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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: 205240" data-attributes="member: 11500"><p>[h=2]FFT-Q Window Analysis[/h] Frequency, group delay, and phase delay response curves can be viewed with varied FFT-Q window settings to remove reflection interference from the frequency, phase, and group delay response graphs. </p><p></p><p>The controls for FFT-Q window analysis are below the lower left corner of the main display screen. A low Q setting represents a narrow time window, better able to keep out reflections. A high Q keeps the window open longer making more detail of the measured response visible. The “FFT-Q” is roughly equivalent in effect to a frequency dependent window in other measurement tools. </p><p></p><p>With Q set to 8, the widest the window can open, we get this detailed picture of the AFR of a 1m measurement:</p><p></p><p>[ATTACH]205241[/ATTACH]</p><p>Figure 11 AFR of the 1m Measurement with Q = 8</p><p></p><p>Reducing Q to 1.8, the narrowest opening, results in this AFR from the same measurement:</p><p></p><p>[ATTACH]205242[/ATTACH]</p><p>Figure 12 AFR of the 1m Measurement with Q = 1.8</p><p></p><p>FFT-Q windowing also affects group delay, GDR, and phase delay, PDR, response curves. With Q set to 8, the curve is affected by the room to a high degree, despite the close in measurement, no doubt because of its use of boundary support in the bass and low midrange.</p><p></p><p>[ATTACH]205243[/ATTACH]</p><p>Figure 13 GDR, Group Delay Response of the 1m Measurement with Q = 8</p><p></p><p>Narrowing the window by reducing Q to 1.8, we see a curve more representative of the group delay of the woofer and its cross over and less of the room:</p><p></p><p>[ATTACH]205244[/ATTACH]</p><p>Figure 14 GDR, Group Delay Response of the 1m Measurement with Q = 1.8</p><p></p><p>The subtract minimum phase option has a further effect on the FFT-Q windowed response. Viewing GDR or PDR with a narrow window and minimum phase subtracted reveals the residual non-minimum behavior of the response, most often due to the crossover and/or equalizer.</p><p></p><p>[ATTACH]205245[/ATTACH]</p><p>Figure 15 GDR, Group Delay Response of the 1m Measurement with FFT-Q = 1.8 and minimum phase subtracted</p><p></p><p>The rising group delay in the bass in Figure 15 is in good agreement with the woofer simulation shown below in Figure 16 over at least the 20 to 100 Hz range. The small bump just past 1 KHz is due to the mid-CD crossover and might be eliminated with further development of the crossover. Using TDA’s GDR and PDR curves with minimum phase subtracted allows one to zero in on crossover phase and group delay using indoor measurements for the final stage of crossover tuning.</p><p></p><p>[ATTACH]205246[/ATTACH]</p><p>Figure 16 Simulated Woofer Group Delay</p><p></p><p>(continued in next post)</p></blockquote><p></p>
[QUOTE="Jack Regula, post: 205240, member: 11500"] [h=2]FFT-Q Window Analysis[/h] Frequency, group delay, and phase delay response curves can be viewed with varied FFT-Q window settings to remove reflection interference from the frequency, phase, and group delay response graphs. The controls for FFT-Q window analysis are below the lower left corner of the main display screen. A low Q setting represents a narrow time window, better able to keep out reflections. A high Q keeps the window open longer making more detail of the measured response visible. The “FFT-Q” is roughly equivalent in effect to a frequency dependent window in other measurement tools. With Q set to 8, the widest the window can open, we get this detailed picture of the AFR of a 1m measurement: [ATTACH=JSON]205241.vB5-nodeid=205241[/ATTACH] Figure 11 AFR of the 1m Measurement with Q = 8 Reducing Q to 1.8, the narrowest opening, results in this AFR from the same measurement: [ATTACH=JSON]205242.vB5-nodeid=205242[/ATTACH] Figure 12 AFR of the 1m Measurement with Q = 1.8 FFT-Q windowing also affects group delay, GDR, and phase delay, PDR, response curves. With Q set to 8, the curve is affected by the room to a high degree, despite the close in measurement, no doubt because of its use of boundary support in the bass and low midrange. [ATTACH=JSON]205243.vB5-nodeid=205243[/ATTACH] Figure 13 GDR, Group Delay Response of the 1m Measurement with Q = 8 Narrowing the window by reducing Q to 1.8, we see a curve more representative of the group delay of the woofer and its cross over and less of the room: [ATTACH=JSON]205244.vB5-nodeid=205244[/ATTACH] Figure 14 GDR, Group Delay Response of the 1m Measurement with Q = 1.8 The subtract minimum phase option has a further effect on the FFT-Q windowed response. Viewing GDR or PDR with a narrow window and minimum phase subtracted reveals the residual non-minimum behavior of the response, most often due to the crossover and/or equalizer. [ATTACH=JSON]205245.vB5-nodeid=205245[/ATTACH] Figure 15 GDR, Group Delay Response of the 1m Measurement with FFT-Q = 1.8 and minimum phase subtracted The rising group delay in the bass in Figure 15 is in good agreement with the woofer simulation shown below in Figure 16 over at least the 20 to 100 Hz range. The small bump just past 1 KHz is due to the mid-CD crossover and might be eliminated with further development of the crossover. Using TDA’s GDR and PDR curves with minimum phase subtracted allows one to zero in on crossover phase and group delay using indoor measurements for the final stage of crossover tuning. [ATTACH=JSON]205246.vB5-nodeid=205246[/ATTACH] Figure 16 Simulated Woofer Group Delay (continued in next post) [/QUOTE]
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