Are all subwoofers equally fast?

Some folks see things better as pictures... so let's look at the rise/fall rates of low vs. high frequencies.

I plotted 10, 20 and 50 Hz sine waves along side of a 1KHz 5-cycle burst.

As you can see, by the time the 1KHz burst ends, only the 50 Hz wave has even reached max amplitude once. Which one will sound "faster?"

Hope the visual helps,
Frank

PS the axis is in units of 2pi/frequency, so the first 10Hz zero-crossing is an pi/10, or 0.314159.

There are pretty big differences in sound quality between drivers and subwoofer type/design. In theory for example ported subwoofers can be made so good so that the transient response difference between that and sealed is not perceivable to the human ear. BUT how many manufacturers have the time, knowledge etc. to implement it like this? Also most of the time enclosure size is made smaller than it needs to be for WAF reasons. In reality I've only heard one or maybe two really good ported subs where I couldn't really tell it was a ported design. But these are few and far between.

And then there is the driver itself which does make a difference (at least to my ears) to the perceived "fastness" but it seems like an outdated term that is too closely related to myths etc. Low(~.30) Qts/Qes seems to be factor here in my tests but I'm no expert. I think it's safe to say at least that different drivers sound differently and certain drivers just sound better in every respect. I usually trust drivers from Scan Speak and Acoustic Elegance.

Edit: Also I'd like to add that even with the best driver you need an acoustically treated room, multi-sub + something like Dirac. This WILL make the bass sound really clean. I've tested a few different drivers with Dirac in a treated room and even cheap drivers can sound a lot better than you think but in the end the more expensive stuff just pulls way ahead under these conditions(treated room + Dirac). When the room is not in the way they have room to shine so to say where you find the limits of the cheaper stuff . A bad driver in an excellent room/room correction can sound better than a excellent driver in a bad room. It makes it really worth it to invest extra in the speakers if you already have a good room.

philipbtz said:

One can have a look at the T/S parameters of really good subs like LMS Ultra, AE etc. and compare those to the cheaper stuff and there are some trends. A lot of people only look at Xmax but imo xmax is way overrated. Find a driver you think sounds excellent and then just buy many of those. My thought is basically if you don't see the drivers moving when you play at reference level then there is probably minimal distortion

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It all depends on what you want, what frequency range you desire, what SPL at what frequency and acceptable distortion levels. Data-bass will hook you up! The Xmax thing tends to come from car audio, very important when you have a very limited space so more Xmax and power tends to work well when space/location is constricted.

For me, I have had a few commercial subs--but tend to build my own to fit my ever changing needs and room response. The problem with consumer/off the shelf subwoofers is you need to sell a lot of them to stay in business, think about it! The huge issues (in consumer reality) is size and looks--performance takes a back seat. The larger/heavier the subwoofer is for the manufacturer, the more expensive it is to build in material costs/finishing costs, higher labor to build the big thing, higher scrap rate as they get bigger, higher storage/warehousing costs and ever increasing shipping costs. Big is very expensive! Big also will kill off sales so I don't blame manufacturers for trying to provide what the consumer wants--as small as possible.

To get small is not a mystery, very easy to do as thousands of teenagers build very small subs for their cars every year and do a decent job. Grab a high Xmax woofer with a big voice coil, slap on a very powerful amp to get the woofer to reach Xmax and slap it in the smallest box you can. EQ to make it flat(ish) so the spec sheet can legally lie to the consumer. Max SPL 115dB and frequency response of 16 to 160 Hz and a THOUSAND WATTS (peak)---see it all the time. Granted, it is just numbers so you can bet that max SPL is probably around 50 Hz or so, no way you'll get 115dB at 16 Hz but as long as you don't give full specifications, all is well. What sells is size, finish, how many watts and how large is the subwoofer--capitalism will provide.

Sealed is preferred (for the most part) in car audio because of massive cabin gain natural in cars, the very small space allowed for subwoofers and weight is your enemy in cars. This "bleeds over" to music/HT systems with poor results due to lack of room gain like you would have in a car--so weird things happen. The good thing about sealed and infinite baffle is you can EQ anything you want--as long as you don't hit Xmech anyway. Infinite baffle works very well real world--IF you know where to install them, have a spare room to seal off behind them (or vent it outside with no neighbors for miles) Since most people don't have that situation, then the job goes to sealed with massive EQ/power and so on.

This creates a problem, if you use say a sealed sub and it has 12dB of boost at the low end (say 16 Hz) The drum hit at 63 Hz requires no EQ boost but the pipe organ demands 12dB more power--basically 16 times the power--yep, 16 times the power! No big deal with huge voice coils, massive amps and so on these days. Well, it is a big deal because with more power and more cone movement breeds more distortion. So as the frequency drops, the distortion increases which is not a good thing. Luckily, human hearing becomes less sensitive as frequency drops below 30Hz so exploit that when designing a sub.

What about ported? Well, IF it is done correctly, it will kick serious butt! However, it goes against marketing because it will be HUGE, very large ports to prevent chuffing and tuned down very low (below 20 Hz or ideally below 15Hz) The ports will be huge--think 6 and 8 inch ports that are many feet long. To get the port resonance higher as huge ports start to resonate low like pipe organ pipes--you need to make the ports shorter. The only way to do that is either make the port smaller in diameter which invites chuffing/overloaded ports or make the box larger. OK, you have your 15" sub tuned to 15 Hz in a 13 cubic foot box--have fun trying to sell something like that!

You can use various modeling software to design your own basic sealed/ported/passive radiator subs using WinISD Pro, Bassbox and others. If you really want to understand subwoofers, playing around with that software will open your eyes to the pain of ports, port compression, port resonances etc. You can also model passive radiators with WinISD, they are a viable option if using smaller boxes when it is impossible to use a massive port. I have a 12" sub that rests in a 4.1 cubic foot box with passive radiators tuned to around 20 Hz--in room response is down to 18 Hz. A viable option but it will cost you both in money, time to build the box and a slight hit in efficiency--read the fine print.

As you mentioned, the other option is to get many subs--the more the better! MORE! Not only will you get more even frequency response around the room, you'll also lower distortion drastically at the same SPL levels. At the most extreme, you can get amazing performance with basic drivers. BassThatHz uses 16 Dayton Audio 18" PA woofers at $90 each in quanitity as "mid bass modules". There are 8 drivers in each 25 cubic foot sealed column running opposed with 4 on each side. He runs them at around 50 Hz to 160 Hz and reaches reference levels of 115dB at his seated position with 2 watts of power total. Go into rockstar mode and throw a stagger 10 watts into them teotal and blow right past 120dB at the seats--not sure if he has tried tube amps with them but you get the idea. Each column can take 4,000 watts of power which he tested--with ear plugs and hearing protection for 150dB+ levels. If you "only" want 140dB or less, the 16 drivers and something like an iNuke 6000 DSP will run you under $2K total (wood is extra!)

Distortion is vanishiingly low--divide up 2 watts amongst 16 eighteen inch PA drivers would equal maybe a shimmer of cone movement? Alas, 99.9% or higher of us don't live in a basement with 2 inch thick plywood/green glue/sheet rock walls and have an entire floor we can play around with. You do get the idea though, as Paul W. Klpsch once said "if it moves, it distorts" so the idea is less movement by either horn loading (Klipsch) using ports/passive radiators down very low in frequency or just using more subs, keep adding them until you have the SPL and distortion level you desire. One of the rare bonuses in audio where more is better.

This rabbit hole can run deep, for me I tend to be a realist. My house is not on a concrete slab, it sits on studs driven into the ground. Works great for getting the floor shaking though The bedroom will resonate when given single digits of bass--my wife informed me of this rather loudly. So I use three subwoofers with in room response down to around 16 Hz (ported and PR) and if I want more, I'll need to get some of those shaker things for my couch for infrasound or simulation of it at least. Realistically, I don't have a need for under 15 Hz at this point, the huge picture window is not a fan either. Sometime you will be structurally limited when digging the depths.

As you know, the lower the frequency goes, the longer the wavelength becomes. Eventually, unless you live in a stadium--that wavelength will not "it" in the longest dimension of your room. It then reflects causing peaks/nulls and room gain. If a person has a pair of speakers that go down to 40 Hz and loves the bass--then adds a subwoofer(s) that go down to 16 Hz, it will sound drastically different. It will sound "slow" because it IS slow! The other issue is also simple, the lower in frequency you go the more the room has a nagative effect on bass response accuracy. The subwoofer is not distorting, being "loose" or "boomy"--the room creates that at low frequencies.

If you want to find out what your room is doing to your bass response, take your subwoofer outside. Another option is to take it to a very large room, think something like a gym, church or factory floor type big. Slap your sub in a bathroom, just move it around and hear how things change.

In summation, "fast" really does not make any sense to me for several reasons. Subs are slow, that is kinda the point. I've never heard of a "fast" midrange or "fast" treble response... I tend to look at distortion readings for subwoofers when tested outside (what data-bass does) I also look at Fs, Xmech, QTS, efficiency/sensitivity, inductance and the thermal capacity of the voice coil along with Xmax when modeling drivers. A big red flag is inductance--it gets really expensive to try to control that with huge voice coils. One way to do that is to NOT use huge voice coils (genius!) This demands using more drivers or more subwoofers if you need more air displacement at the same distoriton levels. For this reason, cost is a factor but not much of one depending on how many subs you can use. A lot of times, the high cost is because of huge voice coil inductance problems (shorting rings etc.) the massive magnets or neo magnets required to get very high motor power to move such heavy cones/heavy voice coils--don't forget the beefy frames to hold all these heavy parts together. Sometimes is costs a TON of money to get smaller--just the way it is! These issues are not really applicable to frequencies over 100 Hz---so the cost of subwoofer(s) is more size related, not performance related. Unfortunately, at least for most of us... we can't have a bunch of walk in freezer sized boxes scattered around the room. We have to buck up and either blow big bucks for smaller subwoofers, use a lot of power to run them or disguise them as furniture (my subs are end tables) Some day, when (if) I move and have a full basement to play with--then I'll build push-pull slot loaded LLT boxes tuned to 10 Hz because I can. LLT = Large Low Tuned and a pair of 18's in a 40 cubic foot box (1,100 liters). Want to be reasonable, four of them should do it.

Of course, it might be easier to build a pair of PPSL infinite baffle subs positioned correctly--make that decision when/if I ever have a basement. The room and patience of my wife will dictate that freak show build--reality sucks.