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Subwoofer Tuning Frequency for Live Sound?
Old 5 days ago
  #1
Gear Maniac
Subwoofer Tuning Frequency for Live Sound?

[Sorry this is redundant. An attempted edit resulted in a second, nearly identical thread. If a moderator wishes to help me clean up the board, please delete the one without the graphic attached.]

A fairly diligent search has not surfaced this question, so I guess I have to start a thread.

I have a decent small PA system that I use for my Sixties cover band (Folk-Rock and British Invasion with acoustic rhythm guitar) and a variety of other community sound gigs. A Soundcraft Ui24R mixer, two RCF ART 325A mains, three RCF ART 310A Mkii monitors (also used as small-room mains), Audix OM vocal mics. In general, it's been a good rig, but we recently added a strong rhythm section, and I haven't had to concern myself with the lower end of the audio spectrum before.

I've heard that the only real knock on the RCFs is that their low end is not a strength, so I want to add a subwoofer to receive the kick and bass channels on a dedicated AUX-out to fill that gap. Being retired and on a tight budget, I can't realistically consider $1500 or so for a good RCF sub, and I don't want to just go buy something like a Behringer sub for $300 and live with its shortcomings, so I've decided to build one.

I've built quite a few decent speaker boxes over the years, and I found a 250w rms subwoofer plate amplifier and a beefy 12" subwoofer (both Hsu, a brand apparently suited to small enclosures) for less than $200. I'm re-purposing an old box (25"x16"x13.5") that's thick and heavy, with lots of bracing on the panels. Online software has gotten me into the ballpark for porting it, but I can't find much discussion online about optimal tuning frequencies for this kind of application.

My goal is a compact box that can fatten up my live sound, scaled to the size and power of my basic system, not a big thump for dance clubs or arenas. The only configuration that's practicable for this box is a rectangular ducted port at one end of the face.

It appears that for every hz lower, the port has to get considerably longer. If I try to tune it to 30 hz, there's not enough space in the box for the partitions. Most of the online discussions seem to be about making one's car doors flex in and out, so that's kind of out in left field for my issue.

What are your opinions about a good tuning frequency for this box? I'm sort of settling in around 45-50 hz after researching the frequency of the lowest bass notes and kick drums. I can easily tune the mixer AUX with HPF and LPF filters so this box only gets the kick and bass down low. I'm also not sure of how to cross this over with the 15" mains. The Hsu sub amp has a crossover that tops out at 90 hz. I can HPF every channel and the main outs to fine-tune it.

Open to your opinions, suggestions, and ridicule

Thanks.
Old 5 days ago
  #2
Gear Head
 

To answer to your question 40hz is a fine tuning frequency for live sound sub but I think you're wasting your time with this driver. A sub has to be capable of producing at least as much SPL as the mains just to be audible(and preferrable quite a bit more) but I suspect this driver couldn't handle enough power to get anywhere near that. There are some pretty impressive small PA subs on the market now but what you will find with them is that they have hugh power handling.. relatively speaking, that is just physics at work a small driver has to work harder to make as much SPL as a larger driver because at low frequencies it's all about air displacement.
Old 5 days ago
  #3
a 250 watt 12" ? ... good for your living room surround system - not going to get you what you're looking for especially at 40hz- ....dual 15 or single 18 at least ...unless its Danley stuff like this
https://www.danleysoundlabs.com/prod...d-horns/th112/
Old 5 days ago
  #4
Lives for gear
 

There's no shortcut to good subs. They can be small, cheap, lightweight, loud, and reach deep - pick any three. I can get you subs that reach deep and pound plenty hard for about $400. They're 180lb dual 18" cabs that stand about 5' high from the 90's.

The best cheap shortcut is to send kick and floor tom through the bass rig. I'll save the tech details because it obviously requires the consent of your bassist. Paying for and hauling around an additional bass cab to prevent frying the bassist's cab is the cheapest and reasonably effective solution. It's a good negotiating point too, that the bassist will have a beefier rig to play through, and hearing the kick on stage generally helps a bassist. Not a pro move but the crowd doesn't care where the low end is coming from.

But it sounds like you've already committed to the sub amp and driver. The cabinet vent tuning should suit the driver, for the purposes of mitigating cone excursion, not the usable frequency range of the cabinet. No matter what you're going to have to put a HPF on the subwoofer send also to mitigate cone excursion because a 250W 12" sub is not going to accomplish much paired with two 400W 15" RCF tops. Assuming you're right that the tops don't put out much low end, it doesn't change the fact that RCF's generally have very good efficiency so they're going to have strong mid and high output that requires a pretty serious subwoofer to keep up with.

Oh man, that Hsu subwoofer driver is for home hifi. Its sensitivity is 86dB @ 1w/1m. Even crappy PA subs are around 96dB sensitivity. Your RCF tops are going to be at least 98dB sensitive. The sensitivity is a rating of the speaker's efficiency. It takes 10x the power for an 86dB sensitive speaker to be as loud as a 96dB sensitive speaker. That sub at full power has the output of a PA sub at 25W. You will fry that sub before it makes a meaningful improvement to the low end. My subs are 105dB sensitive, which is 80x more efficient that yours. They can out-drive your sub at only 4W, and they can handle 1400W each.

I get the $1500 budget being out of reach, because that's exactly what it would normally take. Assuming your bassist has a reasonable rig that can power an additional 15" bass cabinet, there are lots of decent options under $300 on the used market with output down to 40Hz you'll at least notice is there. Not ideal, but cost-effective.
Old 5 days ago
  #5
Lives for gear
 

Quote:
Originally Posted by RightOnRome View Post
a 250 watt 12" ? ... good for your living room surround system - not going to get you what you're looking for especially at 40hz- ....dual 15 or single 18 at least ...unless its Danley stuff like this
https://www.danleysoundlabs.com/prod...d-horns/th112/
Or the Mini'. Doesn't go a low.. but half the weight :>)
https://www.danleysoundlabs.com/prod...-horns/thmini/
Old 4 days ago
  #6
zzounds is also a good option - its a great way to get what you need NOW and gig to pay it off over a years time ..
Old 4 days ago
  #7
Gear Maniac
Most of our shows are pretty small by pro-sound standards, maybe 100 people in a venue that could hold twice that. Anything larger, such as regional festival stages, generally provide a sound contractor.

I did do sound for a pretty big parking-lot party at a craft brewery last summer. We were one of three local bands, maybe 300 people. I mix us from the stage with a tablet on my mic stand (the continuous, simultaneous monitoring of the Ui user interface is a godsend, like having everything on solo at the same time, and we've saved a time-tested snapshot for the band), and I mixed the other two bands (funk and jazzy pop-rock) from the audience. Plenty of sound, plenty of compliments, so it's a good basic system.

I'm enjoying the range of responses.

Yes, I'm aware of Thiele-Small parameters and understand that extremely precise cabinet designs begin there. I'm just not capable of doing all the math and don't want to design a perfect box from scratch, or pay for design software, or spend $500 for a driver, or $1000+ for a pro sub.

Yes, I'm aware that my little project may well not add much to the low end of what the 325s are putting out, but that response curve (on the redundant thread I can't delete--be sure to find both of them, because they're drawing different responses) shows the low end plummeting steeply from about 90hz on down. My reasoning was that if I could feed a narrow range of 30-50hz to the little sub and increase the feel of the very lowest notes, it would be worth carting it around.

My whole system fits into my aging Toyota Previa minivan (seats out), so I really don't want to pick up a good used 18" sub the size of a fridge, plus a power amp and cables and.... If I had the budget, I'd probably start with a pair of RCF 702s and see how that combo worked, but this is a bargain basement DIY project while we're sheltering in place. If it's useless for our PA rig, there's room for it under my DAW desk.

I just wanted some feedback about a good ballpark tuning frequency.

Finally, the SPL spread may well be tempered as applied here because we've never run the 325s at full chat. The usual level control is generally around 1-2 o'clock unless we're in some infinite outdoor space. The LF amp in the 325 is 350w, and it has to drive everything up to the crossover frequency, so 250w dedicated to a small sub isn't a ridiculous proposition (even with 2x350 v 1x250).

We're basically a vocal trio with my acoustic-electric rhythm guitar direct to mixer and a PRS electric lead running through a 25w Vox single-12 amp with a 57 in front of it, so it's not headbanger loud. Our new bassist is basically a versatile jazz guy (probably the best session bassist for miles around), and our drummer has a soft touch.

Not a loud band, because we emphasize vocal intelligibility.

I'm trying to get more of that sub-90hz content from the bass, and the kick thump, into the FOH environment, and the 325s fail off rapidly under 90. I want the audience to hear a bit more of the low end we hear directly from the bass amp and kick on stage.

So a 250w 12" sub running wide-open with only 40hz through 80hz in it might be worth trying.

BTW, we rehearse with vocals and my guitars running through my 6" KRK Rockit DAW monitors (that's how quiet the band is), so if this box ends up in the studio, that'll be fun too.

If I hit tonight's Lotto, gimme a pair of powered Danleys and a bigger van, and a driver, and a coupla roadies, and...

I think I'll go with 40hz.

Thanks, guys, and stay safe, OK?
Old 4 days ago
  #8
Lives for gear
Some thoughts. I design and build the PA systems that I use for my business.


- 250w into a single 12" isn't going to make much noise in a PA format unless you can find a way of making it very efficient.
- WinISD Pro is free, and useful. You can use that to simulate what the woofer will do in different cabinets.
- The lower you try to go, the quieter it'll be before the driver makes farting noises and then stops working.
- 40Hz extension is pretty good for a smallish PA system. For your sort of gig, though, I'd be using a pair of Beyma 15P1200Nd in 40Hz ported boxes, and I'd have a couple of kilowatts on tap for each driver.
- If you have the driver, amp, wood, and want to make a bit of thump, a 4th order bandpass is probably worth a look. You won't get to 40Hz, but those designs give you the flexibility to push for more output in the 60Hz region.

Before you build anything, at least run the numbers on WinISD Pro.

Chris
Old 4 days ago
  #9
Lives for gear
 
Pro Sound Guy's Avatar
 

Consider the used market. You can get way more bang for you buck
over your idea.
Pick yourself up a sub meant for live sound reinforcement or a pair of subs used
that are designed for live sound reinforcement.
Old 3 days ago
  #10
Gear Head
 

Quote:
Originally Posted by OldChico View Post
Finally, the SPL spread may well be tempered as applied here because we've never run the 325s at full chat. The usual level control is generally around 1-2 o'clock unless we're in some infinite outdoor space. The LF amp in the 325 is 350w, and it has to drive everything up to the crossover frequency, so 250w dedicated to a small sub isn't a ridiculous proposition (even with 2x350 v 1x250).

Sounds like you are going to build this anyway and nothing wrong with that it's a good learning experience, but what you will discover is that it won't be useful as a PA sub. The thing you don't seem to understand yet is that the driver sensitivity difference is everything here, this sub running flat out will only be as loud as the pair of 325s running on about 5w each or just barely ticking over. In your practice room it will be fine but out in a venue it won't add anything noticeable even at very modest volumes.
Old 3 days ago
  #11
Gear Maniac
Quote:
Originally Posted by paul o View Post
Sounds like you are going to build this anyway and nothing wrong with that it's a good learning experience, but what you will discover is that it won't be useful as a PA sub. The thing you don't seem to understand yet is that the driver sensitivity difference is everything here, this sub running flat out will only be as loud as the pair of 325s running on about 5w each or just barely ticking over. In your practice room it will be fine but out in a venue it won't add anything noticeable even at very modest volumes.
Thanks. I do basically get sensitivity and spl, and you all are probably right that it won't add enough to be useful. I just don't have budget or storage and transport capacity for proper PA subs, and I thought maybe with our relatively modest FOH levels, something like this might add something. Just really enthused about finally having really good bass and drums after three years of hit-and-miss rhythm support. I love how our dozen or so live tracks sound fattened up in Logic ProX and want the audience to hear it too.

Shame that the recordings are from our last gig before this virus went nuts. We had a great spring schedule lined up.
Old 3 days ago
  #12
Lives for gear
 

Well here's the math on what your sub will produce. Let's say your cab manages to get another dB of sensitivity, for 87dB at 1W. 250W is 24dB of gain, for a max of 111dB.

The 325's I assume are going to be 98dB sensitive or better, so a pair of them are already producing 101dB at 1W. That 10dB difference is only 10W to each of the 325's Furthermore the 325's are projecting in a directional manner, your sub will be projecting omnidirectionally.

That's a losing contest at any practical performance level. Not trying to rub it in, just trying to help you find the right solution.

The cheapest subwoofer option that will produce usable results is a used bass cabinet, maybe you could use the subwoofer amp module to drive it. There's an SWR 18" cab in my area for $250, that would kick any hifi sub's butt any day. Can't speak for your bassist but I'd have no issue with putting an 18" cab under mine.
Old 2 days ago
  #13
Here for the gear
 
Luke V's Avatar
 

You can get a b-stock, scratch and dent RCF 705 ASII for a little over $600 and you get a new warranty. It is a nice sounding sub that I think would work very well with what you have and the type of music you play. 1400 watts peak, 700 rms, 131db spl.
Old 1 day ago
  #14
Gear Maniac
Quote:
Originally Posted by Luke V View Post
You can get a b-stock, scratch and dent RCF 705 ASII for a little over $600 and you get a new warranty. It is a nice sounding sub that I think would work very well with what you have and the type of music you play. 1400 watts peak, 700 rms, 131db spl.
Yes, I'd love to add RCF sub(s). One question, given the size of my mains and our relatively quiet sound, would one suitable sub do the job, or is it usually the rule to match mains and subs on each side? I know that deep bass is heard omnidirectionally, and I see that the RCF ASII boxes have left and right ins and outs.

I see a lot of sub + satellite rigs with 10" or 12" satellites on poles, but I'm assuming that subs and 15" mains would be more powerful? How about clarity vis-a-vis smaller mains?
Old 1 day ago
  #15
Gear Head
 

Of all the portable PA speaker configs the 15" version gets the loudest but when talking about other performance parameters like dispersion(horizontal coverage) and midrange clarity they are often the worst. When subs are added to a system it becomes a 3-way, this takes the load of producing the extreme low frequencies away from the mid driver so it doesn't need to be nearly as big and this has an additional benefit of cleaning up midrange output. If you look at any manufactured 3-way PA speakers you will have a hard time finding any with a mid driver larger than 8" in diameter for example, that isn't an exact apples to apples comparison as the freq ranges covered by each driver is a bit different but you get the point. In a PA system a dedicated sub often only covers up to 80-100hz so the "mid" driver still has to produce a fair bit of upper bass so there is good reason to use a 10" or 12" here but at everything but the largest scale PA systems a 15" mid is really unnecessary. In your case and actually in most cases the sub/s should be the biggest boxes in the PA system, that means you could downsize your tops to 10" and gain back a bunch of transport/storage room while not losing any SPL capability and gain more lowend extension and better mid clarity and coverage.
Old 12 hours ago
  #16
Gear Maniac
That's what I expected. The RCF 310s are a really nice box for smaller rooms. This little 40-seat club where I do most of my paid sound work bought some 15" Altos when they opened and didn't have a clue what to buy. I tried them a couple of times and quickly decided to bring my 310s in when I did shows.

There was also a local duo, really accomplished artists but only average tech knowledge, who brought a 15" JBL Eon in one night. Sounded like crap, really muddy and unintelligible. My sense was that a speaker that big doesn't really open up without more power going through it, and that would have been too loud for the room. So they got the SPL about right but the result was dull and indistinct.

I finally talked the owner into buying the old kit from the coffeehouse where we used to do Open Mic. It had been in storage for two years since they lost their lease. Homemade (by me) boxes with 12" LF and a P-Audio CD horns, QSC power amp, a pair of EV S-40 nearfield monitors driven by a Yamaha monobloc amp.

We replaced the really decrepit EV 12" woofers with new P-Audio drivers and rebuilt the boxes with new crossovers. Flew everything from the ceiling with the amps in the back room. Connection panels in the corner, including an Ethernet jack to a router in the back room for digital mixers, and a long power strip along the baseboard behind the performance space.

The little EVs are overhead between the mains, so the floor is clear of stands, boxes and cables. Wouldn't work at higher sound levels, but the monitoring is quite good for acts that fit the room.

It was really sweet, saturating the room without harshness or dead zones, all for about $400 net once we find a buyer for the big Altos.

We had only done about four or five shows with it before the place shut down for the virus thing. Damn.
Old 12 hours ago
  #17
Gear Head
 

Quote:
Originally Posted by OldChico View Post
That's what I expected. The RCF 310s are a really nice box for smaller rooms. This little 40-seat club where I do most of my paid sound work bought some 15" Altos when they opened and didn't have a clue what to buy. I tried them a couple of times and quickly decided to bring my 310s in when I did shows.
If you already have a set of 310's you just need one good sub to see/hear for yourself what is possible, that sub just needs to have high pass outputs or you need some other way to high pass the signal going to the tops.. a basic analog rack crossover would suffice for example.

Quote:
Originally Posted by OldChico View Post
There was also a local duo, really accomplished artists but only average tech knowledge, who brought a 15" JBL Eon in one night. Sounded like crap, really muddy and unintelligible. My sense was that a speaker that big doesn't really open up without more power going through it, and that would have been too loud for the room.
No the problem with those boxes are that they are dirt cheap entry level things with really poor high frequency drivers. When it comes to reproducing live music the compression driver in a conventional PA speaker is easily the most important component, cheap out there and there is no fixing it but use a box with a really good HF driver and even a total neophyte could get good results.
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