Hello everybody!
This is my first post here, but I`ve been browsing the forum since around 2006. Back then I had discovered an auction for JBL 4311s on ebay and, in an attempt to find out more about them, stumbled upon the forum. Soon after that I got myself a pair of L150`s and a Yamaha PC2002 amp, both of which I still have and enjoy.
About a year ago, inspired by the DIY-section of this forum, I gathered the parts to build some speakers, too. I have always been impressed by the sound of PA speakers, i.e. the “punch/slam”, “effortless dynamics” and “impressive midrange” that some of these have and wanted to recreate this in a home system. The system is supposed to be used for music playback in a medium-sized (20 sqm) living room – mostly Jazz, Blues, Rock and Soul music. During the last year, the project has already progressed quite a bit.
The aim of this thread is to document the build as it progresses further, and to get some advice and assistance from the many experts and experienced builders on this forum. Currently, I`m stuck in a number of places and I am no longer sure whether the setup I have come up with can be made work satisfactorily.
It all started when one evening, after a few beers and while browsing ebay, I saw an ad for a pair of PA-speakers with components that seemed promising – 2447H large-format compression drivers and 2020H mid/midbass speakers, and, in a beer-fueled rush of euphoria , acquired them using “buy now”. In a similar way, a pair of miniDSP PWR-ICE 250 amps soon followed. These amps offer active crossover functionality with IIR and FIR (2300 taps for lf / 300 taps for hf). The 2447Hs got new 2452-SL diaphragms installed and were paired with STX825 waveguides.
In order to save some space and have some more flexibility in placement, the system was supposed to consist of a pair of satellite speakers and a subwoofer. For the sub, an Audax PR380M2 15 inch woofer is available and might be used. Playing around with the TS-parameters of the 2020H in winISD revealed that the sub-sat concept (crossing over to the sub somewhere between 60 and 100 Hz) would not be viable without bass-reflex loading the 2020H, and that accordingly the satellite enclosures had to become a little bit bigger than estimated initially. I settled on an enclosure volume of 45 liters tuned to 65 Hz, and started planning and constructing the cabinets.
Internal dimensions are WxDxH = 28.5cm x 24cm x 95cm, and all panels are 1.9cm MDF, except for the front plate, which is 3.6 cm MDF. To absorb the sound that is radiated into the enclosure, the internal surfaces are lined with felt of 1 cm thickness, and additionally a double layer of polyester damping sheets is used on the side and rear walls in the enclosure segment containing the 2020H. To fight standing waves, the bottom of the enclosure is filled with polyester damping sheets.
To get something up and running quickly, I decided to skip the bass reflex tuning and start with the crossover first, simply inserting the ports the way they were bought (10 cm diameter, 20 cm length). From conducting measurements of the 2020H and the 2447H at different angles, I concluded 1100 Hz to be a good crossover point. Indoor measurements with an IR windowing width of 3 ms served as a basis to EQ the HF and LF flat in the range between about 300 Hz and 20 kHz, and for crossover, 24 dB slopes were used. Delay between HF and LF was adjusted to get the deepest notch possible at the crossover point when reversing HF polarity.
Unfortunately, listening tests turned out to be not satisfactory. There seemed to be a multitude of resonances masking the direct sound. The midrange was not at all as clear as I had hoped for. This became particularly obvious in a direct comparison to the L150s, which sounded a lot cleaner and more detailed in the midrange.
As I found the side and back panels to vibrate considerably at certain frequencies, I decided to try to remedy this possible cause first. In a first attempt, the bracing in one speaker was extended as per the following drawing:
A comparison to the unmodified speaker revealed that this had made things worse - panel vibration seemed to have become even stronger, now shifted to higher frequencies. Thus, in a second attempt, constrained layer damping was tried. Aluminum sheets (thickness 2 mm) were bonded to the sides and back of each speaker with a layer of Sikaflex 292i (thickness 1mm). Additionally, the woofer gasket that had allowed the frame to be in direct contact with the enclosure was replaced with a thick and rather rigid strip of foam to prevent such contact. After these measures, the enclosure walls now appear to be rather dead, with little tangible panel vibration left.
The sound, however, hasn`t improved much. That`s why I`ve decided to start all over, this time doing the internals of the enclosure and the port tuning right before moving on to the crossover. That`s where I`m stuck now and need your help.
The questions that I have relate to the enclosure size and tuning, to the construction of the enclosure and to the lack of midrange clarity that might be caused by resonances/standing waves and/or internal reflections.