There are two basic ways to keep down noise (exclusive of baffle hiss): (a) Run a longish duct with a silencer mounted between the hood and an external or in-line blower, or (b) use an oversized blower operated at partial power to get the desired volumetric flow rate using lowered blade tip turbulence. Without getting into make-up air issues (but please search them out here), an oversized blower means either a residential grade blower having a higher flow rate than you need, or a commercial blower with the motor to fan "gearing" set to a lower than normally used fan RPM for the designed motor RPM.

For example and convenience, assume a hood with a 10 sq. ft. inlet aperture. The intake air velocity goal for good containment is 90 ft/min so the achieved flow rate should be 900 CFM. Typically, this would call for a 1350 CFM rated blower. Depending on hood baffle pressure loss and "goodness" of MUA, a straight shot out the wall might allow 900 CFM to be achieved with a 1200 CFM rated blower. If a 1500 CFM blower is used at partial power, then it will be a bit quieter, but something in the rated 2000 CFM range operated to yield an actual 900 CFM would likely be desirable for significant noise mitigation.

Because such a blower would tend to be large, it may be more practical to duct out the wall, make a turn, and go up inside a chase wherein a silencer could be mounted, connected to a residential blower or small up-blast or down-blast commercial blower at the chase top.

Otherwise, interior ducting to an attic and thence the roof would be needed, with the silencer in the attic. This may be architecturally difficult or impossible depending on residence design.

Either exterior or in-line blowers may be used where appropriate. Blowers need to have a motor type controllable by the hood's motor control circuit.* In-line blowers such as Fantech manufactures are axial type with radial blades, I believe, while Wolf/Broan/Blue Star (Abbaka) "down-roof" blowers are centrifugal style blades mounted on a disk. Some in-line blowers are reconfigured roof type and not axial.

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*Users being in electrical related professions may be able to design an appropriate interface for motors mis-matched to the controls.

What are the relevant differences between external and in line or internal blower? Pros/cons given this kind of setup where we’re venting directly outside the range hood?

I guess noise level is all relative - previously we had a 1200 CFM thermador that was loud but we were fine with it on high setting when we needed it since it wasn’t all the time. The blower I think was mounted on the outside of house right behind the range hood. Can I assume that if I got a similar setup for our new one (blower on exterior of kitchen wall), the sound level will be the same? Not looking to spend a fortune and looking for something relatively simple to install off that back wall, as opposed to out the the roof.

I personally don’t much care if the fan is noisy at maximum hood CFM, since in a residential setting you don’t tend to run max CFM for a long time.

However, I unfortunately got a fan that is still noisy at lower CFM. There is a distinct humming even at low rpm. I have not yet looked into whether I can get a different controller to reduce that noise. You probably won’t get the same fan as I did, but just a mistake to make sure you avoid.

(I have a CaptiveAire DU-HFA 33. It is powerful but not refined.)

A blower in the hood will be louder than one outside (for equal air flow rate), but maybe only by a tad. One has more options in an outside blower than the one or few that can fit in the hood for which the manufacturer has established the correct interfaces. I don't think an in-line blower can be fitted in your application.

I have no way of knowing the relative sound levels of the blower you had re the blower you are thinking of buying. However, I believe Blue Star bought Abbaka, and comments Abbaka wrote w.r.t. their roof blowers in their literature long ago suggested to me that they were aware of important considerations. So an Abbaka roof blower mounted on the wall might be quieter than your legacy Thermador.

I can mount a roof blower to the wall?

So when looking at other brands or generally, I can’t get ones that come with blowers inside the unit, but need to buy a blower separately? Is that usually how it works when doing an outside blower setup?

Does it make a difference if I’m getting an insert?

"Down-roof" blowers can usually be mounted to a wall, whereby they become down-wall blowers. There are limits to what walls can be used, e.g., dumping cooking effluent onto another window, or onto a small deck used for sitting would be unappreciated by someone. Here is some info for a Abbaka one copied from one of their historical blurbs:

The Wolf, and the Abbaka have built-in dampers. You may not seem to need one at the hood, but it might be useful for minimizing cold transport in winter and general wind leakage.

Generally, most hood designs, including inserts (also called liners), are built to accept (mount) internal blowers, as well as electrically interface with external blowers. Wolf's are, as far as I know. You will have to check with the manufacturer of whatever you choose for a hood/insert. So if using an outside blower, then one orders the hood without an internal blower. I think Wolf sells hoods and blowers separately anyway. I don't know about Blue Star.

An insert may be viewed as a hood without a fancy, shiny external skin. They are unfortunately undersized w.r.t. overlapping the cooktop dimensions by integer inches due to having to fit in standard cabinet sizes. Wolf makes a 58-inch liner (58-3/8) that should provide enough width overlap for a 48-inch cooktop. The 22-5/8 deep one will be best.

Inserts are put into cabinets, and there may be many details that must be followed. You would be wise to obtain the installation guides for whatever candidate inserts you have in mind.

My appliance guy has an XO brand insert and a Best brand insert. The XO comes with a blower and the Best one must be purchased separately. We also found a Zephyr that may fit the bill and comes with a blower. Im sure he has more expensive options as well but trying to keep cost down. Any opinions on these?

Our old thermador had no duct - the hood attached right to the wall in the inside, and somehow married to the blower which was attached to the outside wall. Some of these we are looking at now seem to require a duct of some sort to get out the wall and I’m not getting why, since there’s no where really to travel as the hood is right on the exterior wall.

Related question - zephyr had a makeup air unit that we can install somewhere; again not sure why it needs a duct, and it seems too deep to simply install on an exterior wall. Is there a way to have a completely passive air intake system (no pressure sensor or damper motor), with a louvered vent similar to that on the outside of a bathroom or dryer vent, that is mounted on the inside of the home and opens super easily with slight negative pressure?

I can only opine on equipment I have owned, unless there is an obvious design deficiency. Too little information.

You need a duct between the hood/insert housing and the blower. It will be short. However, you haven't clarified whether it goes up and out from the hood housing, or has to go horizontally from the hood housing. Hood/liner port position has to support whatever you have.

And the duct diameter should support the flow rate and exterior blower interface. With a mere 4 to 6 inch wall, this is not too critical, but desirable diameter is that for which full flow rate causes a duct velocity between 1000 and 2000 ft/min.

As Broan/NuTone makes Wolf blowers (or at least mine), if you are going to have the desired flow rate you might consider the Broan/NuTone down-roof 1500 CFM or the smaller model (forget the flow rate) to achieve the flow rate you can calculate using my example and your actual hood entry aperture.

MUA must match hood flow. This means that a rather large hole is needed for passive MUA. Otherwise, an active (blown) MUA is needed. See Fantech for an example. I think Broan also has some activity in active MUA.

Do not assume that you can provide adequate ventilation for a pro style cooktop without a pro scale hood system, including MUA system.

MUA must be sufficient to keep house pressure difference from outside below that which causes back-drafting of any combustion appliances. This whole topic is addressed in various threads here, please search them out. If there are no relevant combustion appliances, then the issues are achieved hood flow vs. MUA constriction and whether code enforcement will be in play.

So I’ve been researching for a hood liner/insert that has an external/remote blower and allows for direct rear horizontal venting (straight out, as opposed to up and out with a 90 degree turn), and the only option I found is the Blue Star Professional, https://www.bluestarcooking.com/ventilation/liners/46-professional-metal-liner/

Anyone have this one? Or does anyone know of other models that fit these install requirements?

We were trying to find an external blower to help with noise. But if the blower is directly outside the vent hood, on that same wall but just in the exterior, is it really going to be quieter than an internal blower? If we do internal, then it opens up the Zephyr options.

I think I addressed the noise issue, so I won't repeat myself. If you use the BlueStar recommended maximum external blower (1400 CFM Abbaka HYEX design) and generally run it at lower than full power, which will support all but the smokiest conditions, noise should not interfere with conversation.

Note that the unit has push-buttons for 3 speeds (plus boost) and not continuous variability, so flow rates and potential resulting noise levels would be guesses. In any case, adjustment for conversation level would seem to be limited.

For a 48-inch range, the 58-inch liner will have better capture. (A 52-inch liner is listed, but not included in the dimension drawings, so its status is unclear.) https://www.bluestarcooking.com/ventilation/liners/58-professional-metal-liner/

I will repeat myself (w.r.t. other threads) in one respect: At a minimum, the MUA plan must precede the hood system purchase.

Last for this message, I only see flanges for the sides of the liner. I'm unclear what they expect of a liner put into a wood cabinet, if that is your plan. Wood facing the burners should be covered with stainless steel, so some local sheet metal shop action may be needed.

Thanks. Still unsure about noise but will wing it.

Looks like this blue star insert doesn’t come with a damper. Do I have enough space to put a damper in if I’m venting straight out the back? If not and I have to vent up and out with a 90 elbow, how can I determine how much height clearance over the insert I need for that ductwork?

See Fantech's site for examples of dampers. If the blower is outside, and is of the HYEX type from BlueStar, a damper is built-in.

Oh good to know! Their manual says it does not include a damper and the hyex manual on the Bluestar site is very sparse so thank you for this info!
We maybe able to fit a 90 elbow above our hood after all so hopefully that will work.