Step outside after dark this week and stand still for a minute without doing anything else. There is a wall of sound out there. It pulses, it layers, it comes from the hedges and the treetops and the long grass at the edge of the lawn, and it has been building since July without most of us noticing that it arrived. In May the evening belonged to birds. It does not anymore. Somewhere in the middle of summer the soundtrack changed hands, and what you are listening to now is the last and loudest few weeks in the lives of several thousand small musicians.

The Handover

Spring sound is territorial and it is about real estate. Birds sing at dawn to announce a boundary and to advertise for a mate, and by late June most of that business is settled. Nests are built, eggs are laid, and a singing bird is now a conspicuous bird with young to protect. The dawn chorus thins out. By August it has largely gone.

What fills the gap is a completely different orchestra with a completely different schedule. The insects that sing in late summer spent most of the year as eggs or as nymphs underground or in leaf litter, growing slowly, silent the entire time. They arrive at adulthood late, and adulthood is short. Everything you hear in August is being sung by an animal in the final weeks of its life, which is the reason it is being sung so insistently.

Once you know that, the sound stops being background. It becomes an announcement about where you are in the year.

Who Is Actually Out There

Three groups are doing nearly all of it, and they divide up the day between them.

The cicadas own the afternoon. That long rising and falling buzz in the heat, the one that sounds a little like an electrical fault in a tree, belongs to the annual cicadas people around here call dog-day cicadas. They emerge every summer, unlike the famous periodical broods that arrive in enormous synchronized waves every thirteen or seventeen years. They sing in the hottest part of the day and stop when it cools.

The katydids own the treetops after dark. If you hear a rasping, two and three-note phrase repeating from high in the oaks, slow and rhythmic and slightly mechanical, that is a true katydid. The old country name comes from the phrase people heard in it: katy-did, katy-didn't, the argument going back and forth across a street all night.

The crickets own everything close to the ground. Field crickets in the grass and under the step, with the familiar chirp most of us learned as children. And in the shrubs, if you are lucky, the snowy tree cricket: a pure, sweet, pulsing trill, all the individuals in a hedge falling into the same rhythm until the whole shrub seems to breathe.

How the Sound Is Made

The two families make their music in ways that are worth knowing about, because it changes what you hear.

Crickets and katydids rub. One forewing carries a ridged file and the other carries a scraper, and drawing one across the other at speed sets the wing surface vibrating, the way a bow sets a string going. The wing acts as its own soundboard. It is a stringed instrument, played on the animal's back.

Cicadas do something stranger. They have a pair of ribbed membranes on the abdomen called tymbals, and a muscle that buckles each one inward with a click, then lets it snap back with another. Hundreds of clicks a second, resonating in a body that is mostly hollow air sac, which is why a single cicada in a tree can be heard across a yard. It is a drum, not a fiddle.

In all of these, it is the males singing. The females are listening, and they are listening with their legs: crickets and katydids carry their ears just below the knee on the front legs, a small oval membrane on the shin. A female walks toward a male by turning until the sound is balanced between her two front legs. The whole August night is a conversation conducted between wings and knees.

The Thermometer in the Hedge

Here is the part that turns a walk into an experiment, and it works on any warm evening.

These insects are ectothermic. They have no internal furnace, so their body temperature is the air's temperature, and every chemical reaction in the muscle that drives that wing runs faster when it is warm and slower when it is cool. The result is that the tempo of the song is a direct readout of the temperature.

The snowy tree cricket is the reliable one for this. Count its chirps for fourteen seconds and add forty, and you have a rough temperature in Fahrenheit. On an evening in the low seventies you will count about thirty. Come back to the same shrub in September when it is in the fifties and the same insect is chirping at half the speed, sounding tired, sounding like the season is ending, because in a real sense it is.

It is an approximation and not a precision instrument, and the field crickets are noisier data. It is still one of the few pieces of science you can do standing in your own driveway with nothing but patience, and children are usually much better at it than adults because they are willing to actually count.

Why It Gets Louder as It Gets Later

There is a shape to the late-summer soundscape, and August is not the peak of it. Late August and September usually are, and that surprises people who assume the sound tracks the heat.

It tracks the population instead. The insects that hatched in spring have been growing all summer through a series of molts, and the last of them reach adulthood late. The chorus is loudest when the greatest number of them have finally arrived, and that is also the moment the clock runs out. Adult crickets and katydids do not overwinter here. The females lay eggs in soil or in plant stems, and the first hard frost ends the adults entirely.

So the sound outside your window in September is genuinely the sound of a generation finishing. And then one night in late autumn you will step outside and hear absolutely nothing, and the silence will be startling, because you had been listening to that wall of sound for four months without registering it as sound at all.

Noticing the date it stops is a small and strangely moving thing to do. Most people have never once noticed it.

What Listening Does

There is a reason a night full of insect song settles a person, and it is not only association or nostalgia.

Attention has two modes. One is the effortful kind you point at a screen or a task, which fatigues and needs recovering. The other is the loose, undirected kind that a natural setting invites: interesting enough to hold you, undemanding enough that nothing is being asked. A pulsing hedge full of crickets is very nearly the perfect object for the second kind. It has pattern without meaning. It does not want anything from you.

Listening also does something that looking does not. Sight is directional and it is a decision. You choose a thing and aim at it. Hearing is a full sphere, arriving from behind and above and underneath, and you cannot select within it the way you select with your eyes. To listen properly you have to stop choosing and simply receive, which is why so many contemplative traditions use sound rather than sight as the doorway. Ten minutes of real listening leaves most people quieter than ten minutes of anything else available on a Tuesday evening.

How to Listen This Week

None of this requires equipment or expertise. It requires standing outside and not doing anything else, which turns out to be the difficult part.

Go out after dark and give it ten minutes. No phone, no porch light on. It takes a good two minutes for the ear to settle and start separating layers, which is exactly the point at which most people go back inside.

Learn three sounds, not thirty. The daytime cicada buzz, the rasping katydid phrase overhead, the ground-level cricket chirp. Once those three separate out for you, the wall of sound becomes an orchestra you can hear the sections of, and you will not be able to un-hear it.

Try the cricket thermometer. Fourteen seconds, add forty. Do it with a child if there is one available. Do it again in three weeks and notice the number has dropped along with the season.

Turn a light off. Outdoor lighting quiets a yard and disorients night insects and the moths and fireflies that share the space with them. A single fixture switched off, or a warm bulb instead of a white one, changes what happens outside your door within a night or two.

Leave a corner unmown. Crickets and katydids need stems to shelter in, leaf litter to overwinter in, and a soil surface that has not been shaved to a putting green. A patch of long grass at the back fence is the entire habitat requirement, and it is the reason one yard on a street sings and the next one does not.

Note the date it ends. Write it somewhere. Then next year you will have something to compare it against, and you will have started keeping the oldest kind of record there is.

The Season Has a Sound

We mostly mark the turn of the year by sight, and by the calendar, and by what the stores have started selling. The ear is a much older instrument for the job, and it is more honest. It tells you the tempo is slowing. It tells you the chorus is at its fullest just before it goes.

There will be an evening in a few weeks when the trill in the hedge is noticeably slower than it was tonight. You will only know that if you stand outside tonight. That is the whole practice, and it costs ten minutes.