so let's go back to basics.
Sound is made of different frequencies and it is transmitted as a wave.
The speaker driver makes those waves (generally) by moving a cone forwards and backwards at a frequency corresponding to the note. The size of the movement dictates the volume / amplitude of those waves and therefore the "loudness" of those sounds. We also don't measure these things linearly; decibels, the unit of loudness, is a log scale. So 10dB more is a doubling in loudness. You'll also need an order of magnitude more power to achieve this, so for a 10dB increase you'd need to up the power from 1W to 10W. Or from 200 to 2000W. Most of the time this isn't really relevant as you're listening at low levels. Your standard radio probably puts out around 14W per channel.
So if you have a specific, pure note, like an A on a piano; that's 440Hz and so the loudspeaker cone moves back and forth 440 times per second to make that sound available to your ears. It is made by pushing current through a coil which moves the coil in relation to a magnet. The amount of current pushed through dictates how much it moves and how fast the current alternates dictates the frequency,
There's a few obvious things when we start thinking about this: one is that lower sounds have lower frequencies, but our hearing isn't linear; two is that more air takes more effort to move. Also: the size of the speaker cone kind of dictates how high a frequency it can cover, and the way it's put together defines its ability to move easily - there's the surround holding the cone itself to the basket of the driver, the driver material itself, and then there's all the magnets and coils involved. These can all be tweaked for different use cases. More on that later.
The fundamental, though, is that a speaker driver is just moving air. And therefore, the first thing you need is seperation between the waves at the front of the speaker driver and the ones at the rear. Otherwise you're not going to hear very much as the waves will cancel each other out.
So the use cases for any particular driver could be one of three options: you've either got a sealed box, a leaky box, or a ported box. Drivers can be optimized for one of these and trying to use it in something else is silly unless you're deliberately trying to do something specific.
These boxes mainly have an effect on the bass response of the speaker; but remember that the car *itself* is essentially also an enclosure. This leads to something called cabin gain, and you'll get it in rooms as well. It has to do with the dimensions of the space and the waves that actually fit into that space. Anything that doesn't fit gets amplified. There's other effects on bass, too, mainly involving flat surfaces in front of, behind and maybe even next to speaker boxes - if you put a subwoofer facing the wall, you'll get different bass effects than if you put it in the corner (more bass there!) but it might be boomy at specific frequencies. This will be to do with the environment.
Back to the car: most car-specific door drivers are designed to work well in a leaky box, as that's generally what a door is. There's some air movement towards the outside world (drain ports) but the space is mostly sealed. So a driver that works well in anything above, say 14l or so, will probably work well in a door.
The thing is, though, that those drivers are of a limited size - call it 165mm for argument's sake - and that's not really compatible with loud volumes and nice bass. Why? because you're trying to move a lot of air with a relatively small driver.
If you've got a sealed box of a good size, a driver designed for that will sound nice and punchy with crisp basslines. If you decrease the size of the box, the bass response (so how low it will go) goes down and there will be a peak somewhere. If you increase the size, your bass may get better or worse again, depending on what the driver was designed for.
What Bose did and does is to tailor the response of the system to the car. They've analyzed the car and added some correction in to the amplifier, so running their system sounds better than stock. Whether it's good or not is another person's call. The thing is, they've taken into account *everything* at that point - the attributes of the drivers they use, the response of the subwoofer in the boot, and the frequency response which is delivered from the radio itself. This is all then hard-wired into the amp. I'd suspect also the "bass bump" at around 60Hz or so - it's only a 6" sub, which can't do much but if it's tuned this way, it would appear people prefer the sound as it "feels" like "more bass". (
See this if you're really interested, page 5) They also use lots of tricks with driver impedance to improve headline power output while not really doing much on the electronics side (for a given load, 200W into 0.5 Ohm is similar to 100W into 1 ohm, 50W into 2 Ohms or 25W into 4 ohms.. which is about double the power of a standard 12V DC radio amp, so therefore you're getting about 3dB more output, possibly).
A better way to deal with this is DSP (digital signal processing); something like the
C-DSP 6x8 isn't expensive (€250, I think), and it allows you to compensate for a lot of the issues inherent in cars. But even that's not perfect - you can polish a turd, if you really want, but it's still polished and it would be better to have a potato first or better still some aluminium to try and polish or you might be disappointed.
@Olerype is correct; removing the deep bass from the doors is a good way to stress the drivers less and it will sound a lot more effortless and clearer as a result. Putting this to a sub, driving those big waves from a big driver (most subwoofers are 8" or larger) is worthwhile.
But then we get to stereo - and there's only one sub, right? Yes. B-ut humans don't really appear to be able to locate low sounds, so up to about 150Hz or so, it doesn't really matter where the sub is and that there's only one. Yes, it can be found if it's too loud, but that's a different problem. Remember the idea that the driver's producing waves of sound? it's much easier for the higher frequencies and hard for larger diameter drivers to reproduce those - so this is why we get tweeters with really small diaphragms for the high stuff. If you listen carefully, you'll hear how little is generally coming out of them, but it's really important for lots of stuff, mainly "air" and spatial cues. The midwoofers in the doors are dealing with pretty much everything else, but they are also designed for a specific size of enclosure and without that, you'll get strange results. The plots above reflect a box size of 3.5l vs much larger. How did I calculate this? All of those parameters I spec'd earlier: the connection between cone and basket, the power of the magnet, the size of the driver, how far it can move - these all have standardized measurements, called Thiele-Small parameters. This will essentially tell you how the driver will behave in a certain type of box of a certain size; there's software for this, such as Bassbox pro, WinISD and a bunch of others. There's some seriously complex stuff involved in horns, which is why I haven't touched on them here.
So with those parameters - which some companies provide - I can model the different response of the different boxes. I have a
CSS SDX10 waiting for transplant into the car. That has parameters for both sealed and ported boxes, and I've just built a 20l box which will fit nicely into the space next to the battery, sticking up a little (so I will lose the carpet). I've modelled that with lots of different boxes and this is the one that fits and seems good.
The filter I used simulates cabin gain; 57Hz is taken from the longest measurement on the inside of an A2 which was 3m something. Anything lower than 57Hz has a wave which doesn't fit and therefore gets amplified.
Additionally, a whole bunch of road noise happens at around 125Hz - and to compete with this, you'll notice GALA really amps up the bass as you drive (try turning it off in a trip and then back on again and listen for the difference!). There's also the problem with reflective surfaces all over the place - glass is bad. The sound in a car without an OSS blind is very different to one with it.
You also need to take this into account when positioning tweeters; I've wanted to put mine right at the corners of the windscreen, on both sides. But I'm hesitant to say it will work well, as there's a great big funnel and that *will* affect the sound, just like listening to speakers placed on a desk will change your perception of treble and bass, because that sound is being reflected around. Your preference is good, here, but I think it's important to understand what the speaker designer was intending to do.
If OP is happy with the sound in these enclosures, great. I simply intend to explain some of the thinking I've used in the past when I rejected the same idea.
But can't you fix all of this stuff with DSP? Sort of. Any processing introduces delays, so it's best to get it good before making it better with DSP.
A quick coda on the subject of bluetooth speakers and "How do they sound so good, then?" Solid enclosures, specifically designed for drivers; use of passive radiators instead of ports and a certain amount of DSP on-board. Add a bass hump, and I think that's most of it. For some entertainment and talking at least a little about how these are wee miracles, I can recommend
this video.
- Bret