What are early reflections?

Early reflections in plain English

Clap once in an empty room. The first thing to reach your ears is the clap itself, travelling in a straight line. A moment later come the first early reflections: copies of the clap that bounced once off the floor, the ceiling or one of the walls on the way to you. Copies that bounced twice follow them, then three times and more, until so many are arriving so close together that they merge into one smooth fade. That fade is the tail, the part most people mean when they say reverb.

The early reflections are the short stretch between the clap and the tail. In a bedroom they're mostly over within 20 ms, and in a concert hall they can stretch past 80. You can't pick them out one by one, because they arrive too soon after the sound, but they carry most of what your ears know about the room's size and surfaces.

Every reverb plug-in that models a room or hall makes them, and most give you a control for their level. The what is reverb? lesson covers the other controls; this page stays with the early part.

Early reflections off the room's surfaces

Sound in air travels about 343 metres a second, which is 34 cm every millisecond, or roughly 2.9 ms for every metre. A reflection's route is always longer than the straight line, so it always arrives later, and the delay is the extra distance divided by the speed of sound. If a bounce off the side wall covers 7 metres and the direct path is 3 metres, the reflection is 4 metres behind and turns up about 12 ms after the sound itself.

It's also quieter. In open air a sound loses 6 dB every time the distance doubles, so a reflection that has travelled twice as far as the direct sound starts out 6 dB down before the wall has absorbed anything. Then the surface takes its share. Concrete and glass reflect almost everything, while curtains, sofas and people soak up a lot of the top end, so their reflections come back duller.

A plain box-shaped room has six surfaces, which gives you six first-order reflections, each off a single surface. Acousticians work them out with the image-source method: mirror the singer through each wall, and the straight line from that mirror image to you is the reflected path. The figure does exactly that.

Each first-order reflection then hits another surface and makes more. A box-shaped room gives you 18 second-order reflections and 38 third-order ones, and the running total grows with the cube of time: a bedroom of about 30 cubic metres has sent you roughly 700 reflections by 50 ms and more than 5,000 by 100 ms. Long before then they're arriving too close together to tell apart, and that's where the early reflections end and the tail begins. A small room gets there quickly because the bounces are short. A big hall takes longer, which is why its first reflections arrive as a handful of sparse echoes before the wash fills in.

Early reflections fuse into one sound

When a copy of a sound reaches you within roughly 30 ms of the original, your brain folds the two into one. You hear one sound, from wherever the first arrival came from, coloured by the copy. That's the precedence effect, which studios often call the Haas effect after Helmut Haas, who studied it with speech in the early 1950s. The Haas effect lesson goes into it properly.

How long the window lasts depends on the sound. It's shortest for a single click, where a copy can be heard as a separate event after as little as 5 ms, and longer for speech and music, where it can reach 30 to 50 ms. Level counts too: a reflection nearly as loud as the original is easier to hear as a separate event than a quiet one.

In a small room every early reflection falls inside the window. In a hall the later ones arrive after it, but they're much quieter than the sound and surrounded by other reflections, so they still read as space. Push a single strong reflection past 60 to 80 ms and you hear it as its own event, a slapback. The slapback delay lesson starts from there.

The cues in early reflections

Room size

The spacing of the reflections is the main clue. Close walls mean short paths, so the reflections arrive bunched up in the first 20 ms or so and the tail starts almost at once. Far walls mean long paths, so the reflections come in later and further apart. Your ears read that spacing as size before the tail has had time to tell you anything, which is why a hall sounds big even on a short, staccato note.

Distance

Stand next to a singer and their direct sound is loud while the reflections are much quieter, because the reflections have travelled several times as far. Walk to the back of the room and the direct sound drops by 6 dB with every doubling of distance, while the reflections, which were already travelling a long way, barely change. The balance shifts towards the room, and that shift is one of the main ways your ears judge distance.

In a mix you can use the same cue. More early reflection compared with the dry sound pushes a part back; less brings it forward. The timing changes too, since a distant source has its reflections arriving close behind it, and that gap is the subject of the pre-delay lesson.

Wall material

Hard surfaces send back bright, strong reflections, and soft ones send back weaker, duller ones. A tiled bathroom and a carpeted bedroom of the same size can have similar reflection times and still sound nothing alike, because the bathroom's reflections are louder and keep their top end. The lab's chamber is a hard-walled room, so its early reflections are dense and bright.

Width

Reflections that arrive from the sides, off the left and right walls, make a sound seem wider than the instrument itself. Acousticians call this apparent source width. In a stereo reverb the early reflections are panned and timed differently in each channel, so a mono source picks up a sense of width from them. That's why a short room on a mono synth or a mono drum machine can make it fill more of the space between the speakers without any obvious reverb.

Reading the lab

The lab is set up for this one part of the reverb. Pre-delay is locked at zero, so the reflections start right after the sound, and the tone controls are locked as well. The type of space, the size, the decay, the mix and the early-reflections level are free to move.

The screen to watch is the impulse response, which shows what the reverb does to one click. The amber spikes are the early reflections and the coloured slope is the tail. Time is stretched at the start, so the first 50 ms take up a lot of the width and you can see each spike clearly. Hover over one and the lab tells you how far it is under the loudest moment.

The Early ref. knob sets the reflections' level compared with what each type of space would normally have, so 0 dB is the room's own balance and −24 dB nearly removes them. When you move the Early ref. knob, the Right now panel says what share of the reverb's energy arrives in its first few tens of milliseconds at your current settings, measured from the impulse response the lab has just rendered. Each amber spike is drawn at that reflection's own level, so turning the knob down shortens them. Plates and springs have no walls, so the knob greys out when you pick a plate. A/B switches the reverb off at matched loudness, and you can upload your own sound or record eight seconds with your microphone; it stays on your device.

The early reflections control

Start with drums, because their short, sharp hits leave gaps where you can hear what the room is doing. At the room's usual balance the reflections make up a big share of the reverb, so the kit sounds like it's somewhere real. Turn them down and the room gets vaguer; you still hear reverb, but it's harder to say how big the space is or where the drums are in it.

Turn them up past the room's usual level and the walls seem to close in. Strong early reflections are a lot of what people like about a good drum room, and on a dry, programmed kit a few decibels more can make the drums sound recorded. Too much and the kit sounds small and boxed in.

On most reverbs, size and early reflections are linked. Making the space bigger spreads the reflections out and starts them later. Switch to a hall and the reflections thin out to a handful spread over more than 100 ms.

Plug-ins name the control in different ways. Ableton Live's Reverb has an Early Reflections section with Spin and Shape controls, plus Reflect and Diffuse levels that set the early part and the tail separately. Other reverbs label it ER level or early level, and some use one early/late balance knob instead.

Early reflections and comb filtering

A single reflection that arrives a few milliseconds behind a sound doesn't sound like space. It sounds like a tone change, because the two copies add together at some frequencies and cancel at others. That pattern of peaks and notches is a comb filter. The first notch sits at 1 divided by twice the delay, so a reflection 1 ms late, from a path 34 cm longer, cuts 500 Hz, 1.5 kHz, 2.5 kHz and every odd multiple above. One that's 6 ms late, from a wall about a metre behind the singer, puts its notches only 167 Hz apart, starting at 83 Hz.

A real room or a good reverb gives you many reflections at different times, from different directions, so their notches land in different places and blur into a gentle colour. Trouble starts when one reflection is much louder than the rest. That's the boxy, hollow sound of a vocal recorded near a bare wall or with the mic over a desk, and it's also what happens in the lab when a tiny room's reflections are turned right up. The comb filtering lesson covers the effect on its own.

Using early reflections in a mix

Putting dry parts in one room

Samples, synths and direct-injected guitars come with no room at all, while a sampled drum kit or a live vocal often comes with its own. A short room reverb with strong reflections and a short decay, set up on a send and return, can sit all of the dry parts in the same space. Send each one in at a low level; the aim is that you only notice the room when you mute the return.

Splitting the early part from the tail

A lot of mixes use two reverbs: a short room for placement and a plate or hall for the long tail. The room handles distance and width with its early reflections, and the long reverb adds the sense of size without having to carry everything. If your reverb has separate early and late levels, you can get much of the same effect in one plug-in by turning the early part up and keeping the tail short, or the other way round.

Front to back

For a part you want close, use little or no early reflection. For one further back, send more to the room and pull the dry level down a touch, since the two together move a part back far more convincingly than turning it down on its own.

How to set early reflections, step by step

1. Pick a space that suits the partA room or chamber for drums, guitars and anything that should sound recorded; a hall or plate for long, smooth tails.
2. Start with the early level at 0 dB and a short decayA decay under a second keeps the tail out of the way while you judge the reflections.
3. Bring the send up with the mix playingStop when the part sounds placed in a room, before you hear the reverb as an effect.
4. Adjust the sizeSmaller for tight, close parts. Go larger if the part should sound like it's in a big space, keeping an ear on how the reflections sit against a busy rhythm.
5. Trim the early level by earUp for drums that need a room, down if a vocal starts to sound boxed in.
6. Check in monoIf the part dulls or goes hollow, lower the early level or narrow the reverb return.

Starting settings for early reflections

These are places to start, written in the lab's terms, with the early level measured against the space's own balance.

Common early reflection mistakes

A boxy lead vocal

A small room with loud reflections puts a vocal in a cupboard. If the voice sounds hollow or nasal once the reverb is in, turn the early level down or switch to a plate, which has none.

Stacking rooms

A drum loop recorded in a real room already has early reflections in it. Adding a second set from a different-sized room muddles the picture, so try a plate or a smooth hall on that loop and save the short room for dry parts.

Judging them soloed

On their own, strong reflections can sound obvious and odd. With the whole track playing they turn into a sense of space you barely notice, so set them with the mix running.

Forgetting the mono check

Reflections timed differently in the two channels add width in stereo and can comb filter when the channels are summed. Many club systems and a lot of smart speakers play in mono, so check before you commit. The mono compatibility lesson shows how.

Training your ears to hear early reflections

The quiz under the lab plays a hidden room, hall or plate at matched loudness and asks you which. The start of each sound gives it away more than the length of the tail: a room has its reflections bunched in tight, a hall has a few spread-out ones before a slow swell, and a plate has none at all. Listen to the first 50 ms after each hit. There are eight rounds, and your best score is kept in your browser.

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