Sound Waves Inside A Car

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Listening to music in your bedroom or living room is not the same as listening to your favorite tunes on your way to work. So what do you do? You turn up the stereo to drown out the cacophony, sacrificing audio quality and your future ability to hear. Vibration dampeners can help minimize the amount of sound waves your audio has to contend with by absorbing vibrations and minimizes acoustical reverberations.

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You don't have to turn your stereo to 11 and damage your hearing to enjoy the wicked riffs of your favorite band. Dynamat is one of the best examples of a vibration dampener. It comes in adhesive sheets made of lightweight elastomeric butyl and aluminum constrained-layers. We recommend installing vibration dampeners in the following areas for a much improved car audio experience:. Hood Your engine is the worst offender to your acoustics.

It's loud. It vibrates the entire car. By installing Dynamat on the underside of the hood so that it faces the engine, you can dramatically reduce the sound and vibrations caused by your engine. Doors Your doors are where you'll likely have between two or four speakers. Placing Dynamat in the door panel, behind the speakers, will greatly reduce vibrations in the door caused by the speakers.

It will also reduce the sound of a slammed door to a mere muffle. Ceiling Your rear deck speaker reflects sound waves off the rear window and off the roof. 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.


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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.

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.


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Often, there are several resonant frequencies in a car, one in the sub bass spectrum typically below 70Hz , one in the midbass region between Hz, 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 frequency 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 frequency 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, frequency response abnormalities, harshness, distortions, and deceptive location cues in the "Up-front-bass" and imaging department just to name a few.

Car audio system uses interior surfaces as speakers

The same principles apply to standing waves that apply to reflections in general. As with "inside-enclosure" reflections, the rear wave of the speaker tries to disperse into an open area. 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.

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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 frequency 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.

Robert Boyle's classic experiment

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. The formula for the frequency that the observer will detect depends on the speed of the observer — the larger the speed the greater the effect. If we call the speed of the observer, V o , the frequency the observer detects will be:.

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Here, f is the original frequency and V wave is the speed of the wave. However, above, we saw that the Doppler effect depends on the direction that the observer is moving. How does that enter into this formula?

If the observer is moving towards the source of the sound the frequency should go up. That is what the formula predicts — so far so good. If the observer is moving away from the source, the frequency should go down. How can we make this happen? There are two ways to understand this. We can say that if the observer is moving towards the source, its velocity is positive, or greater than zero, while if it is moving away from the source, its velocity is negative, or less than zero.

If you put a negative number for V o into the formula above, the result will be that the frequency decreases. Alternatively, we can write two formulas, one for the observer moving towards the source, and one for moving away from the source:. What happens if you are moving away from the source, with a speed, V o equal to the speed of the wave, V wave? What does this mean? The situation where the source is moving is actually a bit more difficult to picture.

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In the following diagram, the source is the red circle moving to the right. At periodic points in time, it sends off a circular wave. However, once the wave leaves the source, it is no longer affected by the motion of the source — the wave just travels on its own. However, when the source sends off the next wave, it will have moved forward a bit. The new wave is a circular wave, just like the previous one, but its center is shifted slightly in the direction that the source is moving.