- Written by Dr. David A. Rich
- Published on 17 April 2013
Measured data using my independent room acoustics measurement system of the individual main channel speakers
We now turn to the key question: Does the acoustic response of each speaker, after ARC electrical correction, show flat response in the passband, the correct -6dB value at the crossover and the 24dB octave rolloff?
The graph below is my acoustic measurement of the left channel alone with the ARC room equalizer enabled.
It is measured using the same technique outlined in Part 1 of this review (9 point spatial average). Each point my measurement microphone was placed corresponds to a place the ARC microphone was placed for the room measurements. In theory, the curve should be down 6dB at the crossover point (80Hz), and since it is a 4th order filter, the final slope should be 24dB per octave. The curve should show no peaking across the full frequency range.
To verify the cutoff frequency and rolloff slope of the left channel, I needed to expand the span of the graph to 33dB and limit the upper frequency to 500Hz to accurately measure the slope. Using 0.33 octave smoothing eliminates small ripples in the graphs that obscure the asymptotic slope. The measured results closely correspond to the expected 80Hz -6dB point and the 24dB octave slope. The asymptotic slope of a low- or high-pass filter with no transmission zeros is quantized to 6dB per order of the filter (6dB for a first-order, 12dB for a second-order etc.). There is no such thing as 3.6 order filter. Since we are looking at the curve of an acoustic measurement in a room from a real speaker, small deviations from the theoretical quantized values is inevitable. No other room EQ I have tested is capable of producing a curve with a near textbook shape as ARC has done here.
Moving on to investigate the electrical inverse curve using the pair of curves below
The top plot in this curve set has an enlarged left channel frequency response from the ARC display. As we demonstrated in Part 1 of the ARC review, the ARC acoustic measurement before equalization (red curve in the Anthem plot above) closely correspond to my independent measurements (verified but not shown), so we will use the ARC graphic. Note the span of the plot has been reduced by cutting the display below 40dB out.
The bottom curve in this set is the electrical transfer response from left-in to preamp left-out. The six-channel input was used for this measurement, but any input will yield the same result. The black curve on the electrical inverse correction plot is the bass management system with no room equalization applied. It has a slope 12dB octave, which is what the specification for a standard bass management system requires as discussed above.
The red curve in the bottom graph is with the ARC room EQ activated. You can see how different it is from the black curve. In addition to the added peaks and dips which fill the corresponding changes in the pre-equalized acoustic frequency response (red curve on the top graph), you can see the high-pass filter slope. Note how much faster the red curve on the lower graph attenuates the signal than with ARC not activated (black curve on lower graph). The exact rate of attenuation of the electrical response alone is hard to see because the speaker itself is starting to roll off at about 50Hz. ARC must account for the intrinsic speaker rolloff if the post EQ acoustic curve is to be correct.
That the electrical inverse correction curve is calculated correctly is without dispute, given the acoustic measurement shown on the previous graphic - my independent acoustical measurement of the post EQ acoustic response is shown in the plot just above this one.
Please note again that the green curve (post equalization) on the ARC plot (upper plot) shows only the slope (12dB / octave) for the bass management system, but the acoustic measurement verified the acoustic response has the required 24dB / octave slope. The reason for this discrepancy was explained above by ARC designer Peter Schuck.
As an aside, I have circled the area around 500Hz on the electrical inverse correction curve (bottom graph) in blue. The Max EQ setting is entered in the advanced dialog box. Note the smooth transition to the reference value above 500Hz with no discontinuity. I placed an orange box on the Anthem graphic above 500Hz. Note how pre- and post-correction curves are on top of each other. This is the expected result, since no electrical correction is occurring in this area. This is the ideal result. This was discussed in greater detail in last section of Part 1 of the review but with a different Max EQ value and different speaker room position.
Below is an acoustic measurement of the right-channel main speaker taken in the same manner as the left channel.
The result is also close to textbook standards despite the significant difference in the right channels pre-equalized response. The curve set below has a copy of the ARC frequency response display on the top for the right channel. The bottom is the electrical correction response from right-in to preamp right-out with the bass management system activated. This pair of plots presents the same data I discussed for the left channel above.
Recall the left and right channels were deliberately placed in the room to produce different response characteristics below 200Hz. Compare the electrical response curve with ARC EQ (red curve on bottom graph) for the left and right channels. Note how different they look responding to the different pre-equalized response of the left and right channels. That ARC can produce such complex yet very different curves dependent on room conditions speaks to the highly advanced algorithms used to build the DSP filters.
Again note the smooth transition to the reference level above the 500Hz Max Eq (blue circle).
Measured data using my independent room acoustics measurement system of subwoofer
My independent measurement of the acoustic response of the subwoofer alone with ARC activated is shown below.
The response was measured using the same procedure as the left- and right-channel main speaker's acoustic measurements shown above. For this plot, I used a smoothing of 0.15 octave to better resolve the flat passband response of the woofer, which extends from 30 to 60Hz. I deliberately expanded the span of the graph to 45dB and set the upper frequency limit to 200Hz to better resolve the slope.
The ARC system has done an estimable job in flattening the response in the subwoofer's passband. The anechoic rolloff of the NHT woofer (-3dB) is about 29Hz. It is not as extended in this graph because of room mode cancellation around 30Hz. The -6dB point, which should be at the 80Hz crossover and asymptotic slope, correspond closely to the expected values. The critical low-pass filter slope is once again near textbook in shape. The small slope offset that occurs -30dB down has no effect, since -30dB indicates the subwoofer's output has already been decreased to 0.03, and its output is so low as to not be adding to the in-room acoustic response above this point (190Hz).
The curve set below has a copy of the ARC frequency response display for the subwoofer as seen in the top graph. The bottom graph is the electrical inverse correction response of the bass management system activated for the LPF (subwoofer path).
This pair of plots presents the same data I presented for the left and right channels above. This electrical inverse correction response measurement (bottom graph of the pair) is made by applying the test signal to the left six-channel input (not the subwoofer input, which is for the LFE channel) and measuring subwoofer preamp output level. An identical result would be found driving any other channel since, as discussed above, only one LPF is used for the subwoofer channel in Anthem products.
Displayed on the bottom graph are the response of the bass management system alone and the response with the ARC EQ in the signal path. The traditional bass management system has the correct 24dB octave slope, but the curve with the EQ active is different, as the room effects are canceled. As expected, the dips in the lower red curve corresponding to each peak in the upper red curve on the ARC graphic. Note these corrections are required well into the crossover transition band. The correspondence is easier to see with the subwoofer, because the subwoofer is relatively flat to ~150Hz. In the corresponding plots for the left- and right-channel main channels, the speakers were rolling off at the same time the electrical rolloff was being applied. This obscured the complete rolloff slope.
Again, I am not showing my own acoustic measurement to verify the accuracy of the upper red curve (Anthem pre-corrected subwoofer response), but I did do the measurement and confirmed that they matched closely.
The acoustical measurements above verified the left, right, and subwoofer crossover frequencies were very close or at the -6dB level, and slopes were very close or at the 24dB / octave value with each speaker playing alone. The key question is what happens when the subwoofer and one of the main channel speakers play together? The PC display for the ARC software cannot show this, since it requires another set of measurement after the filter coefficient have been downloaded to the Anthem Pre/Pro or AVR.
Measured data using my independent room acoustics measurement system of the of the complete system
Below I show my acoustic measurements for the complete system after all the equalizer filter coefficients, bass management filter settings, distance, and level correction have been downloaded to the Anthem D2 Pre/Pro.
This plot highlights the remarkable success of the ARC system. The upper curve is the nine-point averaged acoustical measurement with the left main speaker and subwoofer operative. The lower curve is the right main speaker and the subwoofer playing together. Note the span has been reduced to 21dB, which is the same value I used when verifying the performance of the ARC system with a full-range speaker in Part 1. The smoothing is 0.15dB/octave, and the maximum frequency of the graphs is limited to 200Hz to better examine the response near the crossover. Above 200Hz the subwoofer is down 35dB.
For the left channel only, a -2dB drop at 70Hz is noted. For the right channel, a slightly larger drop, with a maximum of -3dB, is seen from 75Hz to 95Hz. Note that the frequency variations outside the crossover area are of the same order of magnitude. From the natural rolloff of the subwoofer to 200Hz, the frequency response changes only ± 2dB. This is a remarkable result, considering the complex modal pattern of the room, in addition to required blending of the main channels and subwoofer that are many feet apart.
Note that like almost all room correction systems, ARC cannot tell if the woofer is in or out of phase. If the subwoofer inverts phase, you will get a dip in the crossover. A test disc with 1/3 octave bandwidth noise will tell you if things are correct. Only connect the left channel speaker and play the noise on the test disc centered at the crossover. Now reverse the polarity of the subwoofer and listen again. The result with the loudest level is correct.