Dear AVF members
The table below lists CEA-2010 output rated subwoofers in order of their estimated ability to support user listening levels (relative to reference) in rooms of varying sizes.
For some it may be self-explanatory but if you would like to know how to use the table and how and why I created it then take a quick look at it and keep reading further down below where hopefully all is explained.
What’s this thread all about?
Estimating how much subwoofer SPL output capability you need in your AV system in your room and whether or not you currently have it is fundamentally important yet many people (understandably) struggle with this. This thread is all about helping you do this.
THX and Audioholics room size ratings
A home cinema system's subwoofer output requirement is, to a greater extent, a function of listening level, room volume and the low frequency content of the movies being played. Two information sources currently exist that advise on a specific subwoofer model's suitability for particular room sizes. These are THX and Audioholics.
THX has its Select, Ultra and Dominus programmes under which it tests and certifies subs as being capable of achieving cinema reference level in 2,000Ft3, 3,000Ft3 and 6,500Ft3 rooms respectively.
THX’s certification method isn’t published but a requirement is widely believed to be that frequency response must be flat to 20Hz. So to be THX certified, a sub must be able to hit 115dB reference level peaks with minimal distortion at 20Hz. Very few manufacturers choose to participate in this programme these days so unfortunately very few THX certified subwoofers exist. You will however find THX certified subwoofer models from Monoprice, Perlisten and M&K.
In contrast, AV guru website Audioholics (“AH”) assign room size ratings to the many subs it reviews and measures based on its estimate of their ability to hit 115dB at 31.5Hz (and 109dB at 25Hz) in rooms of different volumes from under 1,500Ft3 to over 5,000Ft3.
This is a significantly easier performance target to hit than THX’s as most subs have far more output capability at 31.5Hz than they do at 20Hz. AH’s rating method is simply to add 9dB to CEA-2010 RMS@2M output at 31.5Hz and adjust this result by +/-6dB per halving/doubling of room volume from a 3,000Ft3 baseline due to a change in sound pressure, e.g. 1,500Ft3/6000Ft3 rooms add/subtract 6dB from the result due to doubling/halving of volume.
My maximum listening level ratings
The main limitation of the THX/AH subwoofer ratings is the fact that almost nobody listens at cinema reference level in their homes. As listening between 10dB and 20dB below reference level can require only 1% to 10% of the amplifier power, the THX/AH ratings can lead to seriously over-specifying the subwoofer output needs of some systems.
In order to address this, using THX’s tougher 20Hz lowest frequency hurdle and AH’s CEA-2010 to in-room output adjustment method, I’ve estimated various subwoofers’ abilities to support different listening levels (relative to reference) in room sizes from 1,250Ft3 to 5,000Ft3. The maximum listening level capability is calculated simply by subtracting 115dB from the subwoofer’s estimated in-room output at 20Hz and these are the figures that are shown in the table.
Apparently, due to psycho-acoustics in smaller spaces, listening at -10dB sounds about as loud as cinema reference level (i.e. 0dB) does in large commercial cinemas. This is why I have referred to the silver highlighted >-10dB ratings as "home reference level".
Why I used a 20Hz low frequency hurdle and not 31.5Hz like AH
AH’s rating system is based on 31.5Hz output due to the bass content of >90% of movies being >30Hz. While this is probably correct, I chose to use THX’s more challenging 20Hz standard for the following reasons:
- 20Hz is the frequency at which subwoofers can produce the least output in their typical 20-120Hz audible range of operation in a home cinema system. A sub’s maximum clean SPL output at 20Hz is therefore the limiting factor on maximum volume capability if a flat, undistorted, response down to 20Hz is desired on dynamic peaks.
- A 31.5Hz hurdle is likely to be too high for movies with very low frequency content. As 20Hz is the lowest audible frequency, if a soundtrack contains 20Hz content, then distortion might be audible if a sub lacks sufficient output capability here. Also, <31.5Hz content is not just reserved for minority interest sci-fi movies as even naturally occurring deep bass sound effects (e.g. thunder) can be as low as 20Hz.
- THX uses 20Hz for its certification programme so this method, in effect, provides THX style ratings that are adjusted for different room sizes and listening levels. This also provides a useful source of validation by looking at THX rated subs for which CEA-2010 output data is available.
Alternatively, refer to the table at the end of this post which shows maximum listening level capability on the basis of 31.5Hz output.
How to use the ratings table
First, determine the maximum listening level, relative to cinema reference, that your subwoofers need to support in your system. Provided your AVR/processor is calibrated to 0dB volume being 85dB reference level (not all are), this will be the lowest volume setting you use to generate the same maximum level of loudness at which you listen. As Blu-rays and DVDs are normally mastered higher than streaming sources, this is likely to be the volume setting that you use with these. Most people's maximum listening level seems to fall in the -10dB to -15dB range.
Now you know your listening level requirement, scan down the column that’s closest to your room size until you get to this number. Every sub at or above this row in the table should be capable of meeting your system's maximum low frequency output needs without distorting.
Sub output typically increases by 4-5dB for every doubling of the number of subs if they are time aligned but not co-located. So for systems with two, three or four identical time aligned subs you can increase the table’s maximum listening levels by 4dB, 6dB or 8dB respectively.
Example: If you listen at -10dB in a 2,750Ft3 room then every subwoofer in the table above an SVS SB-3000 [only -11db capable in a 2,750Ft3 room] should provide enough output. Note that dual SB-3000s would be capable as they should support a -7dB listening level.
Accuracy of the ratings table
CEA-2010 output is objectively measured so the accuracy essentially depends on the reliability of AH’s adjustment method to arrive at an in-room output figure. As they’ve stuck with it for over 11 years now I suspect it's fairly reliable and it appears to be in-line with THX's own ratings which I explain further below. Be aware though that AH's 9dB boundary gain adjustment assumes corner placement to maximise in-room output so if yours is not then you may need to reduce the figures by a few dB to be conservative.
Reassuringly, the table shows Monoprice’s Monolith THX 10” and 12” subs to be -1dB and 0dB capable in 2,000Ft3 and 3,000Ft3 rooms respectively which corresponds closely with their respective THX Select and THX Ultra certifications. So it seems that the great minds at THX and AH might think alike. THX is known to measure subs in actual rooms so it is likely that its ratings factor in some room gain.
As these two THX rated subs barely creep into their certification bands, the table's ratings may understate output capability slightly. This would be particularly so in sealed, smaller rooms that might benefit from a significant amount of room gain at 20Hz which has not been factored into the calculations. In small, well sealed listening spaces room gain at 20Hz could add a up to 7dB to a sub's output and its existence would add dynamic headroom to the figures shown. Estimated potential levels of room gain for different room sizes are shown at the base of the table.
At the end of the day, these ratings are still estimates and it would do no harm to add a few dB headroom to your listening level requirements to be conservative.
First I want to stress that the output ratings table is not a best subwoofer league table of any kind. It's simply a ranking of subwoofers based on their measured maximum, clean (per CEA-2010) output at 20Hz and that is it. There’s far more to subwoofer performance and sound quality than maximum output at 20Hz. And don’t let any of this stop you from buying more powerful subs either as there's often much to be gained by having a considerable amount of output headroom as it can add a more explosive delivery quality to a system.
Hopefully though, this thread will help members make more informed choices when specifying the minimum subwoofer output requirement for their systems.
I consider the ratings table a “work in progress” so would welcome any ideas on how to improve it as well as any CEA-2010 output data for subs not currently listed. Obviously, if you spot any errors or omissions then please let me know. I'm also sure this thread will stimulate a reasonable amount of debate which would be great.
If the thread proves popular, I’ll update it whenever new CEA-2010 test data is published.
Brent Butterworth: CEA-2010 - A Better Way to Measure Subwoofers
Audioholics: Subwoofer room size rating protocol
THX: Room size certifications
ADDENDUM: Maximum listening level capability based on 31.5Hz output