I’ve covered the machlokes Rashi and Tos on the shiur of tekiyos before, but want to revisit it because of an interesting Chayei Adam I noticed this year. The Mishna tells us that the length of a teruah is 3 yebavos. Rashi understands this to mean that the total length of a teruah is three very short tu sounds = 3 beats. Tosfos holds that the length of a teruah is three blasts of 3 yebavos = 9 beats.
The gemara writes that the length of a tekiya = that of a teruah. Therefore, according to Rashi a teruah must be at least 3 beats long; according to Tosfos it has to be 9 beats.
Tosfos points out that according to Rashi, each of the shevarim has to be less than 3 beats long. If not, the shever turns into a tekiya. Therefore, according to Rashi you have to blow 3 shevarim of no more than 2 beats each = 6 beats in total. According to Tosfos, you would not be not yotzei with this, since the minimum lenth of a tekiya, a teruah, or the shevarim = 9 beats.
R’ Soloveitchik came up with a solution to be yotzei both deyos: blow 5 shevarim of 2 beats each. Each shever will be less than 3 beats, fulfilling Rashi’s view, but the total will be 10 beats, fulfilling Tosfos’ requirement.
The minhag is to follow Tosfos’ view, but it’s still important to be aware of shitas Rashi because of a chiddush the Chayei Adam (141:12) quotes l’halacha which some poskim (e.g. R’ Elyashiv) take seriously. The halacha (in a nutshell) is that making a mistake blowing a totally pasul non-kol works out better than making a mistake blowing the wrong kol. If the ba’al tokea, for example, blows a tekiya and then blows a sound that is totally pasul, he can just blow the correct note and move on. However, if the ba’al tokea is supposed to blow teruah but instead starts to blow shevarim, even if he realizes his mistake and stops, he already blew it (sorry, I could not resist) and has to start again from the initial tekiya.
Says the Chayei Adam: since according to Rashi a one beat sound (not a 3 beat sound like Tos holds) is the first note of teruah, even a small toot of the shofar done in error is in reality a partial teruah. It’s a valid kol done in the wrong place, not just a bad pasul non-kol. Therefore, to correct the mistake the ba’al tokeah has to go back to the start of the set and blow the tekiya again.
You could argue (as other poskim do) that a kol blown without any intention to be a teruah is not a teruah at all – it’s just a mistake. The Chayei Adam, however, disagrees.