I am thinking about to DIY my audio system and digging into the speaker design as the first step. I am intrigued by open baffle design cause people said it gives a very airy and open sound. Linkwitz Lab has a very detailed analysis and discussion about dipole speaker and open baffle is one of many implementations.
He designed the speaker with equalization in mind thus he operate the speaker BELOW the first resonance frequency. This is a very different approach. I heard people around different discussion boards saying that larger baffle gives a deeper bass. It suggests they are operating ABOVE the first resonance frequency.
Once you operate above the first resonance, the oscillation in the frequency response comes naturally. As the length equals to a half wavelength, the waves from the front and back add up. When the length equals a full wavelength, the waves cancel. This issue is not that severe using a real driver comparing to the piston model analyzed since the speaker cone gives an asymmetrical response in front and in the back. The bracket and the magnet of the speaker also reduces the airflow on the back size, although the bracket and the cone may generate another resonance and a notch filer is required.
For my understanding in antenna, the distance from the observer to the source has to satisfy two conditions for the far field approximation to be valid. First, 2D^2/lambda, where D is the largest dimension of the radiating source and lambda is the operating wavelength. Second, they have to be at least one wavelength apart. Assuming the speakers are 2.4m apart, as suggested in the room acoustics section on Linkwitz lab, and a triangle listening position, the one wavelength criteria is about 140Hz. I believe the acoustic property is dominated by room mode and near field property of the speakers below this frequency. It leads to the conclusion that a sub-woofer should be used with a 140Hz crossover frequency. Please bear in mind that I am assuming a pretty small room and a pretty close listening position. Once you move to a large room, not only the room mode frequency decreases but also the far field is still valid. Things are back to acoustic pattern again.
Art Ludwig's website gives excellent discussion about room mode based on his own image theory code. Mr. Ludwig and Mr. Riley are on the two ends of a spectrum regarding listening room. Mr. Ludwig has a nearly empty listening room with a lot of treatment while Mr. Riley uses a normal living room. The analysis performed by Mr. Ludwig shows that a "bad room" is not much worse than a "good room" but both of them are far from an optimal flat response. I didn't find the suggesting regarding this fact. On the other hand, Mr. Riley believes that human brain can process a "uniformly" excited room with normal furniture and decoration inside. Since I don't have a room dedicated for listening, I can only believe what Mr. Riley said and will choose to use stereo sub-woofers along the two side walls at a distance to the listening position the same as from the main speaker to the listening spot. This placement is learned from the Pluto+ sub-woofer. The position is chosen to lower the excitation of the room mode (instead of placed at the corner of the room to excite the mode to the most level) and keep everything symmetrical. Symmetry is a beauty and it even works for near field. I would love to keep it whenever I can.
I take the advice from Mr. Riley and try to keep the main speakers at least 1m away from ANY wall. I will also keep the room as comfortable as I like.
Thanks for your sharing!
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