Why Car Subs Sound Louder Inside The Cars?

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Hello Everyone!


So I wanted to make a subwoofer for PA, Thought if I can use heavier car subs and re-design the box. I already know the pro's and cons (mostly cons, I know) of doing such, and I know that it's because of low internal volume of a car cabin that the air is more easily pressurized. What caught my attention, though was that car subwoofers are louder when placed inside the car, even if you listen from OUTSIDE of the car. How is this possible? Is car cabin acting like a 4th order box, correcting the phase? I can't get my head around it. How can such a random shape with -relatively- flimsy body do such. If it's not doing so, then what is the fact? and if it is doing so, how can we use the fact to our advantage?


Best Regards, Ali
 
What caught my attention, though was that car subwoofers are louder when placed inside the car, even if you listen from OUTSIDE of the car. How is this possible? Is car cabin acting like a 4th order box, correcting the phase? I can't get my head around it. How can such a random shape with -relatively- flimsy body do such. If it's not doing so, then what is the fact? and if it is doing so, how can we use the fact to our advantage?
Ali,

Unless you build a cabinet the size of a car, hard to make up for the volume that's missing!
The enclosed volume and seats can create horn or transmission line features.
Open windows act like large ports, closed windows make floppy panels act like passive radiators.

Generally, although quite loud, the external level is usually pretty "one note", very narrow bandwidth, similar to 6th order band pass.

Art
 
Thanks for the input Art,


So what I got was that with a big enough 6th order box I can get the boom needed for outdoor use, probably. Three notes:

1- Here where I live It's much easier to find decent car subwoofer drivers than their high efficiency brothers.
2- Where and how the subwoofer is going to be used? For fairly small parties outdoors and used with casual 100W two-way mains (the ones that use 12" woofers and 1" CD's). Up to four of the mains are used in a party and they start to distort at higher volumes. I thought that maybe by giving the 50-100Hz octave to the subwoofer I can lift some weight off the main speakers.
3- It's important for the sub to be live and punchy, not necessarily high quality, flat or deep so a one-note box may even be desirable here.

Based on the conditions above I concluded that maybe I can use car subwoofer for the application. Am I in the right direction? or using car subwoofers for PA is a complete waste of time?


Thanks, Ali
 
In a small space like a vehicle there is an effect called cabin gain or transfer function. Below a frequency with a wavelength of the longest internal dimension the vehicle will experience an approx 12db/octave bass boost.

This is true with any small sealed space.

I believe this is because of the room modes. Below the first mode frequencey whose wavelength is DOUBLE the longest dimention, the boost happens. I suppose it's because the wave phase becomes more and more uniform in the whole internal space. If such, the wave that comes out of the car will be in-phase at all points as well. That can justify why the output sound is so loud. But one question: The transfer function by definition assumes the input wave have equal amplitude the whole frequency spectrum. That isn't true for car subwoofers. Not only the box specs, but the cabin dimensions also force the drivee diaphragm to move a lot less at lower frequencies.

Regards, Ali
 
The cabin gain phenomenon can also tell us why sealed car subwoofer boxes tuned to 60 Hz are popular. In theory they can go flat when used correctly in conjunction with the cabin gain. But if that's true then why should the response roll off anyway? Does the spring rate (stiffness) of air increase with a rate greater than 12dB/oct here? Or am I missing something here

Regards, Ali
 
Ali,

There are a number of car subs that can work well for PA sub applications.
Alpine has good value per displacement, accurate documentation and wide distribution.

Thanks Art,

Can you please guide me what properties in a driver I should look at? (especially with the drivers with no valid sinsitivity data) I think I have to look for high Cms and Qts, and an Fs near 40Hz.

And as for number of drivers, if everything equal (i.e. same brand and make), do you recommend two 12" or one 15"?

Regards, Ali
 
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They can make powerful THs; FWIW/AFAIK from just looking at them, DSL has [may still?] used these 'bad boys': https://www.mtx.com/


GM

Can you please explain more? As there's generally more efficiency and directivity with horns, and I don't think I will have space problems, I'm very interested. I recently saw a man on the forum designing a foldable horn subwoofer, and that's heck of an awesome idea.

Also for TH, what kind of driver should I be looking at?

Regards, Ali
 
Can you please guide me what properties in a driver I should look at? (especially with the drivers with no valid sinsitivity data) I think I have to look for high Cms and Qts, and an Fs near 40Hz.

And as for number of drivers, if everything equal (i.e. same brand and make), do you recommend two 12" or one 15"?
Ali,

The T.S. parameters for drivers of different types of cabinets vary. Horn loading stresses a cone more than a bass reflex loading, so requires a heavier cone (more Mms) for a given Xmax than a BR.
Xmax is perhaps the most important determination for any enclosure type- doubling Xmax results in 6 dB more output, assuming the driver can hold up to four times more power.

A TH can put out around 6 dB more output than a BR, though will require more cabinet volume. A high Bl is required for most horn enclosures for smooth response.

Using the free Hornresp program allows you to plug in the parameters of drivers available to you to designs you find interesting and compare them to the Hornresp simulations for the drivers that the cabinets were designed for.

Art
 
You dont really need a sub. If you place low Qts speakers within a tiny box cornered to a car`s hatch type trunk for example you get a loud bass evened to the speaker other output frequencies. The trunk glass and car hoof act like a fair decent horn. The same is expected if you corner your box in a room. You just got to take care not to excessively heat your speaker with equalization of any sort. Also, leaving a LPF of about 1300Hz -12dB octave WILL IMPROVE the bass through harmonic coupling to the bass frequencies PLUS the high spl generated by these higher pitched frequencies will help the bass notes come out because then the air impedance is more decently matched when both play together.
This is sll truth, but works with one or more identical speaker close together because phase is delicate stuff to really work in the real world.

My regards

edit: dont test the speaker with the then applied power if not in this corner positioning, just to "see" its xmax... it will lose all of the previous benefits and you probably will shred your speakers suspension by "lack of air"
 
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Well, the lower the gain bandwidth [BW] desired, the more massive [higher Mms] ideally the driver needs to be and for woofer apps, the narrower its BW [lower upper mass corner] the relatively weaker its motor needs to be [higher Qts'] for a given pipe tuning [Fp].

To help keep speakers relatively small, the driver's Vas needs to be low at the expense of efficiency, especially in [tapped] pipe [horn] alignments where a high mechanical control of the diaphragm is desirable [lower Qms]. Obviously, with such specs, high SPL only comes with high power handling motors to offset these generally low efficiency drivers. Note too that the normally dreaded high inductance of a 'poor/cheap' design boosts efficiency at the tap point, so series wiring can be a good thing.

For bass up to ~120 Hz, many modern mobile audio and prosound woofers designed for acoustically tiny boxes are ideal for these types of alignments.

Fhm = 2*Fs/Qts'

Qts' = Qts + any added series resistance: mh-audio.nl - Home

GM
 
Heres a car install...8 18s in a 4th order.

Build it and they will come
 

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What a waste!

GM

Why? Building an enclosure with wasted space is a waste. Corner reflectors are a waste (see jbell's SS15 thread). Not trying to hear frequencies below 40hz is a waste if human hearing is from 20hz to 20,000hz. For all you know, the builder of that enclosure could be trying to listen to Tchaikovsky's 1812 Overture as loud as possible in their vehicle.
 
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