Secrets Benchmark Product Review
 

Klipsch RT-12d Subwoofer

Part III

December, 2006

Ed Mullen

 

CEA 2010 Output Scores: The average sound pressure for the 20-32 Hz (defined by CEA as "ultra low bass") bandwidth, and for the 40-63 Hz (defined by CEA as "low bass") bandwidth has been calculated using the above distortion-limited output values. As required by the CEA 2010 standard, the sound pressure levels have been normalized to a 1 meter test distance.
 

Bandwidth (Hz)

Average Distortion-Limited SPL

1 Meter Test Distance (dB)

CEA 2010 Description

20-32

102.8

"Ultra Low Bass" Output Score

40-63

114.9

"Low Bass" Output Score

Bandwidth Linearity Score: The bandwidth linearity score for a given bandwidth is determined by dividing the average distortion-limited SPL over that bandwidth by the maximum distortion-limited SPL occurring within that bandwidth, and expressing the result as a percentage. The Klipsch scores very well, particularly above 25 Hz.
 

Bandwidth (Hz)

Average Distortion-Limited SPL

2 Meter Test Distance (dB)

Bandwidth Linearity Score (%)

20-80

102.5

94

22-80

104.2

95

25-80

105.8

96

32-80

107.3

98

40-80

108.2

99

Transient Response: An evaluation of the frequency response (particularly the F6 bandwidth and the roll-off slope) can be used to predict the system transient response. These variables affect the rate of phase change with respect to frequency, which in turn manifests itself as latency and ringing in the time domain. This phenomenon can be characterized by evaluating group delay, impulse response and spectral decay.

The RT-12d exhibits a down-sloping frequency response from 60-30 Hz, a narrow F6 bandwidth (from 24-21 Hz), and a roll-off slope of ~75 dB/octave below 23 Hz (thus indicating the presence of a steep high pass filter).

The down-sloping FR from 60-30 Hz increases group delay to near 1.0 cycle from 55-32 Hz. As the system roll-off is approached, the group delay curve rapidly rises below 32 Hz, eventually exceeding 1.5 cycles at 22 Hz.

As expected, the impulse response exhibits ringing for approximately 80 ms. The blue line is the gate curve, the green line is the impulse response of the measurement system, and the black line is the impulse response of the subwoofer. The spectral decay 3-D graph shows this ringing occurring at system resonance (22-23 Hz).

While the audibility of latency and ringing at very low frequencies is subject to debate, extrapolations of existing group delay studies conducted at higher frequencies suggest an audibility threshold of 1.0-1.5 cycles. Readers should be reminded that the in-room frequency response and the structural integrity of the listening room can have a large (if not dominant) influence on our subjective impressions of subwoofer transient response. Subjectively "slow" bass with excessive "overhang" can often be greatly improved with parametric equalization (such as offered by the Klipsch Room Correction software) and bass traps.

In-Room Frequency Response

I placed the RT-12d in the front left corner of my 2,000 ft3 evaluation room, which is acoustically treated and equipped with bass-traps.

First I measured the frequency response of the RT-12d in the three EQ Mode settings (Flat, Punch, Depth). As advertised, the Punch setting has the most effect in the mid-bass regions. Conversely, Depth setting had maximum effect in the 25-30 Hz bandwidth, but also added some mid-bass boost.

Then I measured the frequency response before/after the auto-EQ Room Correction feature. I used the Flat EQ Mode setting as the starting point for the auto-EQ routine. As shown in the screen shot, the Room Correction feature reduced the severity of a mid-bass peak, which audibly improved the sound quality. The final frequency response was 22-100 Hz 2.5 dB, which is impressively flat.

As stated previously, I tried the auto-EQ feature at several different subwoofer locations and listening positions. Despite my best attempts to trick the software, the auto-EQ routine always improved the frequency response, and also never tried to boost true room nulls. Klipsch scores big with the Room Correction feature; it's incredibly easy to use and always works well just the ticket for today's plug-n-play consumer.

For music/movie evaluations, the main speakers were set to "Small" with a crossover frequency of 100 Hz. The digital bass management circuit imposes a 2nd order high pass filter on the speakers, and a 4th order Low-Pass filter on the subwoofer. Accordingly, I set the Low-Pass filter on the RT-12d to the "Low-Pass Off LFE Mode" setting. I set the phase control to 0 degrees and adjusted the subwoofer distance setting in the SSP to obtain the best phase integration with the mains.

Click Here to Go to Part IV.

Copyright 2006 Secrets of Home Theater & High Fidelity

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