Mid-range frequencies in car audio systems

Stuck in the middle with you


We recently discussed the case for bass-less driving. We were not of course, suggesting that you drive with no bass, merely that a subwoofer may not always be necessary to aural enjoyment in a car. Responses to this item were, what you may call, “polarised”. Along with many incredulous suggestions that we had lost our minds there were equally those encouraged by the prospect of not having to give up valuable golf club space to a hulking great subwoofer in an ugly box in order to enjoy music on the move. ¬†We were also advised of a certain degree of confusion over what the hell we were talking about and what makes bass “bassy” and mid-range er, “middy?” and so we felt it time we further investigate the all important middle reaches.

Our research (mainly online) and experience (many years of training and working in audio-related fields) suggests that there is no definitive explanation of what mid-range is. It would be very handy to be able to pinpoint exactly what frequencies we are talking about, but many resources quote different figures. A useful starting point is the capability of human ears and the equipment that helps them to work. It is a scientifically confirmed fact that in prime condition, we are able to hear frequencies from 20Hz to 20,000Hz (20kHz). However we are only able to do this for a relatively short period of time, as our auditory organs reduce in effectiveness alarmingly quickly particularly if we expose ourselves regularly to loud noise, such as playing guitar and singing in loud rock bands for instance.

One of the fundamentals we need to understand is how frequency relates to “pitch” – Sing the highest note you can manage and then the lowest. If you are not a trained or naturally gifted singer your highest and lowest notes or pitch will be in the region of two and a half octaves apart if you are lucky. An octave represents a doubling of a frequency so for instance, if you can sing let’s say, 100 Hz as your low note then your highest note will probably be between 400 and 800Hz. Not a very wide range you may think, but if you start calculating octaves above this you quickly find that the numbers escalate dramatically (1200Hz, 2400, 4800, 9600 etc.). A gifted and trained singer may be able to sing four octaves with a following wind whilst a full-sized 88-note piano keyboard spans a little over seven octaves (27.5Hz – 4.18601kHz). A guitar in standard tuning will reproduce notes from 83Hz to 1.318…kHz. I could go on all day but already, some of you may be scratching your heads! If we can hear up to 20kHz, what instruments produce those high notes? Well, if we are talking about the highest fundamental frequency of a musical instrument then not many, if any at all. But, the characteristic DNA of any musical instrument is not solely related to the fundamental frequencies it can produce. The timbre or characteristic sound of any musical instrument is defined by the amount and frequency of “overtones” or “harmonics” that accompany the fundamental. This is where a car audio system’s ability to reproduce as wide a range of frequencies becomes vitally important.

A cymbal has a fundamental frequency but that alone would not enable you to recognise it as a cymbal. It is the myriad of accompanying harmonics that give it its characteristic sound. If this were not the case then setting up an audio system in a car would be easy and anyone who has tried it, knows that this is not the case. The challenge is to balance all frequencies so that the characteristic of each instrument can be easily identified. This is why, despite the highest reaching musical instrument operating at around 7.5kHz, we need equipment capable of reproducing frequencies up to 20kHz. The argument for vinyl actually offers even more mud to the water. A really, really good turntable and cartridge/stylus combo can reproduce up to 50kHz, well above the capability of the human ear. However, when these ultra sonic frequencies interact with lower ones, they produce “beat” or “sympathetic” frequencies which, although cannot necessarily be pin-pointed and identified, do have an effect on the ambience and timbre produced by instruments and their overtones operating within the human sonic capability.

Getting back to the plot. Without wishing to outrage the pedantic amongst us, we can take mid-range to include frequencies from 250Hz to 4kHz. Bass would need to take you from 250Hz down to 60Hz and high frequencies from 4kHz upwards. Given what we already know about frequency doubling and the fundamental frequencies of particular ¬†instruments, this definition gives mid-range a comparatively wide bandwidth and due to the amount of instruments clambering for space within it, we often sub divide mid-range into at least two: High-mids and low-mids (sometimes known as mid-bass). In issue II of Driving Sounds Magazine, the chart below, featured in an article called “Highlife, Midlife”. This illustrates where the fundamental frequencies of a selection of instruments lie (Red) and also the range of significant overtones or harmonics (Yellow). We think it demonstrates very clearly the importance of mid frequencies but also, the need to balance mids with other frequency ranges they effect. This chart also features some very well used words which industry professionals, music system reviewers and the public at large often use to describe sounds and their place within the audio frequency spectrum.

Frequency chart

As you can see, the definition used for mid-range and particularly high-mids expands way beyond the one we are now using. Why? Many designers of hifi equipment use their own definition and largely, the split frequencies are in line with crossover frequencies used to send sound to speakers. The very word “crossover” should ring alarm bells here as it suggests some effort required in the handling or splitting off of different frequency bands (like crossing a road, crossing a river or crossing a space time continuum!). This is an electro-mechanical fact that we are not really concerning ourselves with here. Instead, we are merely drawing attention to the huge importance any system designer should place on mid-range frequencies and in particular mid-bass which covers the area where an electric guitar and snare drum drive rock music. A deficiency in this area in particular leads to a dull, flat, lifeless reproduction of music which has many a poor wretch, lumbered with standard factory equipment, reaching for talk radio stations instead of an uplifting musical experience on their way to their destinations – Sad but true. Even more regrettable is that if they only knew how easy and comparatively cheap it is for a FOUR MASTER to achieve a stunning uplift in performance in this area, they would have happier lives and drive with big smiles on their faces!


Content provided by our friends at Driving Sounds

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