Join Date: Apr 2006
Location: Dover, NH
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Years ago, I had a Subaru GL-5 Hatchback (rest in peace). This was just around the time CD's were hitting the market, so cassettes and radio still were the mainstream for car audio. Anyway, I went to an audio store and bought a Yamaha head unit and a set of speakers made by a company called ADS (Advanced Digital Systems). The speakers were all named after BMW car models. Mine were the 325i speakers. I still have them and the related hardware, amp, and crossovers but haven't found any use for them since.
Anyway, the owner of the store was also the installer and since he was busy, I offered to help out. While we worked over my car installing a 100 Watt amp under the seat, the crossovers behind the glove box, and the woofers/tweeters in the doors...we talked about sound from the engineering perspective. This guy was a sound engineer, was very smart, and a wealth of information. The main points I remembered from talking with him was that sound is energy, so if you can focus that energy, you can do all sorts of things with very little power. Bass has more energy than treble or mid-range and therefore most of the power is used to create low levels of sound. It's also easier for the human ear to locate the origin of bass but nearly impossible to locate treble. Lastly, speakers are designed to perform a certain way, so if you push them with an equalizer, you are basically destroying the speakers and shortening their lifespan.
Final word on all this...a well designed system should sound perfect with the bass and treble adjustments at neutral and no need for an equalizer.
One thing we did in my Subaru and something I've noticed no car manufacture ever does...is enclose the speakers in airtight containers. The point here is to make it so all the sound energy a speaker produces has only one way to go...out. If you have open space behind the speaker, literally half the energy is wasted. Think of a swimmer starting out in the middle of a pool versus one who has the pool edge to push off from. If you can lock the space behind the speaker by making it airtight, the sound can only go one way and the speaker's efficiency is increased dramatically.
We made little boxes out of 1/4" plywood and sealed the cracks with silcone rubber. I had an old pair of coaxial speakers and built two little boxes for them and set them on the floor in the back of my Subaru for a total of 6 speakers. Let me tell you, I didn't need a sub-woofer at all (of course they weren't being made back then either).
I'm wondering if it wouldn't be possible to encase the OEM speakers/subwoofer with a case (fiberglass maybe).
Let me respond to the statement about being able to locate bass vs. treble. This is absolutely, positively wrong. It is very easy to locate the source of high and middle frequencies and much more difficult to locate the source of low frequencies. One of the characteristics of an audio system is known as Imaging. This is the ability of the system to place each instrument in a specific part of the soundstage. A well-designed system will separate each instrument and place each in its own area of the soundstage, just as if you were listening to a live event. This is made possible because it is easy to locate the source of high and middle frequencies.
Let me provide another example. If you hear a bird chirp you can locate the bird very accurately. When you hear thunder it is impossible to locate its origin. It sounds as if the thunder comes from all directions.
With respect to the equalizer statement and tone controls. I am not a fan of equalizers and believe the less equipment in the reproduction chain the better the results, but using an equalizer will NOT destroy your speakers or shorten their lifespan. Passive equalizers add no power and therefore simply attenuate selected frequencies. Active equalizers provide amplification, allowing the user to attenuate or add gain to the selected frequency bands. Using an equalizer at the user's level in the system does nothing more or less to the energy in the signal than an equalizer used during the recording process. Where the damage concern may lie is with the user boosting certain frequencies beyond the speaker's power handling capacity, but this is not due to simply adding an equalizer.
Anoter little tidbit- it is very easy to damage a speaker with a high power rating when using a low power amplifier. If any amplifier is over-driven it runs into a condition known as clipping. Speakers want to see sine waves. During the clipping condition the amp has run out of power and "clips" the tops off the sine waves, creating square waves. A speaker cannot reproduce a square wave and will become damaged.
Sealed enclosure speakers generally provide better low frequency response that ported or open enclosures, however they have LESS efficiency. The analogy to the swimmer is way off base. The driver (raw speaker) does not push off the rear of the speaker cabinet. What we have in this case is a sealed cabinet with a specific internal volume. As the driver moves in and out it is changing the internal volume of the cabinet, but because the cabinet is sealed the volume of air cannot change. This means that the internal cabinet pressure must change. As the driver moves it fights the changing pressure resulting in a LESS efficent system. A good demonstration of this principle is to try to crush a plastic milk container. Placing the cap on the container results in a sealed enclosure with a specific volume. You will find this very difficult to crush... until the cap pops off. With the cap off it is no longer a sealed enclosure and becomes very easy to crush. You use less energy to crush the uncapped container (unsealed enclosure), meaning a more efficent system.
The result of a sealed enclosure is a less efficient design, but much better low frequency fidelity. The increased fidelity with the sealed enclosure is because the out of phase waves from the rear of the driver cannot interfere with the waves form the front of the driver.
Thanks for taking the time to explain the details. I understand all that you said and wish to add a couple more points or questions.
My reference to being able to locate the source of high and low end sound came from discussing where to locate the tweeters in my Subaru. My installer friend told me the best location for the tweeters was about level with my ears and in front of my head when sitting in the car. I remember that clearly but perhaps my memory is mixed up about whether it was high or low frequency that the human ear can pinpoint. What you say makes sense and supports the instructions with home theater systems where they say it doesn't matter where the subwoofer is located.
The discussion about the efficiency of an enclosed case is very interesting. Since reading it, I think a better analogy to demonstrate my point is not a swimmer but rather a cannon mounted on a wheeled platform. When the cannon fires, the recoil causes the cannon to move backwards on its wheels. This is the energy loss I'm speaking of as it could have been preserved as part of the force pushing the shot to an even higher velocity if the cannon was fixed to the ground.
So when the rear of a speaker is sealed, the backward movement of the driver is hindered because, as you explain, the volume of air behind the speaker can only become pressurized. Incidently, the speaker boxes we made were stuffed with fiberglass insulation to minimize the amount of air behind the speaker. What I remember specifically about the effect was having to use maybe a third of the volume control I had previously used to hear the same volume of sound. I'm not an electrical engineer so I'm not sure what the volume knob is actually doing but I always thought it metered the amount of watts going to each speaker.
1) Tweeters at ear level is correct. This applies to the car environment as well as home audio. Tweeters have small diameter drivers and as a result have small dispersion angles. The best performance will result from having the tweeters at ear level.
2) Regarding efficiency. The cannon analogy is an example of the principle that for every action there is an opposite and equal reaction. If the frame of the driver or the speaker cabinet is not secured properly it will move opposite the action of the driver and this will cause loss of efficiency. Movement of the cabinet or driver frame can also result in phase shift, leading to constructive and destructive interference, or in short poor fidelity. This principle also applies to home audio. Avoid placing speakers on flimsy stands or shelving.
As a public service announcement regarding speakers for home audio, and I have seen this many times, NEVER, EVER place speaker in diagonally opposite corners of a room facing each other!!!
3) Speaker enclosures. Not only is the driver hindered as in moves in to (backward movement) the cabinet, but also as it moves out of the cabinet (forward movement). The pressure in the enclosure is changing with both directions of the driver's movement.
Stuffing the enclosure with fiberglass is a valid and sound practice. This attenuates the sound wave from the rear of the driver. Without the sound deadening material the wave from the rear of the driver would be reflected from the inside of the enclosure and then interfere with the driver and the wave from the front of the driver.
The sound level in your situation may actually have increased, but this would not have been directly related to sealed vs. ported enclosure efficency. It may have been that the original system experienced large losses because it was so poorly designed and produced.
Hopefully this has helped. From your last post it seems that you have a better understanding.
Thanks again for further comments. This is a great discussion and I'm even smacking myself in the forehead with some of what you've said about wave interference. It takes me back to my physics classes and how wavelengths will increase or cancel out each other depending on their frequency and size. We used a shallow water tank with wave paddles to experiment visually but the same forces are at work with both light and sound.
You're right...the sealed case would indeed hinder the flexing of the driver as it tries to move in and out and thus compressing or decompressing the space of air behind it. What's most important as you say is disrupting the rear sound wave. Thanks again...I hope everyone picks up a few pointers by reading this thread.
|Oh yes, the old ripple tank experiments. In my high school class I would float a small toy sailboat in the tank, to the dismay of my instructor. One caution, constructive and destructive interference doed not work with light. Beams of light take on characteristics of photons, not particles. Thsi gets to be a very tricky science.|
|Agreed free air speakers just suck. I would look for baffels? I dont know if they make um for our cars but crutchfield should carry them|
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