that this enclosure not only satisfies the requirements of being free-standing, with the drive units at a convenient height, but also provides an enhanced bass response, using the space that would otherwise have been taken up by a stand. Furthermore, only small cone excursions are required in the bass loaded region and this places the minimum of demands on linearity of the cone suspension and the magnetic field in the voice coil gap, allowing reasonably low priced drive units to be employed. The effect on power handling was quite dramatically demonstrated to the author when two nominally identical bass drive units were compared: one in a 10 litre completely sealed enclosure and the other in a quarterwave loading enclosure. Switching from one to the other and slowly increasing the power level, the unit in the sealed enclosure was the first to show signs of distress.

Continuing the quest for a low price design, it is tempting to consider a wide range twin-cone unit for use in this enclosure. As can be seen in Fig.3, the high frequency response of a 165mm diameter paper cone bass unit used in this position showed considerable ripple in the response due to cone break-up modes. Also, when a small tweeter cone is added to the main cone to widen the frequency range, any improvement in frequency response is accompanied by main cone break-up ripple as shown in Fig.4. A smoother performer was found to be a 165mm polypropylene cone bass unit with the frequency response as shown in Fig.5 and this type is recommended for use in this design. (Needless to say the latest polypropylene bass unit adopted for this updated design from FREDO is even smoother!)

HIGH FREQUENCIES

Because of the unsatisfactory response of twin cone units, space is provided in the top of the quarter-wave enclosure to house a suitable high frequency “tweeter”. The speaker unit baffle board has been kept as small as possible both for the sake of rigidity and the need to keep the frontal area to a minimum for good stereo performance. The top of the bass enclosure also serves to stiffen the

unit baffle. The baffle is deliberately sloped to time-align the outputs from the two units and to improve the coupling of the bass unit to the air column in the enclosure.

The advantage of the quarter-wave loading is that it is not sensitive to the bass driver characteristics in the way that the bass reflex cabinet design is.

All that is asked for is a reasonably low bass resonant frequency. One further consideration was that of the type of crossover to employ. The original capacitor feed to the tweeter and direct connection to the bass speaker meant that the bass unit worked up to 6kHz or so before its contribution was taken over by the tweeter. This is fine in some respects, such as maximum damping from the amplifier, but the directivity of the bass speaker increases with operating frequency and the optimum stereo listening position becomes quite restrictive and critical at the higher frequencies. Also, the tweeter capacitor feed with its low rate of cut-off (6dB) with frequency means that the drive voltage at tweeter resonance can be embarrassingly high. Thus an upgrade also meant the adoption of an improved crossover arrangement. At this point, the author sought the assistance of FREDO speaker manufacturer in Delhi , for help in the choice of loudspeaker units . A suitable cross over was chosen which works quite well with this design, however any 2 way 12dB crossover and a 2khz crossoverF1 can be used with this design.

The units finally chosen were the FREDO 6.5” woofer/Midrange and the DS18 PRO-TW120B Aluminum Super Bullet Tweeter 1-inch

Links

FREDO 6.5” woofer

DS18 PRO-TW120B Aluminum Super Bullet Tweeter 1-inch https://www.amazon.in/gp/product/B07BBQYCNV/ref=ppx_yo_dt_b_asin_title_o00_s00? ie=UTF8&psc=1

These were installed in the QWL enclosure. The new drivers and a more recent crossover is readily apparent from the responses. There is also an improvement in power handling from 25W to 80W, which perhaps is not surprising considering that the weight of the magnet of the bass speaker, for example, represents an increase of over a factor of two. The frequency response of the Qwl is surprisingly good for such a simple arrangement and is most encouraging.

At low frequencies the loudspeaker produces sound both from the cone of the bass unit and the rear port.

Taking a view of the frequency response as a whole, it can be seen that the response extends from 40Hz to 20kHz with an SPL(1W/1m) of 89dB, deviating by 3dB from 1kHz to 20kHz and by † +3dB/-6dB from 40Hz to 500Hz. The low frequency properties of the cabinet can be associated with an intriguing mixture of tuned pipe and horn loading, the advantages of which are evident in terms of an improved transient response from the bass unit. The penalty for this improvement is the increase in response deviation below 400Hz. However, these deviations are small compared with standing wave effects in the listening room at these frequencies and the overall response compares very favorably with other speakers of, what might be termed, an audiophile status.

Finally, the impedance characteristic illustrates that the loudspeaker presents a relatively easy load to most amplifiers, producing really loud results with amplifiers in the 50W range.

CONSTRUCTION

Now that we have discussed the various merits and selected our speakers, it is time to undertake the construction of the QWL enclosure. This is made up from 12mm (1/2in.) thick MDF board as shown in Step 0 and the accompanying photograph’s. The enclosure has been designed to make the construction as simple as possible, and if the various pieces shown in Step 0 are cut accurately square then there should be no difficulties in assembly. It can be seen that there are only two angle cuts to be made, those at the top of both the long front and back panels. All the rest are simple 90 degree butt joints, and it is left to the individual constructor to decide whether to attempt the angle joints or simply butt the joints and fill the wedge shape gaps with whatever technique and material is convenient( Wood Putty or Fevicol). Dimensions quoted are not critical, provided that everything is checked to fit as shown in the diagrams so that airtight joints are obtained, particularly in the high acoustic pressure areas in the tapered wedge and around the speaker unit. (It should be noted that the baffle cut-out dimensions in Step 1 are for the new loudspeaker units and existing cabinets will satisfactorily house the new units simply by opening up the cut-out for the bass unit by about 2mm (1/16in.) or so, and the vertical dimension of the right hand cut-out for the tweeter by 6mm. (It is desirable to ensure that the surrounds for the two units almost touch on the vertical axis.)

The front, back, bottom, top, and internal partition members are all made of nominally 12mm (1/2in.) thick MDF board, and should all be matched to the same width of 178mm (7in.). The two side panels can be made of 6mm nominal plywood I desired, however to use 12mm everywhere.

As the assembly progresses check it for squareness and, if necessary, secure one or two cross-pieces of plywood offcuts with pins driven a little way in to hold the assembly square while the glue sets. Being reasonably liberal with the glue should ensure airtight joints, but pay particular attention to the pointed end of the wedge section, and, if necessary run a fillet of glue along this particular joint. Finally, complete the assembly by gluing and pinning the second plywood panel into place. It will be noted that the enclosure is reasonably heavy( If made from MDF and not Plywood) and stiff minimizing the energy storage in the enclosure walls.

Tapping the sides of the enclosure produces different notes at different positions indicating that the internal bracing and asymmetry is working to minimize undesirable reflections and panel resonances. Finishing tasks involve screwing , filling, and sanding prior to painting or covering with material or an iron-on veneer.

LOUDSPEAKER MOUNTING

One final task now remains, and that is to mount the speaker units and crossovers on each completed enclosure. The loudspeaker units are mounted from the outside of the enclosure and the bass unit needs a sealing gasket fabricated from, preferably a rubber-based, draught sealing strip. FREDO does provide a sealing strip but for a small additional cost. ( well worth ordering from their website in my opinion). If you posses a frequency generator, it is recommended the enclosure is checked for any undesirable resonances by very slowly sweeping through the low frequencies with a sine wave. It is also recommended that the tweeter is mounted on a sealing strip to avoid any extraneous noise set up between tweeter surround and the cabinet.

Use chipboard screws and do not over tighten. Matching black screws can be obtained and with their extra length it is advisable to glue small soft wood blocks behind each screw hole location.

The bass speaker signal lead is simply passed up through a hole in the short baffle after sealing the wires through with generous amount of Fevicol. The crossover unit is placed in the tweeter cavity and connected to the Speak-on connector. See photographs.

Connections can then be made to the crossover board fastened to the back of the Tweeter Cavity to the wires coming from the tweeter and woofer/ midrange .Readers may also wish to experiment with the provision of steel or plastic spikes in the base of the enclosure. I have used simple nylon feet as can be seen in the photograph’s.

Materials Needed.

1. 12mm MDF Sheet 2nos 8’ x 4’( standard size sheet) I made 5 Nos of QWL because I needed 2 pairs . However no one would give me cut pieces nor give me quality MDF of 12mm. So I ordered VIR brand MDF with prelam and got it cut to sizes by a carpenter with my supervision.

2. 100 nos ¾” MD screws

3. Wood Glue (250gm Fevicol Marine) The rest of the items mentioned are if you are going to DIY the enclosures. Tools needed are based on your level of skill in woodworking. I have used entry level tools like…

4. Corded Drill Machine.

5. Battery operated Screw Torque Driver.(Used to drive all screws into the panels)

6. Jig Saw attachment for Drill Machine( used to cut the loudspeaker holes)

7. 1 Meter Scale.

7. Corner Jigs for 90 Degree But Joints.(need 8 nos)

8. 12” Clamps -6nos

9. 16” clamps -2-3nos

10. T-square( for checking 90Degree Joints and perpendiculars)

11. Soldering iron and solder.

12. m4 T Nuts (16nos – for 1 pair of speakers-Stereo)

STEP 0

1. Cut the panels from the main sheet of MDF. Refer to the cutting diagram below. The photograph on the right pertains to one speaker. You will need 2 such sets to make one Stereo pair. What you see in the figure is the VIR brand MDF with white frosty veneer.(both sides)

2. Two identical sets of panels cut out from the Large 8’ x 4’ MDF sheet. Stacked ,marked and ready for assembly.

CUTTING DIAGRAM

Note the names of the panels given in my cutting sheet you can use the same or your own. Whatever makes sense . Please note you can make you own cutting diagram based on the sheet size that you get in you market. You can go to https://www.cutlistoptimizer.com/

STEP 1