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Best Lightweight (Under 150g) Trekking T-Shirt

Rab's Sonic Short Sleeve Zip Tee

Rab's Sonic SS Zip Tee


As always, we're looking at the Rab Sonic from the point of view of long distance trekking over tough terrain.

Test subject: Chest 42", Waist 33", Height: 5ft 8"
Test item: 2020 version, size = Large
Kit Tests: Summer
Disclaimer: None required (item not provided by manufacturer)


Materials: Motiv™ fabric (Polyester) 100%
Treatments: Polygiene Stays Fresh odour control -
Weight (Size Large) 94g
Product Sizing Reference: 42" Chest = Large (tighter) to XL (loose fit)
Manufacturer RRP £45.00

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Scramble Review



Introduction: I'll Tell You What We Want, What We Really Really Want

Back in June 2020 we wrote

For some reason, the outdoor industry seems to have forgotten how to make a decent half-zip short-sleeve t-shirt. Montane, Rab and OMM all have offerings, none of them impress us (either too flimsy for hauling backpacks or weirdly cut for wiry ultra-marathon runners). So we're still on the look out.

And to some degree we still are - our ideal zip tee (pic #4, below) remains unmade. The Rab Sonic was on our radar but we felt it was potentially too ultralight for our purposes.  However, bereft of any genuine alternatives we decided to give it a go and, though there are better tops out there (more on this in a moment), and our previous recommendation remains superior (though long discontinued by Ronhill),  the Rab Sonic impressed us enough to warrant a recommendation.

How long it will remain at the top spot is another matter. The real question is, since plenty of manufacturers are making them, why is it such a puzzle to combine just a few specific elements into one garment? If designers were using a random t-shirt designing machine, we're pretty sure they would have hit upon our simple formula long ago. So, are we really asking for something so niche? Are we really so obscure, so different? I don't think so.

So, What Extraordinary Demands Are We Making?

As we outlined in our review (precised below) of the Ronhill Advance:

When you're lugging a heavy pack up a mountain in 30+ degree heat, you'd rather have nothing on, but you need some protection against the sun and your pack, so the next best thing is a shirt with a deep zip to vent; [...] a reasonably high collar for some sun protection, but also to shield the neck against pack straps etc. [In addition] we want a fast wicking, quick drying fabric, a reasonably loose fit that feels comfortable over a baselayer and we want it to be light and reasonably hard-wearing.

More succinctly and precisely, we want an airy, 3/4 zip, loose-fitting, lightweight t-shirt mainly made from polyester; a t-shirt that knows it's a t-shirt (isn't close fitting or tapered) and isn't trying to hedge its bets by pretending it's a baselayer (lightweight, non-winter baselayers should be skin-tight and made from a mix of polyamide, polypropylene and elastane, NOT 100% polyester!).

No need to re-invent the wheel, as we can see below. 

The unwritten law of zip rationing  (right click, view image for full size)

The question is, why are cyclists (pic #1, and triathletes for that matter) allowed full-length zips? Why are gridded fleece baselayer-wearing Alpinists (#3) allowed 3/4 zips? Yet us mountain trekking outdoor types are restricted to zips of a lesser length (#2). Are long zips on t-shirts dangerous for people like us? Did I miss a meeting?

It's funny, because quite clearly Mountain Equipment acknowledge in the design of their Eclipse (pic #3 above) that in sub-zero conditions a hard-working mountaineer may need to vent quite a lot. So why can't design teams' imaginations stretch to a poor soul carrying a heavy load up an arid, rocky mountain in sweltering conditions? They might envision the climber's relief as he unzips to the waist and a rush of cooling air billows the designer's airy polyester masterpiece.

What we want is highlighted in green (pic #4 above). Something like Montura's (polyester-based) Vertical Block zip top with an extended 3/4 zip and priced (full RRP) around £50 (with the hope you'd be able to pick it up at a bargain for £25 or £35 when shopping in penny-pinch mode). If it weren't for its lavish (75 Euro) price-tag, the Vertical Block would be our top pick.

So with all that context out of the way, let's see how Rab's Sonic stacks up.


Sizing & Fit:  Hedging Their Bets

Rab file the Sonic under their baselayer section. Here's their take:

Whether used as a fast-wicking first layer or a technical standalone mountain running top, our Sonic SS Zip is the perfect tee for athletes seeking performance.

It's interesting that Rab veer away from the term "baselayer" and instead refer to it euphemistically as a "fast-wicking first layer".  Let's be clear, it's not a baselayer, it's a zip tee, and this identity crisis seems to have influenced its "hybrid" cut and sizing.

Generally with Rab, a size large is either an ideal fit for me or it's a little larger than ideal (but still do-able).  Rab seem to have designed their Sonic to please a few different groups whose paths probably don't cross that often; the Sonic is what I'd call semi-fitted. Runners may like it but long distance trekking warrants a looser fit in our view.

Sizing up for the Sonic may not be a bad idea, especially if you're close to the upper limit of their sizing range (I'm a 42" chest, Rab's upper limit for size large is 43", yet for the Sonic I'd be okay sizing up to XL). That said, the Sonic (L) provides enough room to slip a Floating Pocket map bag down its front.

As you can see below (especially pic #2), the Sonic seems to taper down to the waist.

The Rab Sonic's cut falls somewhere between a baselayer and a t-shirtRab's Sonic Short Sleeve Zip has a semi-fitted slightly tapered cut.

Rab make a similar product called the Pulse. Perhaps they could have made the Pulse their tight-fitting baselayer (and made it from more appropriate baselayer fabrics rather than lazily re-use their Motiv as employed on the Sonic) and then made the Sonic their loose, boxy fitting ultralight t-shirt. In our view that would have made much more sense.


There's not a great deal going on with the Sonic, a slightly raised collar and a decent zip. There are no venting panels as none are required; the entire shirt is a venting panel and breathes extremely well. On my version, the two-tone fabric is purely aesthetic, the material is the same throughout.

Rab's Sonic from front to backOn the surface zip tops all look alike, yet it's a real struggle to find a good one.

Before we look at the fabric, let's briefly return to zips. The Sonic features one of the deeper zips currently available (a YKK coil half- zip that drops down to the sternum), but how does it compare to the two t-shirts we've mentioned in this review:  Montura's Vertical Block (below centre) and our previous recommendation (below left), Ronhill's Advance?

How the Sonic's zip compares to the Ronhill and MonturaHow the Rab Sonic's zip stacks up against the best (past and present).

As you can see, compared to the old Ronhill Advance (which was an inexpensive item), both the Montura and Rab pieces fall short in the venting department, with Rab's Sonic faring slightly worse than Montura's classic (looser cut*, slightly heavier) design.

* Note: Montura tend to size small, so a standard UK L, would be XL for Montura.


The Sonic is made from Rab's proprietary Motiv fabric. A polyester jersey that's extremely light and breathable. There's no issues in terms of comfort and it doesn't feel like you're really wearing anything much at all.

The seems are interesting. They're strong, densely stitched yet soft and low profile enough for you not to notice them. Rab have done a nice job here and again, though we've been a little tough on the Sonic, it's very comfortable and in use there's not really much to complain about.  

A close-up view of the Sonic's seams and single knit Motiv fabricA close-up view of the Sonic's seams and single knit Motiv fabric.

Rab use a textured jersey made from a single knit polyester and because it's not woven it has an inherent stretch and thus doesn't require any elastane (which would add weight). The Sonic uses a star-shaped yarn (rather than circular) which increases its surface area, aiding moisture absorption and speeding drying time. The Sonic dries out incredibly quickly.

Held up to the light, you can see linear arrays of micro-pores (the light blue squares pictured above) that cover the entire top. Clearly Rab have set out to make an ultra-light and ultra-breathable top, and they've succeeded. The question is at what cost?  There are nearly always trade-offs and it's hard to imagine that such a light top will be as durable as one that weighs 40% more.  

So far we've not seen any wear issues; but there is an important caveat here that should be noted:

For a few years now,we've mentioned that "summer" kit tests have reoriented to testing "lightweight gear in heavy-weight conditions"; we've not had a scorcher (30℃+) since, I think, 2018.

For the first 10 days of the 2021 kit test, which begun in mid-August, it was either raining or threatening to do so. It was unseasonably cold for mid-August (with a max temperature of 20℃ often chilled by a cold wind). Here's a few pics of a modern Welsh summer (don't tell Greta, apparently we're burning up).

Unseasonably Cold Conditions during the 2021 Summer Kit TestHardly ideal conditions to test an ultralight tee designed for a heatwave.

As such, my Sonic has spent much of its time cowering under a lightweight softshell or hardshell or both. So direct wear from my pack will have been substantially reduced. 

If we see any warning signs we'll update this review, but to date none of us has seen anything that would raise the alarm.

Any Negatives?

Due to the way we rate products, with the RRP being factored in to the value score, the Rab Sonic gets the highest rating for what's currently available. It's a fine top and has a number of very positive attributes. But there are some gripes and concerns. 

Firstly, at just 94g it's a little lighter than we'd like (something we don't often say). We have concerns about long term durability (reflected in its rating score) when used with a heavy pack over multiple days and weeks. The current trend seems to target runners (and those who "move fast in the mountains") over distance trekkers who move fast in the mountains but only downhill. Every year seems to yield ever lighter tops, as though anyone will notice wearing 10g less. Weight savings are useful for kit you have to carry; less so for stuff you wear on your person.

Secondly, the collar could be marginally higher. Compared to the aforementioned Ronhill and Montura tops, the Sonic's collar just about makes an appearance. It's the kind of collar you'd come up with if you weren't sure whether you were making a t-shirt or a baselayer.

Likewise, the slightly tapered cut is odd.  It's not really a runner's cut, it looks more cut for an athletic, muscular squaddie. Nothing wrong with that, but again the slimmer fit hints at the baselayer hedging  confusion that runs through our minor discontent with what is still a more than capable and comfortable shirt. 

Finally, did I mention ... we'd like a deeper zip.


Conclusion & Rating

Though this review may come across as rather critical, that criticism is really directed at the outdoor industry as a whole. My guess is that they're so terrified of a product flopping that they simply copy whatever seems to sell and apply that formula to their own proprietary fabrics. Thus you end up with a range of products all uniquely similar.

Rab's Sonic is simply an ultra-breathable, ultra-light zipped tee that self-identifies as a baselayer but has polyester chromosomes and no matter how much it thinks it's a baselayer its fabric will always give it away.

The Sonic is extremely comfortable, has durable, low profile seams, soaks up moisture well and dries rapidly. It packs away small and would make an ideal backup / spare for very long trips and won't weigh you down should you choose to carry it and wear a genuine baselayer instead.

We have some reservations about long-term durability, but that's always the case when going ultralight and at under 100g the Sonic is truly in the UL class.

So, until someone delivers on a reasonably priced Vertical Block with a 3/4 length zip or Ronhill's Advance rises from the ashes, Rab's Sonic is our current top pick in the Lightweight Trekking T-Shirt category.


Product Images


Rating (out of 10)

RRP Value *

* The value score is derived from two factors:
1) Competitive Market Price (CMP). This represents our judgement of a competitive online price point if we were to stock the item. e.g. if we feel we would need to sell an item at 40% off (i.e. 60% of its full RRP) to be competitive, then our CMP score will be 6/10.
2) Customer Value Price (CVP). We then make an honest appraisal of the maximum price we would be willing to pay for the item (and we're mean). So if we'd pay 80% of its RRP our CVP score would be 8/10.
We then average the two scores to get our final value score, which in our example would be 7/10.


Last Updated: 09/03/22

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