explain lobing in lamina terms

[TaylorFade 10+ year member]

nice thread.
i don't have time right now to go into too much depth, but the overall message is that you are interested in the size of the sound sphere at the listening position (knowing that it's not a full sphere due to reflections and boundary surfaces). while you can delay the time the signal leaves the speaker, you cannot change the size of the sphere at the listening position. this is less crucial in a car but a huge deal in a large auditorium or performance hall, and is most applicable when designing rear monitors (like under a balcony).

time alignment will change the way the two spheres interact, and thus change the lobe shape and orientation. with time alignment you can "steer" the lobe. think about what happens as one speaker's sphere is allowed to grow larger before the other speaker starts.

comb filtering got its name because the response graph (dB vs frequency) looks like the fingers of a comb due to phase interference (both constructive and destructive). when two sources arrive at different times you will have both cancellation (at frequencies who's path length differs by 1/2 wavelength) and combining (at frequencies who's path length differs by a full wavelength). you also get that at harmonics of those frequencies - hence the comb shape in the response.

all of this is simple in free space with no reflecting surfaces. each reflection is another source. as sound continues to play, it keeps reflecting and you have more and more sources. increase room absorption and diffusion of sound, and you can stay focused on what is playing now, not what was playing a few seconds ago.

in a car - the door is the main culprit for acoustic distortion. sound inside the door cavity keeps bouncing around, building, until it eventually comes back out through the speaker cone as a muddy mix of crap. this is why songs seem to deteriorate the longer and louder you play them. doors with adequate absorption inside them (by adequate i mean as much as possible) do not suffer the same fate. the same concept is true for the cabin or listening space. lower reverberation time and you improve accuracy of the listening experience.

I love how this ^^^^ is his "I don't have time to get in depth" answer. Lol.

 

[keep_hope_alive Premium Member]

I love how this ^^^^ is his "I don't have time to get in depth" answer. Lol.

I only had 5 minutes... //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[keep_hope_alive Premium Member]

Wow so distortion can beer mistaken as comb filtering?
Can you explain this. Do you mean different frequencies being wavelengths ex 1/2 of 100hz being 50hz, and if the source arrives at different times there will be cancellation of some sort...

phase interference is when sound from the same source arrives at different times (due to driver differences and reflections).

Is this what is referred to as a backwave. So reflected sounds from inside the door can actually come back through the speaker? So really alot of what can be mistaken as distortion could actually be just a bunch of interference from reflections off surfaces.
Sorry I keep posting I just keep taking more in the more I read...

correct - "backwave" is equal in both volume and amplitude to the "frontwave". speaker cones are very light and while able to move air have very little transmission loss (ability to block sound).

Not cancellation of different frequencies, cancellation of the frequencies where the 2 drivers overlap (your crossover point and around there). Of course you're bound to have issues where your sub and mids cross since they're far apart from eachother, but the sub is pretty non-directional so some delay or phase adjustment can fix that easy enough.
Of course in a car you also have the issue of left and right won't hit your ear at the same time so there's going to be some issues there, but this article is concerned with where the two drivers are playing the same freq. As you said, changing listening position (or aiming of the drivers) can make a dramatic difference, and as others have said, coaxial in the kick firing straight at your face is the ideal way to minimize the effect. Next best would be mid and tweet mounted with the coils as equal distant from your ears and as close together as possible.

yes, crossover points are a concern... but moreso are reflections. reflections off glass are specular - angle of reflection is equal to angle of incidence and level is not diminished.

I would bet with the shape of the inside of a car door, that's probably not a huge issue, not enough flat surfaces to build up a good standing wave IMO. Then again, untreated doors will sound like *** anyway.

i disagree - the large flat surface behind the speaker is a perfect reflecting plane. doors are reflective and will allow sound to build and build and then come out through the door speaker (or any other openings). i'm not surprised this concept is new to most people since i'm the first person i've seen discuss it at all. people say to "seal the doors" so sound doesn't come out through the plastic but they neglect the door speaker as "hole". people use "deflex pads" behind speakers to "break up" the rear wave, but they are not large/deep enough to be of any benefit to midbass and lower midrange. that is why i recommend massive absorption inside the door - the difference is amazing and instant.

How far away from the crossover point does interference occur? Say 12DB/oct
And again, interference can be mistaken as distortion right?

an octave is a doubling (or halving) of frequency. So if the HPF crossover point is 1000Hz, at 500Hz the signal is 12dB lower. note that we (humans) perceive a reduction of 10dB as being "half as loud".

yes, what we consider to be distortion is commonly phase interference.

the reason sound changed as you moved your head back is because you changed the arrival time difference between direct and reflected. with dash tweeters you hear both direct and reflected at different times. further back = more off axis so the direct path has more attenuation.

when considering phase interference - level plays a huge role. the louder source will be dominant.

 

[calebkhill 10+ year member]

that is why i recommend massive absorption inside the door - the difference is amazing and instant.

Suggestions?

angle of reflection is equal to angle of incidence and level is not diminished.

So sound levels don't decrease when Reflecting off glass.

when considering phase interference - level plays a huge role. the louder source will be dominant.

So from what I see there are two things to do to get ahold of phase interference. Time alignment or placement of drivers, and trying to avoid any type of reflections.

What do you think about speakers in the a pillars. Maybe with speakers pointed directly at listeners is a good path for sound to travel... Edit: Also, would sound from different drivers that cross paths cause any interference to be worried about, say drivers in pillars pointed at opposite listeners.

 

[calebkhill 10+ year member]

Just out curiosity... Are you stockpiling info or having specific issues you're trying to alleviate? Planning an install an how best to minimize stuff you can control?
Because honestly, you can drive yourself bananas and get lost in the theoretical realm when in reality, you just have to do the best you can, try different locations and axes as tune to the best of your processing ability.

Yes, I think all this pertains to problems I'm having a will have in the future

 

[keep_hope_alive Premium Member]

It is not as simple as just pointing straight back as that affects HRTF...

I am currently playing with speakers (2" mids) in dash/apillars and it is interesting. Certainly different challenges than kicks.

 

[calebkhill 10+ year member]

@keep_hope_alive;

Let me first say that that post was completely awesome. It explains allot of different phenomenon and sheds light on how exactly and why certain things happen to the signals. someone mentioned earlier not to get caught up in theory butt I think understanding the theory can give insight on how to control certain phenomenon.

It is not as simple as just pointing straight back as that affects HRTF...

I can see exactly how this would affect hrtf.

Let me ask. How would you go about counteracting ITS, ILD, and hrtf.

Also, could you explain what the graphs in the bottom picture are showing. I don't know what the lines are saying, its also somewhat hard to see on my phone.

Lastly what all have you learned from playing with your mids

I know it may take some time to respond but I'm patient. Take your time, just whenever you have a chance.

p.s., where did you get that picture from.

 

[calebkhill 10+ year member]

angle of reflection is equal to angle of incidence

Angle of incidence?

 

[keep_hope_alive Premium Member]

[quote name='calebkhill']@keep_hope_alive;

Let me first say that that post was completely awesome. It explains allot of different phenomenon and sheds light on how exactly and why certain things happen to the signals. someone mentioned earlier not to get caught up in theory butt I think understanding the theory can give insight on how to control certain phenomenon.

I can see exactly how this would affect hrtf.

Let me ask. How would you go about counteracting ITS, ILD, and hrtf.

Also, could you explain what the graphs in the bottom picture are showing. I don't know what the lines are saying, its also somewhat hard to see on my phone.

Lastly what all have you learned from playing with your mids

I know it may take some time to respond but I'm patient. Take your time, just whenever you have a chance.

p.s., where did you get that picture from. [/QUOTE]

that picture is straight out of NFPA 72 - national fire alarm and signalling code - appendix on speech intelligibility. one of my tasks is computer modeling (with EASE) to design voice evacuation/notification systems and ensure intelligibility is maintained. i also recommend acoustic treatments and speaker layouts/selection to achieve that goal.

you have a PM to whet your appetite for acoustics knowledge...

ITD is handled with Time Alignment and placement
IID is handled with level adjustment and sometimes aiming
HRTF is handled with speaker placement and aiming

ITD and IID is not an issue when you are equi-distant from the left and right speakers
HRTF is resolved by ensuring the speakers are far apart and have some toe-in to ensure shadowing
the remaining challenges are room acoustics and eliminating/controlling reflections that would otherwise muddy the signals.

the bottom graph illustrates typical attenuation due to source location relative to head placement. i.e. what filtering is expected from sources behind you relative to in front of you.

 

[calebkhill 10+ year member]

Thx for the pm.

I see.

As far as what was mentioned earlier about phase interference and source levels,I think I encountered this today.my oscope came in and I set my hu and amps to highest non clipped volume. Doing this I found my door speakers could handle more power than I thought. before i had a lot of issues with interference and reflections with my my in dash tweets which meant a lot of Eq'ing. Now that my door speakers are louder with a clean signal I believe they are counteracting a lot of the interference from the tweets, considering that before the sound from above the dash and reflections of the glass were the most dominant sources in the front stage, as opposed to now the door speakers are the louder and more dominant source. Also my head position is slightly further back than before so the hrtf comes into play also.

Learning is fun haha.

By the way, my stem sounds better than it ever had after using the oscope, even louder and even clearer. Its awesome.

 

[keep_hope_alive Premium Member]

good deal. experimentation combined with learning more about how and why is not only fun but very productive!

 

[calebkhill 10+ year member]

@keep_hope_alive; @ciaonzo; @TaylorFade; or anyone else.

I don't mean to bring back an old thread or maker a long post....
But, today I don't know what drove me to do this but I put my ear right in front of the speaker and covered up the other ear and it was like I could hear every detail of the song. I sat up normal and covered the ear opposite of the speaker I was listening to and I could hear guitars and drum symbols, vocals everything with such detail. Covered the other ear, same thing. When I listened with both ears the clarity I was hearing completely diminished and some sounds I couldn't even hear any more.

I have an idea of what's happening, but let me ask.
What's happening and how do you achieve this same clarity? I just don't see any way to get the system to show this clarity. It was surprising and amazing.

 

[MutantDreams 10+ year member]

I'm no expert on this at all, but i believe it has something to do with our brain filtering out some frequencies, im not 100 percent sure, but im sure someone can give you an actual answer.

 

[brandonkain82 10+ year member]

Wowwwwwwwwww u guys are amazing. **** that blew me away reading that. Thank u

 

[keep_hope_alive Premium Member]

it leads me to believe that with just one ear you don't hear delayed differences between both ears, and the effect of reverberation is minimized.

with both ears and sitting back in your seat you are hearing more reflections than direct sound. when sitting in the driver's seat, the right ear can be dominated with reverberant sound and the left ear dominated with reflections off the glass.