As technical computing was an enjoyable hobby of mine at University, I was so excited that somebody was willing to pay me for "work" I would have willingly done free. As I thought I was earning easy money, I decided to treat myself to something really special with my first pay, which obviously translated into a high powered music system.
I love Rock and Roll, Heavy Metal and other music that was meant to be played out really loud. This is music that has to be "felt" as much as "heard".
I therefore bought a Nakamichi 505 two head Cassette player, a pair of Bose 501 Direct/Reflecting speakers and some cost-effective Technics 75Wrms amplifier. I am of the opinion that frequency response and distortion of a complete music system depend only on the Speaker and the input source. The electronics in the middle like the Amplifier and the Active Equalizer are so good these days that the distortion they normally produce is totally inaudible.
To my ears, the Bose 501 is a good enough speaker and gave me years of listening pleasure while listening to Audio Cassettes. Bose’s Direct/Reflecting technology of pointing one of the two Tweeters at an angle so that it reflects from a wall does create a realistic large auditorium type of sound.
However recently I was gifted a complete set of Classical CDs by a friend. This tempted me to purchase a Kenwood CD player which instantaneously made me realize how much I was missing out on earlier. The first thing one notices while making the shift to CDs was the much clearer highs. And then you notice the thundering bass (atleast on the friend’s music system). I then decided it was time to change my Loudspeakers to better reproduce what the Electronics was capable of now.
Though America seems to have more loudspeaker manufacturers and hobbyists than Europe, I found it really difficult to even get a quotation from the larger shops. They might have thought that a small order for two Woofers was not worth the effort to work out the cost of freight to Dubai and the problems of exporting it here.
In the end, I located a reputable manufacturer of High powered speakers in Britain. Warfedale/ McKenzie Acoustics makes a wide range of Woofers, Tweeters and Special speakers (for the sixty’s guitar sound).
I chose the mkC15-400lr, a15 inch, 400 Watt RMS, 800 Watt peak driver meant for Woofer and Woofer/Midrange applications. The specifications of the mkC15-400lr are:
fs 29.0 Hz Qts 0.23 Vas 175 ltrs Sensitivity 98 dB 1W -1M AV Cost 170 PoundsFor the tweeters, I decided to use two Motorola Piezo Electric Wide Dispersion horns per channel in the same box as the mkC15-400lr. According to the specifications, they are capable of reproducing upto 40Khz at low distortion levels below 0.5%. Though many enthusiasts feel that Piezo Electrics might not be up to the mark, I was confident of the Motorola name and the three page technical literature they give with the Tweeters.
I intended to drive the mkC15-400lr with a new 150Wrms per channel amplifier. From past experience, I had found that my Bose 501 would distort Bass at loud volumes if I turned up the Bass control on the Amp. This was so even though the Bose 510 is rated at 150 Wrms while my old Amplifier was just 75W rms. I now know that Loudspeakers do not have the same power handling capability at each frequency. I also thought that I would get lower overall distortion if I used a high powered speaker and drove it to a f raction of its capability.
I had originally intended to build the "recommended speaker cabinet" that would come with the speaker and get it over with and enjoy the music. But as luck would have it, I received two differing recommended designs.
The Distributor in UK had a drawing of a 200 Ltr box which claimed an f3 of 30 Hz while the Manufacturer claimed that the best box was 95 Ltrs and gave an f3 near 35 Hz. This got me confused. I then spoke to a "Speaker Guru" who worked something out on his HP calculator and told me that Keele’s alignment dictated a box of only 35 Litres. It was then that I decided to take matters in my own hand and started reading up on Loudspeaker design theory.
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:
(Click on the picture for a full size version)
Vivek Mehta P.O. Box 50194, Dubai, UAE Tel : +971-4-691540 Fax: +971-4-667387 E-Mail : puja_mehta%c2k__me@notes.worldcom.com