PS Audio: LFE (Low frequency extension)

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Paul McGowan writes: I was thinking about these Audiophile terms we use this morning and it occurred to me that perhaps one of the more frustrating and misused terms is Low Frequency Extension.  Frustrating because the group of people who believe if it doesn’t show up on their meters it doesn’t exist must be driven mad by this and misused because it gets bandied about by every writer I know of, including me.

If you were just walking into our community of high-end audio people and heard that a particular piece of gear had “great low frequency extension” you’d be inclined to believe that it produced measurably lower bass notes than what it was being compared to.  In fact, its published measured bass response would probably be identical.  So what the heck does LFE mean when you read it in a review of two pieces who are both “flat” to 20Hz?

Certainly I can’t answer for reviewers and how they use it but I can for myself.  I have, many times, described a product as having extraordinary bass response and subterranean low frequency extension.  By that I mean that the sound seems to plumb the depths of the room and speaker system in surprising ways – it sounds like there’s deeper, lower bass than expected – as if you added an extra octave of bass that wasn’t previously there.

The key here is “sounds like” since even the cheesiest of products you might find at Best Buy no doubt is flat to 20Hz, the supposed limit to human hearing.  But there’s no doubt one of these cheesy products compared to something you and I would consider listenable would most likely not have great LFE – despite the measured response.

What’s up with that?  I can give you a couple of hints.  In my experience kit must be flat to below 10Hz and preferably down to about 2Hz to have this LFE we talk about.  I suspect part of this has to do with phase shift – which is pretty severe in a product with only “flat” to 20Hz.  Another aspect of this is the way specifications are listed.  When something says it is “flat” to 20Hz it doesn’t really mean that.  What it means is that it probably starts to rolloff at 20Hz and is perhaps 12dB down at 10Hz.  A unit that measures like this will have poor perceived LFE and a unit that is -3dB at 2Hz will most likely have excellent LFE.

Lastly, if you have a good subwoofer installed in your system and you can reproduce “flat” bass below 20Hz, there’s much you can feel of the bass notes, even if you can’t technically hear those frequencies.

So the bottom line is this: both camps of people, those that measure and those that listen, can probably be ok with the term LFE to describe bass response on equipment that measures “the same” in their published specs – but only because the real and important specs just aren’t listed.

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