What is ‘LFE + Main’ or ‘Double Bass’ and should I use it?
The short answer is ‘No’ – you should not use these settings and, if you have a subwoofer, you should never set your speakers to ‘Large’. For the more detailed reasoning behind this, read on…
Denon units have a setting called ‘LFE + Main’ and Onkyo units call this ‘Double Bass’. They both set out to achieve the same thing. Before we look at the issues surrounding these settings, we need to clarify what your AVR manufacturer means when they say speakers are ‘Large’ or ‘Small’.
For starters, these designations have nothing whatsoever to do with the physical size of your speakers. In AVR-speak, ‘Large’ means “no bass management” and ‘Small’ means “bass management is used”. For our present purposes, ‘bass management’ means that you have a subwoofer and you want to send bass frequencies to it – usually all the frequencies below a certain crossover level that you have chosen (or which your AVR has chosen when you ran your Audyssey setup routine). Often this crossover will be 80Hz.
If you set your speakers to ‘Large’ then ordinarily no bass management at all is used. This means that your expensive subwoofer is doing nothing other than handling the relatively small amount of content in the LFE channel (the .1 in 5.1).
If , however, you decide to use a crossover to send the low frequencies to your subwoofer, then you will need to set the speakers to ‘Small’. In some AVRs, you don’t specify ‘Small’ – the very act of setting a crossover means that the speakers have been designated as small.
However… what happens if you set your AVR to use the ‘LFE + Main’ or the ‘Double Bass’ setting?
With Denon units, if you set the mains to "Large" and ‘LFE+MAIN’, the mains will receive the full frequency spectrum, and bass from the main channels will also be sent to the sub (LFE) simultaneously. The same thing happens with Onkyo units if you set ‘Double Bass’. In both cases you are now sending low frequencies to both the main speakers AND the subwoofer. The problem is, this is a really bad idea for the following reasons:
First, there is the possibility of phase cancellation when the main speakers and the subwoofer play the same bass frequencies.
Second, in the region where the frequencies overlap between the subwoofer and the main speakers, the bass frequencies are doubled and tend to become bloated, boomy, and exaggerated.
Also, the XT and MultEQ versions of Audyssey apply more correction filters to the subwoofer frequencies. If the same frequencies are sent to the main speakers and the subwoofer at the same time, you will apply higher resolution filters to the same frequencies in the subwoofer and lower resolution filters to the same frequencies going to the front speakers. When the two low frequency sources are combined, we will have unpredictable results to say the least.
Finally, read what Chris Kyriakakis of Audyssey has to say on the subject:
"LFE + Main should not even be an option because it just causes duplication of bass content by sending it to both the sub and any speakers set to Large (Full Range).
A "high ranking" official in a "well-known" AVR company told me that LFE + Main was invented to appease customers that were upset when their speakers were being set to Small. These customers had a complete lack of understanding of what Small means (i.e. turn on bass management and redirect the bass to the subwoofer) and felt... inadequate. LFE + Main allows them to set their speakers to a more manly Large and still have bass management. But it's a compromise that can cause boomy bass if the speaker and subwoofer overlap in the lower frequencies."