- Written by Dr. David A. Rich
- Published on 17 April 2013
Setup for Performance Verification Testing of the ARC System with a Stereo Configuration that has a Subwoofer
To test the ARC system with a subwoofer, I used a subset of the main channel speakers I used for the tests without a subwoofer presented in Part 1 of this review. The subwoofers used for these tests were from NHT and Sherwood. The sample measurements below are for a pairing of the Infinity C336 as the main channel and the NHT B10d as the subwoofer. The latter was recently reviewed in Secrets. The Infinity appears here so as to compare results with my review of the HK 990 in Secrets and the Sherwood R972 with Trinnov printed in the Boston Audio Society Journal Speaker (BASS v33 n2). While I am showing only one brand of speaker in one room I replicated the result shown below other speakers and rooms.
The left and right channels speakers were placed in different parts of the room to produce different frequency responses below 200Hz. This was done to see how ARC worked under different conditions. The positions were both reasonable places a speaker would be set in the room. I did not attempt to create an artificial condition by placing the left channel in the corner.
The target panel settings are shown below for all the measurements presented in this document is part of the ARC review.
You can see 80Hz was used for both the L/R fronts and the subwoofer. Failing to match mains and subwoofer frequencies renders everything below invalid. I did verify that the use of other matched crossover frequencies will yield similar results to what is shown below. The ability of ARC to produce exemplary main channel – subwoofers blending with a user selectable crossover is a significant advantage of this system.
While the test below was for a stereo setup I verified that the center and surround channels produced the same results (5.1). The rear speakers (6th and 7th speakers in a 7.1 system) were not directly accessible from jacks at the back of the unit.
Below I have expanded the view of the target panel to shown two of the optional advanced setting.
A 500Hz Max EQ frequency prevents the EQ from applying any correction above 500Hz. A Room Gain of 0dB produces a flat low frequency target. The room gain setting was discussed in Part 1 of the ARC review. The white space cuts out a section of the control panel containing text that supplies some information to the user setting the panel.
Why the filters Siegfried Linkwitz and Russ Riley invented are important to you
Below is the ARC PC display after making a 9 point measurement with the ARC microphone.
As with the full range speaker tests in Part 1 of this review the points are around an 18 inch square and center as the starting point. This the first time I am describing how the ARC PC program shows multiple speakers. This is a 2.1 system. Eight frequency response plots would be shown for a 7.1 system.
What is shown in the green curves in the three graphs above is the response of the individual drivers based on the correction filtering ARC has calculated for each channel. ARC cannot predict the complete response with the main channel and subwoofer both operative, only the individual drivers.
What we are most interested in with these curves are the characteristics of the highpass filter in the main channels and the lowpass filter in the subwoofer path. These should represent a 4th order Linkwitz–Riley (LR) filter set. With an LR LPF (main channel) and LR HPF (subwoofer) at the same crossover points and filter order, the signal path should sum to constant amplitude. The key attributes of the LR filters are the 6dB down point at the crossover frequency and the slope of the filter below the crossover. The LR filters should show no peaking in the passband, and after the crossover point it should become asymptotic to 12dB /octave (2nd order) or 24dB octave (4th order).
The graph below is from Wikipedia:
It shows a pair of 2nd order LPF filters (LR and Butterworth). A corresponding set of HPF filters are also shown. The summation of the LPF and HPF filters for the Butterworth and LR are the fifth (Butterworth sum) and sixth (LR2 sum) curves. The 2nd order Butterworth filter has a 3dB peak at the crossover point, but the LR filters sum to a flat response.
Since the graph above shows 2nd order filters, the slopes are at asymptotic to 12dB per octave.
Technically a 2nd order LR is two 1st order Butterworth's in cascade. The 4th order LR is two 2nd order Butterworth filters in cascade (often called Butterworth squared). LR filters are even-ordered only. Odd-ordered crossover filters do exist that sum to unity gain but may have issues the even-ordered LR filters do not. This is beyond the scope of this review.
Below I have shown magnified sections (how to magnify an ARC plot was discussed in Part 1) of the frequency response plots for Left and Subwoofer channel.
We need to verify if the post correction -6dB crossover points and asymptotic slopes are correct to form a 4th order Linkwitz–Riley (LR).
The upper curve showing the highpass filter response (upper curve) was expected to match the lowpass filter response (lower curve) in the lower curve (-6dB at 80Hz with an asymptotic slope of 24dB per octave). The principle designer of ARC, Peter Schuck, explains the reason the upper curve displays different data: "ARC displays the target highpass response without the 2nd order highpass filter that is added by the bass manager. Thus it displays a 2nd order highpass target (Butterworth with -3dB point at nominal crossover frequency). The final (acoustic) response with the crossover turned on will be 4th order Linkwitz-Riley (Butterworth squared) at -6dB at the nominal crossover frequency. For the subwoofer ARC displays the target 4th order LR response that is provided by the bass manager. This may seem a bit odd but makes sense from the following point of view: the difference between the 4th order LR subwoofer target and the subwoofer measurement is the ARC target EQ response. Similarly the difference between the 2nd order Butterworth highpass and the speaker acoustic measurement is the ARC target EQ response. The lower graph shows the subwoofer response on the ARC PC display, and has the correct 24dB octave low-pass LPF rolloff".