~20hz Horn Help

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Thanks, that does look pretty close to the measured results with my current box, is that with 2 drivers or just one?

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Just to be clear though (since I don't know your technical level) the measured results actually should not look very much like the sim at all when measured with 3000 watts. This box has a lot of problems (losses) that the sim won't show, as well as different driver specs at this level of power and excursion. The losses and changes due to power and port compression will mostly be down in the 30 hz area of the graph.
 
I can adjust volume and open/close doors to change loading and you've got up to 30 seconds per tone generally, but as things heat up quick you usually just do your best, swing the door til the meter reads loudest and leave it.

This should all be determined before the competition starts. Once the tone starts, thermal compression begins immediately. Your best shot at high spl is within the first second of starting the tone, it's all downhill from there as heat builds up and the driver's t/s parameters change completely.
 
Ah ok, so would you expect the sub 30hz area to be noticeably lower in spl at high power? That would indicate I'm getting quite a big gain from the cabin.

I know ideally you'd only play for as long as it takes to get a reading. The other competition format I do but am not too bothered with is DB Drag Racing. That's just one frequency burp below 60hz. In the same box but a much bigger port I did 147db at 56hz. Below tuning was terrible though. It's simpler as you know your cars resonant frequency and all windows and doors have to be sealed. If you pick the wrong frequency for your first burp you'll never be as loud as you could've been. But the last time I competed in Propper Droppers I was still building the box in the morning during the show, power saw hooked up to an inverter et al. So I had no idea what it was gunna do once it was in the car.
 
Here's a first run at a tapped horn. I spent about 2 minutes on this (so it could be improved) and the goals I focused on were 20 hz tuning and 10 cubic feet net. Shown at xmax at 1900 watts. You can give it more power and it will exceed xmax on the 25 hz tone (but not on any of the other tones, they will all handle a bunch more power). Even with 3000 watts only the 25 hz tone will exceed xmax (by about 5 mm) but no damage will be done as long as xlim is at least 23 mm. I don't have an xlim spec. so I don't know but I assume it's at least 23 mm.

You can change this in an infinite amount of ways depending on your goals. You could achieve much flatter response although I wouldn't recommend it. High power = power compression and will change the curve completely, so I built in a bit of padding to account for parameter drift.

As you can see, you don't gain much from this compared to the first sim (except below 30 hz) since this one is tuned a LOT lower. If you tuned higher you could gain a bunch of spl above tuning but would obviously lose some below tuning.

And remember, garbage in = garbage out. If these specs are not right the sim won't be right. If you can't measure it's all just a guess.

But since your current box has massively undersized ports almost anything would be an improvement, this box should stomp all over it (even though the sims don't really reflect it) but that's to be expected since it's several times larger and your current design is very bad for the intended purpose of sine waves at high power below tuning.

This is a reasonable starting point, you can play with this sim and ask questions but there's no point in me spending too much more time on it without measured specs.

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Thank you so much, you've been a massive help so far :) There's only one or two guys I'd trust on a subject like this over at the forum I usually frequent but they don't have much experience in building horns. I will try and get the ts parameters measured over the weekend. My dad is a retired electrical and broadcast engineering lecturer and now builds studios in his spare time so hopefully he should have everything I need. Is there a prefered guide on how to measure the specs? And which ones do I need?

Thanks :)
 
If your dad has an electrical background and an oscilloscope he can probably do it with stuff he has lying around. Just google the topic.

If he doesn't have a scope build this jig. AudioBlog: A simple loudspeaker measurement jig for ARTA
And use ARTA (LIMP) or Room Eq Wizard to measure specs. Both programs are free. Both include detailed instructions on what to do.

If you can build that jig, buy this woofertester.com or this Dayton Audio WT3 Woofer Tester | 390-804
Both devices come with software and instructions that will tell you exactly what to do.
I don't own either but I hear the WT2 is much better and it's what I'll buy when I get around to it. If you don't have any experience with this and don't want to spend a ton of time on it and want to make sure the results are accurate, these commercial woofer testers are probably the way to go. I always had problems with my homemade jig (most likely a soundcard compatibility issue) and I had to lie to the program about the value of resistor used to get numbers that made sense - and even then I was never 100 percent sure the results were accurate. The finished speakers always matched my simulations pretty well so I was doing something right but if you don't want to worry about that kind of stuff just buy a woofer tester.

You need full t/s specs, same thing as provided by the manufacturer on the page you linked but preferably more accurate than the provided t/s specs.
 
There's only one or two guys I'd trust on a subject like this over at the forum I usually frequent ...

Thanks :)

Which forum? Someone should have warned you about your massively undersized ports in your current box. 110 m/s air velocity is not joke, it's a serious problem. I like to keep it under 10 m/s when possible and practical and would never go above 27 m/s for any reason. Even if you don't end up building a horn we can do a lot better than what you currently have.
 
Here's a first run at a tapped horn.

This is a reasonable starting point, you can play with this sim and ask questions but there's no point in me spending too much more time on it without measured specs.

Oops, forgot to switch to PAR segments. That will make a difference. There may be other errors as well, I didn't have too much time to do this. If you get measured specs I'll spend a bit more time on it, but even so it's still a good starting point for you to play with.
 
Talk Audio, I know the old box was far from ideal, I built it to conform to the class regulations of Db Drag Racing but I'm not too bothered about that anymore and I'll happily enter a higher class if it means I can have a nice sounding sub and still be in for a chance in a different event.

I'll look into getting the specs measured and think about maybe making more room for the enclosure. Ie. Taking out the back seats...
 
You probably won't do well in a higher class with a horn. Ported boxes give the most output for a given volume of space. The key to very high spl is lots of drivers in ported boxes with insane amounts of power. (Or burping at higher frequencies.) Horns are for when you don't have a lot of drivers and power but you have tons of space. If you really want to win get lots more drivers and amps and use ported boxes while staying in the lowest class you can.

Also, if you remove the back seats not only will you be moving up in class but you will have to bolt the sub to the car frame. (Probably should do that anyways.) If you get into a collision the sub will kill you and everyone else in the car. If it will fit through the windshield it will kill everyone in the other car too. Assuming it's not bolted down the back seats will help to slow it down a bit and might even stop it if you are really lucky. I cut my car apart a bit to fit the sub in but it still looks standard. I wouldn't remove the back seats unless I was doing at least 150 db. It makes you look like you are trying too hard but without the numbers to show for it (IMO). 140 db or better with a hidden install is cool, cost effective and much less prone to theft. But I don't compete and I don't know what the kids are doing these days so my opinion on car modification doesn't matter much.

I'm assuming you've already done the big 3 wiring runs? With this amount of power that will probably yield the easiest gains if not done yet.
 
The problem is that I have the drivers and they're good subs but I can't afford to buy another 2 or more for a 150db+ wall or whatever. Hence why I was thinking of ways to get more efficiency out of what I've got.

I crashed my last car with a single 12 in the boot, it was bolted through the floor but it snapped the bolts and smashed half way through the back seats, that was only at 30(ish) mph as well. I only use 2 ton swl ratchet straps now. Tightening the strap gains me over a db too which is a plus. What I was planning was a wall but only behind the back seats, so mdf glued and sealed to the chassis and body panels at the sides, floor and roof with room above the seats for the subs/horn mouth and amps could go under the hatchback away from any vibrations.

I've had 140db+ hidden installs since I started driving, I'm getting a bit bored and I'm just trying to get louder with the kit I've got. The shows I go to, more people seem to have walls than not nowadays, certainly you have to spend thousands to turn heads with a boot install.

Yeah, I've done the big 3, upgraded alternator, 2 agm batteries etc.
 
A full horn no, but a tapped horn which will only play one octave, possibly. It may well turn out, once I've measured the ts parameters, that a bigger ported box or perhaps a bandpass box would be optimum. But I like what I've read about tapped horns and I think it'll be worth it if designed well.
 
Actually........ when the MCM 55-2421 came along I looked at making multiple small THs for use in a ~10.3 Hz HT app distributed sub floor/ceiling decorative 'column' system, but still haven't gotten beyond a sim, which indicates that four with a minor amount of room gain will have a ~120 dB/11 Hz/m rated excursion limited dynamic headroom, so a single in a car will theoretically be at least this good if it’s sufficiently airtight.

At ~98 L/3.46 ft^3 net and an 845.2 cm/332.36” path-length, I figure a many fold layout similar to RCA’s late ‘30s ~20 Hz studio monitor will probably double its size, but still small enough to fit in a typical American vehicle, though it remains to be seen whether this much folding can be configured for a smaller vehicle’s odd shaped storage spaces and still handle this much internal pressure without blowing the driver.

Shrunk down to a ~20 Hz tuning cuts the net Vb in half of course and the path-length nearly in half to 481.832 cm/189.7”, so should fit even many compact family size vehicles.

That said, I imagine there’s some high excursion, low Fs car audio drivers that can at least match, and maybe beat, its performance in a similar net Vb, EQ’d sealed alignment, so probably not worth the effort except for ‘bragging rights’ of having a true sub horn theoretically capable of at least 115 dB/18 Hz and rising to 125 dB/90 Hz [~2.32 octaves] with ‘only’ ~240 W on tap in ones ‘daily driver’/’smooth ride’/’beater’/whatever if the driver can handle it at a ~3.2:1 CR.

GM
 
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At the moment I can average 142dB SPL from 20-40hz on 3kw in a 3.5 cubic foot ported box tuned to 30hz.


Hmm, in theory, to get a dual BL12 TH to do 140 dB/18 mm/m/3 kW in an infinitely rigid 0.5 pi space down to even 30 Hz with a reasonable expectation of not blowing them up from high non-linear excursion requires at least ~1072 L/~38 ft^3, so not a viable option.

GM
 
But you can't say that for sure until I've measured cabin gain, ts parameters of my drivers and remeasured my current output. I'm sure there's more to come from these drivers. It's just a case of getting all the data together to make an educated enough guess at what will be the best way to go about it.
 
I hate to be a bummer, but I've built a pile of tapped horns for the car, and after doing that I'm convinced that sealed subs are more efficient at ultra low frequency.

The reason for this is that you can use the entire volume of the car cabin as a quarter wave resonator if you plan it carefully.

Here's an example of what I mean:

Let's say you build a tapped horn for your car. You tune that tapped horn to 30hz and you put a 12" woofer in it. (This is fairly similar to the tapped horn that I built for my Honda Accord, and it's documented on this forum and also on diyma. Just do a google search for tapped horns on diyma and you'll find me.)

Let's say that enclosure is five cubic feet.

Now let's take a plain ol sealed box and put it in the same car. If the sealed box is in the corner, it's going to resonate at approximately two frequencies. One frequency is based on the width of the car; the other frequency is based on the length of the car.

In my car, those frequencies were approximately 70hz and 35hz.

The cubic volume of my car is nearly FIFTY times larger than that tapped horn, so the efficiency of the in-car resonance is outrageously higher.

Now you might look at this, and ask "if sealed boxes work so great, then why do prosound companies used tapped horns and vented boxes?"

And the answer is simple; their venues are so large, the in-room resonances are below the pass band. For instance, even a tiny 20' x 10' piano bar will have resonances that aren't in the passband. In that example, the resonances would be at 28hz and 14hz, low enough in frequency that we're getting a lot less 'free' gain from the room than we get in a car.


But in a car or truck, you just get tons of free gain at low frequency. IMHO, this is the reason that a lot of the winning SPL cars are the ones that simply have the largest amplifier. The car is so efficient, if you want more SPL, you'll get more bang for the buck from a bigger amplifier than from a complex box. Sealed works great.

And this is coming from someone who's probably written more articles on putting tapped horns in a car than just about anyone.


There is one last piece to this puzzle, by the way. You might look at a measurement of a tapped horn and a sealed box and exclaim that the TH is louder. And it might be, at a narrow bandwidth. The difference will be found when you reach the limits of the box. At low frequency, both boxes are typically limited by the displacement of the driver itself. In a car, the big difference is that you can typically fit four sealed boxes in the space you'd use for a single tapped horn. So that's how you get louder; just simple displacement.

Now there are plenty of other reasons to use a TH in a car, but output isn't one of them. A TH can have excellent phase response and group delay if designed carefully. And a TH can use less power to get to the same SPL. If increasing your power is not an option, and you have a lot of space, a TH might be fun. Having said that, there's a catch-22 to big vehicles in SPL competitions, and that is the same one that the prosound guys run into: The larger the room is, the lower the Schroeder Frequency. Due to that, a very small car can rack up some mind numbing numbers on an SPL meter. I've seen 6000lb SUVs beaten by tiny Honda CRXs in SPL competitions due to the benefits of a small rigid cabin. (Remember, the car basically IS part of the enclosure.)
 
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I hate to be a bummer, but I've built a pile of tapped horns for the car, and after doing that I'm convinced that sealed subs are more efficient at ultra low frequency.

What? I'm convinced that a tapped horn is always going to be louder than a sealed box regardless of the environment they are compared in. Bigger box + vented resonant output is going to win every time.

The reason for this is that you can use the entire volume of the car cabin as a quarter wave resonator if you plan it carefully.

Yeah, same thing goes for a tapped horn. A 1/4 wave resonator inside a 1/4 wave resonator is going to beat a sealed box inside a 1/4 wave resonator.

I've played with tapped horns in cars and there's NO WAY you can beat them with the same driver in a sealed box. Now you you stack a dozen drivers in sealed boxes with insane amounts of power and compare that to a single tapped horn, the sealed boxes will win but that's not a fair comparison and OP already said he can't afford more drivers and amps.
 
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What? I'm convinced that a tapped horn is always going to be louder than a sealed box regardless of the environment they are compared in. Bigger box + vented resonant output is going to win every time.



Yeah, same thing goes for a tapped horn. A 1/4 wave resonator inside a 1/4 wave resonator is going to beat a sealed box inside a 1/4 wave resonator.

I've played with tapped horns in cars and there's NO WAY you can beat them with the same driver in a sealed box.
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I've built a pile of them and I don't see that it can be done.

If we're comparing one sealed box to one tapped horn, the tapped horn will win.
If we're comparing one vented box to one tapped horn, the tapped horn will win.

The advantage of the vented box and the sealed box comes when you use them in multiples.

For instance, a TH-Mini is about five cubic feet and it's good to 40hz.
In the same five cubic feet you can shoehorn four twelves into a big sealed box, and due to the displacement advantage, the sealed box will get louder.

Will it sound as good? Will it sound as effortless? That's debatable. But for sheer output, an array of small subs in a car will exceed a tapped horn due to the 'free' gain you get from the car's interior.

Things get murkier in a large space because we don't get gain from the room to the degree that we do in a car.

Believe me, people gave me this exact same advice five years ago, and I had to find out the hard way. The tapped horn that I built for my car now sits in my living room where it's doing a fine job.
 
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