# How long do early reflections take to become audible after direct sound [closed]

When speaking in terms of reverb, how long do early reflections take to occur/become audible after the direct sound?

Also, do early reflections dictate if a space is long/short or does it also dictate if a room is high/short and/or wide/narrow?

and lastly, why are early reflections more spaced out than late reflections? As shown in the image below.

Thank you!!

When speaking in terms of reverb, how long do early reflections take to occur/become audible after the direct sound?

in addition to the other answers, this also depends on the decay time of the original sound. A long decay time means the direct sound can be louder than the early reflections.

E.g. a single clap will generate audible reflections sooner than a piano tone with the Sustain pedal pressed. Although this depends on the exact decay characteristics of the instrument. In the graph in the question, a slope is drawn for the decay rate of the room. You can draw a similar slope for the decay rate of the instrument. If this is steeper than the room decay rate, you could hear the reflections over the original sound.

Also, do early reflections dictate if a space is long/short or does it also dictate if a room is high/short and/or wide/narrow?

The reflection time says something about all 3 dimensions of the room, but can't distinguish between them. Although the first reflection will generally be straight off the back wall. Later reflections will have bounced off 2 or more walls.

and lastly, why are early reflections more spaced out than late reflections? As shown in the image below.

The direct sound travels more or less as a sphere from its point of origin. When the sphere intersects a wall, it'll create a whole series of reflections all along the wall, smearing out the signal.

For the late reflections, the sound has had time to bounce around multiple times, getting more smeared out each time.

• 2nd para - won't any direct sound be louder than any reflections? – Tim Nov 29 '19 at 11:44
• Mostly yes, not always. I've added some more info. – Hobbes Nov 29 '19 at 13:03
• @Hobbes okay so the reason early reflections are more spaced out is because they are producing less reflections as opposed to late reflections that because of the increased time have had more chance to produce more reflections which as a result gives that reverb wash or tail? – Seery Nov 29 '19 at 21:53
• I found this answer so interesting and useful! – jdjazz Dec 14 '19 at 17:44

It will all depend on the size and shape of the room, and where the sound source and listener/microphone are located in the room. Sound travels in all directions and reflects off of most surfaces, so the width, length, and height of the room all matter. Sound travels at approximately 1 foot per millisecond, so with a little math you can calculate the delay between the direct sound and the first reflection (the pre-delay). If either the sound source or the listener is very close to a wall, there will be hardly any delay, whereas if the sound source and listener are side by side and the nearest reflective surface is 100m away, the delay will be substantial.

why are early reflections more spaced out than late reflections?

Are they? If they are, then I would guess it is because the sound hasn't fully dispersed yet.