Knowledge is Key

[Sencheezy 689]

Since we are all very familiar with this phrase, I though that I, as well as anyone who is interested, to collectively share any real knowledge, related to car audio, into this thread. Preferably information that has been tested and proven and that has data to back it up. Such as formulas, graphs, and things of the like. If no URL is available, then go ahead and copy the material word for word, with the topic in bold letters before your response. I'm in hopes we could use this thread as one big knowledge base.

 

[Sencheezy 689]

Transfer Function

 

As the Polk/MOMO subwoofers are optimized for acoustic suspension enclosures, we suggest you use this type of design. The acoustic suspension cabinet is a sealed airtight box, and is the easiest box to build. It also is a very predictable enclosure with easily calculated parameters, and it has a smooth natural sound. Properly built acoustic suspension cabinets have a flat frequency response that begins rolling off at 12 dB per octave at the frequencies below its cabinet resonance. This works very well inside a car because of a natural phenomenon called "room gain" that gives you roughly a 12 dB per octave increase in bass frequencies. You can roughly calculate at what frequency this gain begins by using the equation F= 565/L, F is the frequency at which bass gain begins, and L is the longest dimension of your "room."

If, for example, you measured the longest dimension of your car as 5.65 ft., the room gain begins at 565/5.65 or 100 Hz. If your goal was perfectly flat frequency response you would design your cabinet for this particular car to have a resonance frequency of 100 Hz. Since most people want more bass than a flat frequency response yields, tuning the cabinet at a lower frequency, say 50 Hz, would give you a gain of 12 dB per octave between 100 and 50 Hz and flat response from 50 Hz down. The larger the cabinet, the lower the resonant frequency, and the lower the efficiency. Two identical systems will sound very different in a Honda vs. a Cadillac. The bigger the car the lower the frequency at which room gain begins.

[Sencheezy 689]

Phase Plug

 

"Phase plugs may be placed in front of woofer cones, especially in horn-loaded loudspeaker designs. In the same fashion as compression driver phase plugs, the intent is to minimize higher-frequency wave interference near the driver. In this case, "high frequency" is relative to the intended bandpass; for example, a 12-inch (300 mm) cone woofer might be expected to reproduce 550 Hz energy near the top of its intended range, however, the wavelength of 550 Hz is approximately twice the diameter of the woofer, so wave energy at that frequency traveling laterally from one side to the other will be out of phase and will cancel. With a phase plug in the center, such lateral wave energy bounces off of the obstruction and is reflected outward toward the listener. Phase plugs for woofer cones are typically solid plugs positioned over the woofer's central dust cap, or in the center of the woofer, replacing the dust cap." - 

 

Sited from The Cook Book

[Sencheezy 689]

Understanding the limits of WinISD

 

This is a short, simple tutorial on how to interpret the results of your typical subwoofer box modelling software such as Winisd.

Here's our good old trusty Ascendant Audio Atlas 12" modelled up using the manufacturer supplied t/s parameters and Winisd. Notice the shape of the rolloff, and an F3 point of about 52hz. Not bad.

 

 

Now take a look at the actual, measured frequency response (semi-anechoic) at 1 watt, 20 watts, and 75 watts of power.

 

 

First off, you can see that while Winisd was fairly accurate, there was still some differences between the actual response and the calculated response. Note that the actual -3db point is around 56hz vs. predicted Winisd 52hz, and the actual -6db point is ~48hz vs. Winisd's prediction of 40hz.

This is due to variance in t/s parameters between the actual sub, and the manufacturer's specifications. We can also see that the frequency response changes with different power outputs. At 400 watts of power I'm sure the response looks quite different, but unfortunately my amp would only do 75 watts before clipping

Also, it's pretty evident that Winisd does not take into account the upper end response of the driver.

Now let's look at the same sub in the trunk of a Honda Accord (Gold line).

 

 

Now that looks nothing at all like what we modelled  So you can see that the car's interior itself and placement has the greatest effect on the frequency response.

So to sum up, what do box modelling programs like Winisd tell you?

They can tell you:

1. The low end frequency response of the box+sub "ONLY", based on the manufacturer's supplied t/s parameters. The actual t/s parameters can vary by quite a bit, and will change the low end frequency response. For a ported enclosure, I wouldn't even think of using the manufacturer supplied t/s parameters, but rather I would measure the actual t/s specs myself. Reliable manufacturers are generally around 10-15% deviation from what I've noticed, while I've seen some t/s parameters that were off by at least 50%.

2. Frequency response changes with power output. The more linear your subwoofer's motor, the less shift you will see as power increases.

3. Frequency response changes DRAMATICALLY with the listening room/car, and placement.

In conclusion, Winisd can be a powerful tool for comparing different driver's low end response in a variety of enclosures against each other, but the actual frequency response in-car is going to depend on quite a few more factors that Winisd(and similar programs) cannot account for.

One last thing that I forgot to mention. If you subtract the in car response from the sub+box response, you can find the transfer function of your vehicle. Applying this transfer function to Winisd's calculated response can give you a much better estimate of how a sub wil sound in your car.

 

-sited from http://www.diymobileaudio.com/forum/how-articles-provided-our-members/19-understanding-limits-winisd.html

[shizzzon 461]

E = MC²

Edited January 28, 2014 by shizzzon

[Sencheezy 689]

Understanding some Thiele and Small parameters:

 

"Cms = The compliance of the suspension--how easily it's moved

Vas = Cms expressed as a volume of air that has the same compliance

These two are a measure of the suspension's restoring force--how much the suspension pushes back when the motor moves the cone.

Q is a measure of the amount of overshoot allowed at resonance. High Q means that the speaker keeps moving after the signal goes away. Low Q means that the speaker stops moving more quickly when the signal goes away.

Qms is the amount of overshoot that the suspension allows
Qes is the amount that the motor allows
Qts is the total and the formula for Qts is the product of Qms and Qes over the sum of Qms and Qes--like resistors in parallel.

Now...if you look at the parameters for 99% of the speakers that are available you'll see that the Qms is always MUCH higher than the Qes. That means that the suspension allows MUCH more overshoot than the motor. Thinking a little further, you'll discover that what that really means is that the MOTOR controls the motion of the cone and the suspension contributes very little control. Both the motor and the suspension work to overcome the inertia of the moving MASS. The motor does a better job than the suspension. essentially, the suspension screws things up. The suspension is there mostly to keep the coil from leaving the gap and should be designed to apply very little force until that is about to happen.

Resonance is the point where the motor and suspension make the hand-off. Above resonance, the motor provides nearly all the control and below resonance, the suspension does more work. Above resonance we say a speaker is mass controlled and below we say it's stiffness controlled. Above resonance the motor overcomes the inertia of the moving MASS and below resonance the motor overcomes the COMPLIANCE of the suspension.

Here's an example: a super-ball is a high Q device and one of those squeezable things in the check out line of Bed Bath and Beyond is a low Q device. When you drop the super ball, it bounces back nearly to the same height as from where it was dropped. That squeezy thing falls with a thud and doesn't bounce at all.

The super ball bounces back because the rubber has a VERY HIGH restoring force--very low compliance or very low VAS. The squeezy thing has a very high compliance--very high VAS. It ABSORBES the force of the impact.

Now, back to the boxes. A woofer by itself with no box is controlled mostly by the motor but the restoring force of the suspension causes it to bounce around a bit at resonance after it should have come to a stop. When we put the woofer in a box, we ADD restoring force because the cone compresses (and rarefies) the air in the box when it moves, causing it to bounce around even more--it's a stiffer super ball. The Q is increased (and so is the frequency of resonance). If the box is bigger, there's less force applied to the cone when the air is compressed and more if the box is smaller. The box RAISES the Q. A small box raises the Q more than a large one. For a small sealed box, you need a woofer with a very low Q. A woofer with a higher Q will need a larger box. The box provides the additional overshoot necessary to achieve the desired Q.

Here's how sealed box design works. The box raises the Qms of the speaker and we choose a box volume that raises it enough to produce the desired response. Qtc is a measure of how much overshoot the whole system of box and woofer allow at resonance. A Qtc of .707 provides the best combination of flat response and low frequency extension. However, there is some overshoot involved. A Qtc of .5 is critically damped or "transient perfect". There's less overshoot.

Before you freak out and decide that you should always build a box with a Qtc of .5 for best accuracy, we have to think a little further:

Choosing .7 is like saying, "well, I know it can't be flat, so I'll choose to make it as flat as possible to the lowest frequency possible and i'll deal with the group delay (inaccuracy) below the cutoff frequency." Choosing a lower Q is like saying, "Well, it can't be flat and I don't care about low frequency extension, so I'll minimize the SLOPE of the rolloff for less group delay. I'll make it more inaccurate at some frequencies and less inaccurate at the lowest frequencies."

Why would we want overshoot? Because if the woofer contnues moving, it makes bass. We want the woofer to continue moving a little bit at resonance, to boost the bass at the bottom of the response for better low-frequency extension and flatter response. Choosing a lower Qtc is essentially managing the compromise a little differently.

The reason we care more about flat response and low frequency extension more than minimizing overshoot is because flat frequency response from the system IS transient accuracy, by the Fourier Transform. Choosing a Qtc of .7 is like saying "I want perfection down to the lowest frequency possible". Choosing .5 is like saying, "I'd prefer to have the transient accuracy be less screwed up at the very lowest frequencies and in order to get that I'll let it be a little scewed over a wider range of frequencies"

The easiest and best rule is to choose flat response and wide bandwidth if it's accuracy that you're after." - site http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/38784-given-cabin-gain-reality-do-we-need-low-fs.html

[lithium 358]

needs a bit of formatting