I started blogging over a year ago and forgot the anniversary. I did post the first posts on 17.03.2013 and wrote them a couple of days before. What was I doing at the same time this year? I hiked the Bodenstein and Harz Mountains and visited Leipzig for almost two weeks to meet old friends and make new ones. So I actually did quite an anniversary celebration without even realizing. Not too bad at all.
German wikipedia provided something interesting. The resonance effect is working best from 0,75*fb to 2*fb (resonance frequency of the enclosure). Subwoofer normally operate from approx. 20Hz to 80Hz.
The previous owner tuned the enlosure to 27Hz, to work nicely from 20Hz to 54Hz. It would have been interesting to adjust the crossover (80Hz, 4th order in my case) to 54Hz and to run it as a deep bass addon with my living room stereo. But since I’m running it with small satellite speakers on my desk 27Hz – 72Hz seems like a good choice…
Friends pointed out to me that they don’t understand my writings about building/optimizing speakers and don’t even know what to google for. Sorry, guys, I was just writing down what I was doing and not thinking about that.
The Variables (Fs, fb, ….) are so called Thiele-Small Parameters, named after two aussie guys doing research on the matter. Wikipedia provides links to sites explaining the stuff in both german and english.
Don’t let the math scare you away, you don’t need it. To build a woofer you just have to look up these values and put them in some online calculator which is spewing out build dimensions. Just make sure to get a second opinion on the results, be it through research, online forums or just asking folks…
“Yeah Science, Bitch” -- Jesse Pinkman
I did some research. The concept of tuning a vented box very low is called EBS (Extended Bass Shelf). This site explains it pretty good, the basic idea is getting more bass on low frequencies. You basically take an enclosure 160-180% the size of a “maximum flat” setup and then tune it close to Fs (Free Air Resonance of the speaker). As I already found out the previous owner did tune the enclosure to about that frequency. But how about enclosure size? The manual suggests “maximum flat” enclosures from 64 to 92 litres, suggesting an EBS setup between 102 and 165 litres. The enclosure has around 80 litres not accounting for the speaker and the vents. Well, I guess I found a problem here.
I tried some more online calculators and plenty of them tune close to Fs without even mentioning it. So it’s more than understandable to end up with this setup. But as always, you gotta trust your senses: If it sounds shitty there has to be a reason… Ah, and trust the engineers. If you deviate far from their suggested path you better know what you are doing…
So to wrap this up and continue other important work I’ll go the humble approach. I’ll measure the actual resonance frequency of the enclosure and tune it to something between 36-39Hz for the suggested “maximum flat” response.
Measuring shouldn’t be to difficult, the impedance of the speaker drops at the resonance frequency, so the current spikes ( I = U/R ). So just rig an ampere meter to the speaker, a frequency generator to the amplifier and play with the frequency knob until you reach maximum current. Well, my ampere meter doesn’t like low frequencies at all making it impossible to get a reading. Using a shunt (resistor in series) and measuring voltage I pinpointed the resonance frequency at 35,7Hz. That’s way lower than the expected 43Hz. Good news: That’s the lower end of my target frequency. Bad news: I’ve got to redo the math…
Some site suggests if the distance of the ports to any wall is less than their diameter the tuning gets lower. That’s definitely the case here… The increase of Volume by damping depends strongly on the material, ranging from 0-20%. If I run the numbers with an estimated volume of 115 litres everything checks out and the estimated resonance frequency with the long vents would be around 24Hz. I guess I have to measure that as well Hmmm, 27Hz, not too bad for an estimate…
Hmm, maybe I should give the 31Hz a shot. 18,5 cm for the vents… Let’s say 20 for good measure… After some try and error 21cm turn out to lead to 30.something Hz. Good enough. Yeah, there is more ooompfff on low frequencies, you don’t hear some but feel it in your guts. I had to adjust the woofer level back to +6db on the amp. But it seems to be a trade-off with quality. Most of the time there’s not enough bass and then suddenly it’s killing you when the frequency drops low enough. For example with “Clint Eastwood / Gorilaz”, some low rumbling for the whole song and then the break with a couple of low beats is just plain awful. Somebody suggested that EBS setups often have cancellation issues on the two octaves above the resonance frequency (so 30-120Hz). Yeah, that might be the issue here.
To cut a long story short: EBS doesn’t work out with this enclosure, even if you do it right. I guess that’s 2:0 for the engineers… Disclaimer: I’m working low volume settings (neighbours…) and the placement of the sub is bad. Maybe it’s getting better with optimal placement and if you crank up the amplifier. Maybe not. At least now the sound setup at my desk is ok. Time to actually do some work…
The new subwoofer has bass, I’ll give it that. But sounds like a trumpeting elephant somehow. Well, it’s probably not the speaker itself. The broken crossover / connection panel is already removed. Hmm, let’s have a look at the mods of the previous owner.
He reduced the volume by adding an enclosure for the connection panel, added some foam isolation and adjusted the length of the vents. Alright, the enclosure lowers the volume (obviously), the insulation increases the (virtual) volume of the enclosure (not so obvious). Measuring inside the box is difficult, I’d estimate the volume about 70 litres so with the insulation it’s probably somewhere between 70 and 80 litres. The original vents have 75mm diameter and are 120mm long. The previous owner extended them to 350mm / 360mm using pipes from the hardware store and claims to have calculated everything with some software.
The vents should be the same length. They are extremely close to the rear wall of the box but I couldn’t find proof that this is an issue. Let’s start with the obvious: Some designer made this enclosure and even if it’s a cheapo they probably put some thought in it. The vents adjust the resonance frequency of the box. Changing them by a factor of three means either the engineer who build it was way off or the one doing the modification. I used an online calculator to run the numbers. With the extensions the resonance frequency (fb) of the enclosure is around 29Hz, without approx. 43Hz. The manual of the speaker comes with 12 suggested enclosure dimensions ranging from 36 to 50Hz fb. The interwebs suggest it makes some sense to go for a “bass shelf” by tuning fb to Fs (30,83Hz) or even lower. One formula I found suggests 31Hz being ideal for this setup.
The manufacturer of the speaker and the manufacturer of the enclosure obviously don’t think so. I removed the extensions and the setup is quite close to the “Standard Tight Bass” box design of the speaker manufacturer. Afterwards I had to reduce the amp setting from +7 to +3 (probably db) and it sounds pretty tight.
I guess I’ll rig up a setup to actually measure the resonance frequency to check my estimates and tune the box to “optimal sound quality / flat response”.
My Desktop Stereo isn’t quite performing as I want it to. The JBL Control One are awesome but have no bass. Bi Amping with a leftover subwoofer from my station wagon didn’t really do the trick, a 25cm speaker is ok for a confined space like a car but sucks in bigger rooms as for example my office. Ebay provided some auctions with interesting items and I got lucky: A Hifonics OLM 1615 38cm Monster with a vamped up enclosure of a much cheaper omnitronics speaker.
DIY at it’s best, so it went cheap. 76€ for something that’s worth a couple of hundreds is more than alright. Then something weird happened:
Seller: I would like to cancel the deal because I dropped the speaker and there’s some rattling noise inside / something broke even though there’s no visual damage. I’m willing to pay some compensation for your troubles.
Me: Well, I can totally understand if you’re not satisfied with the price but it’s not my problem if you don’t keep track of your auctions. These speakers are designed to take quite a beating so it seems highly unlikely you broke it by dropping it. Are you really sure it’s broken?
Seller: Yeah, it’s broken. I’d send you 50€ if you cancel the transaction.
Me: Maybe I can repair the speaker. How about we take the 50€ off the price, so I pay you 26€. If I can’t fix it I still got the enclosure and you got rid of the broken speaker…
Seller: No thanks, I’d rather bring it to my dealership to get it fixed and then sell it on ebay again.
Me (to myself): Gotcha!
Me (to seller): Sorry to say but you raised my suspicions. By german law we have got a contract so you dropped my speaker and not yours. You owe me compensation for the difference between the 70€ and the normal street value of the item. In addition I’ll contact ebay’s customer support and they’ll probably ban you.
Seller: Alright, I’ll get the speaker fixed and then send it to you but it will take some time.
Seller (next day): It’s fixed, a large paperclip somehow got stuck inside. It’s on it’s way.
So yeah, there are quite some dickheads on ebay. The speaker arrived in the meantime, it’s got some minor scratches and the crossover is fried. The capacitor blew and the circuit board got cracks. Luckily I’m using active crossovers anyway, so I realized that while removing it. So I already fixed one problem. The sound is still not where I want it but the vents of the enclosure are loose and possibly to close to the back of the enclosure. I guess I’ll do the math on it as well, just in case the previous owner fucked it up.
I hate nagging about stuff. The book really seems like a good read. Robinson Crusoe stranded on mars. But the scientific stuff drives me bonkers. It’s really not my area of expertise so I’d actually would like to learn stuff. But plenty of stuff just seems wrong.
For example producing water from Hydrazine (rocket fuel) and Oxygen. Yeah, it works more or less the described way by using Iridium as a catalyst to split Hydrazine into Hydrogen, Nitrogen and some Ammonia. Then burn Oxygen and Hydrogen to get water, no biggy…
But… While working in the habitat with earth-like conditions with an open flame (and a hot catalytic converter) he ends up in an Atmosphere of 22% Nitrogen, 9% Oxygen and 64% Hydrogen and flees the habitat because he fears it might explode. If you mix normal air with hydrogen it ignites at 4% H2, explodes around 18% and will stop igniting around 78%. These numbers shift due to the low Oxygen Level but still, these numbers are still way past boom. Maybe his open flame didn’t burn all the time and he really kept to catalytic converter below 560C, at least the book doesn’t mention it.
After fleeing the habitat and figuring out a plan he reduces the oxygen level of his home to 1% and puts on an oxygen mask (quite like the ones in hospitals) which enriches his breathing air to normal oxygen levels. Like this, he ensures to keep everything from exploding while burning the hydrogen with controlled o2 bursts. And then, there is a big explosion. Dafuq? Yeah, his exhaling is claimed to have risen the O2 level high enough for an explosion. Well, standing there with an O2 hose for the short bursts in one hand and a spark generator in the other he managed to exhale enough O2 to get an explosion. More likely he’d literally breath fire due to the proximity of hydrogen, oxygen and ignition source.
But even if he manages to exhale somewhere else, a human only exhales like 12m3 of air per day with a little more than 1m3 of O2. His habitat has 90m2 and I’d guess at least 2m in height leading to 180m3 of volume. In just a couple of hours he can’t make a significant raise, even in a day he would only get from 1% to 1.something %…
I soldiered to that part but then it came to electronics. Fuck. That’s where I decided to do a blog post about it..
Alright… He needs to cover some distance on mars and has two battery powered mars rovers. Each has a capacity of 9000Wh allowing 35km of travel. Well, my van in comparison has 2400Wh to power the fridge, stereo, laptop and such. A Tesla Roadster has 60000Wh allowing approx. 300km of range. So a mars rover has a much smaller battery than a electric car on earth and a much worse mileage while driving much much slower. For comparison the Apollo Lunar Rover had 8676Wh lasting for 90km. Let’s just say future NASA isn’t exactly packing big batteries. Maybe they’re just more efficient in using them… Wrong! The use them for heating the rover as well. It drains 400W per hour for heating. My van is using gas (LPG) for heating because of the immense amount of power needed for the job. My heater produces 3200W of heat when running. Let’s say 11kg of LPG last for a week in Winter. That’s 141570W of heat per week or 842W per hour. So heating my van on earth takes twice as much energy as heating a rover on mars. Future NASA is really good on insulation!
The solar panels on his Habitat have an “astounding efficiency level of 10.2%” (quote)
Yeah, in 1975 this would have been astounding… I guess even the cheap Polycrystalline Panels on my van are more efficient but I couldn’t find some numbers. Good mono cells have around 18% though..
The habitat has 100m2 of solar panels and 700W of sun energy per m2, harvested at 10% efficiency. that’s 7kW or about 84kW/h energy harvest per day (12h of sun). That’s not bad at all…
Alright, I’m loosing it. The 9000Wh battery of the rover is so heavy and huge he can barely move it and there’s no place to store it inside the second rover. Tesla for example packs around 200Wh per kg for their roadster, so 45kg for the rover battery if future NASA shops at Tesla… At martian gravity that comes down to 17kg. That’s a crate of beer. I’m no engineer but that seems manageable.
The rover recharge port of the Habitat runs at 36V / 10A so it takes 25 hours to charge a rover. Since there’s an abundance of solar power this makes no sense. For comparison, my van charges at 20A peak.
He completely fries his radio by electrical current. The housing of the radio is connected to ground (quite a common measure) and the radio is powered by the habitat. By accidentally touching the radio housing with another plus wire he fries the radio. Since electrical current is quite a lazy son of a bitch this seems unlikely. The current would go directly from the housing to ground, not taking any detours through the circuitry.
to be continued…
If you're committed enough, you can make any story work. I once told a woman I was Kevin Costner, and it worked because I believed it. -- Saul Goodman
“Better call Saul” aires in November starring Bob Odenkirk & Jonathan Banks. I’m really looking forward for Mikes Story! Chances are this will become the best spin off ever!
There are these days where you think you didn’t got nothing done. I just wrote some (business) emails, applied for a working project, did half a penetration test and cobbled together a linux bi-amping setup with a digital crossover which works out really nice. And I finally understood what a 4th order Linkwitz-Riley filter is and why I like it. Ah, and I made some nice charts about it. Last but not least I worked another couple of hours on the van.
So today I learned something new, earned some money, did stuff from my ToDo list and produced something which I will enjoy almost every day for the next months/years (decent sound on my desk). Why do I feel bad again? There’s really no reason. I guess it’s not about the stuff I did but about the ToDo pile I didn’t finish off. I think I gotta get used to the reality that there’s always stuff to be done…
"He is dead, Jim" -- Leonard McCoy
(posted a week or so after writing…)
I thought this might have been the first winter (ever?) without getting sick. Didn’t work out Whatever… After even netflix became boring I had to something else.
My stereo hasn’t been the same since I moved home. Today I finally looked into the issue. Some peak performance testing before moving to Hannover fried one tweeter completely and hurt the other one badly. The automatic eq / analyzer of the amp covered this failure up somehow (being a four way system the mid speaker range is overlapping with the tweeters). Since I use the amp mostly for surround sound, the other speakers helped covering it up as well. But after I bought some JBL Control One for my Desk it became quite obvious that some frequencies are missing. I still had some tweeters from the stereo of the 190 series ( a car…) catching dust so I went for a quick fix. It turned out they are too noisy but I got them more or less under control thanks to the equalizer.
Linux provides some great tools for audio testing and they are for free. So I cobbled together an audio analyzer using the mic which came with the amplifier for setting up surround sound. The following are all frequency diagrams, x-axis is frequency, y-axis is output volume. The actual numbers don’t matter, what we are looking for is a more or less straight line indicating that the pink noise is reproduced by the speakers “as is”.
Up to now the audio system + duct tape did a superior performance than fancy (and well fitting) Gradient Tweeters. Since both speakers have different volume levels on high frequencies (which you can’t see on the diagrams) there’s still something wrong. More investigation has to be done I guess. To be continued…