Car Audio

The adage that someone could write a book about a subject certainly holds true when it comes to a discussion of loudspeakers and their parameters. In fact, there are dozens of great books already available about the subject. This article provides an overview of some of the most commonly discussed speaker parameters.

What are Speaker Parameters?

Speaker parameters, often called Thiele/Small parameters, are a set of electromechanical measurements that can be used to define the low-frequency performance of a transducer. Using these parameters and a series of calculations, your installer can predict the performance of that speaker in an enclosure.

What Can We Determine from these Parameters?

Perhaps the most important set of calculations we can create is the output of the system. When we discuss the “system,” we are referring to the speaker itself and the enclosure in which we intend to install the speaker. Every speaker enclosure acts as a high-pass filter and reduces the low-frequency output of the driver. We gain physical power handling in return for this diminished output. Using a set of calculations, we can predict how much low-frequency information the system will produce.

Another important calculation is power handling. As mentioned, we need to control the movement of the speaker cone to prevent distortion and damage. We can predict how much the cone will move for a given amount of power in our test enclosure.

Resonant Frequency of the Speaker – Fs

In terms of analyzing the moving parts of the speaker, we need to know the frequency at which the compliance (springiness) of the spider and the surround combine with the mass of the cone and dust cap to store the most energy. At this frequency, the system alternately stores and subsequently releases the most energy for a given voltage input. If you were to swing a weight on a string suspended from the ceiling, the natural frequency at which it oscillates back and force would be equal to the resonant frequency of a loudspeaker.

Equivalent Compliance Volume – Vas

To understand how stiff the spider and the surround are, we compare them to an amount of air that would exert the same resistance to motion. Because air is easily compressed, a high Vas specification would represent a very softly suspended cone. Conversely, a speaker with a low Vas would have a very stiff suspension.

Electrical Q of the Driver at Fs – Qes

Understanding the Q (Quality Factor) can be somewhat difficult because it is a dimension-less value. In essence, the Q factor describes the damping characteristic of a resonant system. A higher Q represents less energy loss relative to the total energy stored in a system. A pendulum suspended from a low-friction bearing will have a high Q. That same pendulum, submerged in water, will have a much lower Q. An important consideration is that high-Q systems have less damping and, therefore, vibrate longer. The Electrical Q specification describes how much damping the voice coil and magnet assembly invoke on the moving cone.

As the voice coil moves past the magnet, it produces an electrical current. This current reaches its peak value at the resonant frequency of the driver and counteracts the current being provided by the amplifier. The net result is a significant rise in impedance at the resonant frequency.

Mechanical Q of the Driver at Fs – Qms

Just as the electrical characteristics of a speaker cause an opposition to cone motion, we have a similar effect from the mechanical properties of the speaker. Qms describes the mechanical losses resulting from the spider and the surround. A high Qms value describes lower mechanical losses, while a low Qms value describes higher losses.

Total System Q at Fs – Qts

This unit-less measurement is a mathematical combination of the mechanical and electrical characteristics of the speaker. In simple terms, we calculate Qts by dividing the total stored energy of the speaker by the dissipated energy in the speaker at resonance.

Compliance of the Driver Suspension – Cms

The Cms specification describes the stiffness of the driver suspension in meters per newton. A stiffer suspension will move less distance for a given amount of force applied to it.

Effective Cone Area of the Driver – Sd

This parameter describes the effective “size” of our speaker. We all realize that the cone will move air for us, but we also have to take into account the addition of the surround. It is commonly accepted that we can use a value of half the surround as contributing to the output of the driver.

Mass of the Cone and Moving Parts – Mms

The Mms specification describes the mass of the speaker cone and part of the spider and surround. Unlike the Mmd specification, Mms includes the acoustic load caused by the air in contact with the cone. In most cases, the values are similar, but as the surface area of the cone increases, so too does the value of Mms, relative to Mmd.

Maximum Excursion Level – Xmax

This parameter is frequently misinterpreted as being the defining factor in the distance a speaker cone can move. Early calculations used a formula that subtracted the height of the voice coil winding from the height of the magnetic gap, then divided by 2. This calculation describes how far the speaker can move before the winding comes out of the gap.

Subsequent investigation shows that non-linear behavior elsewhere in the driver design could have a larger influence on the motion limits of the cone. This suggests that Xmax should be the one-way excursion distance that represents a distortion level of 10%. This performance-oriented specification is far more indicative of the useful operating range of a driver, but is much harder to ascertain.

Additional Parameters

In this article, we only describe the basic parameters that are commonly used in predicting the low-frequency performance of a loudspeaker. Other parameters, such as inductance, become more relevant at higher frequencies. Addition parameters such as Nominal Impedance (Znom), efficiency, sensitivity and the Efficiency Bandwidth Product (EBF) are derived through equations that use the specifications above.

Proper Design Requires Simulation

A woofer in an over-sized enclosure may bottom out and be damaged easily. A midrange driver crammed into a small speaker pod may have a significant frequency response spike and an associated distortion peak. The result is quite unfavorable.

Before you assume a subwoofer or speaker is suitable for the enclosure or mounting location you have chosen, it is worth asking your mobile electronics retailer to perform a simulation to ensure everything will function the way you want. They can work with you to ensure everything will perform optimally, and your system will sound great!

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

If you spend any time reading car audio discussions on Facebook or in forums, then you will have undoubtedly come across comments involving the supposed drawbacks of using RCA Y-cables. There seems to be a lot of misconception or misunderstanding about how preamp signals works, and this misinformation leads to comments that aren’t always accurate. Let’s take (more than) a few minutes to clear things up.

Understanding Preamp Level Audio Signals

The audio signal that connects your source unit to your amplifier is both very weak and quite small. The voltage of the preamp signal is rarely above 10% of the maximum voltage capability of your source unit for several reasons. Firstly, the signal level is directly proportional to the output of the system. When the volume is low, the signal is low in amplitude.

The second factor that contributes to the microscopic amplitude of the preamp signal is known as the Crest Factor. By way of a formal definition, the Crest Factor is the ratio between the peak signal amplitude and the RMS value of a waveform. For a pure sine wave, this value would be 1.414. For music, the Crest Factor value is much larger.

We analyzed a few different songs to come up with some relatable numbers. The new song Run by the Foo Fighters has a maximum amplitude of +0.15 dB and an RMS amplitude of -12.7 dB over the entire track. To keep the math simple, let’s call it 13 dB, which is a ratio just shy of 20:1. We also analyzed Heathens by Twenty One Pilots and found that it has a Crest Factor of 10.5 dB, or just about 11.25:1.

If we think about the highest voltage possible on our preamp signal as being 4 volts, then the average voltage for the above track would be 200 millivolts and 355 millivolts respectively. The peak of 4 V only happens when the volume is at maximum. Don’t forget that.

Scotty, We Have No Power!

Another characteristic of our preamp signal is that it contains almost no current flow. As with any electrical circuit, the amount of current flowing through the circuit is determined by the voltage in the circuit and how much resistance there is. The output impedance of most head units is between 300 and 500 ohms. The input impedance on most amplifiers is about 10,000 ohms.

Using our maximum voltage of 4 volts, and a resistance of 10,500 ohms, the maximum current in our circuit will be 0.381 milliamps. If we consider that the average signal amplitude is about 275 millivolts, then we have an average current flow of 0.0275 milliamps. That is nothing.

What does an RCA Y-cable Do?

An RCA Y-cable allows you to connect a single RCA output to two RCA inputs. Typical applications for Y-cables are a single subwoofer output RCA on a source unit or processor and the need to feed a pair of inputs on a subwoofer amp. Another common application is a source unit with only a single left and right RCA output; you want to use a four-channel amp that doesn’t include a two-input/four-input switch.

Please Don’t Believe the Hype

The biggest myth about the use of Y-cables is that they dramatically reduce the signal going to each input. To prove why this is not true, we need to understand how a voltage divider circuit works. Yes, it is time for a little physics and math.

In an ideal situation, when we have a signal source and a single load, all the voltage developed by the source appears across the load.

If we have multiple loads, the voltage produced by the source is divided among the loads when they are wired in series. In the image below, we have two loads in series with our single signal source.

If the resistance value of the two loads is the same, then the voltage produced by the source is divided equally across the loads. Half the voltage can be measured across each load. Using our 4 V preamp example, we would see 2 V across each load. However, what happens when the load resistance is not the same? We have to do some math to determine how much voltage is across each.

Let’s label the loads. The load on the left will be called Rs. This is the resistance of our source. For this example, we will use a value of 500 ohms. The load on the right will be our amplifier input resistance of 10,000 ohms, and we will call it Ra1.

We have 4 volts being produced by the source and a total circuit resistance of 10,500 ohms. We can calculate that the current flowing in the circuit is 0.0381 milliamps using Ohm’s law. Knowing the current in the circuit allows us to determine how much voltage is dropped across each resistance. For our source load, we have a resistance of 500 ohms with a current of 0.381 milliamps to produce 190.476 millivolts. The rest of the 4 V source signal or 3.809525 volts appears across the load.

Let’s wire another amplifier in parallel with our first amplifier. This is the same effect as using a Y-cable. Our second amplifier will be called Ra2.

Now it is math time again. This time, our circuit has a total resistance of 5500 ohms, and as such, has a current of 0.7272 milliamps flowing in it. The voltage dropped across the source has increased to 0.363636 volts, and each amp is seeing 3.636 volts. That seems like a noticeable difference, doesn’t it?

The Decibel Scale Changes Everything

Between the two examples above, we have seen a decrease in voltage at the amplifiers by 4.772%. Does that mean our music is almost 5% quieter? No. When we talk about the ratio of voltage to volume, we need to take into account the decibel scale. Our decrease of 4.772% percent in voltage works out to -0.405 dB less output.

Before you get your knickers in a knot, you can fix that by turning the gain on your amplifier up by that amount.

A Worst-case Mathematical Example

This example was a worst-case scenario. What if you have a source unit with a lower output impedance? Some head units have an output impedance of 300 ohms. For that head unit, with the same 10,000 ohm input impedance on the amplifiers, the change in output by using a Y-cable would be -0.2493 dB. If you have a premium line driver in your system, the output impedance may be as low as 50 ohms. In this scenario, the loss is a paltry -0.0431 dB.

What did we learn from this? If you need to connect many amplifiers to a single source, then choose a source with a low output impedance.

RCA Y-cables as a Solution are Not Evil

If your system requires that you use a set of Y-cables to distribute the audio signal to multiple amplifiers, then go right ahead. Once your installer sets the sensitivity controls on your amps, you will never, ever know they are there.

If you have any questions about the design of your audio system or what to know about how your installer will be wiring it, talk to the salesperson and your local mobile electronics specialist retailer – they would be happy to explain things to you.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.