Build Your Own Multi Media Loudspeakers
Build Your Own Multi Media Loudspeakers
As many others I bought a Sound Blaster AWE32 PnP and a pair of Chinese 150W multi media loudspeakers with built-in power amplifier, volume, bass and treble control to extend the use of my PC. (I personally rank the quality of stated output power with increasing inflation: HiFi Watts, CarFi Watts, Multi Media Watts). After listening to these speakers for some months I decided to build a pair of good HiFi multi media loudspeakers as the Chinese one's only sounded tolerable when reproducing the standard windows beeps and boops. As monitor loudspeakers and for normal music reproduction they were no good. Well, what can you expect of 400 DKK.
The Drive Units
First I had to select suitable magnetic shielded drive units which would fit into a reasonable small cabinet. For the woofer/midrange unit the choice fell on the 5" Monacor SPP-135S unit, which gives good bass response in a bass reflex cabinet of 7 to 11 liters. As tweeter the SEAS H623-06 27/92 with 25 mm aluminum diaphragm, soft polyamide surround and neodymium magnet was selected. Aluminum tweeters with aluminum surround tend to give a rather sharp treble, where as the use of soft surrounds often produce a treble of a much higher quality. 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 Monacor production.
The Cabinet
Based on the Thiele-Small parameters a bass reflex enclosure was calculated using the program BOXCALC stating a minimum volume of approx. 7 liters. Consequently, compensating for the volume occupied by the woofer and reflex port, a cabinet with an internal volume of 8 liters was constructed using 16 mm MD board. All edges were cut down 8 mm at an angle of 45 degrees to make the cabinet look smaller and to reduce edge reflections. Both drive units were flush mounted to avoid early reflections from their edges and to give a nicer look. On the back side of the cabinet an 80 mm cutout serves as mounting hole for the speaker terminals.
All internal surfaces except the front plate were covered with 10 mm thick felt and half the cabinet was filled with acoustic batting material. Do not place this damping material directly behind the woofer/midrange unit or close to the port internal opening, as this will over-damp the bass and give a dull sound.
A port a little longer than calculated by BOXCALC was mounted and the actual bass tuning was measured with a microphone placed very close to the port and the bass unit using the program SALSA. The port was shortened until the correct tuning and flat frequency response was obtained.
The Crossover
After bass tuning the frequency response at 0, -20 and +20 degrees horizontal offset for both drive units mounted in the cabinet were measured using the SALSA program. 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 2.5KHz was selected, and to compensate for the slight peaking of the bass/mid range unit from 800Hz to 1400Hz partly due to change in acoustic impedance caused by the front baffle, a serial LC damping circuit was applied. The optimum and final crossover circuit is shown below.
The frequency response is optimized for a slowly increasing level from 2kHz to 15kHz of approximately +2.5dB to give an open sounding treble and to compensate for the slight tweeter roll off when listening a little off axis. If this effect is not wanted the series resistor of 3.33 ohms should be changed to 4.7 or 5.6 ohms and the 1uF capacitor removed.
Good quality crossover components should be selected. Only the 80uF should be a bipolar electrolytic capacitor, the rest are best selected as polycarbonate or even better polypropylene types. As resistors the best choice is non-inductive types, especially for the tweeter damping. Two of the coils are air-coils and the 0.76mH coil houses an iron powder core. The measured coil data are listed below.
The Frequency Response
The total frequency response measured at a distance of 1 meter and the speaker impedance was measured using the SALSA program.
Download TIF pictures of the Frequency Response (43kb) and Impedance (42kb) for a better view.
Instead of just connecting the new multi media speakers to the PC sound system they were placed on 60 cm high stands replacing my normal HiFi speakers. They were tested on many different kinds of music, and after some hours of membrane massage the sound and perspective became stable. The bas is astonishing firm and deep compared to the size of the cabinet and the mid range is clear and detailed. The treble provides fine resolution without any hardness or ringing from the aluminum diaphragm. The stereo sound stage is wide and deep and these speakers were actually used as stereo speakers for some months due to their good quality. The only time they said no was when the volume was approaching party level, but what can you expect from two 5" woofers in a 25 square meter living room.
As multi media speakers connected to your PC reproducing the gruesome sounds of your favorite game these speakers will, at a price of approximately 1500 DKK, give you tenfold better quality than the standard Chinese plastic boxes. Furthermore, they also possess the quality to be used as monitors if you work more professionally with music, and also as small center speakers or rear effect speakers in a surround sound system. Even as compact HiFi speakers best mounted on stands placed at some distance from the walls.
Warning: Do not expect that the Sound Blaster AWE32 on-board power amplifier is capable of driving these speakers much above whispering level. The amplifier is simply too weak. Use an old HiFi amplifier. 20 to 30 good HiFi watts will do the job.
You have been listening to: Chet Atkins & Mark Knopfler, There'll be some changes made.