Definition: A comb filter adds a slightly delayed version of the original signal to itself causing either destructive or constructive interference. Destructive interference lessens or eliminates the frequency and constructive interference adds to it. The additions or subtractions happen at regular intervals giving the graphic response the appearance of a comb.

 

Comb filtering happens when the surface underneath a speaker resonates constructively or destructively due to the natural resonances of the stand (for example) or surface the speaker is placed on.

 

A constructive comb filter can add an artificially high level of a particular frequency, while a destructive comb filter can make certain frequencies seem to disappear. These effects are most noticeable in the bass region.

 

A while back, I had my LS50 Wireless set on a pair of stands in my office. My room is pretty lively, but I’ve got the speakers tuned and positioned for a really excellent flat response throughout the range. Or so I thought. I had been using a pair of wooden stands I’ve had for years. Admittedly they weren’t “audiophile-grade” stands, but they looked nice and had served me well. I never used bookshelf speakers on free stands for serious listening prior to getting my LS50 Wireless so I was never in the market for a stand upgrade.

 

I got myself a pair of our bespoke LS50 stands and what I heard the first time I listened to my heretofore properly tuned and positioned KEF LS50 WirelessLS50 Wireless was nothing short of a revelation. It’s really difficult to hear subtle room differences with a Youtube video, so a couple of pictures and a few words will have to suffice. But trust me, the difference was noticeable – and startling.   

 

It seems my old wooden stands had some serious comb filtering going on. I had gotten used to it, or more properly, since I had never heard the difference I simply didn’t know what I was missing. That’s the real key to understanding why quality audio products are so necessary to the audio experience – you simply don’t know what you’ve been missing until you actually hear what you’ve been missing.

 

Regardless of the song or source I listened to, the fullness of the bass and thickness of the mids was easy to hear and hard to deny.

 

Below is a side-by-side comparison of a marginally scientific RTA (Real Time Analyzer) reading of the spectrum response during the exact same passage of Dire Strait’s So Far Away. For an absolutely clean production with tons of interplay between the different frequency regions, So Far Away is an essential test track.

KEF - Comb Filter Wooden Stands

The first picture shows the spectrum (if you’re counting, this is during the first verse, second line) using the older wooden stands. Keep in mind, I was very happy with how my speakers sounded in my room and I’d heard this song on these stands several times before. At 50, 125, 200, 250 Hz and regular intervals afterward, you’ll notice a great example of what a comb filter looks like – this example showing constructive interference. But also notice the steepness of the slope from 60 Hz downward. 

KEF - Comb Filter Metal Stand

The second picture was taken at the same place and beat. The comb filtering is gone, with a much flatter response throughout the range. The peaks at 250, ~400, ~700 and 1K Hz are most likely vocal peaks and harmonics, and appear more natural. But what’s most striking is the shallowness of the slope below 60 Hz and the overall higher response (~20dB at 60Hz with the old stands as opposed to ~26dB with the new stands). In an algorithmic scale such as audio, every 3dB change is either a halving or doubling of the amplitude, so yeah, I’ll take 26dB over 20dB at the same spot in a song any day.

 

So the conclusion here is: my LS50 Wireless sounded really good on a pair of old, cheap wooden stands, but when I added a pair of metal stands designed specifically for my speakers, what used to sound really good suddenly sounded weak and lame. The speakers I was previously very happy with suddenly exploded with a response and richness I had been completely missing.

 

I get it, you’re thinking yeah well you get paid to write that and you’re right, but that doesn’t change how much more I’m enjoying my speakers!