The biggest difference between the Shure SRH1840 and Shure SRH1540 is that the SRH1840 is an open back headphone whereas the SRH1540 is a closed back headphone.
Choosing the right Shure can for you is determining what and how you plan on using the headphone.
Open Back Headphones
Open back headphones are headphones that have holes in the cups to allow air and sound to pass in and out of the headphone.
Open headphones tend to have a wider soundstage. The open space allows more air to pass through the drivers and diaphragm.
The downfall of open headphones is that they have no noise isolation. This means that outside noise will leak into your headphones and interfere with your music. We usually suggest open back headphones for at home or studio listening.
Many audio engineers enjoy mastering with open back headphones because of the accuracy they provide.
The Shure SRH1840 open back headphone will offer an airy and spacious soundstage often making your music have a more natural and less enhanced sound. It is more like listening to you music through studio monitors or speakers instead of headphones.
Audiophiles tend to enjoy open back headphones because of the airy and natural sound quality. This is why most of the “audiophile” headphones like the Audeze LCD-4 are also open backs.
Closed Back Headphones
The drivers of closed-back headphones are completely enclosed in the cups of the headphones. No air can pass in or out of the headphones creating a more closed and sound isolated environment.
Closed headphones are the best for traveling or for use in places where there is a lot of outside noise interference. Since closed headphones are completely sealed, sound does not easily escape or enter the headphones. This means you will get better sound isolation for outside noise and others will not be disturbed by your music.
Due to their isolated nature, closed back headphones do not sound as airy or natural as open back headphones. The sound is more concentrated in your head instead of being spread out over a wide soundstage.