What is Wavelength | Room Acoustics & Sounds Reflections
- We’ve covered Frequencies and amplitude. Along with phase and how constructive and destructive interference can cause waves to take on different shapes when we combine them. Those lessons will be important prerequisites for this lesson.
Today we’re going to cover wavelength, acoustics, and why your environment matters when working with audio.
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What exactly is wavelength, and how is it measured?
Wavelength and frequency are directly related. In fact, it's safe to say frequency and wavelength are one in the same thing. If you remember from our first lesson (What are frequencies) we discussed how Hertz or Hz is a measurement of Cycles per second. Low Frequency waves have less cycles per second than high frequency waves. Due to our cycle time being fixed to one second, this means that in order to fit more cycles into our one second measurement, we’ll need to shrink them down smaller and smaller the higher the frequencies go. Therefore we can conclude that a Low frequency will have a longer wavelength because its cycle happens fewer times per second. The inverse to this statement would be that the higher frequencies will be smaller or have a shorter wavelength because more cycles happen within the one second period.
With this information we can conclude that a high frequency has a shorter wavelength. This means that a low pitch is equal to a long wavelength, and a high pitch is equal to a short wavelength.
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What is the formula for calculating wavelength?
Wavelength (Lambda) is equal to a Constant that is divided by a frequency. But if we know that the Wavelength changes, and the Frequency changes. What would our constant be? Our constant is the Speed of Sound!
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How does the speed of sound affect wavelength?
The Speed of Sound travels at around 343 meters per second. 343 meters converted to feet is around 1125 feet per second. But even though the speed of sound is our mathematical constant, it actually changes based on the temperature of the air due to molecular density. This being the case, we simply round to the higher number of 1130 to play it safe by trying to avoid temperature fluctuations.
Example equations:
If we divide 1130 feet per second by 20Hz, we get a whopping 56.5 foot long wave!!! If we divide 1130 by 20,000kHz we get just about half an inch. These wavelengths are physically occurring in the air around us. Yes that's right, 20Hz is a physically manifested 56.5 foot long fluctuation of air pressure! 20,000Hz is merely the size of your thumbnail. The lowest frequency a human can hear is 56.5 feet long, and the highest frequency a human can hear is 0.6 of an inch. Wavelength isn't just an invisible thing; it's a physical change in the molecular density of the air around us.
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What role does wavelength play in room acoustics?
Now that we have an understanding of physical wavelengths, let’s get into why this is important for audio work.
I have a question for you, it may sound silly but it’s a serious question.
Why do we have recording studios? Why are they so special?
Well, you might say it's because they have all the recording gear like microphones and the mixing consoles. You would be right in that aspect, but a more precise answer would actually be:
- Because they are rooms built with acoustics in mind. -
While Audio is the science and art of sound as electrical energy. Acoustics is the science and art of sound energy in physical spaces. While audio and acoustics are identical in almost all of their aspects. Acoustics adds one element that no one can avoid, and not one person deals with an identical situation. The Room!
Even though every room is different, Acoustics is just as much an art as it is a science! You might be wondering why we went from wavelength to talking about Rooms. Well my friend, this is probably one of the most important concepts (in my opinion) about audio that the mass majority of the general public and beginners in audio don't understand or even know about.
Room Acoustics!
For Example, walk out into the center of your living room and simply clap your hands while focusing on listening to what you hear directly after you clap. Now go to your bathroom and do the same thing. Notice any changes? Now go outside and clap your hands in the open air. Go clap your hands in the car, and then back inside to your room and clap your hands there.
Did you notice any changes in the way your clap sounded, or how your clap interacted with the environment you were in?
What you’re experiencing is called “reflection” of sound. Sound moves at 1130 Feet per second and it bounces off hard surfaces like a bouncy ball that is traveling at 1130 feet per second! You can just imagine how chaotic that could be. In a studio situation, instead of our hands clapping, the speakers will be making all the noise and if you’re speakers are making noises that are bouncing around your room at 1130 fps and totally untamed, you could bet good money that you’ll have some issues if you don't try to control the reflections.
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How does wavelength influence the way sound behaves in small vs. large rooms?
Not every room is made equal. Some rooms are long and narrow, some are short and wide. Some rooms have tall ceilings, others are short. Some rooms might have windows while others only have a door and 4 walls. What does this have to do with Wavelength? Absolutely everything!
As I like to say - ”You can have a $1,000,000 dollar speaker in a $10 Room, or you can have a $10 speaker in a $1,000,000 Room. Which do you think would sound better?”
If you said the million dollar speaker in the $10 room, This lesson is for you.
If you said the $10 speaker would sound better in a million dollar room, you’ve already got the right mind set.
Just like a singer can have a beautiful voice, if you put them in the wrong environment it can sound just awful.
In a previous lesson we covered phase and polarity. We learned how the amplitudes of frequencies that are in phase are summed together, increasing the overall amplitude as the 2 waves combine. As well as what happens when they are 180 degrees out of phase, and how waves of the same frequency will cancel each other out resulting in no sound.
This doesn't just happen in the computer like I showed you. This happens in the physical world with the changing amplitudes in air pressure caused by sound waves. This means we can actually cause the summing amplitudes or induce phase cancelation depending on 1. where the speaker is placed 2. how long the room is and 3. what frequencies we are dealing with.
Take a look at this illustration of a speaker producing sound waves. The speaker is producing what is called the direct wave. Once the sound hits the wall, it immediately bounces back towards the speaker as a reflection.
The sound waves begin to cross and create ripples with all sorts of constructive and destructive interferences. When a reflected wave overlaps with the speaker's wave, they create what we call standing waves. Every Room has different standing waves because every room is built differently. Standing waves cause variations in amplitudes within the room based on the location of overlapping reflections. Standing waves dont change locations because the wavelength and the room’s dimensions never change sizes. A 20 Hz wave will always be 56.5 feet long, it’s simply physics.
Sound reflections bouncing off the walls at 1130 fps is the exact reason why studios are special. They are designed to get rid of these reflections and standing waves by means of acoustic treatment.
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The difference between soundproofing and acoustic treatment?
Sound Proofing, is the science of keeping sound from both entering and leaving a specific room. It is sound proof.
Acoustical treatment is the science of reducing reflections in a room. We can accomplish this by means of absorption and diffusion. This does not keep sound from entering or leaving the room. But it provides a more accurate listening environment. Meaning what you hear is not affected by your room.
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Why do certain wavelengths (frequencies) “build up” in certain spots in a room?
Standing waves are dependent on the dimensions of the room compared to the wavelength of the frequency you’re working with. Some lower frequencies just can't even be produced in smaller rooms. If your room is only 6 feet long, how do you expect to fit a 56.5 foot wave in it? I don't want this to discourage you at all because we still have tools and analyzers to help us see what we can't hear. Allowing us to compensate.
If your room was exactly 56.5 feet long, and your speaker was against the wall. You would be able to change how loud 20 Hz sounded based on changing your position relative to the waves amplitude. If you’re right in the middle, you’ll actually struggle to hear the frequency because of a near 0 amplitude between the positive and negative.
If you move to the center of either the positive or negative amplitude, the frequency would actually sound much louder and potentially exaggerated.
It’s best to find somewhere in between or around a 3rd.
If your room was half the size at only 25 feet long. We would be forced to stay in a high amplitude area because the wave needs physical space to develop and that's something we just don't have in this situation.
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How does wavelength affect acoustic treatment?
We’re now coming back to my main point and a bold claim. Why would I say “don't buy the black foam”? While I'm also a huge advocate for doing what you can with what you have. It’s also just as important to know what you’re doing before making a decision! That's what I'm here for.
When working with acoustic treatment, we have a rule of thumb. The acoustic panel should be at least ¼ the length of the wavelength you wish to treat. Let's break that down more.
If our panel needs to be at least 1/4th the length of a specific frequency, we can use this equation to find out how effective a specific depth of panel is at treating a specific frequency or wavelength.
If our foam Panel is 1.5 inches deep, converting that to feet, we’re looking at 0.125 feet. Since we are ¼ the depth of the wavelength, we can multiply 0.125 x 4 to find the full length of the wave which is 0.5 feet.
1130 / 0.5 = 2,260Hz meaning that this panel is effective in treating 2,260Hz and up ! This does not cover the full spectrum. And can often lead to a dull sounding room if you were to place these all over. This can alter factors in your room that would provoke you to make a decision you otherwise wouldn't have, because you aren't hearing the sound correctly.
Out of the whole audio spectrum, we are only covering the upper 3rd.
This means that all the high frequency information is being treated while the lower two thirds are left to chaotically bounce throughout your room. (I highly recommend watching the YouTube video for this lesson so you can actually hear the difference in frequency information).
Below is a snapshot of what my voice looks like in the frequency spectrum.
If you were to place these panels all over your room, it could potentially make your recording sound “boomy” or less “Shiny”.
There is less information being treated than there is untreated information
- Why is Acoustic treatment important?
Acoustic treatment prevents unwanted sound reflections from changing the way we hear sound when working with audio. Standing waves can cause the amplitude of varying frequencies to increase or decrease, based on the location we’re sitting at within the room. By adding acoustic treatment we can minimize the effects of standing wave interference. Thus providing a more accurate sound. -
How does wavelength affect Acoustic treatment?
The rule of thumb here is that a panel must be at least 1/4th the length of the wave in order to accurately absorb it. This means the deeper the panel, the lower the frequency the panel will handle. It’s important to be able to cover the entire audible spectrum.
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So what is wavelength?
Wavelength is the physical length of a frequency. It's calculated by the speed of sound divided by the frequency.
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What is Acoustics?
Acoustics is the science of how sound energy interacts with physical spaces. The size of the space will dictate how wavelengths develop and where standing waves form and alter how you hear things.