In this article we will cover:
- Sound basics – sound wave behavior, frequency, pitch and loudness
- Sound waves interactions – reflection, transmission, diffraction
- Reverberations and diffusion
Understanding how sound behaves in our podcasting studio is an essential first step to appreciate the value and importance of soundproofing and implementing acoustical treatment. Both will improve your audio quality, make you sound like a professional and help to build a larger audience.
What is our voice and how is it transmitted?
Sound is a wave – it travels from its source of origin in all directions. A sound wave is a disturbance that travels through a medium, transporting energy from one place to another. Of course, the main medium is the air but since we can hear often outside noises we know that walls, steel, windows, and other objects are also perfectly capable of transporting sound.
We call sound a mechanical wave because it is transported through a medium by the particle to particle interaction. Hence we don’t hear sound in a vacuum as there aren’t enough particles to interact.
Sound waves are generated by a vibrating object. We are generating it with our vocal cords by creating a vibration of air particles. The air particles are vibrating back and forth at a certain frequency. The frequency of a wave refers to how often the particles of the medium vibrate when a wave passes through the medium.
- Humans are capable of detecting sound waves within a wide range of frequencies, between 20 Hz and 20,000 Hz.
- Inaudible sound with a frequency below 20 Hz is called infrasound.
- Inaudible sound with a frequency above 20,000 Hz is called an ultrasound.
- A typical adult male will have a voice frequency from 85 to 180 Hz.
- A typical adult female will have a voice frequency from 165 to 255 Hz.
Pitch of sound
The sensation of a frequency is referred to as the pitch.
- High-frequency sound wave is associated with a high pitch
- Low-frequency sound wave is associated with a low pitch
A rise in frequency causes a sensation of rising pitch and vice versa. If we are not trained professionals our ability to differentiate two sounds that are close in frequency will decrease in the upper and lower parts of the audible frequency range.
On this Wikipedia page, you can see examples of sounds at various frequencies from 32 Hz to 16,000 Hz and feel the sensation of pitch.
Loudness of voice
Since the human ear is more sensitive to some frequencies than to others, the loudness will be affected by frequency.
- If a sound is loud, it has a high intensity. If a sound is soft, it has a low intensity.
- An increase in intensity will cause a sensation of increased loudness.
- Loudness does not increase in direct proportion to intensity.
- Loudness doubles with each increase of 10 dB in intensity. For example, a sound of 30 dB has ten times the intensity of a sound of 20 dB but is only twice as loud.
How sound waves interact with surrounding in podcasting studio – reflection, transmission, diffraction
When recording a podcast your vocal cords emit sound waves. A sound wave then spreads through the air and reaches an obstacle. At this moment a portion of the wave undergoes reflection and a portion of the wave undergoes transmission through the obstacle.
Sound can also undergo diffraction around the obstacle, which involves a change in direction. That is why we are trying to isolate our recording location (locked doors) so the sound from around corners or through open doors won’t reach the microphone.
Reflection of sound waves off of surfaces can lead to reverberation in your podcasting studio
Actually, there are two phenomenons which can happen – an echo or a reverberation. Echoes occur in larger areas with distance from the source of sound to reflecting surface is c.a. 56 feet (17 meters) or more, as then reflected sound reaches us after 0.1 seconds, which is enough for the brain to drop the sound from memory. In this case, we hear reflected sound like a second sound. For simplicity, we can assume echo doesn’t affect us.
Reverberations on the opposite, are affecting us constantly, as most of our rooms have smaller dimensions so the reflected sounds reach us before 0.1 seconds, and create an effect of prolonging the first sound.
Why reverberations are common in small rooms?
Reverberations are common in rooms with dimensions of approximately 56 feet (17 meters) or less.
Since sound waves travel at about 340 m/s at room temperature, it will take approximately 0.1 seconds for a sound to travel the length of a 17-meter room and back, thus causing a reverberation.
- As velocity (v) equals distance (d) divided by time (t), then time (t) equals distance (d) divided by velocity (v).
- t = d/v = (34 m)/(340 m/s) = 0.1 s
Reverberation is the result of sound waves reflecting off of many objects or surfaces in the environment. To minimize the level of captured reverberations use acoustic shielding and a good dynamic microphone (to make a correct choice read our Choosing a Microphone for Podcasting – Buyers Guide).
How reverberations impact the recording of your podcast?
When you speak in your studio to your microphone some of the sounds are transmitted through or absorbed by walls of various objects in the room. The rest is reflected, bouncing off walls, ceiling, floor and most of the hard and smooth surfaces
What can we record in our DAW when working on a podcast episode?
- Direct sound – this is the strong signal captured as the sound wave travels in a straight line from your mouth to your microphone
- Primary / early reflections – are the first few instances of reflected sound. They got reflected and traveled to the microphone by the shortest path.
- Secondary/high order reflections – come after primary reflections and can be potentially quite loud if you are recording in an unprepared location. This is a result of sound waves bouncing off multiple times of multiple surfaces. As a result, they can stack up and create intense background noise. The strongest high order reflections are created in the trihedral and dihedral corners of your location. Those secondary reflections due to stacking are more numerous and closer together.
- Reverberations – for a sound that gets generated only once the reverberations eventually disappear as the sound wave carrying them loses its energy. However, when you are recording a fluent speech you are constantly generating new sound waves which ensure you will always struggle with a level of reverberations.
The strength and example impact of the described phenomena is shown in this diagram. You should monitor reverberations during your recording session using good closed-back headphones. We have reviewed available headphones and collected our recommendations in this buyers guide on podcasting headphones.
How walls and ceiling increase reverb on your podcast?
Trihedral corners of your room are where 3 surfaces meet – 2 walls and a ceiling or a floor.
Dihedral corners of your room are where 2 surfaces meet – wall & wall, wall & ceiling and wall & floor.
Both types of corners are places where sound gest trapped. It can bounce there multiple times, amplify the sound wave and come back to your mic as a noisy reverb.
That is why when you start acoustic treatment of your podcasting room, you should in the first place install bass traps in those places.
Rough walls vs. smooth surfaces – effects in your podcasting studio?
As we wrote before a sound wave is a disturbance that travels through a medium. When a sound wave reaches an obstacle it can be reflected or transmitted through the obstacle. The amount of both effects depends on how dissimilar is the obstacle from the medium in which the sound wave was traveling. This results in:
- Hard materials, concrete or brick walls, ceilings, hardened wooden floors – they are as dissimilar to the air as can be. Upon contact, most of the sound wave is reflected from them and very little gets absorbed. This means we want those hard surfaces shielding us from outside noises (neighbors, traffic sound, nature, etc.) but inside of our recording studio, those walls will damage our audio quality.
- Drywall/plaster wall – is much softer than concrete or brick walls. They are made largely of gypsum and are very common nowadays because they are inexpensive and relatively easy to apply. They will leak sound into your recording studio but they will also transmit some of the sound waves you generate and partially limit the reflections and reverb.
- Fiberglass and acoustic tiles – walls and ceiling of the professional recording studio are laid out with soft material which strongly absorbs the majority of reflections. This is also an ideal solution available for home studios.
What is the recommended solution for the least amount of reverberations?
- Record in a room with hard walls protecting you from external noises,
- If you have drywall add another layer of drywall, filling the wall studs with cheap fiberglass,
- Address potential reflections with the necessary amount of acoustic foam bass traps.
What is sound diffusion?
Diffusion is the efficiency of the environment by which sound waves and more importantly their reflections are distributed evenly.
- If you would have achieved a very high level of sound absorption then in your room and on the recording you would experience an uncomfortable feeling of “dead” silence,
- Diffusion is important especially in large musical aulas and concert halls where the goal is for the sound to be audible in the same way in all places around the location.
Why do you need diffusion in your podcasting studio?
You need a level of diffusion in your podcasting studio so the recorded audio will sound more natural.
Professionally it is achieved by:
- Mounting special acoustic diffusion panel on the walls – those uneven surfaces diffuse sound more randomly and evenly in all directions,
- Installing your foam acoustic panels alternately with leaving parts your wall naked – this solution allows you to absorb a large portion of sound waves but simultaneously allows also for some of it to be spread around the room.
The proper response to address sound behavior is to evaluate your podcast recording room correctly and understand how much of soundproofing and acoustical treatment it needs.
If you want to find out more and start implementing both soundproofing and acoustical treatment as a means to achieve the superior audio quality we recommend our detailed article with guidelines: Silent home podcast studio – detailed and easy soundproofing guide
If you are recording remotely and can’t perform the correct soundproofing and acoustical treatment there are still steps you can take to improve the conditions in your hotel room (or other temporary location). Recommended steps we have summarized in this article: 13 practical Podcasting Remotely guidelines for improving audio quality.