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Oct 22, 2024
Let’s face it – when driving, you can either listen to the symphony of internal combustion from your engine or enjoy your favorite tunes from your car stereo. But have you ever stopped (maybe not literally!) and wondered what makes your BMW speakers sound the way they do? What exactly affects the audio quality? Enter total harmonic distortion (THD).
While it’s not the only thing that influences your music-listening experience, it’s definitely one of the more important behind-the-scenes aspects in this regard. Join us today as we dive deeper into the science of sounds in car audio and learn everything about the THD.
Coming up:
Enjoy the read!
So, what is THD?
Before we dive deeper into its definition, let’s take a small step back. If you are a hardcore audio geek, you can skip this part! For those of you who aren’t familiar with the science of audio systems though, we should answer another question first – what is sound? Well, the sound waves are basically cyclical rapid changes in air pressure. These vibrations can then be traveling through certain mediums:
These vibrations have their frequencies, measured in hertz. For a signal to be audible to the human ear, it has to be within the 20 Hz and 20 kHz range. Everything above that is known as ultrasound, while everything below it – infrasound.
Now for the fun part.
Total Harmonic Distortion (THD) is a measure of the distortion that occurs in audio systems when they are processing signals. In the ideal world that you may remember from your science or math classes, the output signal should exactly match the input signal.
However, in real life, some distortion occurs due to the non-linearities in the components. They are mostly caused by the difficulty of changing the electrical signals to mechanical movement (vibration), which is sound, as we established earlier.
Due to limitations in materials or dome geometries, speakers aren’t too good at operating outside their specific frequency ranges. A tweeter that covers high frequencies simply won’t be able to also cover very low frequencies at the same time. Outside their range, moving or magnetic parts of a speaker start acting in a non-linear way, hence the distortions.
This causes the so-called harmonic frequencies (also known as non-fundamental frequencies) to be added to the signal. THD quantifies exactly how much of the output signal consists of these unwanted harmonic frequencies compared to the fundamental (original) frequency.
Simply put, total harmonic distortion is a calculation of just how much the output signal has distorted from the input signal represented as a percentage. But how is it calculated? Glad you asked!
It’s not like you have to calculate it on a daily basis when dealing with car BMW audio systems… But since we promised you an in-depth look, below is the formula you’d use to do just that.
As mathematical as it seems, there is a very simple correlation between this formula and the definition we presented above.
When it comes to measuring the THD, it’s not as simple as calculating it from a set of variables. You’d need specialized instrumentation, such as a spectrum analyzer or THD meter, so unless you have one at hand, it’s pretty much impossible to do. If you do, however, have the tools, you’d go about it by first measuring the energy at the input signal harmonics and then applying the formula using the measurements you took.
Well, we can’t really see it with the naked eye, but it’s possible to observe its visual representation in a simulated form or on the measurement graphs. Let’s check out an example in simulated THD from the Class A amplifier.
In this instance, the original genesis of distortion is increasing power. When we increase the gain in the amp, our output signal gets much closer to the maximum power supply which is +/-28V here. Because of that, the area free from distortion is from -28V to +28V.
If we were to up the gain, the signal amplitude would stop increasing at these values and the whole signal body would become square-shaped. Assuming an infinite sine amplitude, the signal wave in this small +/-28V window would look like a perfect rectangle.
Below you can see different levels of total harmonic distortion represented in graphs.
The sound we’re hearing comes a long way from its source and the signal path is an important aspect of how the THD comes to be. Throughout all its stages, there are more and more distortions added to the signal. What does the path look like?
In the first stage, there’s not much distortion added. The market standard for modern DAC chips is a stable THD of 0.0005%, which means almost perfect signal repeatability. The next stop increases the THD of the audio signal up to 0.005–0.01% (for solid-state amplifiers) and 1–10% (for tube amplifiers), meaning even more distortions are added.
The last stage (speakers), is the most tricky and complex one.
In the electrical domain, the distortions are much more predictable, since there aren’t many external variables that could affect them. However, when it comes to the speakers where the electromotive force is changed into air pressure, numerous other factors come into play.
Ideally, the movement of the speaker should be pistonic. That’s of course assuming the infinite stiffness of the membrane and infinite motor force, which would allow it to control the membrane with infinite acceleration. Unfortunately, the stiffness and force to control the membrane are both finite.
Total harmonic distortion in the speakers can also be generated as a function of power. The bigger the displacement of a membrane the more distortion increases. The biggest distortion starts generating when the speaker crosses the level of 80% maximum power. Then the coil of the speaker moves out from its linear operation area.
This could be seen especially in woofers and mids. Distortions are audible in mids much sooner because when the speaker is leaving its linear area of operation, it has to generate more sound waves per given period.
It’s getting tough, so let’s try using an example.
Picture this: we are playing a song that has some deep tones in the background and lyrics sung by a female singer. With the bass signal of 100 Hz and women's vocals being audible at 2–5kHz, the speaker will be playing 20–50 times more vocal wave periods in one bass period. That’s why mids are the most crucial speakers in the whole spectrum.
Two other things are the quality of the materials and the design of the speakers. Properties of the membrane, size and shape of the cone, or the speaker housing – all of that will affect the amount of distortion added to the signal at the end of its journey.
You’ll be surprised to learn that the answer is NOT “It depends!”
Many people will start noticing distortions only after it reaches 1%. On the other hand, those with trained or sensitive ears and audiophiles can spot them at much lower levels – this is why true Hi-Fi systems go as low as 0.1–0.01%.
Is it possible to have a distortion of more than 100%? Yes, it is. But since this means there will be more distortion than the original signal, we probably wouldn’t be able to recognize the sound anymore.
Not having any distortion at all also isn’t that great for the audio quality. While it’s true that it makes the sound much cleaner, no THD can get too close to sounding too sterile. Moreover, in certain situations—i.e., specific music genres or tracks—a pinch of distortions is desirable for some added warmth or overdrive growl, in the case of rock tracks.
It’s safe to assume then that, as a rule of thumb, a total harmonic distortion acceptable range is around 0.01–1%, depending on the application.
Prevention is basically the most important part of low total harmonic distortion.
By that, we mean using high-quality BMW speakers and amplifiers for your audio system. If you already have a complete setup, the only thing you can do is to start tweaking its layout or configuration.
For example: if there are some tones that the previous speaker could not handle, resulting in more THD, you should try out a speaker with a much stiffer membrane or just a smaller size. Such as the tweeters, which have a smaller membrane and work perfectly as a piston from 2.5kHz up to 25kHz or, when it comes to diamond cone ones, even 100kHz.
One thing you could do as somewhat of a remedy is to turn down the volume to around 80% or lower since that’s where most of the distortions start.
Naturally, due to the way the Alpha One BMW Audio Upgrades are designed and tuned if you get our entire audio upgrade kit, you won’t have to worry about any of that. All of the Alpha One Speakers will work perfectly with the UP8/10 or Lightwave Amplifiers (depending on your car model). This setup has been tried and tested numerous times – the reviews by our satisfied customers are a testament to that.
…Not really.
Ultimately, the distribution of the distortion in the speaker spectrum is like a fingerprint and it directly corresponds to the sound quality. To quote one of our Alpha One audio engineers, Jacob: “It’s similar to how mineral water has its own taste. Water that is mineralized less does not have a taste and isn’t that healthy.”
Translating that to the world of BMW sound systems: there should always be a small level of distortions in the audio setup or it would be characterless. What are your thoughts on that? Are you struggling with THD in your speakers?
Generally speaking, a THD of 0.01–1% in BMW audio systems is acceptable (depending on the application and your desired goals).
Materials and shapes of speaker components are designed to handle specific frequencies – outside their range, they become non-linear and start producing distortions.
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Generally speaking, a THD of 0.01–1% in BMW audio systems is acceptable (depending on the application and your desired goals).
Materials and shapes of speaker components are designed to handle specific frequencies – outside their range, they become non-linear and start producing distortions.
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