Car Audio Tuning - Fundamentals Of Car Audio

Last issue, we talked about that ambiguous but appealing specification known as power, as applied to a loudspeaker. We explained that with car audio loudspeakers, the maximum power input specified by the manufacturer is really of very little use, because a loudspeaker's actual limits depend on how we adjust the crossover, build the box and how many decibels (dB) of gain overlap in the amplifier we are willing to use. That was on the loudspeaker side.

On the amplifier side, once again, things are not as clear or direct as most of us may think. First and foremost, the amplifier output power is not a given. It's not something readily available. What do I mean? I mean that power is not like voltage. In a typical 12.6-volt car battery, you "have" that voltage sitting there, waiting if you wish. You can take a voltmeter, measure it, and it will be there. You can safely say that your battery has 12.6 DC volts. What about current? Current is not always there because it is supplied under demand. The current specification of a typical car battery is known as its cold cranking amperes (CCA), defined as the amount of amperes that a battery can deliver for 30 seconds, at a temperature of 0F, sustaining a voltage between terminals of 7.2 volts. This also means that, as current output increases, voltage between terminals decreases. This is due to the battery's internal resistance, its limitations.

What I would like you to grasp is that current is only delivered by demand. If you connect a small lightbulb to a battery, you may get 1 amp of current flow that then, and only then, you could measure with an ammeter. If you connect 15 of these lamps, all of them in parallel, you could get 15 amperes of current from that battery. Now, since power is voltage times current, power is also delivered on demand. Take an amplifier with no woofer connected to its output terminals. How much power is it producing? Nothing! Nevertheless, you would be able to measure perhaps 40 Volts RMS from those terminals. So, the amount of power the amplifier outputs depends on the load, which depends on how many woofers we plan to connect to it and how.

All this goes to say that it is extremely important to make smart woofer arrangements for the bass-dedicated amplifier in order to actually make use of its power-output capabilities, the power we paid for. Please note that the same does not apply to higher frequencies. One of the many reasons for this is that we don't need very high pressure levels at mid and high frequencies. Our ears are extremely keen to those ranges, and therefore it is easier to permanently damage them at those frequencies. But with low bass, more is generally better, either because we want to be the loudest on our block or simply because we want our woofers to play cleanly at realistic levels with lots of headroom.

So let's make some real life applications. Suppose we work at a car audio shop and a client walks in wanting us to sell him and install a woofer for this amplifier he just found in his garage. It is a 2-channel model, at 75 watts x 2 @ 4 ohms, stable to 2 ohms. When talking about loading an amplifier, 2 ohms stereo is the same as 4 ohms bridge, as in bridge configuration, in which both channels work for the same load, each channel working with half of that load.

So, how will we wire this woofer to the amplifier and how many decibels can we expect to get? The following rules will apply:

1. The starting point is the sensitivity of the woofer, the amount of decibels it can produce when receiving one watt of input power (typically used as 2.83 volts), measured at a distance of one meter.

2. Each time we double input power, we gain 3dB. Three decibels is also accepted as the minimum change in level necessary to detect an increase or decrease in loudness.

3. Each time we double the number of woofers, we can ideally get 6dB of extra output. This assumes that each woofer receives exactly the same signal, the same input power (double the power of that of a single driver) and that the woofers are working very close to each other, separated by less than one fourth the wavelength of the given frequency.

However, we have some ambiguity with Rule 1: the sensitivity of the woofer as published by the manufacturer. The problem is that we are saying that the input power must be 1 watt (typically used instead as an input voltage of 2.83 volts, as I explained last month) and the measuring distance, 1m, but that is all that is stated. It is not explained which audio signal should be used, the kind of enclosure the driver will be working in (we could get very different values switching, say, from a sealed enclosure to a bandpass design) or the characteristics of the measuring chamber (it could be big and live or small and dead, or it could even be measured inside a car). All these additional characteristics of the test would be valid but would produce very different results, and as long as we don't have an international approved standard, each manufacturer is free to use the one it thinks is best and even use different ones for each line carried.

Fortunately, there is a method for calculating this sensitivity, known as the Reference Efficiency, such that all contestants would be competing on a level field. We would need to apply the following formula:

[A calculator is online at www.mhsoft.nl/Reference.asp.]This formula uses the published or measured Thiele-Small parameters of the driver as Fs, or the resonant frequency of the woofer in free air; Vas, or the compliance of the driver expressed in cubic feet; and the electrical Q of the driver, or Qes.

Thiele-Small parameters are normally included in the driver's specification sheet or can be obtained through the manufacturer's website. The idea is that the results obtained from this formula should give numbers very useful for comparison purposes.