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A New Way of Looking at Speakers and Rooms - Review of APL TDA
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<blockquote data-quote="Jack Regula" data-source="post: 205250" data-attributes="member: 11500"><p>[h=2]Probing the Limits of TDA’s Ability to Discriminate Reflections from the Direct Response[/h] To be useful for evaluating crossover timing using indoor measurements and in guiding equalization, TDA must be able to discriminate against reflections. What are the limitations on TDA’s ability to do this?</p><p></p><p>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. </p><p></p><p>The speaker has 4 ways with crossovers at 200 Hz, 1.6 kHz, and 5 kHz that use 4[SUP]th[/SUP] 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.</p><p></p><p>[ATTACH]205251[/ATTACH]</p><p>Figure 18 Reflection-free TDA pl</p><p></p><p>[ATTACH]205252[/ATTACH]</p><p>Figure 19 Reflection free FFT-Q AFR</p><p></p><p>[ATTACH]205253[/ATTACH]</p><p>Figure 20 Reflection free GDR, minimum phase not subtracted</p><p></p><p>The AFR and GDR above 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 speaker’s response.</p><p></p><p>[ATTACH]205254[/ATTACH]</p><p>Figure 21 Crossover GDR obtained by subtracting minimum phase</p><p></p><p></p><p>Crossover frequencies are apparent in the PFR, phase frequency response:</p><p></p><p>[ATTACH]205255[/ATTACH]</p><p>Figure 22 Reflection free phase response</p><p></p><p>(continued in next post)</p></blockquote><p></p>
[QUOTE="Jack Regula, post: 205250, member: 11500"] [h=2]Probing the Limits of TDA’s Ability to Discriminate Reflections from the Direct Response[/h] To be useful for evaluating crossover timing using indoor measurements and in guiding equalization, TDA must be able to discriminate against reflections. What are the limitations on TDA’s ability to do this? 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 4[SUP]th[/SUP] 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. [ATTACH=JSON]205251.vB5-nodeid=205251[/ATTACH] Figure 18 Reflection-free TDA pl [ATTACH=JSON]205252.vB5-nodeid=205252[/ATTACH] Figure 19 Reflection free FFT-Q AFR [ATTACH=JSON]205253.vB5-nodeid=205253[/ATTACH] Figure 20 Reflection free GDR, minimum phase not subtracted The AFR and GDR above 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 speaker’s response. [ATTACH=JSON]205254.vB5-nodeid=205254[/ATTACH] Figure 21 Crossover GDR obtained by subtracting minimum phase Crossover frequencies are apparent in the PFR, phase frequency response: [ATTACH=JSON]205255.vB5-nodeid=205255[/ATTACH] Figure 22 Reflection free phase response (continued in next post) [/QUOTE]
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