APL_TDA has become my favorite tool for evaluating my DIY speaker efforts in room, the listening position itself, and the effectiveness of various room treatments Iíve been experimenting with. It has helped me see issues that I missed or misinterpreted with conventional tools. I still use those programs for simulation and crossover development but TDA is where I turn to see (as opposed to hear) how well Iíve done in speaker and system design and implementation and integrating the speaker into my room.
Well, that was the end of my review but I want to continue to probe the limits of TDA's ability to discriminate between the direct response and reflections. That is what its utility depends on and we need to understand its limits.
To explore this topic, we will look at a speaker both alone and in the presence of a strong reflection, delayed by 5 ms. and down only 3 db. For most small rooms, a 5 ms. delayed reflection would have traveled more than twice as far as the direct sound and would be down by more than 6 db. Thus, it is a worse than worst case situation and has been created artificially rather than by measurement.
The speaker has 4 ways with crossovers at 200 Hz, 1.6 kHz, and 5 kHz that use 4th order LR filters. Measured close up and free from room effects, TDA shows a clean pl graph, with group delay increasing towards low frequencies and an impressively flat frequency response.
The AFR and GDR below were taken with FFT-Q = 8. The GDR below is plotted with minimum phase subtracted, leaving the crossover group delay. The slight group delay ripple at 50 Hz correlates with the European AC mains power and is an equipment artifact, not part of the speakerís response.