What are the advantages and disadvantages of deploying subs on carts? Well, it would seem pretty obvious that having them on carts makes them much easier to move around, and that’s what carts are primarily for. But are there really significant negative consequences to having a sub on a cart when you're using it, when you're actually trying to produce sound with it?
Every pathway through which sound pressure can escape is a potential pathway for it to go away from the direction that you want it to go. So a five-inch gap underneath a subwoofer is a potential loss pathway. It is measurable, it isn't that significant, and the more cabinets you bring to bear, the less of a significance it has.
In reality, though, it really isn't down to sound pressure level at that point. It's down to workload. Do you want to be lifting and hauling subwoofers, some of them over 250 pounds? There’s no way around that. If you can roll them into a venue, lock the wheels, and use them on the cart, it has advantages that I think outweigh the disadvantages of that slight energy bleed underneath them.
Some of those advantages include not having your subwoofers sitting directly on a potentially dirty or wet floor. Another advantage that some people don't consider is that when the subwoofer is sitting on its feet, it has that relatively direct coupling to the floor, which is not a bad thing. But if its mass isn't sufficient—like if you don’t have another one on top of it—it’s not really heavy enough. Then the movement of the cone, and the equal and opposite reaction that the cone’s movement has on the box, will cause it to walk or shift slightly depending on the floor’s angle or where the holes are. This happens less with a box on a cart because that slight give lets the wheels stay in contact with the ground, allowing the cabinet to shift ever so slightly.
Yes, you bleed a little bit of energy there, but the boxes stay where you left them, and generally speaking, they’re cleaner and easier to move back out of the venue. From my point of view, I don’t mind deploying boxes on a cart. In a purist world, you could go to the trouble of taking them off the cart. If you are going to do that, you should also take a big cargo strap, put it underneath however many subs you're going to stack together, loop the strap over, and lash them all together so that they are all coupled as one mass. If you’re going to do that, you could do it with them on the cart.
You still have that mass and that tiny little energy bleed path, but you’ve addressed most of the advantages of taking them off the cart. Visually, if you want them on the ground and you don’t want the visual of the cart and are willing to do the extra lifting, there are those advantages. Yes, they’re going to get more detritus on them, but that will wipe off.
Some people will be concerned that having your subs on a cart will make them less loud due to this energy bleed. The challenge is that we’re not really that sensitive to hearing that difference. If you did a blind test where you were listening to a sub on a cart versus not on a cart, you’d be hard-pressed to hear the difference. You can measure it, but the larger the number of subs, the more directivity you get, and the less of a difference it makes. Also, where you place the microphone matters.
If the box is on the cart and the microphone is on the ground, the added distance from the acoustical source to the microphone, plus that energy bleed gap, will make it measure slightly louder when the box is not on the cart, but it’s on its feet. The feet give you three-quarters of an inch of elevation, while the cart provides about five inches.
Yes, that’s a significant difference in terms of gap, but if you move the microphone up to where the distance is the same, you’ll have a hard time measuring a significant difference. It’s a fraction of a decibel, and the audience generally isn’t going to notice.
Is it going to change the frequency response? Negligibly. It’s pretty much so many environmental factors will influence the difference from one venue to another that you won’t have a significant acoustic cost to having it on the cart compared to the workload and potential damage from lifting and moving. There are just so many other reasons why it might not be worth taking them off the cart. Everyone has their priorities—visual, aesthetic, or absolute performance. Personally, I’d rather have slightly more than I need and not be overly concerned with the absolute last tiny bit of performance from putting the box on the floor versus having it on the cart.