Response At its Best
By Michael Severn
15 Apr 1999
The Response is a three way loudspeaker system featuring Vifa's D19TD-05-08 tweeter, Jaycar's 30mm dome midrange and Jaycar's 8" poly woofer. The need of this system was bass below 40Hz but giving the best of a crisp mid and tweeter. This is why the dome midrange was chosen for its speed and softness. The budget was set of $600.00NZ dollars per pair (approximately $300US) Fig. 1. Picture of the Response.
The first step was driver selection. The drivers needed to be high quality if they were to deliver sweet sounding music. After considering many drivers I narrowed it down to three finalists, Vifa's D19TD-05-08, Jaycar's 30mm dome midrange and Jaycar's 8" poly woofer. A lot of tweeters these days are delivering "tinney" sounding music and I was aiming for a reasonably flat response. This Mylar 19ml tweeter had good write ups and I had seen this tweeter used in professional Hi-Fi before. My next choice was my bass driver, Jaycar's 8". This driver is not very well known but I believe from looking at the driver's response, that it was high end. The response of this driver ranged from 39Hz-3500Hz this would tie in nicely with my next choice, Jaycar's mid. This 30mm dome midrange had an extremely smooth response as well as a very high sensitivity of 93db. Ferro fluid cooling as well as a Mylar dome would ensure the midrange response to be good. Mid's response ranged from 1200Hz-5kHz.
This part was probably the most stressful part in the whole construction of these speakers. With the much-needed help of some very good friends a design was decided upon with the help of the computer aided programmer CALSOD. The cabinet was going to be a major decider on how these speakers were going to sound. After calculating the inside volume of 33 liters the cabinet dimensions had to evolve around this measurement. Next we had decided upon a sloping baffle that would supposedly keep all speakers in phase where the listeners, us were siting or standing. The baffle sloped back at an approximate 13.8 degrees. This would ensure that the acoustic centers would be in line.
Two divisions as well as a brace were placed in the cabinet. These were for the following reasons. Division 1 was to separate the woofer and tweeter so the compartment would match the measurement of 33 liters in the woofer's cab. Division 2 was placed at the bottom of the cabinet for two reasons to strengthen the cab as well as a suitable mounting deck for my crossover. Finally the brace was placed in the center of the cab, this was purely to strengthen the cabinet and eliminate any resonance going on inside the woofers home. The Dimensions of the cabinet are as follows.
Depth at bottom: 300mm
Depth at top: 213mm
Width of cab: 248mm
Slope of baffle: 13.8 degrees
Height of sloping baffle: 366mm
Height of flat baffle: 495mm
The crossover for this speaker system was designed with the aid of CALSOD. The target acoustic response was set to a 4th order Linkwitz Reilly. The crossover frequencies are 1500Hz and 5000Hz. Designing the crossovers proved to be the most difficult and time consuming part of the project, this was because we had a limited amount of information on the Jaycar speakers and this meant a lot of fine-tuning by ear had to be done. The CALSOD graph shows the overall result, which is reasonably flat (+/- 1.0DB).
The crossover used air core inductors and polypropylene capacitors in the most critical parts. The finished product proved to be a great success with a great overall response present on the Calsod graph. The crossover board was mounted under the speaker cabinet itself so it would allow easy access if additional tweaking were needed.
Graph 1. Shows mid and tweeter response without woofer present Fig. 2. Mid and tweeter response without woofer present.
Graph 2. Shows the response with the woofer added Fig. 3. Response with the woofer added.
Graph 3. Shows overall responses of the drivers Fig. 4. Overall responses of the drivers.
As you can see the overall response of the loudspeakers is pretty good just +/- 1.0 db which allows a smooth response to flow through the Hz to kHz range.
In conclusion these loudspeakers are great quality for the amount of money spent with a power rating of 120W RMS and 8 ohms, the speakers handle very well at high or low volumes. I enjoyed designing and building this system, but do remember a lot of thought and time should be taken in order to get the right sound you are looking for. Good luck if you are looking to build a pair.