Acoustic loading and wall explanation

[adulbrich 5,000+ posts]

This is kind of long, so please bare with me..

Many trunk cars have the sub and port facing the back of the trunk. Because of acoustic loading, the sound is amplified. One of my friends moved his box to different spots in his trunk and turned it to various angles. What turned out to be the loudest was right up against the trunk lid.

Why is it that lots of people in SPL competitions have 12's walled at the trunk? I know there's a science behind acoustic loading, but what is best?

In my car, my only choice is to wall off the front. I also invert my subs to gain every last bit of box volume I can. Is there enough loading off my windshield? Sound waves off the top half of the cone should be hitting it.

My port is faced towards the passenger door. When sitting in the passenger seat, it is loudest with the driver side door open, or the driver side window down a little. I know this is common, but can anyone explain it?

I'm trying to find out the best sub/port orientation for one or two 12's. I want it to sound the best in the passenger seat, but still allow for foot room.

I know this is a lot of questions, but input is appreciated!

Pic of my setup:

Click to read more...

 

[adulbrich 5,000+ posts]

I was thinking about putting my port behind the gauges, pointed to the left. Its hard to explain

 

[keep_hope_alive Premium Member]

you're misunderstanding acoustics.

no worry, it's not commonly discussed.

sound spreads spherically from both the sub and the port. don't think in terms of (top of the sub directs to the roof). imagine a bubble that continues to grow. as the bubble hits a surface, another bubble starts to grow, and so on. except the expansion of sound is a wave that oscillates. the oscillations are pressure changes due to air molecules vibrating.

when you measure sound you measure direct + all reflections. some reflections arrive in-phase, some out-of-phase, and some in-between. wavelengths can be easily calculated using the speed of sound (1125ft/sec) and either frequency (1/sec) or distance (ft). divide speed of sound by frequency to get wavelength. 1/2 wavelength differences cancel.

the trunk causes reflections and by putting the sub near the rear you get a reflection that is mostly in-phase with direct.

walling off the trunk eliminates losses in the trunk, so you don't waste energy. it also eliminates reflection (phase) issues in the trunk.

your port should effectively combine with the output from the sub. putting your port opening a long way away is purely trial and error and may or may not be helpful for a narrow band of frequencies. changing port tuning changes overall response.

the main reason people are louder with the windows open/cracked is because it alleviates pressure that the sub was fighting against. this allows for more excursion and thus more output. doors open also eliminate cancellation due to reflections. vehicles that are loud (155+) when sealed have hit on a specific combination of enclosure, power, vehicle configuration, and frequency.

 

[gckless 5,000+ posts]

Holy crap, what's going on in your car. More pics please.

 

[adulbrich 5,000+ posts]

you're misunderstanding acoustics.
no worry, it's not commonly discussed.

sound spreads spherically from both the sub and the port. don't think in terms of (top of the sub directs to the roof). imagine a bubble that continues to grow. as the bubble hits a surface, another bubble starts to grow, and so on. except the expansion of sound is a wave that oscillates.

when you measure sound you measure direct + all reflections. some reflections arrive in-phase, some out-of-phase, and some in-between.

the trunk causes reflections and by putting the sub near the rear you get a reflection that is mostly in-phase with direct.

walling off the trunk eliminates losses in the trunk, so you don't waste energy. it also eliminates reflection (phase) issues in the trunk.

your port should effectively combine with the output from the sub. putting your port opening a long way away is purely trial and error and may or may not be helpful for a narrow band of frequencies. changing port tuning changes overall response.

the main reason people are louder with the windows open/cracked is because it alleviates pressure that the sub was fighting against. this allows for more excursion and thus more output. doors open also eliminate cancellation due to reflections. vehicles that are loud (155+) when sealed have hit on a specific combination of enclosure, power, vehicle configuration, and frequency.

Thank you for your input! Your explanation was way more in-depth than anything I had read earlier.

Should I try to angle my subs and get more reflection, or would a regular wall be better like I have? Also, should I point my port towards the driver side A-pillar or how it is now (towards the passenger door)?

I'm looking to make a good park and pound demo car, so passenger headrest is where I want it to be loudest

 

---------- Post added at 05:28 PM ---------- Previous post was at 05:28 PM ----------

 

Holy crap, what's going on in your car. More pics please.

My build log!

http://www.caraudio.com/forums/car-audio-build-logs-cars-trucks-suvs/611930-dd-z-series-build-log.html#post8651554

 

[keep_hope_alive Premium Member]

changing the orientation slightly won't make a difference since the spherical spread doesn't change. you're not "directing" bass frequencies. by moving the sub you're just changing arrival times and phase relationships.

flex is lost energy. a rigid cabin with controlled losses is a good place to start.

 

[adulbrich 5,000+ posts]

Ok, I was wondering how cancellation would be if I moved the port opening to the opposite corner of the cabin.

I sound-deadened one of my doors, but have not had time to do the other one. I sealed up my sunroof with tape and sealed all but one area off from the front trunk. My panels are fiberglass, so they don't flex like metal. I know flex is a bad thing.

I was thinking about putting fiberglass resin on the floor and door carpet to reduce dampening.. Is this a stupid idea? I know my builds don't look clean. Its about how loud the bass is to me

 

[adulbrich 5,000+ posts]

As a side note, my car is really quiet from the outside. On the inside, it is louder than any other setup I have heard yet (I haven't heard real crazy setups like some people on here have).

My friend's type R 12 sounds louder when we stand outside our sealed vehicles

 

[keep_hope_alive Premium Member]

don't worry about being loud outside the car.

stiffening as in adding wood ribs to large surfaces. thin panel carpet isn't making a difference.

consider that a perfect doubling of sound energy is a 3dB gain. changing the port location will only make a 1-2dB difference, if anything. changing the port length will have larger effects.

if bass output is your primary goal, why limit your enclosure volume? wall the car.

 

[adulbrich 5,000+ posts]

What kind of large surfaces are you talking about bracing? The floor or roof? My box has lots of really small faces.

The port is as long as I can make it. It is also difficult to add port area because of my car's internal setup. I could maybe squeeze an 8" aeroport.

A couple people have told me to wall the car, haha. I still want to be able to have a passenger. What good is a loud car when you can only bump on your own? //content.invisioncic.com/y282845/emoticons/crying.gif.ec0ebefe590df0251476573bc49e46d8.gif

 

[keep_hope_alive Premium Member]

wall behind the seats, silly. still allows for two passengers.

what is the port velocity currently? your port area may be too low. your box may be too small. going longer on the port lowers tuning and may be costing you output. shorter ports end up with peaks. a small car will have a higher resonant frequency and the cabin modes will be higher in frequency. so playing lower won't translate to more output.

 

[keep_hope_alive Premium Member]

course, since you've cut through the firewall already...

maybe baffle the rest of the dash so it's flat with the front of the sub enclosure. custom dashes are cool.

wall behind the seats to eliminate the rear wall as losses. mount amps on the rear wall you make.

 

[adulbrich 5,000+ posts]

My engine is behind the seats, haha. Its a Fiero

My box is way too small. Its something like 1.5 cubes, but I can't make it bigger until it warms up. Then I can fiberglass an enclosure with about 5 cubes of true internal volume.

If you're interested, my build log is in my sig. Please excuse the mess. I only had a few days to do the install in a heated shop before winter break.

I made this thread to gain more knowledge for my next build. I'm doing a pair of 9512's.

It is really challenging to get low frequency response out of a small cabin.

 

[adulbrich 5,000+ posts]

I'll do a custom dash when I fiberglass the new box //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif

 

[keep_hope_alive Premium Member]

yea, i saw the build log. i know the engine is behind the rear seats but the factory body is thin and you're losing output to the rear. walling off behind the seats with MDF will gain you overall level.

if anything, incorporate some 8" or 10" midbass there

How deep are the rear firewall mounted sub