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What does a speaker’s ‘waveguide’ do, and why does it matter?

If you’re in the market for a new set of hi-fi speakers or studio monitors, you might have come across the term ‘waveguide’ before. You might’ve seen a photo or illustration in marketing materials pointing to a fancy trim ring or horn-looking-thing around a speaker’s tweeter, as in the header image above. Sometimes a waveguide is even directly integrated into the speaker’s cabinet design itself.

This is more than just an aesthetic frivolity. You might’ve used your big brain to surmise that a waveguide has something to do with ‘guiding’ the sound ‘waves.’ If so, congratulations — you’re a smarty pants. But why, exactly, do you want to guide sound waves?

Why would waveguides guide waves?

In general, a speaker designer will use a waveguide to improve the integration between the different drivers on a speaker — to help the sound of a tweeter better match the sound of a woofer.

As you’ve probably noticed — but perhaps never thought much about — woofers and tweeters are usually different sizes (and shapes and materials). That’s because different designs are better suited to handling certain frequency ranges, with the tweeter almost always being smaller than the woofer.

The tricky part then becomes the handoff from one driver to the other. The chosen frequency range for this handoff is called ‘crossover,’ but it’s not a perfect cutoff.

Rather, there’s a gradual transition from one driver to the other, and in a good speaker, you want this transition to be as smooth as possible. Typical crossovers for bookshelf speakers with one woofer and one tweeter are centered somewhere in the higher midrange, between 1 and 4 kHz.

Things are then further complicated by the fact that woofers and tweeters don’t just radiate sound in a single direction. It’s one thing to smooth the crossover region in the forward direction — the speaker’s ‘on-axis’ response. It’s a lot more difficult to smooth it out in all directions, as each driver will radiate sound differently depending on the frequency. Rarely does a speaker get it perfect.

This relationship between a speaker’s direct and ‘off-axis’ sound is called a speaker’s ‘directivity.’ Having smooth directivity helps create a strong soundstage and maintain even tonality when speakers are placed in a room. Which, you know, is most of them.

Directivity matters because we don’t just hear sound in a line of sight from the speakers to our ears; the sound that reflects off our walls makes a huge perceptual contribution. So in the best speakers — those with the most realistic soundstage and most even tonality — the reflected sound will be similar in character to the direct sound.

It’s also worth noting that while horizontal directivity affects both soundstage and tonality, vertical directivity mostly affects tonality. (Our ears are on the sides of our heads, after all, not on our scalp and chin. Thank god.) As such, most speakers will optimize for horizontal directivity while compromising on the vertical; even the best speakers rarely perform perfectly on both fronts.

What does good directivity look like?

A picture is worth a thousand words, so the frequency response graph below shows what a theoretical speaker with ‘good’ directivity might look like at 0º (the direct sound) and 60º off-axis horizontally (what might be reflecting off your walls).

The sound gets quieter off-axis and tilts down a bit, but you can see the line maintains the same overall shape (I drew these curves with my touchscreen so no real speaker actually looks quite this smooth).

In an actual great speaker like the Neumann KH80 (which does have a waveguide), you can see how for the most part, the response changes gradually as you move further off-axis: