Let's talk acoustics

[Neo_frog]

So you all know my design I've been using.

The subs are in and the system is operational. It is by far the loudest I've had, and the last one was hitting a 154 on a burp. I haven't metered it or anything so don't ask about it just yet.

I built it to have 1 ft³ per sub, and 5.9 in² of port per cube for control. The baffles are at 14° with a 7" separation in the back. Port is tuned to 34.7hZ.

This gives me close to 12 ft³ inside the clam as well for a reference. I figured matching the air space would play a role in the response of the horn.

Now here's what I don't understand. Since the port would technically cancel the subs movement with inverse compression, why is it MUCH louder? Or is it that the port creates a compliance effect? I even play flat from 25-40hZ before it drops off.

And my subs don't get hot, and won't bottom out. I've tried. Even with the doors open, the subs continue to act like they are in their own environment independent of the car. (Thanks to Boon for that recommendation) This probably means I have a lot of scalability for power...

[Forevrbumpn]

My boss was always about the horn effect, as I posted in the Quarterwave thread ( I think) get your hands, and cup over your mouth, make a sound, like a ahhhh or something- make sure no one is looking.. lol

start your hands off in a cylinder/tube shape, and slowly open them to a bull horn shape, the wider you go the louder your voice gets, until your hands are open to wide, and there is no more compression built up in your hands

I always wanted to do a clam shell, my way, where maybe there is 24- 10" speakers, the beginning of the horn throat equals the cone area of just the 4 or 8 subs

it would end with the area of all 24- 10" subs

So basically the horn throat at the back wall, or port would be about 220 - 440sq inch assuming the 10" subs have a cone area of 55sq inches ( the area would be a test and re test method, first try with the area of half the speakers, then try with the full area of the subs)

The ending result would be 660 - 1320sq inches

if the opening goes to wide, say 1500 - 2000sq inches, there would be loss of compression at the horn/clamshell exit

I cant say this is why your clam shell is louder now, than before, but I'am willing to bet compression built up in the horn of the clamshell is key

[STEvil]

air movement has latency to the movement of the driver diaphram, thats why the cancellation doesnt "seem" to be there. It is there, just not perceptible.

[Neo_frog]

My boss was always about the horn effect, as I posted in the Quarterwave thread ( I think) get your hands, and cup over your mouth, make a sound, like a ahhhh or something- make sure no one is looking.. lol

start your hands off in a cylinder/tube shape, and slowly open them to a bull horn shape, the wider you go the louder your voice gets, until your hands are open to wide, and there is no more compression built up in your hands

I always wanted to do a clam shell, my way, where maybe there is 24- 10" speakers, the beginning of the horn throat equals the cone area of just the 4 or 8 subs

it would end with the area of all 24- 10" subs

So basically the horn throat at the back wall, or port would be about 220 - 440sq inch assuming the 10" subs have a cone area of 55sq inches ( the area would be a test and re test method, first try with the area of half the speakers, then try with the full area of the subs)

The ending result would be 660 - 1320sq inches

if the opening goes to wide, say 1500 - 2000sq inches, there would be loss of compression at the horn/clamshell exit

I cant say this is why your clam shell is louder now, than before, but I'am willing to bet compression built up in the horn of the clamshell is key

See, I was going to attempt a cross-sectional area ratio like that, but the port threw that off. That's why my taper is a little looser. I have the port area close to one sub Sd while the whole back face is about 5 subs total. The opening was adjusted to drop my internal airspace and meet my tuning/handling goals. I didn't even think about the area of the opening. I was going for volume matching to attempt a resonating effect.

air movement has latency to the movement of the driver diaphram, thats why the cancellation doesnt "seem" to be there. It is there, just not perceptible.

At this point, sealing it off or attempting a port adjustment would be difficult. I know there is some minimal cancellation, but I don't understand how the incredible amount of cabin compression happens.

The behavior of air is way different than I've felt in the past. I can't do a can trick like my last wall, but it gags me and destroys my door guts. And it's the first system that is bowing my windshield so bad that I've considered reinforcement.

[Neo_frog]

Bump for more info

[skittlesRgood]

ill admit rite now that you seem to have a better grasp at all this than i do but ill give it a shot.

there is a delay from the back wave coming out of the port. if the back wave come out at the same time the subs push then everything is working together. since the horn is so big, you dont have a lot of pressure built up on the face of the subs so cancellation would be minimal. but the small box, port and a little pressure on the face of the cone will all help you not bottom out.

just sounds like everything is working together. i dont like the port being so small but i think you built a great box.

[Neo_frog]

ill admit rite now that you seem to have a better grasp at all this than i do but ill give it a shot.

there is a delay from the back wave coming out of the port. if the back wave come out at the same time the subs push then everything is working together. since the horn is so big, you dont have a lot of pressure built up on the face of the subs so cancellation would be minimal. but the small box, port and a little pressure on the face of the cone will all help you not bottom out.

just sounds like everything is working together. i dont like the port being so small but i think you built a great box.

I made the port smaller to achieve Helmholtz resonance (once again, experimental). The volume of air inside the port against the internal airspace would theoretically achieve this in a practical application. This is another reason I adjusted the baffles in the design to match everything up with my projected scenario, but I wasn't going to mention anything about it in case I screwed it up lol.

Maybe it is the delay that's creating compliance.. I was really hesitant to try this because an oscillator would have an outside force acting on a set volume both inside the system AND the opening. Every time the subs move, this changes the internal volume and would affect the pressures inversely since the port is inside the horn. But maybe since the angles are loose, it's not actually resonating like I am thinking...

I think I'm a bit over my head on this one lol. Hopefully this discussion continues though.

[skittlesRgood]

I made the port smaller to achieve Helmholtz resonance (once again, experimental). The volume of air inside the port against the internal airspace would theoretically achieve this in a practical application. This is another reason I adjusted the baffles in the design to match everything up with my projected scenario, but I wasn't going to mention anything about it in case I screwed it up lol.

Maybe it is the delay that's creating compliance.. I was really hesitant to try this because an oscillator would have an outside force acting on a set volume both inside the system AND the opening. Every time the subs move, this changes the internal volume and would affect the pressures inversely since the port is inside the horn. But maybe since the angles are loose, it's not actually resonating like I am thinking...

I think I'm a bit over my head on this one lol. Hopefully this discussion continues though.

just when i think im getting a solid grasp on car audio i run into this. i have no idea what helmholtz resonance is lol. at least not by name. looks like ill be doing some google searches tonight.

[STEvil]

helmholtz resonance is vibration of air. A port is a helmholtz resonator

Blow across the top of a wine glass.. helmholtz resonator!

edit - I dont like how "; )" gives

Edited June 21, 2010 by STEvil

[skittlesRgood]

i dont like how "eighty" give you 80

Edited June 21, 2010 by skittlesRgood