Writing a FAQ for the audio guys?<===Attn: Admin

[gtphill]

Hey guys,

 

I just sold my W-body, so my days of lurking/posting here are pretty much over. As a going away present I thought I would try to write a FAQ for the car audio section on what the community feels are the 10-15 most commonly asked techie questions. Questions would be like "What is an Ohm, and is it important?" or "Why does a car produce more bass than a living room"

 

I have almost a decade of experience in professional loudspeaker system design and tuning, including advanced FFT and room deconvolution techniques. I also did some loudspeaker design consulting for a major pro audio manufacturer in college.

 

I also have installed modest, but functional stereos in my cars, including my old w-body. Most of the principles from professional audio apply well to the car environment.

 

Let me know what you guys think.

[slick]

I'm definately up for helping out any if possible. I'm been messing with car audio for some years now, and I am an audio installer myself. In the last 2 years I started to get a lot more serious than what I was in the past.

 

Need any help, lemme know.

[FordBoy]

Sounds like a plan Dido slick

[gtphill]

Question: What is an Ohm, why are speakers given an Ohm rating, does it matter?

 

This is the Basic Answer:

Electricity has two components, voltage (V) and current (I). Voltage is like the pressure of water that is flowing in a pipe, and current is like the amount of water flowing in a pipe. In a smooth straight pipe more water will flow at a given pressure than in a curvy pipe, because there is less <i>resistance</i> for flow. In a similar way the less <i>resistance</i> an electrical conductor has the more current will flow for a given pressure (ie voltage). This is defined by a basic equation called Ohm's Law. Ohm was a scientist who discovered the property of electrical resistance. Ohms law says: V(oltage)=R(esistance)*I(current) or V=I*R for short.

 

Different speakers have different resistances. More current will flow through a speaker with lower resistance than one with higher resistance. Resistance is NOT a performance characteristic of a speaker driven with modern (voltage source) car audio amplifiers. Speakers move because of the voltage, not because of current. That means that 4 Ohm speaker is not better than a 2 Ohm speaker, just different.

 

Ohm ratings matter in choosing an amplifier. If your amplifier is only rated to drive 4 Ohm speakers, and you run 2 Ohm speakers on it at high output, you risk causing the amp to overheat, and possibly shutdown to protect itself. This is because the amplifier cannot put out enough current. Pick your amplifier so that it can supply enough current for the speakers you choose.

 

 

This is the intermediate answer expansion:

Speakers do not have a simple single value for resistance. Instead they actually have an <i>impedance</i> which changes as a function of frequency. Therefore it is an oversimplification to give a speaker a singular "Ohm" rating. The speakers impedance is very high at the natural resonance, and then goes to a minimum, and then goes up again at higher frequencies due to the storage of energy in the voice coil's magnetic field. This latter effect is known as inductance.

 

Resistances in series (ie plus to minus to plus, etc.) add linearly. Resistances in parallel (ie all pluss' tied together and all minus' tied together) add by the inverse: 1/Rtotal=1/r1 + 1/r2 + 1/r3, etc.

 

A loudspeaker responds to the (changing) voltage applied to it because of what is known as Ampere's law, which basically says a changing electrical field causes a magnetic field to be formed. A modern voltage amplifier always provides the "correct" amount of current at any given frequency, regardless of the resistance at that frequency. This effectively removes the effect of current from the response of the loudspeaker, and makes it depend only on the voltage that the amplifier produces.

 

 

And the final advanced expansion:

Let's think about what electricity in a cable really consists of. First, and

foremost, it consists of a propagating electric field set up by the rotation of

the generator windings through the magnetic field (Wikipedia Faraday's Law of

Induction). This field is represented by a net displacement of charges (i.e.

electrons). The electric field propagates very rapidly (approx. 0.8 the speed of light depending

on the dielectric constant of the cable)

 

The second thing electricity consists of is the drift velocity of electrons in

the cable, due to the net electric field causing a slow movement of the average

position of an electron in the cable. The drift velocity is slow, only a few

cm/second. Some of the energy the cable dissipates due to resistance contributes

to causing the drift velocity.

 

The net drift velocity for a pure sine signal, of course, is zero.

 

Next we must consider the definition of work, because the work-energy theorem

says that energy is the ability to do work.

 

Work=Force (dot product) displacment, or in integral form,

 

Now, the rubber meets the road for this situation in the dot product. Linear

algebra tells that the dot product equals Force*displacement*cosine(theta) where

theta is the angle between the Force and displacment vectors. (Please no one

jump on me for not rigoursly defining the vector magnitudes)

 

So, why does this vector math matter? Cosine of 0degrees is one, and cosine of

90degrees is zero. That means that the amount of work done is function of the

angle between the direction of the force and the direction of the displacement.

That is why no work is done to keep an object moving in a circle, because the

centripetal force is normal to the displacment at every point.

 

So, in your speaker, you have some current that flows in phase (i.e.

cos(theta)=0) with the voltage, and that in phase current is doing WORK. You

also have an out of phase (i.e. cos(theta)=90) current component for power

factors other than 1, and that component's energy cannot, by definition, do any

work.

 

So, for half of the AC cycle. your amplifier is creating a voltage that

is trying to push electrons towards the speaker. In spite of that, some of the

electrons are not moving in sync with this electric field, but are doing their

own thing on a phase lag. These electrons are not following your amplifier's

gentle reminders to head towards the speaker, and therefore part of the electric

field is "wasted" on them because they aren't doing any work for you at the speaker end.

 

Instead these electrons are content to bounce back and forth to the beat of

their own drum, and therefore the electric field they produce does the same.

Since it lags by exactly 90 degrees it's not doing any work. In a perfect system

this energy would just be stored, but in a real system there are dissipations

due to resisitive losses in the source and load.

 

Those resistive losses (see my blurb about the drift velocity) result in joule

heating of the components involved (cables, speaker voice coil windings, etc.) but this

energy cannot be tapped to perform any work because of its phase relationship to

the amplifiers electric field.

....

 

That is a flavor of what i had in mind. They don't have to be that detailed, but potentially could be.