Physical and Acoustic problems affecting Sound Quality in a Car

It is common knowledge that a car is the absolute WORST place to critically listen to a sound system. Reflections, standing waves, resonations, uneven interior surfaces, resonant frequencies, and less-than-adequate space for proper speaker placement all play a role here. The goal is to design a system AROUND these limitations, and since there is no real way to measure all of these ahead of time, we must (this is imperative!) design our systems by trial-and-error listening in real time. That means, before you build it, try it. For example, the Oz S10 I did with dash pods didn't get it's impeccable sound by chance. I was going for something new in the lanes, defying conventional thinking and trying to use the car's reflections and a dash location to get the stage height right. It took just over 5 hours SOLID of listening just to get the right angle on the mids! Another 3 hours for the tweets. 30 minutes for the midbasses, and the subs just fit where they could. If I merely built the pods how I "thought" they would sound right, I'd have MAJOR time-alignment/phase, EQ side-biasing, and wacky gain setting issues to deal with if there was ANY chance it was going to sound "OK". When I first fired-up my "mock" pods angled how I thought they'd sound good, I got the EXACT opposite. No center focus, poor width, and a "layering effect" of the instruments all caused by the geometry of the dash and windshield reflections. This example shows how important it is to design the system in real-time. But first, we need to discuss the problems we should keep in mind while designing the system initially, and then continue to look for them when tuning the system over time.

Reflections:

Reflections happen when a speaker's sound waves strike hard, non-permeable surfaces as the waves radiate into the listening area. These reflections can also occur AFTER the listener hears the sound, and it reacts with hard surfaces behind said listener. Reflections cause the sound waves to bounce off the surface and travel in a direct fashion, rather than a radiant fashion, after the interaction. This can cause the sound to seemingly have two point sources, detracting from the imaging department. Reflections can also cause frequency cancellations if they force the sound waves to interfere with the direct sound coming from the speakers. This will give you frequency response errors in the form of "peaks and dips" on a real-time analyzer (RTA). Since every human being perceives sound differently to begin with, reflections can also cause these problems WITHOUT causing frequency cancellation. I am trying to keep this simple, so we won't go into the physiology of hearing. Understand that all cars have reflective surfaces. Center consoles and windows are the biggest culprits here. But think about all of the surfaces in your rides...the vinyl dash, the vinyl headliner, the glass sunroof, Vinyl or leather seats, plastic or vinyl door panels, and the list can go on and on.

Glass is #1 on the list of degree of sonic reflection, followed by hard plastic pieces at #2, then vinyl and other impermeable upholstery. I took the liberty of excluding metal on that list b/c most cars have little exposed metal surfaces inside them, but metal would tie with glass. One saving grace of cars is the carpet on the floor. Imagine if the floor were reflective as well.... NOT good. Plush materials have sonically absorptive properties and consequently can be used to help control reflections. Some guys even modify their car's internal structure to have better-flowing surfaces, smaller consoles and dashes, reformed floor board geometry, and who knows what else. Some even go so far as to re-upholster ALL the surfaces in their cars (accept the windows, of course) with absorptive materials such as foam-backed velour, headliner material, plush tweed, etc. It is all because they are trying to diminish the effects of sonic reflections in their cars. BUT, as in Bob's S10, once you figure out how the reflections present themselves in your vehicle, they *can* have benefits. We'll cover that in speaker placement.

Reflections are also a problem INSIDE speaker enclosures themselves. Speakers produce sound on BOTH sides of the cone. In an enclosure, the rear wave of the speaker is playing directly into a small, solid-walled chamber. When the sound waves reach the enclosure boundary, they can either be absorbed or reflected (we'll get to that). In a solid enclosure with no absorptive material, reflections from the back wave will occur, and the most devastating thing that can happen when they reflect happens when these sound waves bounce directly back at the cone. Since sound waves possess energy, when they strike the backside of the cone, their energy causes undue forces to be exerted on the speaker cone. This makes for distortion (both mechanical and sonic) as the reflected wave tries to force the cone backward. Side effects are resonation in the actual speaker itself, decrease in resolution of detail, cancellation at certain frequencies, and a basic "muddiness" of the overall sound (to name a few). Many guys use different tactics to battle reflections within enclosures. Some line the enclosure with fiberglass insulation, some use polyfill, wool, AcoustaStuff, or a derivative to "fill" the enclosure. Others use Acoustic Foam panels (you know the stuff(it's on the walls of recording studios) to line their enclosures. Cascade Audio even has special discs of damping material specifically designed to absorb the back wave of the speaker. They are called "DeFlex Pads" and work very well. In case you were wondering, I usually use deflex pads and/or polyfill to ensure absorption of the rear waves. No matter how you do it, the key is to absorb and dissipate the back waves of the speaker cones so they don't have a chance to alter your sonic performance in a negative way.

Vehicle Resonations:

It is no secret that resonation in the presence of sound is a no-no in a high-end system. Ideally, we want to be in a listening environment where we are surrounded by "acoustically dead" surfaces listening to speakers in "acoustically-dead" enclosures. By acoustically dead, I mean a surface that remains vibration-free in the presence of sound waves. A prime example is the hooptie box-chevy you likely passed on the road today with the loudest "trunk" you ever heard. I mean you hear the rattles OVER the bass, that's "trunk" bass! The sound pressure is causing loose stuff to vibrate and the metal to resonate. This also can happen at ALL frequencies, but as the freq goes up, it becomes less noticeable, almost to the point of us not needing to be concerned with the highs in this fashion. To expand on this a little further, body panels have their own "resonant frequency" and when that particular frequency is played at enough volume, these panels will vibrate and likely be heard. Even actual door panels and headliners can be affected this way. This is the very reason Dynamat and other like products were developed. Ever hear an interior trim piece rattle, but only on certain bass notes? This is exactly what I am talking about. It is a good idea to dampen as much of the vehicle as possible, and you will likely (I do!) continue to find little "rattles" here or there that need some sort of dampening to calm them. There are several types of dampening materials out there, and each type suits a different purpose. Resonations are also a BIG problem with the enclosures of the speakers. It is absolutely, positively VITAL that a speaker's enclosure resonations are dampened. When the enclosure vibrates at it's resonant frequency, it will emit sound waves of it's own. These waves can do several things. They can react with the backside of the speaker cone, cause cancellation, and even change the tonality of the speaker therein. On the outside of the enclosure, we can even hear the enclosure's "note" along with the speaker's note in severe cases. For this reason, enclosures and speaker baffles are often built extremely solid. Some guys even go to double-layers of MDF with Dynamat sandwiched between them. Others have even used sand and/or concrete to fill the space between the layers of double-walled enclosures for added resistance to resonation. In fiberglass applications, guys can add dampening layers to become part of the mold. Sometimes, thicker and stiffer enclosure walls can STILL resonate, so great care should be taken to use some sort of damping material inside the enclosures used in a high-end system. We can't get into all of these materials here, but these materials MUST be in direct contact with the inner surfaces of said enclosures. Sprays are the easiest to use, but self-adhesive damping materials work well too. If enough damping material is applied, virtually all unwanted resonations can be conquered inside an enclosure. Personally, I like to use non-hardening clay to deaden enclosures, as it sticks in direct contact with said enclosure and is easily pliable while offering excellent damping characteristics.

Resonant Frequencies:

Every vehicle has it's own resonation characteristics. In cars, the listening area is much smaller than the recording studio, and the phenomenon known as "transfer function" serves to reinforce frequencies with long wavelengths (typically 40Hz and below). However, the transfer function works independent of vehicle resonance, which consists of the frequencies in the audio spectrums that occur and/or are reinforced naturally in the car. In a car, the vehicle resonance reveals itself as "peaks" in the frequency response, as the car only needs a small amount of audio energy to produce these notes. Often, there are several resonant frequencies in a car, one in the sub bass spectrum (typically below 70Hz), one in the midbass region between 150-350Hz, and possibly others at higher frequencies, but their effects are less prominent, depending on speaker location and reflections. SPL guys actually search for the car's resonation freq in the bass region, and build their enclosures "tuned" to that specific freq. This gives them added output, as the car's interior itself actually helps reproduce their "note". We needn't consider resolution freq until the tuning section, so we will move on.

Standing Waves:

Standing waves are often a result of reflections of some sort and happen when a sound wave "lingers" in the listening area by being bounced off of reflective surfaces. They can cause frequency cancellations, freq response abnormalities, harshness, distortions, and deceptive location cues in the "Up-front-bass" and imaging department just to name a few. The same principles apply to standing waves that apply to reflections in general. BUT! Standing waves mostly occur INSIDE enclosures, whereas reflections often occur OUTSIDE enclosures. As with "inside-enclosure" reflections, the rear wave of the speaker tries to disperse into an open area. But, since the air space in the enclosure is finite, the sound wave encounters abnormal airspace resistance AND/OR reflects off the walls. This can cause the sound wave to slow down or simply bounce around in the enclosure until it loses its energy. Several technical factors affect how sound wave energy is lost over time as well as the duration of time a standing wave can exist, and these are out of the scope of this article. In other words, its boring techie talk folks. Let's move on. The worst affect of enclosure standing waves happens when the sound wave interacts with the backside of the cone AFTER the initial note (very similar to the way reflections cause the same thing, but the standing wave has a longer duration and thus can interfere with the cone for longer time duration). Many audio companies specifically design their pre-made sub enclosures in a wedge, trapezoid, or other obscure shapes to combat standing waves inside the box. Standing waves are very common when an enclosure is present with equilateral parallel walls.

If we set out to make our own enclosure, we should consider trying to avoid a perfectly square box. A good way to do this with the standard "behind the seat, subs firing back" enclosure is to slope the front wall to form-fit the backside of the rear seat. Simply by having one large wall off-axis with the subs, we take away the "perpendicular" surface most responsible for standing waves. In any sealed enclosure, standing waves can occur at virtually any frequency, as ALL speakers produce second and third order harmonics (sound artifacts or notes which affect adjacent octaves in the freq spectrum) when they produce a sound, and for the most part, this is regardless of crossover frequency. Steep slopes do indeed help restrict these "unwanted" harmonics, however they should not be considered the "cure". Proper enclosure design is vital to combat standing waves, and the use of absorptive damping material is how I tackle them. I know there has been some debate lately on the topic "to polyfill or not to polyfill", but I believe strongly that ALL sealed enclosures can realize the benefits of using loosely stuffed polyfill throughout the interior. Polyfill serves to dissipate the rear sound waves of the speaker cones and hinders the chance of reflections and standing waves re-striking the cone. This results in greater detail resolution and the elusive "clean" sub bass, absent of artifacts and "sloppy sound". Polyfill is also known to make subs seem like they are in a bigger enclosure, but I believe the rear wave dissipation character of the stuff is responsible for the phenomenon. I'd like to see someone research the effects of polyfill inside sub enclosures to see if I am right. I believe I am.

Path Length Differences:

In any SQ system, best results are obtained from getting speaker path lengths as identical as possible. Path length refers to the actual distance the sound source is from the listener's ear. Ideally, we want the left and right speakers the same distance away from us, but this is nearly impossible, as we do not sit in the center of the car. Path lengths firstly affect imaging, and secondly affect other characteristics such as height, width, depth, etc. We need imaging as the foundation of all these, so it is vital that the system is designed for best possible image placement. We get our imaging location cues typically from frequencies between 150 Hz and 2 KHz, making our midbass and/or midrange drivers the most important speakers in terms of imaging. Bass frequencies below 100Hz aren't as easy to locate and don't detract from the imaging of the front stage. High frequencies should ideally have path lengths similar to the midranges, but it is not as critical as freqs above 4KHz are mainly for spatial and ambient cues. Let's discuss possible speaker locations for a second.

Speaker Location:

Lets first start off with door mounting locations. Doors are acceptable for midbasses playing BELOW 200Hz, but are poor for midranges of any kind. Doors are ok for tweeters, depending on off-axis response of the tweeter and the tweet's dispersion characteristics. Obviously, higher on the door is better for tweeters, as legs and seats actually "block" their frequencies. Doors are GREAT for subs and since they are in front of the listener, subs in doors can generally be run up to about 150Hz tops without any ill-effects, and they can negate the need for additional midbass speakers and amp channels altogether. Kicks, the reason kick panels are so popular is that they offer the best possible path length equality between right and left w/o major modification to the car. We will get into how kick panel speaker angling and "path length/ sound intensity trading" play a BIG role in the effectiveness of kicks in part 3. For now, realize that by virtue of placing the speakers as far away from the listener as physically possible, kick panels can get right and left path length differences to within a couple inches (best case scenario). For this reason, kicks are great locations for midbasses and midranges, and good locations for tweeters. As for subwoofers, that depends on the rest of your system, but it will definitely get the bass/midbass in the front of the car w/o any sonic smearing toward the rear (BAD!) Dash-- Great for a sub or subs at or near the center, bad for subs at far left and right (unless the x/o freq is below 75Hz at 24dB/oct or higher) In both cases, the subs should not exceed about 125Hz. Dashes are ok for midbasses, but as in the doors, path lengths will be very different from right to left, so a low x/o point MUST be used. Dashes are Horrible for midranges! Especially when firing up at the windshield, as the reflected sound will draw virtually all of the stage to that point. If midranges are to be attempted on dashes with any sort of good imaging being sought, you must get into the pathlength/sound intensity trading" that was mentioned earlier and discover how the dash and windshield are affecting the soundstage in terms of reflections. THEN, you must adjust angles accordingly for proper imaging. Likewise, firing the main tweeters directly at the windshield will draw the stage to the near-side tweeter location. Most tweeters are best used in a cross-firing manner, either 90degrees from the listener, or slightly more "on axis"(toward the listener). This is why many guys choose to do tweeters on the A pillars, because it keeps them high positionally, keeps sonic obstructions away from their sound waves, and offers good imaging for the most part. We shall cover all these locales in depth shortly.

So, to sum the path length section, getting your front main speaker's path lengths as close to the same as possible from right to left is a critical part of getting the elusive "sound stage with good imaging". We will dissect this topic shortly.