If you have purchased a set of premium car audio speakers from a respected mobile electronics retailer in the past few years, then you should be familiar with the concept of sound deadening. If you aren’t familiar with this, or want to know more, then read on! We think you will find sound deadening is an often-overlooked upgrade that has more benefits than most people are aware of.

What Is Sound Deadening?

Automobile manufacturers apply small sheets of dense asphalt or butyl-based material to the floor, firewall or door panels of their vehicles. This damping material adds mass to the panel, making it more difficult for sound and vibration to move the panel and transfer sound into the interior of the vehicle. Automakers walk a fine line between adding weight to a vehicle to reduce noise versus losing fuel economy and handling characteristics due to this added mass. For this reason, most don’t go overboard with sound deadening. They are missing out on a great opportunity.

In spite of what they say in their marketing materials, manufacturers don’t really put that much emphasis on their audio systems. Even when vehicles include multichannel systems with well-recognised namebrands like Bose, Lexicon or JBL, little effort is put into maximizing the performance of the speakers. Proper application of sound deadening can have a dramatic effect on the performance of an audio system.

Aftermarket Deadening Materials

One of the first companies to actively promote sound deadening was Dynamat. Dozens have since followed suit with different approaches to controlling noise inside the vehicle. All of them work on the same principle of absorbing sound energy in one fashion or another and preventing it from being transferred to the interior of the vehicle. Sound deadening has two main benefits when it comes to car audio – exterior noise blocking and audio system performance improvement by preventing backwave cancellation.

When you look at the inside metal skin of a car or truck door, you can see that there are openings to allow access to power window motors, door handles and other components in the door cavity. These openings are typically covered with a thin sheet of plastic. The purpose of the plastic is to keep water away from the interior door panel. That’s important, of course, but these openings work against your efforts to get good sound from your new speakers. There is just as much sound energy being produced from the rear of the speaker as there is from the front. If this rearward-facing sound is allowed to mix with the sound coming from the front, they cancel each other. The result is poor bass and midbass response. Sealing up these openings with a layer of sound deadening means the energy being produced by the rear of the speaker cannot mix with the frontal energy.

Just how dramatic can this cancellation affect be? We have seen instrumented measurements of a factory 6×9” speaker where the difference between having sound deadening or not produced an increase in output of up to 8 dB at several frequencies between 100 and 500 Hz. If you think about how much additional amplifier power it would take to produce the same increase in output, that’s more than six times are much. To be clearer, if you put 10 watts of power into the speaker and measured the response, you would need 63 watts of power into the same speaker to get the same output without the sound deadening. As you can see, that’s a significant difference, and the benefit is not just in efficiency, but in improved low frequency output. The speaker doesn’t have to work as hard, and that alone will improve the overall sound of your system.

It is well worth noting that an upgrade in speaker quality will not produce the same improvement in performance. With a properly sealed and damped door, an inexpensive speaker can easily outperform speakers costing five to 10 times as much money. Sound deadening is critical to the performance of an audio system.

Signal To Noise

The second benefit of sound deadening is in keeping the interior of the vehicle quiet. When you make the interior quieter, the benefit is two-fold. Driving is more comfortable, since you hear less road, wind and tire noise. This reduction in noise also makes it easier to hear your audio system. You don’t have to turn it up quite as loud to drown out the remaining noise. You can hear the quiet parts of your music more easily. Your Bluetooth hands-free system will also sound better. In the same way that controlling backwave cancellation reduces the need for a speaker to work hard, having a quieter interior does the same.

Kinds Of Deadening

There are many different kinds of sound deadening. The most popular are butyl sheets bonded to a thin aluminum layer. The combination works well to span large openings, but is thin and flexible enough to adhere to complex shapes. Other materials are made of vinyl and asphalt-based.

There are three key considerations when looking at different sound deadening products: How flexible is it? How thick is it? How well does it stay adhered once installed? On the engineering and development side, testing the damping characteristics at different temperatures can show quite varied results. Some materials don’t work as well in high or low temperatures. We have seen many people attempt to use materials not specifically designed for automotive applications. When the material melts and ends up as a gooey, black mess at the bottom of your door or leaks onto your carpet, the cost to repair the damage can be significant.

There are also several products on the market that add a layer of foam to the top of the aluminum layer. This foam is great when used between the inside door skin and the metal door because it eliminates buzzes and rattles.

See Your Specialist Car Audio Retailer To Learn More

The next time you are driving by a specialist car audio retailer, drop in and ask about sound deadening. Many people have chosen to apply sound deadening to otherwise stock vehicles. We guarantee the difference in performance from the audio system, combined with the increased comfort while driving, will be well worth the investment.

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 were able to grasp the concepts outlined in the first article about audio distortion, then this one will be a piece of cake. If not, head back and have another read. It can be a bit complicated the first time around.

Undistorted Audio Analysis

When looking at the specifications for an audio component like an amplifier or processor, you should see a specification called THD+N. THD+N stands for Total Harmonic Distortion plus Noise. Based on this description, it is reasonable to think that distortion changes of the shape of the waveform that is being passed through the device.

The two graphs below show a relatively pure 1kHz tone in the frequency and time domains:

A Look At Harmonic Distortion

If we record a pure 1 kHz sine wave as an audio track and look at it from the frequency domain, we should see a single spike at the fundamental frequency of 1 kHz. What happens when a process distorts this signal? Does it become 1.2 or 1.4 kHz? No. Conventional distortions won’t eliminate or move the fundamental frequency. But, it will add additional frequencies. We may have a little bit of 2 kHz or 3 kHz, a tiny but of 5 kHz and a smidge of 7 kHz. The more harmonics there are, the more “harmonic distortion” there is.

You can see that there are some small changes to the waveform after being played back and recorded through some relatively low-quality equipment. Both low- and high-frequency oscillations are added to the fundamental 1 kHz tone.

Signal Clipping

In our last article, we mentioned that the frequency content of a square wave included infinite odd-ordered harmonics. Why is it important to understand the frequency content of a square wave when we talk about audio? The answer lies in an understanding of signal clipping.

When we reach the AC voltage limit of our audio equipment, bad things happen. The waveform may attempt to increase, but we get a flat spot on the top and bottom of the waveform. If we think back to how a square wave is produced, it takes infinite harmonics of the fundamental frequency to combine to create the flat top and bottom of the square wave. This time-domain graph shows a signal with severe clipping.

When you clip an audio signal, you introduce square-wave-like behaviour to the audio signal. You are adding more and more high-frequency content to fill in the gaps above the fundamental frequency. Clipping can occur on a recording, inside a source unit, on the outputs of the source unit, on the inputs of a processor, inside a processor, on the outputs of a processor, on the inputs of an amplifier or on the outputs of an amplifier. The chances of getting settings wrong are real, which is one of the many reasons why we recommend having your audio system installed and tuned by a professional.

Frequency Content

Let’s start to analyze the frequency content of a clipped 1 kHz waveform. We will look at a gentle clip from the frequency and time domains, and a hard clip from the same perspective. For this example, we will provde the digital interface that we use for OEM audio system frequency response testing.

Here are the frequency and time domain graphs of our original 1 kHz audio signal once again. The single tone shows up as the expected single spike on the frequency graph, and the waveform is smooth in the time domain graph:

Low Distortion Analysis

The graphs below show distortion in the audio signal due to clipping in the input stage of our digital interface. In the time domain, you can see some small flat spots at the top of the waveform. In the frequency domain, you can see the additional content at 2, 3, 4, 5, 6 kHz and beyond. This level of clipping or distortion would easily exceed the standard that the CEA-2006A specification allows for power amplifier measurement. You can hear the change in the 1 kHz tone when additional harmonics are added because of the clipping. The sound changes from a pure tone to one that is sour. It’s a great experiment to perform.

High Distortion Analysis

The graphs below show the upper limit of how hard we can clip the input to our test device. You can see that 1 kHz sine wave then looks much more like a square wave. There is no smooth, rolling waveform, just a voltage that jumps from one extreme to the other at the same frequency as our fundamental signal – 1 kHz. From a frequency domain perspective, there are significant harmonics now present in the audio signal. It won’t sound very good and, depending on where this occurs in the audio signal, can lead to equipment damage. Keep an eye on that little spike at 2 kHz, 4 kHz and so on. We will explain those momentarily.

Equipment Damage From Audio Distortion

Now, here is where all this physics and electrical theory start to pay off. If we are listening to music, we know that the audio signal is composed of a nearly infinite number of different frequencies. Different instruments have different harmonic frequency content and, of course, each can play many different notes, sometimes many at a time. When we analyze it, we see just how much is going on.

What happens when we start to clip our music signal? We get harmonics of all the audio signals that are distorted. Imagine that you are clipping 1.0 kHz, 1.1, 1.2, 1.3, 1.4 and 1.5 kHz sine waves, all at the same time, in different amounts. Each one adds harmonic content to the signal. We very quickly add a lot more high-frequency energy to the signal than was in the original recording.

If we think about our speakers, we typically divided their duties into two or three frequency ranges – bass, midrange and highs. For the sake of this example, let’s assume we are using a coaxial speaker with our high-pass crossover set at 100 Hz. The tweeters – the most fragile of our audio system speakers – are reproducing a given amount of audio content above 4 kHz, based on the value of the passive crossover network. The amount of power the tweeters get is proportional to the music and the power we are sending to the midrange speaker.

If we start to distort the audio signal at any point, we start to add harmonics, which means more work for the tweeters. Suddenly, we have this harsh, shrill, distorted sound and a lot more energy being sent to the tweeters. If we exceed their thermal power handling limits, they will fail. In fact, blown tweeters seem as though they are a fact of life in the mobile electronics industry. But they shouldn’t be.

More Distortion

Below is frequency domain graph of three sine waves being played at the same time. The sine waves are at 750 Hz, 1000 Hz and 1250 Hz. This is the original playback file that we created for this test:

After we played the three sine wave track through our computer and recorded it again via our digital interface, here is what we saw. Let’s be clear: This signal was not clipping:

You can see that it’s quite a mess. What you are seeing is called intermodulation distortion. Two things are happening. We are getting harmonics of the original three frequencies. These are represented by the spikes at 1500, 2000 and 2500 Hz. We are also getting noise based on the difference between the frequencies. In this case, we see 250 Hz multiples – so 250 Hz, 500 Hz, 1500 Hz and so on. Ever wonder why some pieces of audio equipment sound better than others? Bingo!

As we increase the recording level, we start to clip the input circuitry to our digital interface and create even more high-frequency harmonics. You can see the results of that here:

Now, to show what happens when you clip a complex audio signal, and why people keep blowing up tweeters, here is the same three-sine wave signal, clipped as hard as we can into our digital interface:

You can see extensive high-frequency content above 5 kHz. Don’t forget – we never had any information above 1250 Hz in the original recording. Imagine a modern compressed music track with nearly full-spectrum audio, played back with clipping. The high-frequency content would be crazy. It’s truly no wonder so many amazing little tweeters have given their lives due to improperly configured systems.

A Few Last Thoughts about Audio Distortion

There has been a myth that clipping an audio signal produces DC voltage, and that this DC voltage was heating up speaker voice coils and causing them to fail. Given what we have examined in the frequency domain graphs of this article, you can now see that it is quite far from a DC signal. In fact, it’s simply just a great deal of high-frequency audio content.

Intermodulation distortion is a sensitive subject. Very few manufacturers even test their equipment for high levels of intermodulation distortion. If a component like a speaker or an amplifier that you are using produces intermodulation distortion, there is no way to get rid of it. Your only choice is to replace it with a higher-quality, better-designed product. Every product has some amount of distortion. How much you can live with is up to you.

Distortion caused by clipping an audio signal is very easily avoided. Once your installer has completed the final tuning of your system, he or she can look at the signal between each component in your system on an oscilloscope with the system at its maximum playback level. Knowing what the upper limits are for voltage (be it into the following device in the audio chain or into a speaker regarding its maximum thermal power handling capabilities), your installer can adjust the system gain structure to eliminate the chances of clipping the signal or overheating the speaker. The result is a system that sounds great and will last for years and years, and won’t sacrifice tweeters to the car audio gods.

If you want to learn even more about audio distortion, go back and read Part 1 of this article!

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.