For subwoofers, how should room size and pressurization be calculated in an open living area?

[chris-lukowski]

Now that I’ve upgraded my 5.1 setup to a full 7.1.4 Atmos configuration, next up is replacing my old Polk PSW505 subwoofer. One concept I’m REALLY struggling with when it comes to subwoofers are the notions of room size and pressurization. Almost every subwoofer has a recommended room size, and in the case of the 1723 1S it recommends a room “up to 30m²” (or 322 ft²). For an open floor plan, how do you define a “room”? In my attached example, I have a 11x21ft (231 ft²)living room on the left that branches out into a dinette area on the top right and an undoored hallway on the bottom right. This combined area is about 22x21ft (462 ft²). Both these areas have standard undoored entries into an 11x21ft kitchen (to the right, unpictured), making the overall first floor footprint 33×21 (693 ft²). ALSO, at the bottom of the floor plan you see there is an undoored staircase leading up to the second floor. Finally there is also a doorway to the basement in the hallway that is always cracked open a bit so the cats can get in and out. If a “room” for this purpose is rigidly defined as a near-airtight area sealed off by doors then my entire townhouse could be considered a single room. Am I thinking about this right?

That particular calculation aside, I need help to understand why room size matters when it comes to subwoofers and what the phrase “pressurizing a room” really means. So let’s say I listen to movies at 65dB. The LFE track by design is 10dB higher which baselines it at 75db. I then throw on a house curve of +10 which brings the LFE baseline to 85dB which needs to burst +20 to 105dB peaks for full fidelity. Is this correct so far?

If so, why does it matter what size the room is and how is the Arendal recommended room size calculated? The CEA 2010 specs say the 1723 1S can do this at 2m down to 25Hz in what I assume is an open-field environment with no nearby walls. Should the CEA 2010 specs be considered the worst case scenario, and that in smaller rooms the 1S would play louder than the CEA specs due to room gain which is required if listening at 80dB reference level which generates 120dB LFE peaks (assuming my prior calculation is correct)? If that’s right, then does the recommended room size matter less if I don’t listen to maximum volume (or Reference Level 80dB, or whichever dB is being considered here)?

OR does room size and pressurization pertain to something else besides CEA 2010 measured dB output? For example, does it mean the ability to get my target 105dB peaks evenly across the entire room vs just a designated listening area? If that’s the case then maybe “pressurizing the whole room” isn’t important if I don’t have a dedicated multi-row theater room.

Another example would be “Does a 40Hz LFE effect @ 105dB feel different in a 20m² room that’s pressurized vs a 60m² room that isn’t?”, or would the correct way of thinking about this is “Pressurizing a room means simply attaining 105dB for a 40Hz effect, and saying a room is not pressurized means it’s just not possible to get 105dB in a room that size without larger equipment or more subs.”?

Finally, how does adding a second subwoofer impact these considerations? If I go the dual-sub route and place them at the front wall about 5ft from each other, would that 30m² single-sub specification then cover 45-60m²?

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[Kristoffer Svendsen]

Hello Chris, and thank you for the great question!

The subwoofer basically “sees” the entire open space, as the bass frequencies are so physically long. That means they’ll bounce around the room multiple times before they dissipate. The uncovered staircase will “break up” the bass a little, so I would only consider the main floor as the room the subwoofer “sees”. Some sound will still travel upstairs, but it’s basically impossible to predict.

When we talk about pressurization, we’re basically talking about SPL (sound pressure level), and the subwoofer’s ability to fill the room with sound and excite the air in the room. A larger room will have more air, which means a smaller subwoofer will have a more difficult job moving that air compared to a larger one. That’s why our dual-driver subwoofers are recommended for larger rooms, as they can achieve the SPL with less excursion, which in turns means less distortion, especially at louder volumes.

Even though the 1723 subwoofer 1s might be able to achieve the levels you want on paper, it would be quite close to the limits of the subwoofer. The 1v, which is larger and has a port, will be better at filling the room with bass, especially below 40Hz, where the port is especially effective. The 2v, with an additional driver and a more powerful amp will be even better.The CEA2010 measurements are made in an anechoic environment, so in-room response will most likely be a little better.

Your calculations are not wrong, but you must also remember that if you use a crossover for your speakers, you might be looking at considerably more output being redirected to the subwoofer. The LFE track is 105dB by itself, but if all the speakers are also being fed that same low-frequency signal that’s then being redirected to the sub, it would need to be able to handle considerably more. Let’s say in an action scene with explosions: Sure, the LFE track is one part, but there will still be lower frequencies routed to the front and surround speakers, that is then crossed over to the sub. That’s why it’s always nice to have a subwoofer with additional headroom, especially if you like to listen loud.

Multiple subs will give you some additional output, but that’s not the main advantage: The main reason for going dual, triple or even quad subs, is to get a more even frequency response. All rooms have room modes that will either amplify or cancel out certain frequencies. By having multiple subs, you target different room modes, which in turn will provide a smoother frequency response as one sub can “cover” where the other might hit a null. That means it’s very difficult to get “perfect” response with a single sub, even in a single location. Having a larger room will definitely help in this regard, but then you run into the whole sub size vs. room size issue again.

Smaller rooms tend to have more difficulties with room modes, and having too large subs in these rooms can make them sound overpowering. Placing a large sub in a small room is also more difficult than placing a large sub in a large room. Placement is key when it comes to subwoofers, regardless of room size, so this is definitely a step that should be considered closely.

This has become quite long, so I think I’ll cap it here. Please let me know if I missed something, and I’d love to try and answer!

[chris-lukowski]

Thanks Kristoffer for all the helpful information! It sounds to me like room gain (or the lack of it in a large space) is playing an issue here even if the seating distance is identical, correct?

It sounds like a single 1723 1S isn’t going to cut it for this area. Do you feel the same about dual 1S’s or even a 2S (single or dual)? It sounds like a vented option is all but required here. If so, leaving issues of seat-to-seat consistency and room mode nullification aside, would there be much audible difference between a single 1723 1V or dual 1V’s positioned on either side of the TV about 2 meters apart?

[Jules Jeanselme]

Hello Chris,

Dual subwoofers will be better for different reasons. It will allow you to have more “better seats” and will pressurize the room better, with less distortion. So louder, cleaner, with more good seats.

Dual 1S could work, and they are pretty small so easy to place, which is a great thing as placement is key and you will want to try as many spots as possible to find the best ones. But as Kris mentioned, the 1V sounds like a better option in your case.

If you leave the seat-to-seat consistency and room modes aside, there will still be differences as the subwoofers will need less power hence less distortion to get the same bass output when you have 2 compared to 1. Would it be audible? Probably, depending on your room.
Finally, from a psychoacoustic point of view, some people can “feel” where the bass is coming from and will enjoy 2 subwoofers to reduce this effect. We’re not equal to this and some people won’t hear a difference with 2 subs versus one. So would it be audible? No idea, it depends on your ears and brain.

I would recommend starting with 1×1723 1V and see if you feel like you’re missing anything.