Hey all, I hope this is the right forum to ask, since this will be a boombox with separate woofers and tweeters.
I'm building a battery powered boombox, something along the lines of the Boominator (there's an instructable by "lutkeveld") but more reasonable. The general idea is to make it 1) loud, 2) have acceptable low frequency response, and 3) have at least 6 hours of battery life at high volume. I've been reading up quite a bit on the electrical side of things and I think making the enclosure as acoustically correct as possible is necessary to maximize battery life and low frequency response. Stop me here if that's wrong. The boombox will most likely have 4 drivers - 2x 8" or 10" woofers and 2 tweeters. Parts Express has "optimal cabinet dimensions" for their woofers, and I understand what cabinet size means: this many internal cubic feet will give you the best response. Vented boxes let the speakers hit lower frequencies, so my plan is to build a vented cabinet.
What I'm a little stuck on: "Vented F3". My understanding is this is effectively the corner frequency where the sub starts rolling off. I know the physical dimensions of the box determine F3. What I don't know is how do you design a box to have a specific F3? Or if you already know the measurements, how do you calculate F3? And regarding the vent: do I need to have a tube on the inside (like in most bass reflex ports) or can it just be literally a hole cut in the side of the speaker as it is in the Boominator (in lutkeveld's instructable)? And how do you calculate the size of the hole or the size and length of the pipe?
Going further with the vent, I've heard of needing a *high* pass filter on woofers to prevent them from damaging themselves at inaudible frequencies (say 40Hz for a portable system like this). Would it be a good idea to include this in the design of a fairly high-output system like I'm trying to build? Or would it just be a waste of time because these drivers will be running from either a 2x15W or 2x25W amplifier?
Google has led me to good explanations of what everything means, but not much in the way of "how do I actually calculate these dimensions". Also if you can offer any pointers on portable speaker design, I'd be happy to listen.
I'm building a battery powered boombox, something along the lines of the Boominator (there's an instructable by "lutkeveld") but more reasonable. The general idea is to make it 1) loud, 2) have acceptable low frequency response, and 3) have at least 6 hours of battery life at high volume. I've been reading up quite a bit on the electrical side of things and I think making the enclosure as acoustically correct as possible is necessary to maximize battery life and low frequency response. Stop me here if that's wrong. The boombox will most likely have 4 drivers - 2x 8" or 10" woofers and 2 tweeters. Parts Express has "optimal cabinet dimensions" for their woofers, and I understand what cabinet size means: this many internal cubic feet will give you the best response. Vented boxes let the speakers hit lower frequencies, so my plan is to build a vented cabinet.
What I'm a little stuck on: "Vented F3". My understanding is this is effectively the corner frequency where the sub starts rolling off. I know the physical dimensions of the box determine F3. What I don't know is how do you design a box to have a specific F3? Or if you already know the measurements, how do you calculate F3? And regarding the vent: do I need to have a tube on the inside (like in most bass reflex ports) or can it just be literally a hole cut in the side of the speaker as it is in the Boominator (in lutkeveld's instructable)? And how do you calculate the size of the hole or the size and length of the pipe?
Going further with the vent, I've heard of needing a *high* pass filter on woofers to prevent them from damaging themselves at inaudible frequencies (say 40Hz for a portable system like this). Would it be a good idea to include this in the design of a fairly high-output system like I'm trying to build? Or would it just be a waste of time because these drivers will be running from either a 2x15W or 2x25W amplifier?
Google has led me to good explanations of what everything means, but not much in the way of "how do I actually calculate these dimensions". Also if you can offer any pointers on portable speaker design, I'd be happy to listen.
F3 = 0.28*Fs*Qts^-1.4
Vent formula/calculator: Ported Vented Box Equations Formulas Design Calculator Lv Port Vent Length
As you can see, for a given net Vb, Fb, the vent's diameter and number of vents sets how long it is. There's also a minimum vent diameter required to keep it from 'chuffing': https://www.trueaudio.com/st_ventq.htm
Maximizing battery life requires minimizing current draw, so you want the highest practical impedance and box acoustical efficiency, especially if you want it loud for any length of time. Unfortunately, drivers that perform well down into the bass in small cabs have abysmal efficiency.
Yes, you'll want a high pass filter to protect the drivers.
In short, you ideally want woofers with a low Vas, Qts along with some weather protection, so folks tend to compromise by using [cheap] mobile audio or weatherproof motorcycle drivers in high tuned lossy sealed/~aperiodic damped alignments since the vents are otherwise too big/long, i.e. only efficient down to maybe 100-120 Hz with a significant 'hump' in the response before rolling off ['thumping'/'booming' bass].
FWIW, back in the '70s I built my daughter a mono boombox radio using a 'curb queen's' 1950s Motorola console's 12" 'full-range' driver + small compression tweeter horn with just a cap for an XO in a damped open back [~aperiodic'] cab just big enough to hold it all, which drowned out the huge stereo BBs just starting to hit the market due to the driver's relatively high efficiencies. A pity such drivers are no longer available AFAIK.
Anyway, search DIYaudio and other DIY audio forum sites for boombox build threads.
GM
Vent formula/calculator: Ported Vented Box Equations Formulas Design Calculator Lv Port Vent Length
As you can see, for a given net Vb, Fb, the vent's diameter and number of vents sets how long it is. There's also a minimum vent diameter required to keep it from 'chuffing': https://www.trueaudio.com/st_ventq.htm
Maximizing battery life requires minimizing current draw, so you want the highest practical impedance and box acoustical efficiency, especially if you want it loud for any length of time. Unfortunately, drivers that perform well down into the bass in small cabs have abysmal efficiency.
Yes, you'll want a high pass filter to protect the drivers.
In short, you ideally want woofers with a low Vas, Qts along with some weather protection, so folks tend to compromise by using [cheap] mobile audio or weatherproof motorcycle drivers in high tuned lossy sealed/~aperiodic damped alignments since the vents are otherwise too big/long, i.e. only efficient down to maybe 100-120 Hz with a significant 'hump' in the response before rolling off ['thumping'/'booming' bass].
FWIW, back in the '70s I built my daughter a mono boombox radio using a 'curb queen's' 1950s Motorola console's 12" 'full-range' driver + small compression tweeter horn with just a cap for an XO in a damped open back [~aperiodic'] cab just big enough to hold it all, which drowned out the huge stereo BBs just starting to hit the market due to the driver's relatively high efficiencies. A pity such drivers are no longer available AFAIK.
Anyway, search DIYaudio and other DIY audio forum sites for boombox build threads.
GM
@GM: Excellent, thanks for the info.
Another question: on the Parts Express "Build a Speaker" resource page, they recommend (basically say it's required) that you use sound dampening material on the inside of the cabinets to prevent standing waves and reflections on the woofer cone. Additionally they say the effective volume of a cabinet increases by 10%-25% depending on the damping material used. Is this necessary/worth it in a portable speaker? And is that requirement (and effective volume) the same for vented boxes?
Going off the assumption it is, I think these are the general interior dimensions I'll build the boombox: 23" wide, 13" tall, 7" deep. Two ports, 1.8" in diameter each, which should tune the box to 58Hz. These won't have a tube on the inside; I figure if I use 1" thick MDF (or whatever building material) then the vent length will be ~1" -> no need for extra parts. I checked that link to prevent chuffing, with his equation the diameter of the vent needs to be more than about 0.2 inches, so 1.8" vents should be fine.
I'm a little fuzzy on if the vents actually set the F3 frequency to 58Hz or if the dimensions will come into play. Every resource I've seen seems to say that "if you build a vent of X area and Y length, while your box is V volume, then the F3 will be [equation]", regardless of the specific length, width, and height.
So those are the inside dimensions and assume a ~17% effective volume increase (that's what Parts Express used) from dampening material that'll go on the inside. Here are the woofers I'm looking at: (Celestion TF1018 10" Professional Speaker 100W) and here are the recommended box specs: vented volume 1.47 cubic feet, vented F3 58Hz. Here are the tweeters I'm thinking of using: (Tymphany BC25SC06-04 1" Textile Dome Tweeter). Those woofers can reasonably get up to ~2.5kHz, and I believe the tweeters would be fine being running down to 2.5kHz, so that's where I'd set the crossover point (probably 2nd order). Additionally the woofers and tweeters have the same rated sensitivity so I'm hoping there won't be much need for an L-pad (if that's the right thing for attenuating a tweeter).
Edit: derped and forgot my possible layout: http://i.imgur.com/0NH5Vha.jpg Forgive the crudeness, this was just to check if those dimensions would actually fit the drivers and ports. I'm thinking the tweeters might be better placed outside the woofers rather than between, but I don't know if it'll make any real difference. For structural integrity I could make the box less deep and wider/taller to have more material surrounding the speakers on the front, but every inch I take off the depth will make it harder to work on the inside. I don't know if that'll be an issue.
Now the next two issues:
1) How do I construct the HPF for the woofer? Do I just stick the HPF in series with the crossover LPF? And would I calculate it using the same equations as for a HPF for a tweeter?
2) I've been talking to lutkeveld, and he highly recommends using a baffle step compensation circuit (really, a dipole mounting scheme for the woofers, but a BSC will effectively do the same thing). Do I put this before the crossover? I have a feeling having this many capacitors and inductors off the same line (left speaker/right speaker) will lead to some unintended consequences.
Another question: on the Parts Express "Build a Speaker" resource page, they recommend (basically say it's required) that you use sound dampening material on the inside of the cabinets to prevent standing waves and reflections on the woofer cone. Additionally they say the effective volume of a cabinet increases by 10%-25% depending on the damping material used. Is this necessary/worth it in a portable speaker? And is that requirement (and effective volume) the same for vented boxes?
Going off the assumption it is, I think these are the general interior dimensions I'll build the boombox: 23" wide, 13" tall, 7" deep. Two ports, 1.8" in diameter each, which should tune the box to 58Hz. These won't have a tube on the inside; I figure if I use 1" thick MDF (or whatever building material) then the vent length will be ~1" -> no need for extra parts. I checked that link to prevent chuffing, with his equation the diameter of the vent needs to be more than about 0.2 inches, so 1.8" vents should be fine.
I'm a little fuzzy on if the vents actually set the F3 frequency to 58Hz or if the dimensions will come into play. Every resource I've seen seems to say that "if you build a vent of X area and Y length, while your box is V volume, then the F3 will be [equation]", regardless of the specific length, width, and height.
So those are the inside dimensions and assume a ~17% effective volume increase (that's what Parts Express used) from dampening material that'll go on the inside. Here are the woofers I'm looking at: (Celestion TF1018 10" Professional Speaker 100W) and here are the recommended box specs: vented volume 1.47 cubic feet, vented F3 58Hz. Here are the tweeters I'm thinking of using: (Tymphany BC25SC06-04 1" Textile Dome Tweeter). Those woofers can reasonably get up to ~2.5kHz, and I believe the tweeters would be fine being running down to 2.5kHz, so that's where I'd set the crossover point (probably 2nd order). Additionally the woofers and tweeters have the same rated sensitivity so I'm hoping there won't be much need for an L-pad (if that's the right thing for attenuating a tweeter).
Edit: derped and forgot my possible layout: http://i.imgur.com/0NH5Vha.jpg Forgive the crudeness, this was just to check if those dimensions would actually fit the drivers and ports. I'm thinking the tweeters might be better placed outside the woofers rather than between, but I don't know if it'll make any real difference. For structural integrity I could make the box less deep and wider/taller to have more material surrounding the speakers on the front, but every inch I take off the depth will make it harder to work on the inside. I don't know if that'll be an issue.
Now the next two issues:
1) How do I construct the HPF for the woofer? Do I just stick the HPF in series with the crossover LPF? And would I calculate it using the same equations as for a HPF for a tweeter?
2) I've been talking to lutkeveld, and he highly recommends using a baffle step compensation circuit (really, a dipole mounting scheme for the woofers, but a BSC will effectively do the same thing). Do I put this before the crossover? I have a feeling having this many capacitors and inductors off the same line (left speaker/right speaker) will lead to some unintended consequences.
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