This *may* be caused by the hatchback. I'm not sure if you are complaining about the same thing as other owners have noticed (a low frequency pressure buffeting feeling/sound over bumps). This can be at least partially fixed (some have had better success) by adjusting the hatchback resting points (large quarter sized adjustable pylons where the hatch contacts on closing) to make a tighter fit.
You could always pull the door and trim panels off to see just how much material is/isn't in there for sound control... But yes the absence of engine noise will really exacerbate other noises that you didn't notice before. Quieting a car requires a little more science than most people would think, especially considering that the similarly important goal of weight control will negate most brute force options (just adding lots of dampening material).
I believe its a drawback of the hatch form factor. There's the additional space for resonance. But you also essentially have the rear suspension sitting in the cabin. In a sedan you have noise from the suspension resonating in the trunk but that is isolated from the cabin. Not so in a hatch or wagon. I wonder how the Model S stacks up compared to the A7 or Panamera in this regard.
In the NVH world, we use the phrase "when you drain the swamp, you start to see the stumps". Anyway, I'm sure Tesla has minimized the sound abatement package in the car in order to maximize the battery range. Electric cars present a whole new set of challenges for body NVH engineers .... I'm sure through experience manufacturers will learn how to better minimize cabin noise in electric vehicles, without adding excessive weight.
I know someone who was part of the Volt engineering team (on the drivetrain side). He said they spent a lot of effort to reduce noise for exactly the reason wdrauch points out. The result is interesting - the car is very quiet despite not having much sound insulation. On the highway, external noise (passing a truck) often dominates. If noone is around, it's weird how quiet 90mph is and the effect is quite different from, say, a heavily insulated ES350.
Is/was the cargo cover in place? Although I don't own a Tesla, owners have commented the cargo cover helps buffer the boom in addition to hiding "cargo".
The hatch contributes to the tendency toward boominess because it makes a big cabin volume that much bigger. With one person rattling around alone it's a bit like a new house with wood floors and no furniture. Add a couch and some throw rugs and it quiets down. Along the same lines, the Model S seems less boomy with three or four persons aboard and some luggage. And yes, the cargo cover would help a smidgen by reducing the volume. It wasn't in place when I drove it recently.
Eh I really don't think that the hatch is contributing to the increased noise as much as it is being credited for. Anyone sit in a luxury wagon or SUV lately? An Escalade ESV, or even to Suburban to a lesser extent, dwarfs the Tesla when it comes to interior volume, and I wouldn't describe either as "boomy", especially not the Escalade. In other words, the Tesla doesn't have this loud resonance because a hatch was stuck on it, it is loud because of one of either two things. First, it could be because of inadequate effort towards sound insulation. Or secondly, it could just be because the lack of engine noise finally brings such things to our attention. Actually its probably a combination of both.
Eh I really don't think that the hatch is contributing to the increased noise as much as it is being credited for. Anyone sit in a luxury wagon or SUV lately? An Escalade ESV, or even to Suburban to a lesser extent, dwarfs the Tesla when it comes to interior volume, and I wouldn't describe either as "boomy", especially not the Escalade. In other words, the Tesla doesn't have this loud resonance because a hatch was stuck on it, it is loud because of one of either two things. First, it could be because of inadequate effort towards sound insulation. Or secondly, it could just be because the lack of engine noise finally brings such things to our attention. Actually its probably a combination of both.
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