Ask an Acoustic Engineer (me)

[dgage]

If Maxmil982, regarding the drivers, I’ll start with my home speakers and then use that to discuss the driver selection. My home theater speakers are JTR 212HTR (sealed version, should have gone with ported) and they have 2 12” MIDRANGE drivers to try to keep up with the sheer efficiency of the horn-loaded compression driver. The 12” midrange drivers only play down to about 80 Hz (70 if ported) so they are definitely not home midbass speakers. Now my speakers, since they are so efficient can play extremely loudly. I often tell people with most speakers if you turn them up too much, they’ll start to become unhappy (distortion), with my speakers you’ll become unhappy before they will.

So using that as the basis for the car audio discussion, you’ll want to focus on higher efficiency drivers and skew your crossover points to the upper end. So maybe the tweeter will be crossed at 4 or even 5,000 Hz. But if you’re looking at a 2-way, this isn’t going to work well for even the best 8-inch drivers so you’ll need to go with a larger format tweeter (1.5”-2” tweeter) so you can cross them closer to 2,500 or 3,000 Hz. You’ll want to choose a tweeter with an Fs closer to or even below 1,000 Hz.

For the midbass you’ll want to play it as high as you have to to integrate with the tweet but preferably bring the tweet down to 2,500Hz or so because the beaming (directionality) of an 8” starts at 2,000Hz. So you might say then maybe you should cross the tweeter at 2,000 Hz so you don’t get any beaming but then you’ll put more strain on the tweeter, which means more distortion. So with a high volume system, you want to stay away from the limits which means crossing the tweeter, crossing the 8” a little higher than maybe they’re capable of. So the midbass you might want to cross at 90 or 100 Hz instead of 80 because 80 will take more effort to play and interfere with getting loud. Then you’d just bring the subwoofer up to 90/100/110 to integrate smoothly with the midbass.

I think the biggest problem people have in car audio is they want to play super loud and make each driver play super low. You can usually have one or the other. A high efficiency driver won’t play as low as a normal efficiency driver. If you just figure out what your goals are, which it seems you have, just choose the drivers that will fit that design goal. I think the MB8 will probably work well for you but you’ll want to go with a larger, high-efficiency tweeter to ensure the 2-way will work well and play loud.

The only other comment I have is that a single 12” sub of moderate excursion may or may not handle the volume you’re trying to run. You may want to consider a higher-efficiency ported design (won’t play as deep as sealed but will be louder), adding a second sub, or going with a higher excursion design with closer to 30mm of excursion.

Sounds like the makings of a very good system. I think you’ll learn a lot.

[spwath]

How did you get into car audio acoustics? I am majoring in mechanical engineering with an acoustics concentration, going into my senior year. Car audio acoustics is certainly an interest of mine. After I graduate I might consider going into that, or to grad school.

[Justin Zazzi]

@Justin Zazzi. First off man, major kudos on all that you've shared. I've been reading the thread and the troves of knowledge and excellent questions asked and answered has been truly remarkable.

You're welcome! Thank you for being a part of all this fun.

Looks like you've had some great feedback in your other thread about the MB8 drivers.

Part 1:
As a total beginner to car audio I have a few basic questions.
I am installing a 2way active system in a 4door Nissan Altima. The sub will be a Sundown SA-12 powered abdundantly with an Orion HCCA 2400 amp that will provide 1200rms clean. I listen to bass heavy music and will be listening very loud at times. The front stage will be powered with a ppi phantom p900.4. The total system will be around 2500w rms.

I understand the basics of efficiency and how the system will not be using the entire 2500w playing music. I plan to just monitor voltages and current once the system is up and running and then plan out electrical upgrades. My stock alt is 110a. It is also time for a new battery and I'm considering a Napa Legend Premuim AGM with 55Ah and 750cca, it's just a mid level East Penn Mfg battery and of course ill be doing the big 3 upgrade.

I've heard so many conflicting tips. Some people say stock alt AND stock battery will be fine. Some people say get caps some say caps are trash. Some say get an XS power AGM and get HO alternator from the beginning; and every imaginable combination of those things.

So my question is just simply, if I have voltage drops, dimming lights etc. once the system is installed, should my next steps be a 2nd AGM in the trunk or a HO alternator?

I admit large and powerful systems are not my specialty so listen to other folks here too. There are many talented people that can help you with this question too.

I'm not a fan of capacitors. Amplifiers have capacitors already.

I had dimming headlights on my 800w system for a while and I thought about upgrading my electrical system then my battery failed completely and I had to replace it. Turns out, my battery at end-of-life was the problem and when I replaced it I didn't have dimming lights anymore. I swear my bass output increased too but it's hard to compare since I don't have the old battery anymore. Take that anecdote for what you will, but I found it really interesting to experience a battery "upgrade" firsthand.

I'm a big fan of installing stuff and seeing if the electrical system is adequate, *then* finding what the weak link is. Could be the "big 3" power/ground cables, could be a battery, could be an alternator, could be none of those things. Can't tell until you get the gear and turn it on otherwise you might have a solution in search of a problem.

When you get the gear installed and if you notice unacceptable dimming (emphasis on unacceptable!), post a new thread and we can help you find the weak link by using a multimeter and stuff.

[Justin Zazzi]

I thought this might be a great thread to ask this silly question. I am almost embarrassed to ask and have done a lot of googling but cant seem to find an answer that i understand.

Is there a rule of thumb when designing an enclosure in regards to a drivers Fs? How far below the Fs can you tune a ported enclosure? What about a sealed enclosure's F3 response, can it be below the Fs of a driver? And lastly... If designing any enclosure (not and IB) is there any weirdness that happens if the F3 is well below the Fs of the driver...

Thanks in advance

I love silly questions! We usually don't approach a problem from silly angles and so I find I learn more this way.

For a sealed system the F3 will only increase. The F3 of the woofer by itself will always be lower than the F3 of the woofer in a sealed box. This can also be seen because the Q of a woofer will always be lower than the Q in a sealed box. (Qts < Qtc)

For a ported system I don't think there is a hard rule against tuning below the woofer's resonant frequency. If you do, it looks like the tradeoffs are not very good. A lower tuning frequency can have some of these:
-group delay increases which is really objectionable to some people
-vent air velocity increases which can make noise
-output could decrease in the range just above tuning so you might get less output above Fb (see below)

Here is a simple simulation of a 6.5" subwoofer with an Fs of 35hz. The three curves are boxes of various sizes tuned to 25hz (green), 35hz (red), and 45hz (orange). Look at the dip in output in the 40-100hz region as the tuning frequency decreases.

[mlekk]

Cutaway , how does it sound ?
As you play above the point ( fs ) where it resonates the most easily . . . Then through the point where it moves the most easily , back to an area where more control is excersized on subwoofer ?
Really Great ? Or just not quite right ?

[Cutaway]

Cutaway , how does it sound ?
As you play above the point ( fs ) where it resonates the most easily . . . Then through the point where it moves the most easily , back to an area where more control is excersized on subwoofer ?
Really Great ? Or just not quite right ?

I havent done it yet... I'm currently looking at shallow subs that i can run under the front seats. A lot of speakers i am coming across have higher Fs than what i am wanting the speaker to play down at. I have always been under the impression that if you want to play down low you need a driver with an Fs in that area or below. BUT i didnt know if this was a valid belief so i thought i would ask those with more knowledge...

Supper appreciate the feedback

[Justin Zazzi]

How did you get into car audio acoustics? I am majoring in mechanical engineering with an acoustics concentration, going into my senior year. Car audio acoustics is certainly an interest of mine. After I graduate I might consider going into that, or to grad school.

Two major things in my favor: I absolutely love the science of sound, and I found a mentor while I was still in college.

I started in car audio because I always lived in someone else's house and couldn't put speakers and wires in the living room wherever I wanted. When I had my first car I quickly realized I could do anything I wanted with speakers so it became my laboratory. Car audio was how I experimented and learned and grew, and I soon found there was a whole community of people who loved car audio too. I joined a couple forums and started making friends.

A big breakthrough was taking the judge training for MECA sound quality competition and then becoming an active judge. This further trained my ears to listen for things, and trained my mind to communicate what I was hearing into a score rubric or into plain words that a non-competitor could easily understand. Becoming a judge is the second-most important thing I've done to further my career because it gave me a new network of people to interact with which is where the first-most important thing happened: I met my mentor.

My mentor has helped me find many ways to connect with the professional audio community, starting with the ALMA organization
https://almaint.org/

ALMA puts on an annual conference where all the engineers at all the serious audio companies come together to talk about all the cool stuff they're working on and the things they learned. This is a place to share knowledge and help eachother. There is some business that happens which is also a draw for professionals already in the field but the best part is the people staffing the company booths are literally the engineers who created the products. If you have a question about microphones, engineers from GRAS and PCB are there to help. If you want to learn about Beryllium as a tweeter dome material, Materion's engineers are there. Wolfgang Klippel is always there and usually brings a few of his employees and they always present something amazing.

Professional networking is something I always shrugged off as "yeah ok that's what old people do" but when I found the right group of people to talk to (the people at ALMA's events) then I realized it was actually amazing. This is where I met Seigfried Linkwitz (from the Linkwitz-Riley crossover) and he invited me to his home to audition his personal audio system where I immediately bought plans to build my own. This is where I met Wolfgang Klippel whom I later had a short internship with at his company in Germany. This is where I met Jerry McNutt who is the head engineer at Eminence Speaker Company where I had a summer internship and was later hired and I began my acoustics career. This is a magical place for someone like you and me.

The neat thing is ALMA's event is virtual this year and admission is free if you register here: https://almaint.org/elementor-6293/
I cannot recommend this highly enough. ALMA also has a student initiative which is getting stronger. Send me a private message here with your email address and I'll introduce you to Barry Vogel who runs ALMA.

As for starting work after college or going to grad school, I can only tell you what works well for me. I've had a lot of practical experience with audio and a lot of self-taught acoustic knowledge. When I graduated I was ready to never be in school ever again, I hated it with a burning rage I cannot put into words. I ran full speed to Kentucky to work at Eminence and never looked back.

Then I did something unfathomable that I still can't believe: I started grad school. I have been a terrible student on paper my whole life and I'm routinely in the 2.0 GPA range. I have a 3.7 in grad school right now and I'm on track to get another A with my current class. I'm enrolled in the "world campus" at Penn State and taking one class a semester with their distance education program (before distance education was cool like today haha). My company is reimbursing the tuition and I'm learning the most amazing stuff and my favorite part is I'm applying the knowledge in a very serious way before the final exam. Right now I'm taking the 2nd of the signal processing classes and I'm applying literally everything I know in that class to a project this week. It's incredible.

So for me, the combination of practical knowledge and experience with acoustics was powerful and let me start my career. If you don't have as much, you might benefit from grad school and if you go full time then you can get it done in a year or two instead of the 5-year pace that I'm at. Plus you don't have to worry about homework for the next five years while you're also working full time (that part sucks hard).

I guess my advice is do what you love and find two communities: a casual one to make friends and a professional one to start a career (and also make friends). ALMA is the best professional one I know of, so lets start there. Send me a message with your contact info.

[Justin Zazzi]

I havent done it yet... I'm currently looking at shallow subs that i can run under the front seats. A lot of speakers i am coming across have higher Fs than what i am wanting the speaker to play down at. I have always been under the impression that if you want to play down low you need a driver with an Fs in that area or below. BUT i didnt know if this was a valid belief so i thought i would ask those with more knowledge...

Supper appreciate the feedback

I forgot something important.
A ported box can be manipulated with an equalizer to some degree, but only above the tuning frequency. If you want output below the Fs of the driver you could build a box that is tuned really low and then attempt to boost it with EQ. However if you built a box that is tuned higher then you should not try to boost frequencies below the tuning frequency otherwise the woofer will just run out of excursion without making any more noise and it'll fail.

[mlekk]

If you "unload" a sub "play lower than frequency tuned at , No Control of sub , other than what is provided by spider/surround , the sub will over excurse ( known as 'bottoming out' )

[Maxmil982]

@Justin Zazzi. Thanks for the reply and the tips about the electrical upgrades. I'd like to send you a PM, exchange emails and in the future ask your opinion on some home audio options... I just have to do a ton of research first lol. I'm also interested in the ALMA event, sounds like a great opportunity to learn.

And again, this thread has been great! Tons of cool people here and on the whole forum. You, and people like you who have a genuine passion for their craft and are willing to invest in themselves and others are our national treasures. It's a real inspiration and motivates a lot of people to pursue their own interests to the fullest.