Frequency Response in Car Audio

Frequency Response in Car Audio

Every piece of audio equipment specifies a frequency response. Here we investigate what it means, what makes a good one and how it should be stated.

First off, it is useful to understand how frequency response relates to a human’s ability to hear. As a general rule, fresh new ears and the brains attached to them can hear frequencies from 20Hz – 20,000 Hz (20kHz) – Hz or Hertz is the unit used to measure cycles per second or frequency. However, the ear does not necessarily hear all of those frequencies at the same level even in the young.  As you get older and are exposed to loud noise, your ears and supporting equipment deteriorate quite quickly. It can actually be quite alarming to think about the noise we expose ourselves to while journeying through life. Much of this can cause slight yet irreparable damage to your listening equipment. Loud music is often blamed for early hearing loss, but everyday exposure to anything from aeroplanes to pressure cookers or pneumatic drills can produce noises above and beyond the safe 89dB limit! Generally, the damage will occur over a specific frequency band and as a result everyone has sightly different hearing. There are also primeval relics related to our natural hearing response curves, for instance, females tend to be more sensitive to frequencies present in a baby’s cry (also a cat’s “mew”) – Annoying for you ladies, I know!

Audio equipment manufacturers aim to reproduce frequencies between 20Hz and 20kHz with as little variation in level as possible. As can be seen in the arbitrary graph at the top of this piece, as well as bandwidth (range of frequencies) the amplitude needs to be specified too. It is very unlikely that a set of speakers would have the same response at 20kHz as it has at 15kHz. Generally for such graphs, a nominal level of 0dBV is the base point and the curve should accurately plot the amplitude at each frequency. It is usual for most speakers to show a “roll off” in level at both the bottom and top extremities, as to make the response completely flat is difficult/expensive and for most listeners, undesirable. The important thing when looking at speaker frequency response is more to do with unwanted level fluctuations in the middle orders. These suggest that the final sound could be “coloured” by erroneous peaks or dips in level at specific frequencies.

A flat or linear response tends to be much easier to achieve in modern audio electronics such as amplifiers. Here, designers can be wholly accurate in their statements and will generally add an amplitude variance to their spec. For instance,  this is from the spec list of an Audison Prima AP8.9 bit amplifier (pictred above): Bandwidth (-3 dB, 2 V RMS, 4Ohms): 10Hz – 22kHz. This tells us that the frequency response is stated from 10Hz (below human hearing threshold) to 22kHz (above human hearing threshold) with a maximum level variance of 3dBV. Measurements were carried out at 2Volts. Although a 3dBV variance can be heard under stringent listening conditions by those blessed with great hearing, it is negligible compared to the natural variance that will result from in-vehicle use.

As mentioned before, it is far harder to achieve a linear response from a speaker and in any case, the response is far more likely to be effected by environmental elements such as mounting position, grille design and material and even number of passengers in the car and whether they are wearing coats or T-shirts! This is why we always recommend you listen to a pair of speakers before buying them. If possible, mounted in a car of similar shape and size to your own. Fortunately, sophisticated sound processors such as that present in the Audison Prima AP8.9 bit, are able to remodel sound and so, a system can be accurately tuned to your preference these days .

Incidentally, it may seem unnecessary to have a frequency bandwidth (range) that exceeds the generally accepted limits of human hearing. However, frequencies outside of human hearing will interact with other frequencies and can add or subtract and even cause “beat frequencies” to present timbral differences. A really good turntable/arm/cartridge combination can reproduce frequencies up to 50kHz and it is felt that the interaction between frequencies outside of the audio range and those within are one of the prime reasons that many music aficionados “prefer” the sound of vinyl renderings.

As always, there is no substitute to a visit to your local FOUR MASTER to receive a recommendation and preferably a demonstration before you buy.

 

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