Why do bridge and neck pickups sound different?
This is a good question and one that has implications for pickup selection both in terms of the sound and tone that you are trying to achieve as well as if you are considering replacing pickups.
So why do bridge and neck pickups sound different?
The answer lies in the way that the string moves relative to the pickup when it is plucked. However, before we consider this in more detail it is probably worth doing a little bit of revision on how electric guitar pickups work.
Electric guitar pickups work through the process of electro-magnetic induction. This is a phenomena first discovered by Michael Faraday and reported in 1831. Electro-magnetic induction is the production of an electro motive force across an electrical conductor due to its dynamic interaction with a magnetic field. Sounds very scientific but how does this apply to the guitar pickup?
The guitar pickup works because of the movement of the guitar string through the magnetic field of the pickup. When a guitar string is plucked it vibrates in the magnetic field of the pickup creating a small electric current which is then amplified by your guitar amp.
But how does this phenomenon give rise to a difference in sound based on where the pickup is sited on the guitar? And therefore to answer the original question why do bridge and neck pickups sound different?
In essence the guitar string moves differently across its length. For example when plucked the amplitude of vibration of the string at the bridge is smaller than the amplitude at the neck. This difference in amplitude can be heard if you unplug your electric guitar or have an acoustic guitar handy. If you pluck the string closer to the bridge the sound will be quite bright, sharp and metallic; whereas if you pluck the string closer to the neck the sound is mellow and resonant. This is down to the different amplitude of vibration created in the string. The pickup in your guitar will therefore create a different electric current due to the different amplitude of vibration. This has an impact on the sound that your guitar will produce and therefore answers the question why do bridge and neck pickups sound different. For me neck pickups always sound more warm and mellow whereas bridge pickups sound thinner and more harsh. Bridge pickups can also have a lower output.
Pickup designers can get around this by balancing the output of the pickups which will help to reduce the impact of the position of the pickup. One easy way to reduce the problem is to design the bridge pickup to have a higher output than the neck pickup. That is why you will often see pickups either sold to be used in a particular position i.e. neck or bridge, or sold as a balanced pair.
If you are looking to replace your pickups it is therefore generally advised that you buy either a balanced set of pickups or pickups specifically designed for either the bridge or neck position. However, there is no reason why you have to stick to this. If you like experimentation then it can be fun to mix up the pickups to see how they perform in different positions. For example a higher output bridge pickup in the neck position can give some interesting results!