Your recording studio is never going to work in a noisy environment. As soon as you turn up the signal to hear the vocals adequately you will also be turning up the noise floor. The noise floor is the sound of everything in your recording environment other than the signal you are trying to capture. Every electronic device produces noise. The noise floor will also include any sound that you have not isolated from outside of your booth. Low frequency rumbles can easily show up within your recordings. The grumbles of a lorry going past or a plane flying over head, These lower frequencies can be cut out using a high pass filter but try your best to eliminate as many sound issues as you can at their source. The more work you put into the design and location of your recording studio the less work you will have to do in post production.