|20th March 2017, 08:43 AM||#1|
Join Date: Nov 2014
12v Party Bike
I'm looking to combine my two passions (audio and bike building) and would really appreciate some assistance. I've recently built my own bakefits style cargo bike, and now I would like to kit it out with a 12v sound system so I can take it to friends houses/the park and crank some tunes (both on the way and while we are at our destination).
I have some 12v components from an old Car sound system that I salvaged and would like to put to use. Namely a 12" pioneer car sub (TS-W308D4), some JL audio 6.5's (with tweeters), all run off a Sony Xplod 4 channel amp (XM-GTR4A). It is unlikely I will be able to find a suitable deep cycle battery where I am living (Laos) so will likely have to use a 2nd hand car battery. I already have a battery charging unit that I will include in the install so I can plug it into wall mains where possible (house parties).
My current thinking is that a simple slot ported sub tuned to the 35hz region would be a sufficient cornerstone for this application. That being said I have no experience with this style of sub!
Here are the TS parameters for the 12" pioneer
Revc- dual 1.5
Levc- 2.18 (4ohm)
Cms- 1.2x10 (to the power of minus 4)
I have had a go at modelling something in Horn Response and came up with a 58l box with an Ap of 35 and Lpt of 10. That gives the box a 35hz corner and about 92db sensitivity. The thing i'm not sure about is if I am going to have issues with port velocity/chuffing. I was using an online tutorial on Home theatre shack and there was no mention of how to calculate/view this.
Also, are there any dimensional minimums i should be considering when designing the port? Ie would a 35cm tall, 1 cm wide slot port, perform in the same way as a 10cm tall, 3.5cm square port?
Would be great to have some experienced folks along for the ride on this build!
Last edited by Mark Communifi; 20th March 2017 at 10:09 AM.
|20th March 2017, 11:42 AM||#2|
Join Date: Oct 2013
WinISD can calculate air speeds. A general rule of thumb is that < 10 m/s will not result in chuffing. You can calculate the maximum air speed with this calculator: Port Flares - Evaluation of noise
Slot ports should have an aspect ratio no larger than 9:1 (it's a bit arbitrary, I know), or they will be too lossy. Closer to round is better.
Last edited by TBTL; 20th March 2017 at 11:45 AM.
|20th March 2017, 01:27 PM||#3|
Join Date: Feb 2008
A 35cm^2 port is going to have serious problems. Hornresp can check vent velocity, but I predict it'll be well over 50m/s, even with modest power inputs.
FWIW, I found a single 3" port to be just about enough for a pair of 8" PA midbass drivers with modest Xmax. You've got similar cone area and twice the potential excursion, so I'd be looking at a pair of 3" ports (~88cm^2) minimum.
|22nd March 2017, 03:10 PM||#4|
Join Date: Nov 2014
Thanks Chris661 for the advice!
A few evenings on Horn Response later and i've come to a solution that seems to take port velocity into account (took a while to figure out I had to switch the Acoustic power output to 'port only' to unlock the particle velocity tab in the menu
73L gross volume (50L chamber+18L port+5L bracing etc)
225cm2 port cross section
23m/s port velocity @ driver xmax.
35hz extension (120db half space at full gas).
Working on some draft layout's of the equipment on the cargo tray now! Also might have a lead on some deep cycle batteries (saw a shop selling solar panels, LED lights and other electronics while out on a bike ride earlier this week and saw they had batteries on their promotional screen out the front).
|23rd March 2017, 04:27 AM||#5|
Join Date: Sep 2005
Location: the leafy west of Brisbane
A couple of sealed lead-acid batteries may be all you need.
A speaker-builder's parable: "That burned down, fell over, then sank into the swamp. But the fourth one stayed up. And that's what you're going to get, Lad, the strongest castle of all."
|24th March 2017, 05:41 AM||#6|
Join Date: Nov 2014
OK so time to talk batteries and run time! So I have found a shop in town that sells 12v AGM deep cycle batteries. These are attractive because they do not release gas when they are being charged and they can be mounted in any orientation. The thing that I am now trying to make my mind up about is unit size (AH) and thus weight/run time etc.
The batteries that are available at the shop are
100AH/10hr ($190 USD)
65AH/10hr ($124 USD)
40AH/10hr ($76 USD)
The 100AH battery is just too large and heavy for me to really consider for this project. Once the drivers, ply, battery, head unit and all the other gear gets stuck on the bike I still want to be able to ride it!
According the the manual for my 4 channel amplifier it says that with all channels being powered to their rated output there is a 48a current drain. Now it is unlikely that I would ever be thrashing this thing off the battery at full steam for very long (when it is parked at someones house party and plugged into the wall is another consideration all together ).
So i'm really just thinking about the 'sound on the go' component of this project (picnics by the river/lake, critical mass events, nude bike ride, protest events etc). So it would seem that at 1/2 power the battery would provide
40Ah = 1.6hrs
60AH = 2.5hrs.
I do realise that I could reduce significant weight, increase run time and output with the combination of a more efficient amp and more sensitive speakers- but the point of all this was to re-use existing components. If i do find I want more 'oohpf' my mate and I have some 2 way passive synergy horns that I can use instead of the little 6.5"s.
So at this stage it looks as though tomorrow morning i'll be heading out to buy a 65AH battery. Anyone with dissenting views?
|24th March 2017, 08:07 AM||#7|
Join Date: Aug 2002
The battery should only be drained at most to 50%, otherwise it won't last very long. I'd consider going for a 2 way mono setup with a tiny amp on a horn / CD combo paired with a good efficient bass unit run off a modest ~30w digital amp. (Basically use the smallest output amps you can get away with to ease the load on the battery)
A small solar panel may help to extend the time the system could be used.
Edit: Just re-read your post and noticed the bit about using existing speakers so ignore me
|24th March 2017, 10:16 AM||#8|
Join Date: Feb 2008
Battery drain is a complicated subject, but here goes.
There's a few concepts to get your head around, and then I'll tie it all together with some equations that you'll be able to use to calculate battery sizes.
First up, peak-to-average ratios (aka duty cycle) and how they apply to music.
Peak-to-average ratios describe the ratio between the average power (which is related to how much is being pulled from your battery) and the peak power (which is when your amp clips).
- square waves have a 0dB peak-to-average. All of the signal is at the peak voltage.
- sine waves have a 3dB peak-to-average. If a 100w amplifier is just clipping, the speaker is receiving 50w of continuous power.
- pink noise has a 9dB peak-to-average. That's why PA amplifiers are often rated for power draw at 1/8th power - that's what they'll pull from the wall when they're just clipping pink noise into the rated load.
It's important to note here that the RMS power rating of a driver is found with a pink noise signal that's clipped to 6dB peak-to-average. So a 1000w rated driver will be at its thermal limits with a 1000w amp playing that pink noise, or a 500w amp playing sine waves, or a 250w amp playing square waves. The heating power for all of those is the same, but the peak voltages are different.
Now, someone high up in some organisation decided that pink noise driven just to clipping would be representative of real-world use. Generally speaking, it's not far off. However, we're on diyAudio and we're going to do things properly.
Here's an example piece of music (caution - NSFW lyrics from the start): https://www.youtube.com/watch?v=UnQY3swM4tA
I've chosen this because it's very very demanding of subwoofers. When the bass drops, it's a 36Hz compressed sine wave, so the peak to average ratio is less than 3dB below 100Hz. That's evil. The 1000w rated driver earlier would burn if a 500w amp was playing the LF signals from that track at clipping.
This has a knock-on effect for our power draw, which we'll look at soon.
Note that the mid-high range above 100Hz or so would have an nice easy time of things.
Here's a different example track: https://www.youtube.com/watch?v=h0ffIJ7ZO4U
There's not much bass content, and what is there is very peaky (kick drum). The mid-high range is pretty busy in comparison, though.
So, the music you're going to play will have a serious influence on what your battery life will be.
Lets work the examples. We'll take a 100w low-frequency amplifier, and a pair of 50w mid-high, and were going to run at clipping.
The dubstep track above will have around 50w average power from the LF amp (3dB peak-to-average), and probably more like 1w/ch average power (17dB peak-to-average, which is an educated guess) from the mid-high amps. Total average output around 52w.
The other track above will have much lower average power for the bass, but higher for the mid-high range. Lets say it's 10dB peak-to-average for both. The LF amp will be putting out 10w average power, and the HF amps will be doing 5w each. Total average power: 20w.
For most music, good working figure would be 10dB peak-to-average overall. If run-time is critical, bet on a 6dB peak-to-average overall, and you're likely to have some juice left.
If you're going to be playing dustep/EDM, bet on 3dB peak-to-average for the LF amp, and 10dB for the mid-highs.
Remember that speaker sensitivity comes into play - if you gain 3dB in sensitivity, you've halved your power requirements. That's a Very Good Thing. Battery size is usually proportional to the number of watt-hours it'll hold, so if you halve your power requirements, chances are you can cut the battery size in two.
Next up, amplifiers
There's two components to calculating an amplifier's power draw:
- idle draw
- converstion efficiency - how much power going in comes out the other end
This is a little complicated, since the conversion efficiency has the idle draw built-in, and also the efficiency of switch-mode amplifiers (which have a lot of advantages that make them perfect for battery systems. I'm just going to assume you're using one, because you should be) is not linear with output power.
At high power, switching amps will approach 90% efficiency. At lower power, that can go below 50%.
Now, while conversion efficiency ratings will have the idle draw built-in, I usually pretend it doesn't. It makes the calculations easier, resulting in a slightly bigger battery. No big deal. Otherwise, you've have to work out how long music is playing for, how long it's not, etc, resulting in lots of "bits" of equations that you'll have to string together with a stopwatch.
So, you'll need to either do some measurements on your head unit, or take some data from the datasheet.
I'd bet on 75% working efficiency on a class D amp, which is an average over the useful power region.
Power draw = idle power + (power output/efficiency)
You can convert between volts, amps, etc, later.
SPLs and power
As I alluded to earlier, 3dB is a doubling of amplifier power. If I have a speaker running on a 10w amp, and I change out to a 20w amp, I'll gain 3dB of volume, and also increase the draw from the battery according to the equation just above.
10dB is a factor of 10 in amplifier power. +10dB sounds twice as loud.
There's a little misinformation online regarding decibels, so here's the run-down:
1dB difference is noticable to most listeners.
3dB is obvious
6dB sounds half as loud again
10dB sounds twice as loud
SPL falls off at 6dB per doubling of distance, 20dB for a factor of 10. Most SPLs are calculated at 1m.
Portable speakers often use large PA speakers with tiny amplifiers and batteries. PA speakers often have high sensitivity, so require smaller power inputs for a given SPL.
For example, a 15" 95dB@1w speaker needs 10w to achieve 105dB. A 5" 85dB@1w HiFi speaker will need 100w. The PA speaker will be loafing along comfortably, while the smaller HiFi speaker might well be getting to the end of its rope.
Finally, batteries and electronics
There are lots of different sorts of batteries out there, but they all have something in common - they do not like to be shorted.
As soon as your battery arrives, connect a fuse holder to one side, and then insulate that terminal. Seriously, just do it.
I've seen a single C-cell NiCd battery go off, and it went off like a stun grenade. NiCd isn't particularly energy-dense, either. I wouldn't want to see anything lithium-based, or anything big and lead-acid go off.
Note that if you drop a spanner across a car battery, it will weld to the terminals and the spanner will burn you if you touch it. This isn't some hollow warning, this is serious. You're looking at major injuries if someone happens to drop something that connects the wrong terminals together and the battery explodes, so get a fuse on there and make sure the battery is protected.
Battery storage (in watt-hours) = operating voltage x rating in amp-hours.
Let's do some equations
I'm going to do the maths for an imaginary system so you can do your own maths on yours. I'll happily sanity-check your work, but I'm not going to do it for you. What I've written here should be enough to help you calculate your own system.
I'm going to work out how much power/speaker sensitivity I'll need for a given SPL target. If you've already got your speakers and amplifiers, you can skip ahead.
So, here's the run-down of numbers:
- Target: 90dB at 10m peak SPL from all bands
- I'm going to be playing dubstep and some chart stuff
- Amplifier idle draw will be 20w per channel (you need to measure yours).
- I've got a bluetooth module and some other input electronics that run up 10w total.
- I'm going to call the amplifier at 75% efficiency.
- I want to run at full power for 10 hours.
I can work out that 90dB at 10m means I need 110dB at 1m.
I fire up the simulation software, and find my subwoofer has a sensitivity of 87dB at 1w, so it needs 200w peak power to get there. The mid-high range have 93dB@1w each. You'll get some combining at the lower midrange, but if anything in the music is hard panned left or right, then all the power needs to be delivered to that speaker. Each one, therefore, will need 50w.
My music choice gives me a 3dB peak-to-average ratio in the bass, and I'm going to play it safe and say 6dB peak-to-average ratio for the rest of the range.
Note that all of these decisions lead to an over-built system. There are places you can compromise. It's up to you to work out where you can safely compromise.
So, we can work out the power draw for each range:
Bass: 20+(100/0.75) = 153w. The 100w is taken from the 200w peak power and the peak-to-average ratio.
Mid-high: 2x(20+(12.5/0.75)) = 73w.
Total amplfier power draw: 226w
Add in the 10w total for bluetooth, crossover, whatever, and you've got 236w draw when music is playing.
I'm going to run a 24v system, since the higher voltage often plays nicer with kit amplifiers (the higher rail voltage means they won't need voltage converters to get high power output). You could run 12v, but I want to run 24v.
So, 236w average draw on a 24v system. That's about 10A continuous. I want 10 hours of that, so that's a 100Ah battery. It's gonna be big!
As I mentioned earlier, though, there's a lot of over-building in there. This system I made up will definitely run for 10 hours of continuous dubstep at 90dB at 10m.
Now you can work out your system!
My work: www.grimshawaudio.com
|24th March 2017, 11:34 AM||#9|
Join Date: Nov 2014
Amazing post Chris! Really nailed the issues down for me. I must say I was a little surprised when I saw the run times that I had calculated as I remember first hand using the sound system in my car at a bush doof at moderate levels and getting about 4 hours of run time before I needed to start the engine to charge the battery again- and that was a regular high draw car battery! I had not thought to delve into the issues around standardised amplifier ratings and how that might relate to the music genres the system would be playing.
Hopefully all your advice will not only help me but also be of use to some other sucker who stumbles on this thread while researching a hair briained 12v sound system project.
I must admit I am a bit of a bass head myself so your hypothetical calculation about battery size and SPL is hitting close to home .
My calculations pan out in much the same way as your hypothetical one does, about the same speaker sensitivities, amp draw and duty cycle expectations. However my output and longevity expectations are somewhat more modest. 100Db at 8m and about 3 hours or run time would suit me just fine.
It appears as though the 65AH battery will be sufficient for my needs in this regard. I'll head out in the morning and pick one up! (and immediately attach my fuse kit to it and insulate the terminals- safety first right!)
Down the track I could look at getting a more efficient amp, or drivers to deal with some deficiencies in the current gear I have to play with. I am also interested in looking at some rather nifty looking 50w 12v flexible solar panels that would fit on the top of the cargo tray quite nicely and help keep things ticking over on a pleasant day!
|24th March 2017, 02:48 PM||#10|
Join Date: Jan 2008
you will probably have to make a 50w class D amp to get 10 hours of play time.
2 * 50w RMS for left and right channels and 2 * 50w Bridged to give 200w for the Subwoofer
idle power would be around 1.5w per amp with 96% power going to the speakers at all power levels greater then 5 watts
2 15 AH batteries could do the job.
You will also need a 12v switching regulator to power the head unit (2 - 4 amp should work), or use 1 20 AH battery and 1 15AH
if you use the 12v regulator , you can charge the batteries in series with a 28 volt 2 amp power supply.
Now if you are really thinking outside the box,
get an Electric bike kit and add a second Lipo4 battery pack to it
getting to the gig is easier and the 40v 5 AH batteries can be recharged 2000 times
2 packs would allow 1 to help you get there and 1 to power the system. both packs charge off the LIPO4 battery charger
you would need 40 to 12v switching regulator and run the class d amps off of the 40 volt supply
3 lead acid = 36v charging = 14 * 3 = 42v which means they should charge off the LiPO4 pack charger
Last edited by stocktrader200; 24th March 2017 at 03:17 PM.
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