Before starting a headphone design, it is important to know what we are aiming to achieve in order to get awesome sound.
Some aspects of headphone performance (such as frequency response) can easily be measured and quantified, but other aspects are more qualitative and require careful listening to pick up.
Sound is basically slight variations in pressure travelling through the air. A speaker or musical instrument produces this ‘pressure wave’ and the ear measures it and converts it into a signal that our brains interpret as sound. The human ear is capable of detecting sound from as low as 20Hz (Hz or Hertz is the number of wave cycles per second) up to 20,000Hz. This chart shows the range of various instruments.
It then stands to reason that we want headphones which produce as much as possible of this audible range, otherwise ‘information’ is being lost, along with sound quality. This is called frequency response, and is quoted as a range, such as 30Hz – 20kHz.
In addition to our headphones reproducing the entire audible range, we don’t want any big dips or peaks in how loud particular frequencies are, that is we want a flat frequency response curve. The graph below shows the frequency responses of one of my designs (red) vs my Grado SR225’s (blue) as tested on my basic test rig. The Grados sound better, and we can see why – their frequency response curve is much flatter, particularly at high frequencies.
NOTE: the above results were measured using my basic test rig, which uses a flat plate rather than human ear interface, so is measuring the response of the headphones rather than what is perceived by the human ear. As such it is just a rough indication.
The following are some descriptions of the qualities of the sound of a pair of headphones – they are things to listen out for when evaluating a set.
Bright: the high frequencies are emphasised and clear
Coloured: not natural sounding, usually due to peaks and dips in the frequency response curve
Detail: the presence of fine details in the music which are lost with poor quality headphones
Soundstage: the ability of the sound to convey the location of the instruments, eg drummer in the centre, guitarist to the left etc. If you close your eyes and listen, you should be able to pick the locations of the instruments on the ‘soundstage’.
For further definitions, see this page.
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