Build Your Own Rear Effect Surround Loudspeakers
Build Your Own Rear Effect Surround Loudspeakers
Every home surround system needs a pair of rear effect loudspeakers to produce the stunning sound effects surrounding the listener. Their role range from reproducing the faint sound of falling leaves and rain drops to the roaring noise of a jet fighter approaching from behind, all to create the illusion that the listener is placed in the middle of the action.
On the market there is a great variety of rear effect surround speakers available ranging from the tiniest cube squawker to the mighty full range floor standing speaker. The well known Dolby Pro Logic system launched in 1987 state somewhat limited requirements for the rear effect speakers, as the mono signal produced by this system is band limited to the frequency range 100 Hz to 7 kHz. The new surround systems Dolby Digital and DTS both produce individual full frequency range signals for the left and right rear effect channels.
The aim of this project was to build a pair of rear effect speakers capable of producing a very detailed, low distortion sound field with full dynamic capability required by both Dolby Digital and DTS. They are designed as medium sized wall mounted speakers with as large and linear frequency range as possible. The drive units have been selected mainly according to their sound quality, so they are not the cheapest available. The crossover components are selected to match the quality of the drive units. All in all these rear effect speakers are in the top end of the scale, but when you build them yourself, you will get them at an reasonable price.
The Drive Units
As tweeter the Dynaudio D260 dome tweeter was selected due to its linear frequency response, detailed sound reproduction and very high dynamic capabilities. This tweeter has been and still is the chosen unit for many renowned HiFi speakers including the Dynaudio Audience series. The voice coil is immersed in magnetic fluid for best cooling properties and the vented pole piece together with the shaped rear chamber are acoustically damped for optimum impulse response.
The woofer/midrange unit had to provide a detailed midrange, a long linear stroke and good Thiele-Small parameters for good bass reproduction in a reasonable sized bas-reflex cabinet. These properties were found in the German Visaton unit AL130. It has an aluminum cone, low loss rubber surround, vented pole piece and spider, a long linear voice coil throw of +/-6.0 mm and further more a copper ring mounted on the pole piece to reduce distortion just like the Scan Speak and Dynaudio units. The frequency response is very linear up to 5 kHz, but at approximately 8.2 kHz, which is outside the usable range of the unit, the aluminum cone possess a resonance peak which has to be damped effectively by the crossover. All in all a very good high end design.
The woofer Thiele-Small parameters were measured and found to be very close to those stated by the manufacturer, which indicates good quality control in the Visaton production.
The Cabinet
Based on the Thiele-Small parameters a bass reflex enclosure was calculated using the program BOXCALC stating a box volume of 12 to 15 liters as optimum. Consequently, compensating for the volume occupied by the woofer and reflex port, a cabinet with an internal volume of approximately 14 liters was constructed using 19 mm MD board with a double board as main baffle to form a rigid low resonance mounting support for the drive units. Two vertical support boards with several air ducts, see photos, are mounted to further strengthen the main baffle. Another two side-baffles forms the overall cabinet trapezoid shape. All angles are easy 45 degrees.
In the bottom a round hole is cut to allow for air tight mounting of the reflex port. On the back side the middle board is made removable for easy mounting of units, port, crossover and damping material. Remember to make an airtight assembly applying rubber strips. On the back side an 80 mm cutout serves as mounting hole for the speaker terminals and wall mounting fittings are integrated into the cabinet as well. The cabinet is assembled using good wood glue.
If wanted the units can be hidden/protected by a plate covered with acoustically transparent fabric to give a nice professional look. See photos.
Use light stuffing of sheep wool or acoustic batting material in the two side chambers and in the air ducts leading to these chambers. Do not stuff tightly as the air flow must not be restricted in the holes.
Cover the back board with one plate of foam (see photos) or acousti-lux approximately 3 cm thick. Cut away a rectangular piece from the bottom and up so that there exist a 3 cm free space above the reflex port. In general, to get a good and dynamic bass response do not place damping material close to the internal opening of the port or the woofer itself.
Cover the top of the cabinet with one plate of foam or acousti-lux.
The reflex port was calculated using the program BOXCALC and later optimized by microphone measurements using the program SALSA. The port tuning was selected to give a maximally flat bass response with a free field -3 dB frequency of 42 Hz. When mounted on a wall the low frequency cutoff will be even lower.
The Crossover
As the typical mounting position for rear effect speakers is on the side walls a little behind the listening position and at a height of approximately two meters, the frequency response at 0, -20 and +20 degrees vertical offset for both drive units mounted in the cabinet were measured using the SALSA program. In this way it was possible to optimize the frequency response focusing on the response at 0 and -20 degrees. Also the impedance of both units were measured, all to form the input files for the CALSOD program, which was used to optimize the crossover components for best frequency response. As crossover network a 12dB/oct filter with a crossover frequency of 2.2KHz was initially selected to get a simple filter, but still providing reasonable good protection for the tweeter. To remove the 8 kHz peaking response of the AL130 aluminium cone a notch filter was added to the woofer crossover. The optimum and final crossover circuit is shown below.
The frequency response is optimized to be as linear as possible both horizontally and in the vertical -20 degree direction, which will approximate the standard listening direction when the speakers are wall mounted above the listening position.
Good quality crossover components should be selected to match the quality of the drive units. The best choice for the capacitors are polycarbonate or even better polypropylene types. For resistors the best choice is non-inductive types such as metal film resistors. Especially for the tweeter damping it is important to choose a good resistor. The two coils have an iron powder core for low DC resistance. This property is most important for the woofer inductor.
The crossover components can be mounted on a PCB or a plate of thin chip board. Glue the components onto the board using silicone rubber or a hot glue gun to avoid any rattling. Mount the finished crossover just behind the tweeter using four stand-off's to avoid the crossover touching the backside of the tweeter rear chamber, again to avoid rattling sounds.
Note that the tweeter polarity is reversed for the 12 dB/oct filter!
Suggested Crossover Components, 12 dB/oct.
The Frequency Response
The total frequency response measured a distance of 1 meter and the speaker impedance both measured using the SALSA program. The graphs show the results of both left and right speaker with very little variation between the units indicating narrow tolerances.
Download TIF pictures of the Frequency Response (45kb) and Impedance (46kb) for a better view.
The Improved Crossover
After long listening tests it became clear that the very detailed mid range could possibly become better if the high frequency range of the AL130 driver was attenuated even more. Consequently a 24 dB/oct Linkwitz-Riley filter was designed and optimized using CALSOD. Again a notch for the mid range aluminum cone resonance at 8 kHz was included. This 4. order filter provides much higher damping of the unwanted resonance peak, approximately 60 dB, and at the same time protects the tweeter from low frequency signals resulting in lower distortion.
The frequency response for this crossover is also optimized to be as linear as possible both horizontally and in the vertical -20 degree direction, which will approximate the standard listening direction, when the speakers are wall mounted above the listening position.
Note that the tweeter polarity is not reversed for the 24 dB/oct Linkwitz-Riley filter!
Suggested Crossover Components, 24 dB/oct Linkwitz-Riley Filter.
The Frequency Response with 24 dB/oct Linkwitz-Riley Filter
The total frequency response measured a distance of 1 meter and the speaker impedance both measured using the SALSA program.
Download TIF pictures of the Frequency Response (44kb) and Impedance (45kb) for a better view.
As said before these speakers are not the cheapest rear effect speakers available, but good sound quality doesn't come for free. When adding it all up: drive units, crossover, wires, speaker terminals, wood, glue and paint, it amounts to a total of DKK 4,600 for both speakers corresponding to US$ 660 or DM 1,150.
The Dynaudio D260 tweeter is available in many countries and the Visaton woofer/midrange is mainly available in Europe. One good place to order your drive units and crossover components is at the German hifisound company. They have a great variety of units and components and they will export based on a valid credit card.
To break-in the speakers they were used as standard HiFi stereo speakers for a couple weeks. During this period sound became smoother and more coherent. They possess a very detailed and firm bass extending lower than normally would be expected for a 130 mm woofer. The lowest octave is however missing, which is a small penalty to pay when talking about a wall mounted speaker. The middle frequency range is reproduced with a very high degree of detail and clarity not surpassed by many speakers of today, and the treble is at the high and well known Dynaudio level. The 24 dB/oct Linkwitz-Riley filter is worth all the extra money. The mid range gets more natural and smoother and lifts the speaker one class up.
The sound stage is deep and wide enhanced by the trapezoid cabinet shape reducing early reflections from cabinet edges.
When included in a matching surround system they contribute positively to what its all about - to surround you with realistic sound effects as intended by the movie makers.
If you would like to build your speakers and need a pair of good wall mounted HiFi speakers, this project might be something for you.
You have been listening to: Music From Twin Peaks, Twin Peaks Theme.