SALAD BOWL SPEAKER PROJECT

IKEA BLANDA MATT SALAD BOWL

Salad bowl speaker concept has been spreading around the internet. This Silicon-Chip article describes a fully functioning pair of desktop / bookshelf speakers and gives some suggestions for tweaking the design to suit your needs. What initially attracted us to this idea was the mix of an old-school spherical speaker with extreme ease of construction. While the initial motive for building these was style and looks, it quickly became apparent that these little cuties had more to offer than that. Those who make speakers will be quick to comment that a sphere should be highly resonant; however, our tests show this is not the case. The fact that the driver forms a significant portion of the surface area of the sphere results in the Q of the internal resonance being relatively low. As a result, our measurements don't show resonant peaks in the response. Another benefit of a spherical speaker is that it has no edges. Or is it all one edge? Either way, concerns like edge diffraction and baffle effect are avoided. The fact that these speakers are spherical makes them extremely rigid. Edge diffraction is the effect of sound waves propagating from the driver across a speaker's front panel, then hilling the edge, which forms a discontinuity from propagation in 'half space' to 'free space·. This change causes diffraction at the speaker edges, affecting the frequency response and off-axis behaviour. There are many ways a spherical speaker can be mounted. Without creating a solution to this, they will tend to roll around! We have come up with a couple of options, including feet for the desk version and "rocket" Floor stands, both shown in the photos. The desktop version uses three small doorknobs as feet. The loudspeaker driver used is the SB Acoustics SB12PFCR25-4-COAX, a bass/mid driver with a coaxial tweeter (mounted in the centre). This allows us to achieve really good performance from about 70Hz upwards. These work brilliantly as desktop speakers and would also match well with many subwoofers crossed over at 80-1OOHz. We chose this specific SB Acoustics driver because it incorporates the tweeter, and neatly addresses the challenge of finding somewhere to mount the tweeter. The only other solution we could think of was to mount the tweeter externally, which we did with the Floor-standing version, but it was a real hassle. We have added a port to our enclosure. This allows us to extend the lower frequency response to about 70Hz, with some useful output below that. That is a good result for such a small speaker and is reasonable in its intended applications of desktop usage or placement in a small room. Don't try lo run a dance party using these speakers, though. There is a bit of a hump in the frequency response in the 100-200Hz region. This is a result of the port and helps fill out the bottom end, given the roll-off below 80Hz. The black line in Fig.2 shows the low frequency response you will achieve if you simply omit the port. If you use these on a desk backed up to a wall, om it the port. We used a 25mm port from Wagner Electronics, cut to 90mm in length. This tunes the system to resonance at 74Hz. In practice, the vent ends close to the driver magnet, so its effective length is over 90mm. This tuning gains us a couple of decibels of extra bass in the roll-off region. In an ideal world, this port would be 160mm long, tuning the enclosure to 58Hz, but there is not enough room in the enclosure for that.