Klipsch SW-311 Subwoofer Review Measurements and Analysis

The Klipsch SW-311 was measured outdoors sitting on the ground with the microphone placed 2 meters from the front lip of the cabinet pointing at the main driver which was forward firing and with the passive radiators side firing. The low pass filter and ARC II processing were both disabled and the subwoofer volume was set to 0 for all testing, except for those tests purposely conducted to examine the effects of the built in DSP functions.

The overall approach to this testing along with the equipment and software used is outlined in the article here:

Powered Subwoofer Testing Outline and Procedures Overview

   

Klipsch SW-311: Effect of Low Pass Filter Settings

The effect of the low pass filter settings on the SW-311 top end response can be seen in the graph above. The Klipsch has a wide variety of available settings with the frequency selectable from 40-150Hz and the roll off rate variable between 12, 18, 24, 36 and 48dB octave slopes. I recorded the frequency response with the low pass filter set to 40, 65, 90, 120 and 150Hz settings along with the response with the filter bypassed. I used the 24dB / octave slope setting. The depth EQ setting was engaged during these measurements.

   

Klipsch SW-311: Effect of Flat, Depth and Punch EQ Settings

The effect of the SW-311’s built in EQ presets was rather mild and mostly consists of slightly boosting the overall output level or a slight adjustment in low frequency strength relative to the top end. I ran through them briefly listening to some music and the differences were noticeable but not huge. The punch setting seemed to boost the overall sub level while the depth setting also boosted the level but was most apparent in the lower bass register of music. The overall changes appear to be no more than about 3dB at most. None of them should have a profound effect on the performance of the SW-311 at the limit which is good.

 Klipsch SW-311: Basic Frequency Response as Tested

I decided to use the punch response setting for the rest of the SW-311 measurements due to it rolling off just slightly towards the low end. In most rooms this setting with a very gradual roll off towards the deeper bass would probably complement the natural boost given to the lower bass region by the room acoustics better than the settings that are flatter or have a more exaggerated low end. Also subwoofers almost always run into their output limitations in the low bass frequencies first and the small roll off of the low frequencies will allow just a bit more of the upper bass headroom to be used before the low end limits the output. Either way the differences in the EQ settings for the SW-311 are not drastic to begin with. With the punch EQ setting and the low pass filter defeated the SW-311 fits within a +/-3dB window from 28-180Hz. With the depth setting engaged the response is within +/-3dB from 27-185Hz. In flat mode the SW-311 stays within the same window from 28-194Hz. Klipsch rates the response at 22-120Hz +/-3dB. The measured SW-311 response is fairly close to that. It easily exceeds the 120Hz spec on the top end and could probably crossover to other speakers in the 150-180Hz region successfully. I see no way that it will meet the 22Hz extension spec though. As you can see from all of the measurements, at about 28 or 29Hz it starts what appears to ultimately be a 48dB/octave roll off, which is a function of the standard 24dB/octave roll off of a bass reflex alignment below tuning, plus what is likely a 24db octave rumble filter applied just below tune as well. This type of thing is standard with most bass reflex alignments to prevent unwanted cone excursion below the system tuning. The SW-311 is down over 24dB by 22Hz already. The effective useful extension of the SW-311 in room appears to be somewhere in the 25-30Hz range.

Klipsch SW-311: Waterfall Decay

Klipsch SW-311: Group Delay

Looking at the waterfall and group delay plots for the SW-311 shows that it is well behaved. There is nothing of note until down near 30Hz where the energy begins hang around a little until it peaks at 25Hz. It does exceed 1.5 cycles for a bit on the group delay chart but this is not an uncommon occurrence for bass reflex alignments with steep high pass filters applied at tuning. Both the reflex tuning of the system and the filter will contribute to this.

 Klipsch SW-311: Long Term Power Compression

In the chart above we have the response of the SW-311 as it is driven increasingly louder with a long duration sine wave sweep. The base level is referenced to 90dB at 50Hz at 2 meters ground plane and each sweep is increased by 5dB in level until the subwoofer exhibits severe compression or distress. Afterwards another nominally 90dB sweep is taken to observe the long term heating effects on the system. The SW-311 exhibited negligible after effects on the repeat 90dB measurement.

Klipsch SW-311: Power Compression Comparison

This graph is another way of presenting the data collected during the long term power compression test. It references the data to the 90dB sweep which is the flat black trace in the graph and shows only the amount of variation in output from the input signal increase. Ideally each other sweep would fall right along the black line up until as high an output level for the subwoofer as possible. The SW-311 performs excellently up until the 105dB sweep where it exhibits clear compression and barely manages any increase in level at 30Hz. The 110dB sweep produced little extra output except above 60Hz indicating that the SW-311 was being limited heavily at that point. Through out the power compression testing the SW-311 never made a distress noise or had issue. Even with the final sweep which caused the SW-311 to be heavily limited it simply coasted through still sounding clean. The protection circuitry is very good on this sub.

Klipsch SW-311: Maximum Long Term Output Level

The maximum sweep level attained by the SW-311 falls into the lower range of all subwoofers I have ever tested below 50Hz, but above there it does better and manages to make it to middle of the pack. That is a pretty good result considering that this is a miniature subwoofer that takes up less than 1.4 cubic foot of space and weighs below 40lbs. Most other subwoofers I have tested are at least two or three times the size of the SW-311.

 

Klipsch SW-311: 95, 100, 105, 110dB Sweep THD Comparison

The distortion performance of the SW-311 during a few of the power compression sweeps is presented above. I’d give the SW-311 an A rating here. It is well below 5% THD for the 100dB sweep and even during the 110dB sweep the THD is still well under 10% for the majority of its entire effective range of operation. The distortion does get out of control below 25Hz but as we have already covered previously that is below the passive radiator tuning and outside of the SW-311’s effective envelope of operation anyway.

 Klipsch SW-311: CEA2010 2 meter Groundplane RMS Results

 Klipsch SW-311: CEA2010 2 Meter Groundplane RMS Comparison

CEA2010 Results

Looking at the CEA2010 test results for the SW-311 we can see that at the 25Hz band and above are all amplifier limited and that the maximum output is in the lower middle range of all units that have been tested. At 50Hz and above the output was commendably strong for a unit of this size. At 20Hz the SW-311 could just eke out a passing result of 84.2dB. With the restriction on distortion removed the maximum output at 20Hz jumps to 90.6dB but with very high THD. Below 20Hz the SW-311 could not produce a passing result or any meaningful output for that matter. The distortion from the SW-311 was dominated by the 3^rd harmonic for the most part. The SW-311 did exhibit good bandwidth uniformity of maximum output over its range of effective operation, staying within a +/-5db window 31.5-125Hz.

ARC II Overview

The SW-311 comes with a built in auto room correction system developed by Klipsch called ARC II. The supplied microphone has a small aperture for the element at the top of a small dome that is roughly the size and shape of a cookie. Unfortunately I forgot to take pictures of the microphone for the review. The mic is simply placed where your head would normally be at the listening position and plugged into the 1/8^th inch port on the back of the SW-311. Using the on board DSC interface you tell it to run the routine, which consists of a slow frequency sweep, a quite loud one I might add, followed by a few seconds of EQ calculations and that is it. The whole process takes a matter of maybe three minutes or so in total. I decided to see how effective ARC II is by placing the SW-311 at a random spot in a room that I don’t normally use for audio and running the auto EQ routine with the microphone placed at three random positions that could potentially be listening positions in the room. Luckily out of these three random microphone placements one of them turned out to have a good natural response, one was a little worse and one was really bad, which is what I was looking for. I was then able to measure the before and after of the SW-311’s response at each position. The results for the position with the worst response are shown in the chart below. The other 2 positions had much better basic responses requiring far less EQ correction. The results for the worst position more clearly show the corrections made by ARC II when faced with a less than ideal scenario.

Klipsch SW-311: ARC II Auto EQ Results for 3 Random Listening Positions

ARC II appears to do a very nice job of addressing the biggest response issues without overdoing it. It does not EQ below 30Hz where the SW-311 isn’t strong and deep high Q nulls are left alone. Any big peaks in response are what are addressed primarily. It didn’t make a silk purse out of this sow’s ear but the response afterward would be a large improvement in quality. It is always a good idea to find the best available placement for the subwoofer to begin with so that the amount of EQ correction needed is minimized, but if you are stuck with less than ideal placement and response, ARC II will help improve the situation. Unfortunately ARC II’s auto EQ correction only applies to a single listening position. I think this limits its usefulness overall. You could account for multiple listening positions or at least the general area where your ears will be located, instead of a single isolated point in room, if averaging of multiple measurements were possible. Then there is the fact that most modern home theaters will have some sort of auto equalization built into the processor or receiver already which will use spatial averaging of multiple measurements.