2/4 Band Eq Pedal for Bass Fundamental Boost

[gedeon]

First thing, I'm completely new to the world of DIY pedals, so forgive any and all on my ignorance please

I'm a bass player that tunes my 5 string down to a G#, which is something around 26hz. I have great eqs, but nothing that boosts lower than 80hz at the moment. I would really like a bass boost eq pedal, something that just boosts the fundamental frequencies. Those are something like 26hz for the G#, 36hz for the C#, and 46hz for the F# and 60hz for the B. I'm not sure if it would be best to do a 4 band graphic eq at each of these frequencies, or maybe a 2 band with a specific Q or something. And yes, I know how hard it is for speakers to reproduce these frequencies, I'm building my own cab tuned to 34hz for just such a purpose

I think 12db boost would be fine, but more isn't bad. Most important to me is sound quality. I would prefer only top notch parts and design so the signal stays as clean as possible.

So, what would would people advise me to do? I don't mind building something myself, but I am totally new to this diy stomp box thing. If anyone wanted to build one for me to purchase, I'd be very open to that too.

Thanks, and glad to have joined your community.

[gedeon]

The more I think about it, I think a 1 band, something like 35hz, with a 2/3 octave Q would work well. Any thoughts?

[aron]

This might help you:
http://www.geofex.com/article_folders/eqs/paramet.htm

[R.G.]

Also the Rane document on constant-Q equalizers and possibly some speaker crossover designs.

[Gus]

2nd harmonic of 26Hz is 52Hz, 2nd of 36Hz is 72Hz etc.

Also the output from a bass is a good signal level often higher than a guitar output.  You might have a good level but the rest of the chain might need to be changed

Why do you want to boost the lows that you are asking about?  Often low frequencies need big speakers and lots of power to move the air just boosting the lows might not help if the speaker(s) amp(s) can not reproduce the sound level you want.  The boosting might hurt the speakers or shutdown the amp.

If you are using a vented box speaker be careful of the power handling at low frequencies

FWIW I built a closed box speaker cab it uses a 18" and was modeled for a -3dB at 60Hz but being sealed(it is airtight I sealed the cab with flexible construction adhesive and checked for leaks) and having a Q of .7 something the roll off is gradual.     A friend testing it for me used their 1,500 watt amp and I asked them to try to blow it up it survived.
Sometimes a sealed box can handle more output a lower frequencies than a vented box because of the different slope of the rolloffs.

[gedeon]

Thanks for all the info, I'll start reading.

I understand the difficulty/danger of reproducing notes that low, but there is a certain sound/feeling I'm going for, so I'm hoping to eq to compensate for the falloff in db at lower frequencies. I'm building a cab with a 15" speaker with a resonant frequency of 34hz itself, so with a well designed cab, 26hz wouldn't be too much of an issue. I've been playing around with bassbox 6, and I can get a vented design that is -3db at 46hz and about -10db at 26hz (It's a big cab), so I was thinking if I had an eq that could do a 12db boost in that neighborhood, I'd be able to make up that -10db to some degree. Obviously I'd have to have plenty of headroom to do this, but I care less about overall volume, more about being able to accurately reproduce those frequencies. So that's why I want to do it Still, I'm new to much of this, so please correct me if any of my thinking is wrong, which likely some of it is.

[Gus]

good luck with a single 15" in a vented box and low frequencies.  What is the power handling of the vented box above and below the resonate frequency. Does the software give you that important information.

You might want to read thishttp://sound.westhost.com/project48.htm

this might be interesting with a sealed box
http://sound.westhost.com/project71.htm

[gedeon]

I'll give those links a read shortly. The software does give that info, and it was interesting to look at. So it's all great above 200hz, but it drops off badly, only 150w at 36hz, and 1.25 watts at 24hz, lol. So, I'm learning

[PRR]

> 26hz for the G#, 36hz for the C#, and 46hz for the F# and 60hz for the B
....4 band graphic eq at each of these frequencies, or maybe a 2 band with a specific Q or something.
> ...I am totally new to this diy stomp box thing.
> ....I'm building a cab with a 15" speaker with a resonant frequency of 34hz
> ...a vented design that is -3db at 46hz and about -10db at 26hz (It's a big cab)

A "good" bass speaker is a 15" in a 4 cubic foot (19" cubed) box. This can be fairly flat and fairly high-output to 50Hz.

To reach a lower "low good limit", scale all dimensions linearly. For 50Hz/26Hz, double all lengths. Therefore a 30" cone (four 15s is reasonable), box 38" cubed or 32 cubic feet.

I have a closet 32 cubic feet. You can put such a box in the Econoline, but it's a hump.

Vented-box is good to a point and then it really sucks. If you apply power below the box resonant frequency, the cone is totally unloaded, it just flaps, messes-up all higher tones in the sound.

There's also real problem tuning a reasonable box to extra-low frequencies. The low pitch means a small hole, but at any reasonable output a small hole "chuffs". Pipe can be larger area, less chuff, but has to be quite long to make-up the difference.... at some point the vent is more cubic space than the box.

And through it all, you are fighting Fletcher-Munson. Your neck/head resonates around 20Hz. When you walk, this thud would deafen you, except human hearing rejects low-mid level bass. While you can caress midrange notes as quiet as 30dB SPL, you just can't hear 26Hz at less than 65dB SPL, and really need over 80dB SPL to hear 26Hz well.

Meanwhile your speaker is fighting the air. The 15" cone is far too small to develop a 520" (43 feet!) wave. This is a massive loss of efficiency. And all speaker drivers are dipoles. To have a hope, you need to keep the front-wave and back-wave apart. Unless you have a handy sewer (it's been done), you put back-wave in a box. But now you fight air's compressibility, a problem which is well understood because it also limits pneumatics and engine performance.

IMHO:

Are you EQ-ing the bass? Or the (inevitably) undersize speaker? An e-bass is just an e-guitar with larger parts tuned to 42Hz. If tuned to 26Hz the parts (neck and strings) should be larger yet to maintain similar sustain and harmonics, but already the e-bass's neck is a long reach. (You might read-up on "octobass" which used two players.)

Anyway the idea of 26hz-36hz-46hz-60hz semi-tone EQ quickly gets awkward. An octave-band EQ can use one opamp per band. 1/3rd octave really needs 3 opamps per band to get the balance of passband flatness and enough stop-band rejection that it makes sense to have another band just 6% over. So a 1/12th octave (semitone) EQ wants 12 opamps per band or 48 for the four notes you mention; more if you need to smooth the transition into the "normal" bass range.

And you don't need to EQ each note. You have finger-touch, you'll learn how much pluck gives how much thud for each note, and control that dynamic. There may also be pickup adjustments to quasi-compensate for the fact the low string is really too short and too loose for its pitch compared to the other "normal" strings.

What you want to EQ for is the small speaker's fall-off. And aside from problems getting high output at super low frequencies from small boxes and vents, the 18dB/octave roll-off of a vented box is really difficult to EQ out.

Again IMHO, the sealed box is the only sensible way to go an octave down from the natural performance of a practical 15" 4cf device.

And this isn't really a "stomp-box". It logically goes between preamp and power amp.

For small bass extension, EQ is easy. The seal-box speaker acts like a 2-pole high-pass filter. 2-pole HPs are simple circuits. When Q is near 1, they are flat to a point then bend toward a 12dB/oct slope. But at higher Q they tend to a 6dB/oct rise to a peak, then 18dB/oct fall, relaxing to 12dB/oct.

There is a whole world of filter theory. Any response can be synthesized, though some need an impossible number of parts. I leave this math to others.

At first blush, two 2-pole HPs can be aligned to give a 12dB/oct rise in the range where the speaker is dropping 12dB/oct. However if trying to stretch nearly a whole octave, the incomplete cancellation causes large "ripple" in the response. like +/-5dB. Also you get into extreme sensitivity, where 5% part tolerance gives unacceptable response error. Some clumsy guesswork led me to a four 2-pole network. This gives couple-dB error 25Hz up. Not worth getting crazy reducing that error because the speaker will vary a dB or so and the room varies many dB. It's still moderately sensitive: 1% resistors, 2% caps is fine, 2% resistors 5% caps may need tedious shimming.

I assumed a sealed-box with Q near 1 and -3dB at 50Hz.

http://i.imgur.com/bGSYvI7.gif

While it is unity-gain in to out, there's internal gain of 2.45 so the power supply must be well above the signal level. For 1V-2V typically taken by power amps, +/-15V rails seem good.

Note 19dB peak boost, 17dB at 26Hz. This is 50:1 power ratio. If 50 Watts at 100Hz makes a nice sound, then you need 2,500 Watts at 26Hz. While the thermal load is massive, I suspect that displacement demands (slapp) will be the real limit. While you can find long-throw woofers, very long throw with wide-range music ("bass" is not just bass) get large intermodulation. You want to hold displacement to a few mm, and increase cone area to get desired acoustic power.

Alternatives:

Headphones "can" deliver huge levels to low frequencies. Many don't. Sealing is a problem. But the real issue is that 26Hz is not just in your ears, it slams your whole body, which headphones don't.

If the room is not much bigger than the speaker enclosure, many of the bass problems get much easier. A couple 6X9s that I would not ask 100Hz in a room can do well to 20Hz in a small closed car. (When I mastered an organ recording with 32Hz pipe, I did the main mix in room but checked the sub-bass in the car.)

[gedeon]

Whoa, +1 for the Dr. Venture photo, and +1 for an awesome reply.

So what I'm gathering from all this is that there really isn't a good way to get substantial output below 40hz with a 15" speaker, without it being huge, or heavy, or expensive, or just plain crazy in some way or another.

So I think I'm going to take a step back from my "goals" and just see about building the best 15" cab I can. Whatever it can do on the low end will just have to be good enough. I still have to be able to transport it to gigs, sometimes via public transportation, so I can't go too crazy there (no 4x15" setups).

Thanks for all the input

[PRR]

> just plain crazy in some way

"Crazy" relative to reasonable size/weight/cost, yes.

It's not crazy if you accept speakers the size of a truck. 10Hz is "easy" with speakers the size of a building. No magic; you just need acres of cone. (Perhaps custom-built because the market for 10Hz speakers is a dozen a decade; the last large project I know is back in the 1960s-1970s when NASA was testing rocket bunkers.)

BTW, this is why we do not fear a "super-weapon" where sub-sonics make soldiers poop their pants and upchuck uncontrollably. The speaker would be so large, so slow to assemble, that they could just walk away before it was ready to rumble.

[Mike Burgundy]

Just an addition: 26Hz fundamental is not needed to "hear" your 26Hz string be big and fat. You're listening to a pretty complex signal, not a sinewave, and your brain tends to "recreate" the fundamental if the overtones are in the right proportion. This is a trick employed in many church organs to eliminate the need for huge, huge pipes for the lower registers.
I play 5 and 6-strings, tune to low B, and yes, have designed and built my cabs to go low enough for the fundamental of roughly 31Hz (well, 29Hz from a 1x15" and 34 Hz from a 4x10"). Cool exercise, but in hindsight maybe not worth the trouble. I got into it when I found that cabs that go that low are incredibly hard to find, especially if you use hifi norms instead of music industry norms (they often state +/15dB or +/- 10dB figures without mentioning it). It works. It makes the glasses rattle in the studio next to ours. But we, playing in our studio, don't hear it. At volume the sub-lows are inaudible, except when youre a long way away and have less noise around. The only thing I notice is that I might like them a bit more controlled, so I'm contemplating a redesign upping f3 a bit on both cabs. I think 40Hz should do just fine.