Ask an Acoustic Engineer (me)

[Maxmil982]

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.

Many thanks man! You've been a great help.

Part 1:
As it pertains to the MB8, I've taken what you said and am considering getting the 2ohm which does have 2.83v SPL of 101.67 db, but I talked to Eric Steven's and he said that that output is when feeding it 450rms and that at half the watts the output will be ~3db lower.

Which works out because the amp I have for it does 225w @2ohm. "Works out", meaning I'm thinking of addressing what you said about the single SA12 possibly not being loud enough for the full output of the MB8, by feeding the 8" half the rated juice and porting the SA12 in a custom box around 1.5 cubes to help it play louder.

I hope this is all correct, and then the system will have some expandability to double the juice on the 8"s and add a 2nd sub in the future.
Please lmk if you see this as a viable solution.

Part 2:
For what you said about needing tweets that can play low to meet the mids and have higher sensitivity, what would be your pick out of these...or none?

https://www.madisoundspeakerstore.co...ring-radiator/

https://www.madisoundspeakerstore.co...-dome-tweeter/

https://www.madisoundspeakerstore.co...-dome-tweeter/

[spwath]

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.

Sounds somewhat similar to me. I have tried to shove a fair amount of speaker in my bedroom here at home, but ran into space issues. Biggest I had was a pair of altec a7s, but those never left the garage. Only had them there a few days until I sold them so my parents could get their garage back...
At my apartment at college though, I have nice 15" jbl speakers with altec horns.

But at home, my only system is my car. I do drive a lot too, so its nice.

Anyway, thanks for all the info, I registered for the event, and sent you a PM.

[geolemon]

...Regarding a capacitor, realize that amplifiers have built-in capacitance and capacitor construction has (slightly) improved over time. A capacitor is just a really short term and very fast “battery” using the generic term of a battery being an electrical charge storage device.

I agree with everything except this part - and it's EXACTLY why the people who are anti-capacitor aren't even having the right conversation, much less making the right points.

A battery and alternator are voltage sources - they simply have different voltage levels:
An alternator is 14.4v.
The only reason a battery is at 12.5v is so it charges.
It's actually not a fundamental problem for voltage to drop or headlights to dim - that's how the system works:
Say your alt is making 110a of current and you put on a track that has the bass hit for a second, every ten seconds. Let's say you have a huge sub amp, every time the bass hits, the amp sucks 150a on that huge bass hit.
What happens?
The alternator can only supply the first 110a, so voltage instantly falls and when it reaches the 12.5v battery level the battery supplies the additional 40a for that second. Headlights aren't as bright on 12v vs 14v so yeah, you'll see them dim. That's just the system working 100%as it should. And 1 second later, current demand ends, voltage rises back up - and the battery charges just about as fast. Plenty of time before the bass hits again. Not a problem.

The point is - like the springs on your car, these are voltage sources. They are like the springs on your car - they hold it up, they fundamentally support the voltage level, and supply the current.

And capacitors are like the shock absorbers on your car - you wouldn't want to drive without them... Let's add a little more reality to this-

Realistically, batteries are chemical devices and they are relatively slow to respond to current demand. They have thin plates and chemicals inside, and big sudden demands can actually physically stress them. It can crack battery plates which will kill a battery.
Just as importantly IMO, is that speed issue - let's go back to that bass hit:
Voltage drops fast, and when it reaches 12.5v, the battery tries to supply current but it's slow to respond... Voltage keeps falling.. 12v... 11.5v...11v? Could happen (and that makes for more dramatic headlight dimming). That stress on the battery plates as it pulls the voltage back up can hurt the battery, as mentioned.

Let's add a capacitor to the mix:
Bass hit, voltage drops. But a capacitor isn't a voltage source, it's at whatever system voltage is. So, as voltage drops, it discharges current that the system can use. That isn't SUPPOSED to stop voltage drop - it's supposed to SLOW voltage drop, so the battery can respond without drama, without damage.

It's not a spring, it's a shock absorber. It's definitely NOT a small battery.

EDIT: Being a capacitor, it also functions as a noise filter - it smooths the power line, by absorbing and filling noise, which appears as ripples - which are low level voltage fluctuations.

EDIT 2: I should also mention ESR, which really is a misnomer since we're using them in a DC capacity. Internal resistance is everything - the lower, the closer to the theoretical perfect capacitor. The higher, the slower to respond.
In the 90s, we saw a bunch of those square "giant farad rating" caps hit the market... Those had high internal resistance, functioned more like batteries as a result - and thus were born the myths of "capacitors don't do anything" and "install a second battery if you want a cap".

That's my only correction. Hope that helps.

Personally, it's a cheap upgrade that I think every system should have - first. Well, second, after power wire. Possibly even before the "big 3", since those are simply about a relatively minor efficiency improvement.

Sent from my LM-G710 using Tapatalk

[dgage]

Part 2:
For what you said about needing tweets that can play low to meet the mids and have higher sensitivity, what would be your pick out of these...or none?

https://www.madisoundspeakerstore.co...ring-radiator/

https://www.madisoundspeakerstore.co...-dome-tweeter/

https://www.madisoundspeakerstore.co...-dome-tweeter/

Out of those I like the SB29 based on the frequency response graph, sensitivity, and helpful reviews. It also has a lower Fs to allow crossing a little lower; maybe a little lower than 2,500Hz depending on crossover slope and volume level.

[dgage]

I agree with everything except this part - and it's EXACTLY why the people who are anti-capacitor aren't even having the right conversation, much less making the right points.

A battery and alternator are voltage sources - they simply have different voltage levels:
An alternator is 14.4v.
The only reason a battery is at 12.5v is so it charges.
It's actually not a fundamental problem for voltage to drop or headlights to dim - that's how the system works:
Say your alt is making 110a of current and you put on a track that has the bass hit for a second, every ten seconds. Let's say you have a huge sub amp, every time the bass hits, the amp sucks 150a on that huge bass hit.
What happens?
The alternator can only supply the first 110a, so voltage instantly falls and when it reaches the 12.5v battery level the battery supplies the additional 40a for that second. Headlights aren't as bright on 12v vs 14v so yeah, you'll see them dim. That's just the system working 100%as it should. And 1 second later, current demand ends, voltage rises back up - and the battery charges just about as fast. Plenty of time before the bass hits again. Not a problem.

The point is - like the springs on your car, these are voltage sources. They are like the springs on your car - they hold it up, they fundamentally support the voltage level, and supply the current.

And capacitors are like the shock absorbers on your car - you wouldn't want to drive without them... Let's add a little more reality to this-

Realistically, batteries are chemical devices and they are relatively slow to respond to current demand. They have thin plates and chemicals inside, and big sudden demands can actually physically stress them. It can crack battery plates which will kill a battery.
Just as importantly IMO, is that speed issue - let's go back to that bass hit:
Voltage drops fast, and when it reaches 12.5v, the battery tries to supply current but it's slow to respond... Voltage keeps falling.. 12v... 11.5v...11v? Could happen (and that makes for more dramatic headlight dimming). That stress on the battery plates as it pulls the voltage back up can hurt the battery, as mentioned.

Let's add a capacitor to the mix:
Bass hit, voltage drops. But a capacitor isn't a voltage source, it's at whatever system voltage is. So, as voltage drops, it discharges current that the system can use. That isn't SUPPOSED to stop voltage drop - it's supposed to SLOW voltage drop, so the battery can respond without drama, without damage.

It's not a spring, it's a shock absorber. It's definitely NOT a small battery.

That's my only correction. Hope that helps.

Personally, it's a cheap upgrade that I think every system should have - first. Well, second, after power wire. Possibly even before the "big 3", since those are simply about a relatively minor efficiency improvement.

I was trying to simplify but thanks for the detail. I’ll follow up by saying I read a lot of posts where people think they need this or that and they haven’t even tried their system yet. There are a lot of people with successful car audio systems using the stock electrical system. No capacitors, no big 3, etc. So this is why I suggest installing the system first and seeing how it goes, especially when the poster in this particular case is only using a pair of reasonable size class D amplifiers. But that is just my opinion and worth the price paid.

[Justin Zazzi]

Capacitors are one of those never-ending debates like "is class d as good as ab".

Unless we can share measurements and experiments and data, these discussions are rarely productive.

[Ge0]

I have a question for an acoustics engineer that is brewing over on another thread.

Why does 125dB at 4KHz damage your ears but 125dB at 40Hz does not? I used to know the answer back in the day. But now my brain is filled with crap like Tiger King. Had to eject some knowledge to make room for trailer park reality shows

Ge0

[Justin Zazzi]

I have a question for an acoustics engineer that is brewing over on another thread.

Why does 125dB at 4KHz damage your ears but 125dB at 40Hz does not? I used to know the answer back in the day. But now my brain is filled with crap like Tiger King. Had to eject some knowledge to make room for trailer park reality shows

Ge0

What a complex question. I think you need the "ask audiologist" thread!
This is at the limits of my expertise, so don't quote me too hard.

The question can be interpreted many different ways such as:

why does 4khz hurt more than 40hz?
We are more sensitive to this region due to speech or some evolutionary thing or maybe just because. Maybe predators in the bushes make noise in this region right before they ate our ancestors so we became tuned to that region.

why does 4khz get damaged more than 40hz?
The 4khz frequency range seems to be the region most commonly damaged by noise-induced hearing loss. More thoughts below.

why does 4khz cause more damage than 40hz?
More thoughts below.

It is hard to find a consensus on what exactly causes hearing loss. There is some agreement that very high intensity noise can cause permanent damage immediately. There is some agreement that long-term exposure to medium and high intensity noise can cause damage. There is little agreement on exactly what composition of noise causes more damage.

There are also many mechanisms for hearing loss including mechanical conduction loss in the eardrum or the tiny bones between it and the inner ear, or damage to the tiny hairs in the cochlea, or damage to the nerve endings or a degenerative condition and so on.

There is also the Acoustic Reflex to consider which can act like an attenuator to reduce transduction of sound from the outer ear (eardrum) to the inner ear (cochlea and such). This reflex is activated during very loud sounds as a reflex and it is, interestingly, activated involuntarily during speech and you can contract the muscles voluntarily such as during a yawn (I think?). I've read that the acoustic reflex has a reaction time on the order of 10ms to 100ms which is in the frequency range of 100hz and lower.

I have two guesses (and only guesses!) why 4khz can cause more damage than 40hz:
First, and I think my better guess, is the ear canal acts like a peaking filter that increases the intensity of sound in the 4khz range. Thus, any noise in that frequency range that enters the ear is going to be amplified and therefore have a better chance of causing damage. You can see this when the transfer function of the ear canal is measured such as the graph below.


image borrowed from Clinical Verification of Custom-Fitted Musicians' Earplugs

Second guess, and this is pure dreaming on my part, is the acoustic reflex that can attenuate a sound before it reaches your inner ear has a "slow" reaction time so that impulsive noises like gunshots or firecrakers (or 4khz tones) start so suddenly that the acoustic reflex doesn't have enough time to react and give you what little protection that it can. Again, pure conjecture on my part!

---

Kinda related but really cool:

Typical foam earplugs have a good noise attenuation and they are good at protecting the ear from damage. However, they change the frequency response of what we experience and they tend to sound muffled like the higher frequencies are missing.




I'm going to borrow two more pictures from the really neat article below:
Clinical Verification of Custom-Fitted Musicians' Earplugs



The blue line is frequency response of a typical ear, open, without an earplug. The orange line is frequency response measured with foam plugs in place. Notice the bass is attenuated a little bit and the treble is attenuated a lot? This is why foam plugs sound muddy or dark.

The image below is a product from Etymotic called the ER-20 musician earplugs.



The article above examined the ER-15 model which is very similar. The musician earplugs have a few specially-tuned air chambers inside the clear shell that act liked tuned resonators to allow the sound to pass through the earplug with a "flat" frequency response so that your experience with the earplugs in is much more like with the earplugs out, but overall quieter and much safer.



Notice in this measurement that the orange line more closely follows the blue line? This is what the musicians earplugs are all about.

I use these are the office all the time and they are spectacular! I highly recommend them since you can still talk to people and enjoy loud things at the same time while not having this muddy/bad experience. I can hear all the little telltales of distortion and buzz/squeak/rattle and other anomalies we need to listen for at high volumes to make sure the speakers are not being over-stressed and so on ... but while protecting my ears. Super cool stuff.

[dgage]

I have a question for an acoustics engineer that is brewing over on another thread.

Why does 125dB at 4KHz damage your ears but 125dB at 40Hz does not? I used to know the answer back in the day. But now my brain is filled with crap like Tiger King. Had to eject some knowledge to make room for trailer park reality shows

Ge0

I couldn’t find a definitive reason for why high frequencies cause more hearing loss but an audiologist mentioned bass didn’t cause hearing damage when I was exhibiting my 24” subs at a show. I just tried finding an answer and couldn’t but this was the most interesting paper I could find for our audience and it gives several clues.

https://courses.physics.illinois.edu...6POM_Lect5.pdf

I’m ignoring Presbycusis, which is age-induced hearing loss that primarily affects the high-frequencies, likely due to the finer hairs required to hear (be excited by) the short high-frequency audio waves.

After skimming this multiple times, I wasn’t able to come up with a single answer but I learned a ton about how the ear works. A couple things stood out.

* The ear is designed to boost sensitivity to higher frequencies (above 4,000 Hz). My guess is because these frequencies are more directional and our survival has often depended on identifying where a threat is.

* if you look at the figure at the bottom of page 19, notice how less sensitive humans are to lower frequencies while being more sensitive to frequencies above 10,000 Hz. I guess this would be my best guess for why low frequencies don’t cause hearing loss as readily, the human ear is simply less sensitive to the deep bass region. And I wonder if that is due to evolution since how many deep bass frequencies occur in nature that also affect human survivability? I can think of volcanos but not many others. And with volcanos, you’ll get many non-audible clues.

* As an aside, it is interesting that only 1,000Hz is equal for actual loudness level and apparent loudness level. Maybe this is why 1,000Hz has been selected for so many audio related specifications such as amplifier power.

[dumdum]

If the mechanism is to lower the rail voltage like I think RIPS does, then I imagine it would affect all channels since they usually share one set of rails.

If the mechanism is something else, then .... maybe?

I’m presuming each ‘half’ of the amplifier has its own power supply section so it can control both sides voltage rails individually in a four channel... so 1+2 would have its own rails and 3+4 would have its own power rails and it can adapt the power rails for each to suit