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Building Bass guitar secondary cabinet, need port length/size advice

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Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Wed Sep 04, 2019 4:13 pm

Need to eat this fish, really don’t want to learn to fish.

I am building a second powered cabinet to work with my bass amp. It has its own power. I’m using stuff I already have, not looking to buy another speaker, amp or cabinet.

My main bass guitar amp has a preamp output that splits the the preamp signal. That signal is going to a second cabinet with its own power section.

The cabinet is 18” H X 18” W X 16” D = 2.28 interior volume (3/4) 11 ply birch

Here is the 12” driver specs:
4-OHM
2" BASV VOICE COIL
LARGE ROLL FOAM SURROUNDS
POLE AND AERO VENTED COOLING cooler woofers mean better bass and handling
DESIGNED FOR SMALL SEALED OR PORTED ENCLOSURES which include most pre-fabricated boxes
Power Handling Capacity (Peak) : 600W
Power Handling Capacity (RMS) : 300W

Nominal Impedancel : Dual 4 ohm

Voice Coil Size : 2"

Frequency Response : 29Hz - 450Hz

Sensitivity : 91 dB/W/M

Motor Structure : 124 oz

Mounting Depth : 5.5"
S12D - Powerbass 12" 600 Watt Subwoofer S 12D

What I need to know is what size/length to make a port?

Power section is 380 rms / 550 peak
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Wonks » Wed Sep 04, 2019 4:57 pm

To do that you need to know the Thiele & Small parameters for the speaker. However, that particular speaker's not made any more and I couldn't find a spec sheet for it on the PowerBass web site.

Without those T & S parameters, you can only make a guess.

I'd try it out as a sealed enclosure first. It will probably give you all the bass you need and have a tighter bottom end than a ported enclosure.

Also, the speaker is 300W RMS/600W peak, so you'll need to be a bit careful with the amplifier and don't turn it up all the way.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Hugh Robjohns » Wed Sep 04, 2019 5:04 pm

funky54 wrote:What I need to know is what size/length to make a port?

You can't calculate it with information you've given, almost all of which is completely irrelevant to calculating the required enclosure parameters.

This is why learning at least the basics of fishing is necessary...

There are lots of online calculators for ported speaker enclosures, but they need a set of basic acoustic (Theile-Small) parameters for the drive unit to be able to calculate anything.

Assuming the bass driver you want to use is the Powerbass car subwoofer, I found the required specs here:

https://www.sonicelectronix.com/item_42463_PowerBass-S-124DX.html

Specifically, these are the relevant specs:

Free-Air Resonance (Fs) -- 31 Hz
Equivalent Compliance (Vas) -- 59 liters
Total Speaker Q Value (Qts) -- 0.58
Mechanical Q Value (Qms) -- 7.29

Plugging those numbers into a ported cabinet calculator with your box dimensions suggests a side port of 5.35-inch diameter and 15.1-inches in length -- that's assuming a box resonant frequency of 36Hz.

But I've not checked any of this and absolutely do not guarantee the result is correct or will work correctly... -- I just plugged in the numbers and got a result. You'll need to double check the specs and calculations for yourself.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Wonks » Wed Sep 04, 2019 5:13 pm

That may be a different speaker as this is the S-12D (it's been superseded three times). But no TS paramaters listed. :(

https://www.sonicelectronix.com/item_7017_PowerBass-S-12D.html
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Hugh Robjohns » Wed Sep 04, 2019 5:40 pm

Wouldn't be at all surprised... but I was trying to show why it's worth learning to fish rather than getting someone to go down the chippy... :-)
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Wonks » Wed Sep 04, 2019 6:48 pm

And at a sensitivity of 91dB, it really is quite an inefficient speaker. Even with 300W through it, you are only going to get a maximum of 115-116dB from it.

Go for a modern efficient proper bass guitar speaker, e.g. this Celestion, https://celestion.com/product/165/bn12300s_8/ and you'll get 8 more dB (at 124 dB) from the same amp. That's over double the volume.

I also noticed that the frequency response of the S-12D is very limited indeed at 29 - 450Hz. it really is just a car-audio sub-woofer designed just for 'thump' and no definition. It would be like playing your bass with the tone control wound right down. It is simply not a bass guitar speaker, which you need to have go up to between 5-6kHz.

You may want to use what you already have, but it simply won't work well. I suggest that you either pick a proper bass speaker or forget about it.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Wed Sep 04, 2019 9:10 pm

I really appreciate all the replies. A couple of pieces of info to answer some of your questions.

I’m hoping to use this cabinet “WITH” an existing bass combo. The existing combo is an SWR LA15. It sounds nice. It’s enough to keep up with a small drum kit, but it’s just on the cusp of not being enough. Hence using up existing left over parts to compliment the existing bass amp. Hopefully just adding a little more stage volume.

This cabinet will be fed with the preamp signal from the SWR bass amp, so I’m using a left over power amp. It produces 380 watts at 4 ohm and peaks at 550. So it should be just fine with the powerbass speaker.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Wed Sep 04, 2019 9:54 pm

Hugh Robjohns wrote:Wouldn't be at all surprised... but I was trying to show why it's worth learning to fish rather than getting someone to go down the chippy... :-)

Seems in life I have learned to fish so many things to ever get a straight answer. Just this once , I was hoping for the easy quick answer.

Using the calc’s I find on line, I’m thinking I might just do 3” adjustable length port. I’m pretty sure it will wind up somewhere between 2.5” and 6” in length.

Cubic of cabinet = 2.28 (interior)
Speaker fs guess is 30hz (full range is 29 to 45 and previous versions were 30hz)

That’s gotta be close. If the port is adjustable, I should be able to come up with a decent compromise.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Wonks » Wed Sep 04, 2019 10:28 pm

It will sound bad with that speaker. If you've got most of the parts, then you won't waste much money, just your time. You'll just get a low powered rumble that will be quieter than your existing amp.

It's not really about watts, it's about speaker efficiency. The speaker in the SWR LA15 will have an efficiency of close to 100dB for 1W at 1m distance. Your car sub speaker is 91dB for 1W at 1m distance.

100W of SWR amp power will produce around 120dB from the combo's speaker. 300W of amp power will produce around 115dB from the S-12D. That's just over half the volume of the SWR. It's also produced over a very limited frequency range.

120dB + 115dB = 121.2dB You really won't hear any noticeable difference except a slightly thicker bass-end. You simply will not get the louder rig you really want.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby ef37a » Wed Sep 04, 2019 11:15 pm

I agree with all that has been said so far, that speaker will not add much if anything to the exiting rig.

But there is, IMO another issue, a FOAM supsended speaker for bass guitar? The instrument is THE most destructive device known to mankind and speaker! Even within its 300W rating I think the speaker will be ripped to bits in hours if not minutes.

If you do go ahead and use these components I would not go for a ported cab. Unless you get them exactly right the bass response will resemble the Alps. Many years ago I built a bass cab for a Fane 15" 200W driver. The results were pretty good but only because I had the cab and port dimensions from Fane for THAT exact driver.

Ported cabs also do not protect the driver from VLF signals. Does not matter that much for music signals at "studio" volumes. Bass guitars? Another much nastier annimal.

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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Thu Sep 05, 2019 2:54 am

Wow, very interesting. Sounds like I should just toss it all in the trash.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Wonks » Thu Sep 05, 2019 7:15 am

I'd put the speaker and the amp up for sale rather than just throw them away. Someone might want them.

That speaker is designed to be a car audio subwoofer, not a 'full-range' bass speaker (as you have in the SWR). So it really needs to be fed a signal from a crossover before its amp which limits the frequencies passed to the amp to between say 30Hz and 120 Hz. But as ef37a says, if going in a ported cab, it does need to be correctly sized and tuned cab for the speaker parameters (which we don't know). Otherwise the tuning could be way out and could damage the speaker as a result (as well as sounding bad).
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Thu Sep 05, 2019 1:25 pm

ef37a wrote:I agree with all that has been said so far, that speaker will not add much if anything to the exiting rig.

But there is, IMO another issue, a FOAM supsended speaker for bass guitar? The instrument is THE most destructive device known to mankind and speaker! Even within its 300W rating I think the speaker will be ripped to bits in hours if not minutes.

If you do go ahead and use these components I would not go for a ported cab. Unless you get them exactly right the bass response will resemble the Alps. Many years ago I built a bass cab for a Fane 15" 200W driver. The results were pretty good but only because I had the cab and port dimensions from Fane for THAT exact driver.

Ported cabs also do not protect the driver from VLF signals. Does not matter that much for music signals at "studio" volumes. Bass guitars? Another much nastier annimal.

Dave.



Trying to learn here, so thanks for everyone’s patience. I’m a little confused on some things. First let’s talk bass guitar, I don’t get why 29hz to 450hz doesn’t cover 75% of bass guitar? What’s over 380hz for bass guitar according to Fender is just harmonics. I’m just trying add not create. I may be absolutely wrong.. I really am just trying to understand. Second I’m tired of bass players showing up with stacks of tens and 5-7 string bass guitars... GET OUT OF EVERYONES FREQUENCIES. Stay low, stay where they dance and stop creating mud trying to be a guitar player. I don’t want to add or support the higher frequencies.


Last I keep saying this part... this is like running a stereo rig in some way. TWO power sections with ONE preamp. The cross over work is already done isn’t it? Is the SWR preamp out sending a signal beyond? There is no crossover in the SWR LA15 between driver and amp. It could be the preamp section goes to its own power section and then there is some sort of upper choke hidden in the plates amp?

Anyone know these answers?
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Thu Sep 05, 2019 1:34 pm

Wonks wrote:It will sound bad with that speaker. If you've got most of the parts, then you won't waste much money, just your time. You'll just get a low powered rumble that will be quieter than your existing amp.

It's not really about watts, it's about speaker efficiency. The speaker in the SWR LA15 will have an efficiency of close to 100dB for 1W at 1m distance. Your car sub speaker is 91dB for 1W at 1m distance.

100W of SWR amp power will produce around 120dB from the combo's speaker. 300W of amp power will produce around 115dB from the S-12D. That's just over half the volume of the SWR. It's also produced over a very limited frequency range.

120dB + 115dB = 121.2dB You really won't hear any noticeable difference except a slightly thicker bass-end. You simply will not get the louder rig you really want.


I can’t find any of that info published from SWR. Is there somewhere I can go to learn more about it? It’s their cheap little “big” practice amp. My last band was a trio, the drummer used a 20” bass drum with blankets and moongel all over the cymbals and snare. In that setting the SWR amp was enough for the bassist. We just ran di to the board and barely cracked on the bass amp through PA. (Bag End TA12-c mains with 1x 15” sub)

Now it’s a larger drum kit, two guitars and still 3 vocals. The SWR is enough if we’re careful with dynamics and play as softly as possible.. We just need “a Little” more. I’d be shocked if the SWR puts out 120 db’s. That driver probably weighs half what this car sub weighs.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Thu Sep 05, 2019 1:44 pm

Wonks wrote:I'd put the speaker and the amp up for sale rather than just throw them away. Someone might want them.

That speaker is designed to be a car audio subwoofer, not a 'full-range' bass speaker (as you have in the SWR). So it really needs to be fed a signal from a crossover before its amp which limits the frequencies passed to the amp to between say 30Hz and 120 Hz. But as ef37a says, if going in a ported cab, it does need to be correctly sized and tuned cab for the speaker parameters (which we don't know). Otherwise the tuning could be way out and could damage the speaker as a result (as well as sounding bad).

Hmmm... that speaker’s frequency Response is 29hz to 450hz. Why do you say to limit it to 120hz? Tying to understand.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby funky54 » Thu Sep 05, 2019 1:58 pm

http://www.mobileinformationlabs.com/Ho ... th%201.htm

Tuning 30 hz (I found that speaker tuned at 28 and 30 from other threads)
Port diameter 3”
7.09” length

Interior 2.28
Tuning 30hz
4” diameter
13.5 length

Interior 2.28
Tuning 28 hz
Port 3”
8.46 length

I don’t know what I’m doing, but guessing about the Tuning Frequency?
Bass guitar bottom end is about 50hz, so say I use 48hz for tuning..

Interior 2.28
Tuning 48hz
4” port
3.5” length

Not sure if I should use a known tuning frequency others on line have used or a frequency closer to the needed frequencies?
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Wonks » Fri Sep 06, 2019 5:13 pm

funky54 wrote:I can’t find any of that info published from SWR. Is there somewhere I can go to learn more about it?

I admit its a bit of a guess, but when searching I did find a 200W Eminence bass driver used by SWR that had a 100dB @ 1W efficiency , so I'd thought it would be pretty similar. Even if it was just 95dB, then it would still produce 115dB, which is the same as what you'd get from that car sub speaker with 300W of power. 115dB +115dB = 118dB, so in that case it would add a little, but you really would need a fair bit more than that to make a definite improvement in volume.

That 120dB is at 1m/3 feet from the centre of the cone. Drop your ear down to speaker level about 3 feet away and it will seem noticeably louder. And that 120dB will also be the peak level at the attack on the initial note and the level will quickly drop down from there, so the average note level will be a fair bit quieter. And our ears/brain tend to judge volume by average levels, not peak.

You also have to remember that the ear doesn't respond to bass sounds as well as it does to mid frequencies (refer to Fletcher Munson curves). You'll need a 40Hz note to be between 10 and 20dB louder to sound the same volume as a 1kHz note (the louder the overall volume, the less the required difference is). Which is partly why bass amps need so much more power than guitar amps to compete in perceived output. Also, the low frequency response of a speaker drops off quite quickly as the frequency reduces, so you need more power to overcome that extra loss of efficiency.

Plus because of the inverse square law for sound dispersion, standing six feet away instead of three feet drops the sound level at that point by 6dB, and nine feet away the level would be about 10dB down. So you don't have to be that far away from the amp for the sound level to be significantly lower.

funky54 wrote:I’d be shocked if the SWR puts out 120 db’s. That driver probably weighs half what this car sub weighs.

Well, its a 100W speaker and not a 300W speaker like the sub. So you will certainty need a much bigger magnet for the sub and magnets weigh a lot. I know a little bit about driver design, not a huge deal, but I do know there are all sorts of things that affect sensitivity and weight. Magnet size, voice coil gap size, flux density, chassis thickness, suspension materials etc. Going by weight alone won't tell the whole story.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Wonks » Fri Sep 06, 2019 6:42 pm

funky54 wrote:Hmmm... that speaker’s frequency Response is 29hz to 450hz. Why do you say to limit it to 120hz? Tying to understand.

Because it's designed just for reproducing bass frequencies. I find that 29Hz to 450Hz rating a bit strange, because a normal 12" speaker would produce sounds up to 5-6kHz before the output drops off quickly by around 20-25dB, but then still put out frequencies up to maybe 18kHz (mainly just from the central dome area alone, not the whole cone. So there may be some low pass filter already built-in to the speaker. Also frequency ranges are normally given at a certain point below a nominal maximum output point. As 12" speakers don't normally have a huge amount of bass extension, I'd suggest that those frequency points may well be at -20dB, instead of a more usual -10dB, in order to make the bass extension more impressive. Without a proper data sheet and frequency response, we'll never know.

But regardless, a sub is supposed to be part of an integrated system of speakers. Take a PA speaker. A typical 10", 12" or 15" PA 'top' will consist of a high-frequency driver and a chassis speaker. With passive speakers, there will be an internal passive crossover that splits the full-range input signal and lets the chassis speaker do the bass and mid-frequencies up to about 1.5-2kHz, and the HF driver the treble frequencies above that point. With an active speaker, the crossover is before the internal power amps but the general principle is the same.

The chassis speaker may have a low frequency response that tails off quickly below 50-60Hz, so will produce a fair amount of bass, but nothing that will really hit you in the chest. To do that, you add a sub. Now the sub doesn't have to produce any middle or high bass frequencies at all. The top's chassis speaker will happily produce a near-flat output down to 100Hz or so, and then start falling off a bit, until it then falls off a lot.

So you design a sub to just provide those lower frequencies, and you can boost its LF output by using ports and tunings. Obviously the bigger the speaker, the lower its frequency response is. So the LF limit is really determined by the speaker and its enclosure. Its upper frequency limit is normally set for passive speakers either by an external crossover before the power amps, or (often for lower powered subs) by a crossover within the sub itself (that often then passes on a high frequency signal to the passive tops). For active speakers, there is generally a low pass filter in the sub, and a switchable high pass filter in the tops.

For passive subs, the low pass filter frequency will be fixed, but on active speakers it's often selectable. But the selected frequency will be between 80Hz and 120Hz. It's around this point that sound stops being directional in nature and is radiated pretty much in a circular pattern. So one sub can be used with two tops with stereo material and it's fine. Two subs helps increase the bass level but you don't really need it for stereo reproduction, especially with an 80Hz crossover point.

What a small frequency range also does is allow the amplifier to supply more power across that small frequency range rather than spreading its power across a full-frequency signal. This works when you have a passive system with crossovers before the power amps, or on active systems with high and low pass filters.

Remembering that it takes a lot more power to make bass frequencies sound as loud as mid frequencies, this is the best way to get as much 'thump' into the bass end as possible.

You can see from this frequency chart that the bass produces frequencies at least up to 10kHz:

Image

Whilst those very upper frequencies aren't that important for the majority of bass duties, the mid range is very important in defining the sound, which is why you need a bass speaker that can cover them. A speaker that does just cover the bass frequencies will, as I've previously said, make a very woolly and indistinct sound. Having an extra speaker that just produces more bass won't do a great deal to make the overall bass sound louder, as a lot of that energy is really in the mids.

Now, if you could make it part of a bi-amped system, so that the sub worked from say 300Hz downwards, and the SWR 300Hz upwards, then that would make more overall sense. However you'd need to be able to put a crossover in the SWR's signal path. Unfortunately it doesn't have a series effect loop that would allow that. So at best you'd have to have a crossover before the amp, which effectively means needing a pre-amp that can send an output to a crossover, which then spits the signal to the SWR and the sub unit. Which is getting complicated and expensive.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby Sam Spoons » Fri Sep 06, 2019 7:02 pm

That chart shows the frequencies to cut or boost to get certain effects on the bass tone rather the represent the range of the instruments (the low E on a bass guitar has a fundamental frequency of 41.7Hz, but if you drop to D that becomes 36.7Hz and a low B is down to 30.8Hz).

As Wonks says much of the 'sound' of a bass is in the mid range (indeed if it wasn't you would not hear it at all on laptop or phone speakers).

I'd say the cabinet may well be usable as might the amp (depending what it is designed to drive, what actually is it?) but the speaker is likely to be a disappointment. However, since you have it all, why don't you simply bolt it into the box, connect up the amp and see if it does do anything useful. Do this as a proof of concept before spending time fabricating a port you are unlikely to damage anything (except possibly the speaker which would at least prove that it is unsuitable). Try it with the master on the SWR off to see what it sounds like on it's own and If it sounds like it might work but lacks 'oomph' then it's probably worth buying a proper bass guitar speaker to bung in there to continue the experiment.
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Re: Building Bass guitar secondary cabinet, need port length/size advice

Postby ef37a » Fri Sep 06, 2019 9:36 pm

+1 Wonks. We put a Strat through the aformentioned Fane driven from a Dominator clone (2x el84) and the sound was definitely not muddy! Quite remarkable in fact becuse of the massive speaker headroom the OD sound of the amp never became harsh.

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