It is unusual to think of a city as a sound. We picture cities visually, from photographs or maps, and we describe them in terms of their buildings. Yet every urban place has its own acoustic signature, assembled from traffic, machinery, footsteps, birds, voices and the reflections of all of these from walls and pavements. The study of this signature, known as urban acoustics or soundscape research, has developed rapidly since the 1970s and now informs planning decisions in many parts of the world.

The foundational work was done by the Canadian composer and academic R. Murray Schafer, who in 1977 introduced the idea of a 'soundscape' by analogy with landscape. Schafer argued that many modern cities had become acoustically impoverished, dominated by low-frequency mechanical hum from engines, ventilation and electrical systems, which he described as 'flat-line sound'. Louder events, such as a delivery truck or an aircraft overhead, became harder to notice against this background, a condition Schafer called masking. His writing encouraged a first generation of researchers to listen to cities as carefully as they looked at them.

Modern measurement has confirmed his intuitions with instruments. Urban noise monitors installed in many European cities continuously record sound levels across a range of frequencies. The data show that a busy avenue has a very different acoustic profile from a quiet square, even when the average decibel level is similar: the avenue contains more steady low-frequency noise, the square contains more intermittent high-frequency sound from footsteps and voices. Residents distinguish between the two even when they cannot put the distinction into technical words. Acoustic engineer Dr. Helena Varga has shown, in studies across several European cities, that residents rate areas by their soundscape almost as strongly as they rate them by their visual appearance.

Particular sounds have outsized effects. Church bells, for example, are often remembered as defining a neighbourhood, even when they sound only a few times a day. The continuous rumble of a nearby motorway, which residents may consciously tune out, nevertheless produces measurable stress responses in the body, including elevated blood pressure and disturbed sleep. A 2021 review by the World Health Organization estimated that long-term exposure to traffic noise contributes significantly to cardiovascular disease across Europe, with the effects heaviest in working-class neighbourhoods close to major roads.

Design can change this. Pavement surfaces, building facades and street furniture all affect how sound propagates in a street. Trees and shrubs absorb sound at higher frequencies without necessarily reducing the total decibel level, which is why a planted avenue can feel quieter than a treeless one, even when meters disagree. Architects increasingly work with acousticians from the earliest phases of a project, designing courtyards and setbacks that shelter residents from traffic while preserving the variety of sound that makes a place feel alive. A well-known example is the refurbished Parc de la Villette in Paris, where strategic placement of walls and vegetation produces pockets of quiet within an otherwise busy urban park.

Loudness is not the only dimension. Dr. Varga argues for the concept of 'acoustic diversity': a soundscape that contains a balance of natural, social and mechanical sounds, each recognisable but none overwhelming. Her surveys suggest that residents prefer neighbourhoods with a rich but moderate soundscape to those that are either monotonously loud or eerily silent. Total quiet, which might seem desirable, is often experienced as unsettling in urban settings, particularly at night.

Policy has begun to catch up. The European Union requires cities above a certain population to produce 'strategic noise maps' every five years, and to identify quiet areas that deserve protection. Japan has long prohibited certain levels of sound at particular times near residential districts. Enforcement, however, is uneven, and the rules tend to target only the very loudest sources. The more subtle question raised by soundscape research - what the overall balance of sound should be in any given neighbourhood - rarely appears in law. Dr. Varga sees this as the next frontier. Cities already regulate what a place looks like; they are slowly learning to regulate, or at least to guide, what it sounds like.