Audio FAQ

MrEs
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Audio FAQ

Postby MrEs » Tue Oct 16, 2007 1:02 pm

Most of this information is stolen from other several locations and glued together... but would be important/relevant to somebody. Feel free to CONTRIBUTE information...



How do I determine a speaker's polarity?
If you have a speaker and the terminals are no longer marked, you can do a simple test to determine which terminal is positive (+) and which is negative (-). This test is useful for midrange/midbass/subwoofers, but not for tweeters.
Use a 1.5V battery (AA, C, D) and connect the (+) terminal on the battery to one terminal of the speaker, and connect the (-) terminal to the other terminal of the speaker.
If the cone moves OUT, then the battery is connected "properly," i. e., the (+) terminal of the battery is connected to the (+) terminal of the speaker, and the (-) terminal of the battery is connected to the (-) terminal of the speaker.
If, however, the cone moves IN, the battery has been connected "backwards," i. e., the (+) terminal of the battery is connected to the (-) terminal of the speaker, and the (-) terminal of the battery is connected to the (+) terminal of the speaker.




What's worse for a speaker, too much or too little power?
Problems occur (in everyday operation) when distortion is fed to a speaker. This occurs MUCH more often when you are dealing with an underpowered system - typically the owner will turn up the volume too much or set the amplifier gains too high to try and get more volume from the system. These introduce distortion to the signal - this will destroy *any* speaker.
When a speaker is overpowered, however, it is not nearly as common to have these kind of problems, so speakers aren't blown as much. Of course, it is certainly possible to destroy a speaker (thermally) by overpowering it, but you'll have a pretty hard time doing this on your own, especially with standard car audio amplifiers.




Why is distortion harmful to my speakers?
Distortion is hard on speakers for two reasons.

Reason 1: Distortion causes the power spectrum to shift upwards in frequency. A bass note, when distorted, will have lots of high frequency energy. This will cause mid-ranges and tweeters to fry, if the amplifier is operating full range. It doesn't harm woofers, necessarily.

Reason 2: Distortion causes the average power to be much higher. Typically, a music signal that never clips has an average power level of 1/4 the peak power level for even the most compressed speed metal or pop. More dynamic music will be 1/8 the peak level or less on average.

When you clip the amp hard, the average output moves up to the full-rated output of the amp or more. The peak to average ratio can be less than 2 to 1, with the peaks being at double the rated power of the amp, and the average being at the rated power of the amp or higher.

Thermally, the speaker can handle the average power being 1/4 the rated power of the amp (little to no clipping), but it will have a much harder time with the average power being the amp's rated power or more (massive clipping). As you might expect, this is pretty hard on the amp, too.

For transients, most speakers can handle a ton of power. But for long term signals, the power handling is much less.




What do I need to hook up my amp? Do I need to buy anything?
It depends on what you're hooking your amp to. If you're hooking it up to a factory car stereo, you either need a line-out converter to step down the output of the deck to a low-level signal or an amplifier with speaker-level inputs. If you are adding the amp to a system featuring a deck with RCA preouts, all you need for signal transfer is a run of shielded cable with RCA connections. Either way, you will need a heavy-duty power wire (with a fuse or circuit breaker) and a ground wire of the same size. The size of the wire you need depends on the size of the amp you're installing. A great option to simplify matters is to buy a prepackaged amp install kit designed for the size of the amp you want to install. You'll also need some heavy-duty speaker wire to pass the amplified signal to the speakers or subwoofer that you're powering with the amp. Finally, you'll also need a small gauge remote-turn-on wire that leads from your deck's turn-on lead to the amp's remote terminal. This wire will turn your amp on when your radio receives power.




How much power does it take to get good bass from my sub?
When matching up an amplifier to a subwoofer, it's important to look at continuous power. Many subwoofers and amplifiers are rated at both continuous and peak power; however, continuous power is the best indicator of what your gear can really handle. Try to match the continuous (or RMS) power rating of the sub to an amplifier with a similar continuous (or RMS) power rating. Err on the side of too much power. It is actually better (for the amp and the sub) to have too much amp and run it below its maximum output than to try and push a sub with an amp that is a little underpowered. Using an underpowered amp causes it to clip, which causes distortion that can damage your sub and amp.




What is "rear fill", and how do I effectively use it?
"Rear fill" refers to the presence of depth and ambiance in music. A properly designed system using two channels will reproduce original rear fill on the source without rear high frequency drivers. Since recordings are made in two channels, that is all you will need to reproduce it. What is captured at the recording session (coincident pair mics, Blumlein mic patterns, etc.) by a two channel mic array will capture the so called rear fill or ambiance. Many of the winning IASCA vehicles have no rear high frequency drivers. Also a lot of this has to do with system tuning. If rear high frequency drivers are added, however, the power level of the rear fill speakers should be lower than that of the front speakers, or else you will lose your front-primary staging, which is not what you want (when was the last time you went to a concert and stood backwards?). The proper amount of amplification for rear fill speakers is the point where you can just barely detect their presence while sitting in the front seat. Separates are not a requirement for rear fill; in fact, you may be better of with a pair of coaxial speakers, as separates may throw off your staging.




How do I set the gains on my amp?
The best way to do this is with a test tone and an oscilloscope (*Note Oscilloscope::.) Since most people have neither item, the following will work approximately as well.
* Disconnect all signal inputs to the amp
* Turn all sensitivity adjustments as low as possible
* Turn head unit on to around 90% volume (not 100% or else you'll have head unit distortion in there - unless you've got a good head unit) with some music with which you're familiar, and with EQ controls set to normal listening positions
* Plug in one channel's input to the amp
* Slowly turn that channel's gain up until you just start to notice distortion on the output
* Turn it down just a wee little bit
* Disconnect current input
* Repeat steps 4-7 with each input on your amp
* Turn off head unit
* Plug in all amp inputs, and you're done

If by some chance you do have an oscilloscope (and preferably a test disc), you do essentially the same thing as above, except that you stop turning the gains up when you see clipping on the outputs of the amplifier.

Note that if you are paralleling multiple speakers on a single amp output, you need to set the gains with all of the speakers in place, since they will be affecting the power and distortion characteristics of the channel as a whole.
Last edited by MrEs on Fri Dec 28, 2007 9:12 am, edited 4 times in total.
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Postby MrEs » Tue Oct 16, 2007 1:08 pm

What is meant by "frequency response?"
The frequency response of a device is the range of frequencies over which that device can perform in some fashion. The action is specific to the device in question. For example, the frequency response of the human ear is around 20Hz-20kHz, which is the range of frequencies which can be resolved by the eardrum. The frequency response of an amplifier may be 50Hz-40kHz, and that of a certain speaker may be 120Hz-17kHz. In the car audio world, frequency responses should usually be given with a power ratio range as well, such as (in the case of the speaker) 120Hz-17kHz +/-3dB. What this means is that given an input signal anywhere from 120Hz to 17kHz, the output signal is guaranteed to be within an "envelope" that is 6dB tall. Typically the extreme ends of the frequency range are the hardest to reproduce, so in this example, the 120Hz and 17kHz points may be referred to as the "-3dB points" of the amplifier. When no dB range is given with a frequency response specification, it can sometimes be assumed to be +/-3dB.




What is a "soundstage?" What is an "image?"
The soundstage is the position (front/back and high/low) that the music appears to be coming from, as well as the depth of the stage. A car with speakers only in the front will likely have a forward soundstage, but may not have enough rear fill to make the music seem live. A car with both front and rear speakers may have anything from a forward to a rear soundstage, with an accompanying fill from the softer drivers depending on the relative power levels and the frequencies reproduced. The high/low position of the soundstage is generally only obvious in a car with a forward soundstage. The music may seem to be originating in the footwells, the dash, or out on the hood, depending on how the drivers interact with the environment.

The stereo image is the width and definition of the soundstage. Instruments should appear to be coming from their correct positions, relative to the recording. The position of the instruments should be solid and easily identifiable, not changing with varying frequencies. A car can image perfectly with only a center-mounted mono speaker, but the stereo placement of the music will be absent.




My system "pops" when I turn it off. What is happening and how can I get rid of it?
This kind of problem is often caused by transients in the signal processor as it powers down finding their way into the signal path, which the amplifier then transmits to the speakers.

Usually this can be solved by adding a little turn-off delay to the processor. This allows the processor to stay powered on for a short time after the amplifiers have powered down, thus preventing the pop.

Many components sold today (such as crossovers, equalizers, etc) have delays built-in. Read your manual to see if it is possible to set this delay on your piece of equipment or be sure to look for this feature during your next car audio purchase.

If your processor does not have this feature, you can build your own delay circuit with a diode and a capacitor. Add a 1N4004 diode in series with the processor's turn-on lead, striped side towards the EQ. Then add a capacitor in parallel, the (+) side of the cap connects to the striped (processor) side of the diode, the (-) side of the cap goes to ground (not the radio or EQ chassis - connect to the car chassis).

Experimenting with the cap value will give you the right amount of delay before the EQ shuts off. You don't want it too long, just long enough to make sure the amp is off before the EQ powers down. 220 - 1000 uF is about right, and make sure the cap is a polarized electrolytic, 16V or higher. Also keep in mind that the diode will introduce a 0.7V drop on the remote wire, which can cause the processor to power down before the rest of the system.




What is the best power wire to use?
There is much debate over the benefit of certain wiring schemes (oxygen-free, multistranded, braided, twisted, air core, you name it). However, most people do agree that the most important factor in selecting power wire is to use the proper size. Wire is generally rated in size by American Wire Gauge, abbreviated AWG, or commonly just gauge. To determine the correct wire size for your application, you should first determine the maximum current flow through the cable (looking at the amplifier's fuse is a relatively simple and conservative way to do this). Then determine the length of the cable that your will use, and consult the following chart, taken from the IASCA handbook See section 6.1 What is IASCA, and how do I get involved? [JSC, HK, IDB],

Length of run (in feet)
Current 0-4 4-7 7-10 10-13 13-16 16-19 19-22 22-28

0-20A 14 12 12 10 10 8 8 8
20-35A 12 10 8 8 6 6 6 4
35-50A 10 8 8 6 6 4 4 4
50-65A 8 8 6 4 4 4 4 2
65-85A 6 6 4 4 2 2 2 0
85-105A 6 6 4 2 2 2 2 0
105-125A 4 4 4 2 2 0 0 0
125-150A 2 2 2 2 0 0 0 00

If aluminum wire is used instead of copper wire, the next larger size (smaller number) should be used. You should also consider the installation demands: will you need to run the wire around corners or through doors or into the engine compartment? These sorts of problems in the car audio application require some special care in cable selection. You will want to have cable that is flexible; it should have thick insulation as well, and not melt at low temperatures. You don't want to install wire that is rigid and prone to cracks and cuts, or else the results could literally be explosive.




What do all of those specifications on amplifiers mean?
Frequency response refers to the range of frequencies which the amplifier can reproduce within a certain power range, usually +/-3dB.

Continuous power output is the power output of the amplifier into one channel into a certain load (usually four ohms) below a certain distortion level (usually at most 1%THD) at a certain frequency (usually 1kHz). A complete power specification should include all of this information, e.g. "20W/ch into 4 ohms at < 0.03%THD at 1kHz" although this can also be stated as (and be assumed equivalent to) "20W/ch at < 0.03%THD". The amplifier should also be able to sustain this power level for long periods of time without difficulties such as overheating.

Peak power output is the power output of the amplifier into one channel into a certain load (usually four ohms) below a certain distortion level (usually much higher than the continuous rating level) at a certain frequency (usually 1kHz). A complete power specification should include all of this information, e.g. "35W/ch into 4 ohms at < 10.0%THD at 1kHz" although this can also be stated as (and be assumed equivalent to) "35Wch at < 10.0%THD". Consumer warning: some manufacturers will state the "peak power output" rating by including the amount of power which can be drawn from "headroom", which means power supply capacitors. They usually will not tell you this in the specification, however; indeed, they tend to prominently display the figure in big, bold letters on the front of the box, such as "MAXIMUM 200W PER CHANNEL!!!" when the continuous rating is 15W/ch and the unit has a 5A fuse.

Damping factor represents the ratio of the load being driven (that is, the speaker - usually four ohms) to the output impedance of the amplifier (that is, the output impedance of the transistors which drive the speakers). The lower the output impedance, the higher the damping factor. Higher damping factors indicate a greater ability to help control the motion of the cone of the speaker which is being driven. When this motion is tightly controlled, a greater transient response is evident in the system, which most people refer to as a "tight" or "crisp" sound. Damping factors above 100 are generally regarded as good.

Signal to Noise or S/N is the ratio, usually expressed in decibels, of the amount of true amplified output of the amplifier to the amount of extraneous noise injected into the signal. S/N ratios above 90 to 95dB are generally regarded as good.




What is a crossover? Why would I need one?
A crossover is a device which filters signals based on frequency. A high pass crossover is a filter which allows frequencies above a certain point to pass unfiltered; those below that same point still get through, but are attenuated according to the crossover slope. A low pass crossover is just the opposite: the lows pass through, but the highs are attenuated. A band pass crossover is a filter that allows a certain range of frequencies to pass through while attenuating those above and below that range.

There are passive crossovers, which are collections of purely passive (non-powered) devices - mainly capacitors and inductors and sometimes resistors. There are also active crossovers which are powered electrical devices. Passive crossovers are typically placed between the amplifier and the speakers, while active crossovers are typically placed between the head unit and the amplifier. There are a few passive crossovers on the market which are intended for pre-amp use (between the head unit and the amplifier), but the cutoff frequencies (also known as the "crossover point", defined below) of these devices are not typically well-defined since they depend on the input impedance of the amplifier, which varies from amplifier to amplifier.

There are many reasons for using crossovers. One is to filter out deep bass from relatively small drivers. Another is to split the signal in a multi-driver speaker so that the woofer gets the bass, the midrange gets the mids, and the tweeter gets the highs.

Crossovers are categorized by their order and their crossover point. The order of the crossover indicates how steep the attenuation slope is. A first order crossover "rolls off" the signal at -6dB/octave (that is, quarter power per doubling or halving in frequency). A second order crossover has a slope of -12dB/octave; third order is -18dB/octave; etc. The crossover point is generally the frequency at which the -3dB point of the attenuation slope occurs. Thus, a first order high pass crossover at 200Hz is -3dB down at 200Hz, -9dB down at 100Hz, -15dB down at 50Hz, etc.

It should be noted that the slope (rolloff) of a crossover, as defined above, is only an approximation. This issue will be clarified in future revisions of this document.

The expected impedance of a passive crossover is important as well. A crossover which is designed as -6dB/octave at 200Hz high pass with a 4 ohm driver will not have the same crossover frequency with a driver which is not 4 ohms. With crossovers of order higher than one, using the wrong impedance driver will wreak havoc with the frequency response. Don't do it.




Should I get an active or a passive crossover?
Active crossovers are more efficient than passive crossovers. A typical insertion loss (power loss due to use) of a passive crossover is around 0.5dB. Active crossovers have much lower insertion losses, if they have any loss at all, since the losses can effectively be negated by adjusting the amplifier gain. Also, with some active crossovers, you can continuously vary not only the crossover point, but also the slope. Thus, if you wanted to, with some active crossovers you could create a high pass filter at 112.3Hz at -18dB/octave, or other such things.

However, active crossovers have their disadvantages as well. An active crossover may very well cost more than an equivalent number of passive crossovers. Also, since the active crossover has separate outputs for each frequency band that you desire, you will need to have separate amplifiers for each frequency range. Furthermore, since an active crossover is by definition a powered device, the use of one will raise a system's noise floor, while passive crossovers do not insert any additional noise into a system.

Many people find it advantageous to use both active and passive crossovers. Often, a separate amp is dedicated to the subwoofers, to give them as much power as possible. The other amplifier is used to power the mids and tweeters. In this scheme, an active crossover is used to send only the sub-bass frequencies to the sub amp, and the other frequencies to the other amp. The passive crossovers are used to send the correct frequencies to the individual speakers (e.g., mids and tweeters).

Thus, if you have extra money to spend on an active crossover and separate amplifiers, and are willing to deal with the slightly more complex installation and possible noise problems, an active crossover is probably the way to go. However, if you are on a budget and can find a passive crossover with the characteristics you desire, go with a passive.



How do I wire speakers "in series" and "in parallel?"
Wiring speakers in series involves connecting at least two speakers so that the first speaker's positive lead is connected to the amplifier's positive terminal, and the negative lead is connected to the positive lead of the second speaker. If there is a third speaker, its positive lead will be connected to the second speaker's negative lead ... and so on. The last speaker in the chain will have its negative lead connected to the amplifier's negative terminal.

Speakers that are wired in parallel are all connected to the positive and negative terminals of the amplifier. So, when two speakers are wired in parallel, you'll connect each speaker's positive lead to the amplifier's positive terminal, and you'll connect each speaker's negative lead to the amplifier's negative terminal.

Be careful when wiring multiple speakers in parallel or series so that you do not exceed your amplifier's rating. To calculate the effective impedance of a number of speakers, use the following formulas:

Series Connections:
Z(t) = Z(1) + Z(2) + Z(3) + ... + Z(n)

That is, add up all of the impedances for each speaker to get the total impedance. For example, with 3 4-ohm speaker in series, the total impedance is 4 + 4 + 4 = 12 ohms.

Parallel Connections:
1/Z(t) = 1/Z(1) + 1/Z(2) + 1/Z(3) + ... + 1/Z(n)

That is, add up the inverse of the impedance of each speaker and invert the sum to get the total impedance. For example, with 3 4-ohm speakers in parallel, the total impedance is 1 / ( 1/4 + 1/4 + 1/4) = 1 / (3/4) = 1.33 ohms.




How do I turn a stereo signal into a mono signal?
Creating a mono signal is often necessary when you are powering a subwoofer by bridging the amplifier. Many people do not realize that bridging an amplifier does not always provide a mono signal - many amplifiers will simply use only one input channel, which means that the subwoofer won't be receiving the full signal.

Some amplifiers have a switch that will allow you to combine the left and right channels into a mono signal. Some signal processors and head units provide a subwoofer-out channel that can be switched between stereo and mono.

If you don't have this feature on any of your equipment, you will need to provide a mono signal to the amplifier. The common thought is to use a Y-adapter to "combine" the left and right channels. However, by using a Y-adapter, you are actually summing the line voltages and directly shorting the left and right channels at the head unit, which could cause problems.

The correct way to create a mono signal is to cut off the ends of the RCA cables, combine the signal grounds (the outer shield), and then use a 1 kOhm (1/4 watt, 5% tolerance) resistor to each of the center conductors. Solder and insulate the resistors so that you don't short them prematurely, and then connect the two resistors together. Connect the summed signal ground to the shield of the new RCA plug, and the summed center conductor to the center pin of the RCA plug.
Last edited by MrEs on Tue Oct 16, 2007 1:18 pm, edited 2 times in total.
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Postby MrEs » Tue Oct 16, 2007 1:11 pm

Which enclosure type is right for me?

This answer is not designed to tell you exactly what kind of enclosure to build, but rather to give an idea of the advantages and disadvantages to the simple configurations (Infinite baffle [1st order], Sealed [2nd order], Ported [4th order] and basic bandpass). Building and designing more complicated systems (order > 4) is not for the light at heart.



Infinite Baffle ("free-air")

* Advantages...
o No box necessary!
o This means it's usually cheaper to design and implement in your system

* Disadvantages...
o Requires that a good seal be obtained between front and rear of driver. In a car, this can be quite difficult and may require the installer to remove trim panels to plug any holes that would let energy "bleed through".

o The responsibility for damping cone motion rests solely on the driver's suspension. As fatigue sets in, this becomes a critical issue in infinite baffle set-ups.

o Less efficient in the sub-bass region than above mentioned enclosures.

o Potentially more expensive drivers than good boxable woofer - The suspension must be extremely hearty and long-lasting to withstand high power applications.



Sealed Box

* Advantages...
o Small enclosure volumes
o Shallow (12 dB/Octave) roll off on low end
o Excellent power handling at extremely low frequencies
o Excellent transient response/ group delay characteristics
o Easy to build and design
o Forgiving of design and construction errors
* Disadvantages...
o Not particularly efficient
o Marginal power handling in upper bass frequencies
o Increased distortion in upper bass over ported design
o When using high power and small box, magnet structure is not in an ideal cooling environment



Ported Box

* Advantages...
o 3-4 dB more efficient overall than sealed design
o Handles upper bass frequencies better with less distortion
o Magnet is in good cooling environment
o When properly designed, a ported box will slaughter a sealed in terms of low frequency extension

* Disadvantages...
o Size (not so critical outside the mobile environment)
o Woofer unloads below Fb
o More difficult to design/ can result in boomy, nasty sounding bass if misaligned




Bandpass Box

* Advantages...
o When properly designed and implemented, can provide superior LF extension and efficiency.
o Cone motion is controlled more and therefore mechanical power handling is increased.
o Cones are physically protected from contents of trunk flying around.
o Output is easily channeled directly into the interior of sedans.

* Disadvantages...
o Difficult to build (not recommended for newbies), and very sensitive to misalignment due to calculation or construction errors.
o Their characteristic filtering often masks any distortion that occurs as a result of amplifier clipping or overexcursion and thus will give the user no warning that the driver is over-stressed and about to fail.
o Need substantial mid-bass reinforcement to make up for narrow bandwidths in efficient alignments.
o Transient response is largely dependent upon the alignment chosen....wider bandwidths will result in sloppier performance, narrower bandwidths (and thus higher effiencies) result in better transient performance.
o They can oft times be quite large.
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Postby deafwish » Tue Oct 16, 2007 2:53 pm

I can't add anything! :o :giggle:
Top find, Denis!!! :thumbs:
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Postby MrEs » Fri Dec 28, 2007 9:10 am

re-stickied this!! :chair: :chair: :chair:
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Re: Audio FAQ

Postby Random Dude » Sun Dec 21, 2008 8:45 pm

Found this one that's pretty good.

Parametric Equaliser Amp Controls
ie Q factor, Centre Frequency control, Boost/Gain

Fantastic little flash display showing what it affects:
http://mobile.jlaudio.com/support_pages.php?page_id=144
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Re: Audio FAQ

Postby wokka wokka » Mon Dec 22, 2008 7:36 am

Good for stuff from the basics (both electronics and car audio), up to a bit more advanced stuff (not sure how advanced, I'm not much of a car audio person).

http://www.bcae1.com/
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...but.......but......what if Google is wrong?

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Re: Audio FAQ

Postby Random Dude » Mon Jan 19, 2009 1:58 am

Cable Gauge requirements vs current draw (amps)

http://www.crutchfield.com/learn/learni ... chart.html
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Re: Audio FAQ

Postby TBOACE » Mon Aug 24, 2009 3:53 pm

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Re: Audio FAQ

Postby L1TEW8 » Wed Feb 23, 2011 11:12 am

hi guys,

what is ment buy Ohms? eg in the example below what do the numbers with the ohms mean?

500 Watts x 1 Channel @ 4 Ohms
800 Watts x 1 Channel @ 2 Ohms
1000 Watts x 1 Channel @ 1 Ohm
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Re: Audio FAQ

Postby Nitropro » Wed Feb 23, 2011 2:05 pm

http://www.prestonelectronics.com/audio/Impedance.htm

very simple. google search 'ohms speakers' 2.6m hits.
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