ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2] 3   Go Down

Author Topic: APL's Time Domain Analysis - Update?  (Read 2232 times)

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #10 on: February 18, 2017, 11:57:14 am »

It’s easier to determine the frequency ranges affected by room modes and reflections by looking down on the 3D map from above.  The TDA “pl” display option is used in the attached figure.

It’s important to be able to distinguish between the properties of the speaker and properties of the room in these displays.  To do that we must first understand the speakers; in particular, that they are corner speakers and rely on supportive reflections from nearby floor and walls to support the bass and to a lesser extent the lower midrange.

The wedge of red level response embracing the black vertical line of the direct response from 50 Hz up to 1000 Hz is the result of reflections from the floor at a nominal delay of 2.8 ms and from the ceiling as well where the delay is greater than 6.9 ms.  For the corner woofer, this is floor support and does indeed seem to be propping up the direct response line in the 3D display.  The crossover between the woofer and the multiple entry horn that sits on top of it is at 350 Hz and shows in the PL graph as an abrupt narrowing of the boundary support wedge.  This narrowing continues gradually as frequency rises due to increasing directivity of the speaker and the increasing absorption from the carpet and temporary floor absorber used for this measurement.  Unfortunately, what was boundary support for the woofer becomes boundary interference for the midrange resulting in nulls in the 400 -500 Hz range in unsmoothed measurements taken at the listening position.   

Above the bass, we are seeing the speaker and not the room; that is abundantly clear in the 3D display.  The color shading close in to the direct response line at higher frequencies is due to speaker imperfections and can be improved by minimum phase equalization.  Strong room resonances appear as the red streaks at 50 and 70 Hz and a weaker one at 180 Hz.  In a steady state, the standing waves of a room mode would develop but in a transient sine sweep or impulse response they can only be called resonances.  The vertical white streak at 38 ms. has a timing consistent with a reflection from the back wall of the room.
Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #11 on: February 18, 2017, 12:02:05 pm »

TDA can also show frequency response curves.  While the 3D and pl displays are normalized, the AFR is the un-normalized frequency response recorded along the black direct response line of the 3D display.  Room resonances, or their effects, visible in the 3D display can also be seen in the AFR. To wit, the room resonances at 50 and 70 Hz result in a peak at 50 Hz followed by a shallow null at 70 Hz. 

To the extent that the processing succeeds in separating the direct response from reflections, the effect of reflections won’t show up in the AFR.  In this measurement, supportive floor reflections have elevated the bass and lower midrange but no reflection nulls are visible.
Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #12 on: February 18, 2017, 12:05:11 pm »

Before we go any further, let me tell you about the speakers whose measurements are being used in this review.

These 3-way corner speakers use active, DSP crossovers.  They each consist of a multiple entry conical horn containing a compression driver tweeter and four 4” midranges sitting on top of a 15” sealed, slot loaded woofer nestled tightly into each of the room’s two front corners.  Time alignment was achieved by setting DSP delays to align the IR peaks of individual drivers with their crossover filters and PEQs enabled.  Time alignment was then confirmed with TDA.  The delay bend just past the mid to CD crossover at 950 Hz is due to the mid-tweeter crossover; it increases with increasing crossover filter slope.  A room resonance or reflection also affects the response there.  The only hint in the 3D display of the woofer-mid crossover at 350 Hz is the narrowing of the floor support wedge there, indicating excellent time alignment.  Both XOs use only 12 dB slope electrical filters, achieving 24 dB acoustical slopes. Neither crossover shows a phase wrap in its conventional frequency/phase response graph.
Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #13 on: February 18, 2017, 12:11:08 pm »

The next step is to look at the speaker's response out at the listening position.

When the mic is moved out to the primary LP, 4m from the corner speakers, the room dominates the measurement, especially in the lower registers.  The TDA display isn’t as pretty a picture there but then it’s an untreated room.  The low frequency red and black horizontal streaks are room resonances.  Vertical streaks and  points or blobs are reflections. Despite the resonances and reflections, the direct response stands out down into the modal region, which is encouraging. 





Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #14 on: February 18, 2017, 12:14:21 pm »

To diagnose the room modes and reflections evident above, we again look at the TDA PL display and at the AFR graph to see their effect on the frequency response.  The pl display below is configured to high dynamic range mode with a 20dB logarithmic scale and annotated to correlate the 3D display with the corresponding AFR.

In this room, modal effects dominate below 200 Hz and a general lack of bass damping is evident.  Without TDA, seeing nulls in the 100-200 Hz region in conventional frequency response measurements and misled by SBIR and room mode calculators that assumed a rectangular room, I initially believed those frequency response nulls were due to ceiling reflections. With TDA, I at first thought it was a more distant reflection. Gradually, I came to understand that it was a longitudinal room mode and that the listening position just happened to be sitting in a null of the mode’s standing wave.   Moving the microphone just 2’ forward and out of the null gave a cleaner measurement, confirming the diagnosis. 

Several reflections and resonances in Figure 6 were annotated and traced back to their sources by iteratively moving absorber panels and re-measuring.  Marker 1 shows the impact of the longitudinal room mode just discussed.   In general, the bass is elevated in part due to the modes and floor support but also as part of a house curve.  That voicing needs to be redone based on a set of measurements taken over the listening window. Marker 2 points at reflections from objects on the front wall between the speakers, the flat panel TV and audio equipment rack.  Marker 3 points at reflections from the unterminated conical horn mouth itself.  Marker 4 shows multiple reflections or modes between 400 and 500 Hz, where floor bounce nulls occur.
Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #15 on: February 18, 2017, 12:16:23 pm »

Eventually, I placed bass traps in the corners above the speakers, which reduced the room modes dramatically.  Using the AFR as a guide, I attenuated the modal peaks at 80 Hz and 180 Hz and obtained the vastly improved measurement below in which the direct response stands out well down into the bass.

Prior to TDA, I found even windowed measurement data taken at the LP in room overwhelming – too many frequency response nulls and impulse response peaks to make sense of easily.  TDA helped me distinguish between room modes and reflections, to locate the sources of reflections, and immediately showed me the effectiveness of treatments I applied.   With the speaker’s direct response now prominent in the measurements taken at the listening positions, I’m confident the room equalization process will result in a more than satisfactory listening experience.
Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #16 on: February 18, 2017, 12:19:12 pm »

The discussion so far has been focused on the unique2D and 3D maps of APL_TDA.  Let’s look at its controls and see how else it can present measurement data.

In the upper left hand corner of the main screen, TDA has shown me all the audio devices it has found on my system.  Below it, I tell TDA which ones to use for input and output.  Then I just click in the green “RUN MEASUREMENT” box and, after a delay for computation, get a measurement and a result display.  Below the run button is a column of 4 grey buttons that allow the saving and subsequent importing of recorded measurement data and the impulse response computed from it.  TDA automatically saves measurement impulse responses in a “HISTORY” directory below its executable.

Results appear in the large window to the right. Display options, as shown in the column of buttons immediately to the left of the main display window are: recorded measurement data, IR normalized, IR Log, the 3D and pl displays we’ve seen, AFR – the frequency response, DFR – delay vs frequency and non-linear distortion (NLDA). If the “SW” box is checked then pressing any of these buttons shows the curve in a new window from which the graph can be saved to memory in a variety of formats.
Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: APL's Time Domain Analysis - Update?
« Reply #17 on: February 18, 2017, 12:21:31 pm »

Impulse Response Display
The figure in the previous posted shows the IR Log display.  The height of the peak above the noise floor shows the SNR ratio of the measurement.  Reverberation time can be determined from the slope of the response envelope following the main IR peak.  Zoom in/out controls are available.  A linear IR graph is also available.

Visualization of Reverberation Time
Another way to visualize reverberation decay is with the PL display zoomed out to 300 ms. and configured for 60 dB dynamic range as in the picture below.

Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
FFT-Q Window Analysis
« Reply #18 on: February 18, 2017, 12:24:40 pm »

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 a detailed picture of the AFR of a 1m measurement in the first attached figure.

Reducing Q to 1.8, the narrowest opening, results in this AFR from the same measurement in the second attached figure.

 
Logged

Jack Regula

  • Newbie
  • *
  • Offline Offline
  • Posts: 20
Re: FFT-Q Window Analysis
« Reply #19 on: February 18, 2017, 12:29:45 pm »

FFT-Q windowing also affects group delay, GDR, and phase delay, PDR, response curves.   

With Q set to 8, the curve in the first attached figure is affected by the room to a high degree, despite the close in measurement, no doubt because of the speaker's use of boundary support in the bass and low midrange.

Narrowing the window by reducing Q to 1.8 in the second attached figure, we see a curve more representative of the group delay of the woofer and its cross over and less of the room.

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 in the third attached figure reveals the residual non-minimum behavior of the response, most often due to the crossover and/or equalizer.

The rising group delay in the bass in the third figure is in good agreement with the woofer simulation shown below in the fourth attachment 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.

Logged
Pages: 1 [2] 3   Go Up
 


Page created in 0.085 seconds with 32 queries.