Decent Floor standing Speakers for Home use

By Vivek Mehta
04 Apr 1996

The Cabinet

After my research, I wrote a Speaker Design program in Visual Basic 1.0 for the MS Windows environment. It is simply called "Speaker Designer" because all the more apt names like "Perfect Box" and BoomBox" and "Woofer" are already being used by programmers earlier than me.

I ran the parameters of the mkC15-400lr on Speaker Designer and by playing around with Box volume and vent size, did come to an acceptable frequency response for a Floor standing speaker for home use. A box of 105 Ltr internal Volume with a port tuning the box to 38.8 Hz would give an f3 of 38.6 Hz, acceptable for the humble use these speakers were going to be put for.

In the literature, there is talk of needing upto 30% more volume than that predicted by the mathematical models. But then they say that adding damping material in the speaker box can increase effective volume by 1.25 to 1.4 times. I decided to build my box exactly as per the mathematical model, hoping that I could "tune" it acceptably by varying the amount of damping material and port length till it sounded just right.

It was not easy to design a box with dimensions of exactly the "Golden ratio" even though I am fascinated by the history behind that number. It seems to be one of Mathematics’ closest links with Art.

After much juggling around, I chose to build a box with the following internal dimensions

___________
/          /|
/          / |
/__________/  |
|          |  |
|  T     T |  |
|          |  | 75cm
|          |  |
|   ---    |  |
|  / W  \  |  |
|  \    /  |  |
|   \--/   |  |
| P      P | /
|__________|/ 30 cm
50cm

After you discount the volume of the internal bracing and the 12 thick "glue blocks" on every corner of the box, this gives an internal Volume of very close to the desired 105 Litres. The material was 18mm thick MDF which is very easily available in these parts.

According to the output of my design program, I used two nos 3 inch plastic ports with a length of 8 cms.

When I first assembled the cabinet, I was not too happy with it. Maybe I was using the Bose as my yardstick, and they are good. So then began the process of tuning the cabinet. I bought a Test CD called " Sound Check" mastered in the Abbey Road studios made famous by the Beatles. This is available from Maplin Electronics, UK for 15 Pounds. I decided not to use high technology methods like measuring impedance at each frequency. My aim was to have speakers that sound about right to my ears instead of speaker s that fit a formula. In the end, I am going to have to listen to my speakers, not my slide rule!

I tried altering the Box volume by placing objects within the box and listening for differences in the "Bass" and "Complete Drum kit" tracks on the test CD. It was soon obvious that the volume was OK but damping material was needed to smoothen the midrange. I used industrial Fibre Glass insulation that comes in about 4 inch uncompressed thickness, about 1 inch when I pinch it between my fingers (with gloves on, the fibre glass can pierce the skin leading to itches) This proved too much as the midrange seem ed to go dead. I then decided to split the insulation material into half i.e. 2 inch loose, 0.5 inch when compressed tightly. I loosely mounted this material on all surfaces except the front piece.

Interestingly enough, the final results were what I would call "acceptably good". The speakers are good for reproducing Music as it was meant to be, not the steps of a Dinosaur or the sound of Rambo sub-machining down the enemy.

But then the speakers started sounding even better after about 15 days of use. I did read somewhere that Speakers need to be worn in just like your favorite shoes or the engine of your new car. There seems to be a theory that the paper cone changes subtly after the first few hours of use till the speakers sound like they are actually supposed to.

Or maybe I just got used to the sound of my new speakers!

Speaker Designer screen for the mkC15-400lr in a 105 Ltr enclosure Fig. 1. Frequency response.

Vivek Mehta

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