Characteristics of Sound waves in venues & broadcast

Sound waves

There is nothing more fundamental to audio reinforcement and mixing than the concept and properties of sound waves. In order to fully empower your approach and control of a mix, you must focus on refining your understanding of sound waves.

What sound waves are and how they are produced

Sound waves are created by vibrations and disturbances in materials which then causes compression and rarefaction in the surrounding environment. A guitar string which is plucked causes vibrations in the strings, which carry into the body of the guitar, causing air to compress in sync with the strings, translating the audio from a vibration to a form of air pressure. The vibrations and air pressure waves create identical patterns which, in their purest form, have the form of sine waves.

Sound waves have two primary characteristics, amplitude and frequency. Literally all of audio engineering rests on these two concepts. Amplitude is the height of the wave from peak to x-axis. The frequency is the number of times a full wavelength will occur in 1 second. If a wave has frequency of 20 Hz, then it will repeat 20 times in a second. Sound travels at roughly 1125 feet per second. If you divide the distance sound travels in a second by the frequency, you can get information on how long the wavelength of sound is as it travels through the air. For a 20 Hz sound wave, 1125 ft/20 ~ 56.25ft per wavelength. 

Having an understanding of the “length” of a soundwave helps with the next concept, how sound waves interact with their environments. Low frequencies have very long wavelengths as they travel through the air, which causes them to wrap around objects and morph and bend as they interact with obstacles. They can also create standing waves against walls. The higher the frequency, the smaller the wavelength. For instance, a 1k Hz signal will have length of 1125/1000~ 1.125ft. This is a much shorter wavelength than 20 Hz and will tend not to wrap around objects when it collides with them. It will simply reflect and be absorbed to some extent. 

The material which the sound waves impact determines the way in which they are absorbed and reflect. Hard surfaces, like stone, tend to reflect sound waves as they don’t easily absorb the energy in their dense and rigid composition. Materials which are flexible in nature allow for the energy of sound to be easily translated to movement in the material itself which, due to friction in the atoms, causes heat and dissipates the sonic energy. Because lower frequencies have longer wavelengths, more sound deading material is typically needed to capture the energy from wavelengths that “take up more space” in air. This is a somewhat informal approach and perspective, but it gets to the fundamental idea of wavelength and frequency. Typically foam pads are used for acoustic deadening applications, but there are also Mass loaded vinyl products which have higher density than traditional foam pads while maintaining flexibility to easily translate sound pressure to kinetic energy and then to thermal energy. MLV can be used to greatly reduce sound waves from around 125 Hz to 4k Hz. It is important to remember that literally everything has a dampening effect, to some degree, on sound. Even air itself resists and absorbs sonic energy to some extent, causing sound to “fall off” as energy is used up. Traveling sonic waves move new air particles as they progress through the air, exhausting energy and dampening wave amplitude as it extends through the air until you can’t hear the sound.

As mentioned before, low frequency sounds can cause standing waves, especially in what we call resonate frequencies. Standing waves are characteristic of low frequencies hitting walls with poor sound dampening treatment. The waveform bounces of the wall and, while colliding with the incoming sound waves, can cause an increase in volume and sound pressure against the back walls. In mid-sized rooms you can emit a single frequency with a tone generator and actually walk around and experience the maximum points(antinodes)  of the standing wave and other points where it is strangley quiet (nodes). This can be very dangerous at high SPLs, such as in rock concerts, where you may already be at 120 dBs but against a back wall, the low frequencies can become dangerously intense, interfering with your chest and breathing. For this reason, all audio engineers working at high dBs with strong bass in their mix must pay special attention to measuring dBs against backwalls, and using spectrum analyzers to identify troublesome frequencies that need to be addressed.

Resonant frequencies are simply frequencies where standing waves naturally arise in speaker or bass units themselves and in rooms. They are particular frequencies that require treatment to achieve a flat response in your system.

The importance of soundproofing and deadening a space

Soundproofing a room helps to isolate exterior noises from your room or venue. This is important in getting an accurate signal into your ears and being able to enjoy sound without  competing noise. The louder the noise, the lounder the overall sound level needs to be to reduce the perceived noise in the mix. The louder the mix, the less you can listen to it without causing permanent hearing damage. Even at only 90 dBs.

Incorporating audio deadening in a space helps to reduce unwanted reverberations, standing waves, artifacts and copies of undesirable sounds within the space. In a perfectly tuned room, the sound is clearly disseminated from a well designed audio system. The reverberations and echos heard in the room are generated by the audio tech in the mix and are controlled, allowing the audience to only hear what they are supposed to hear one time as intended, not reverberations and reflections off walls that muddy the original sound.

However, with this level of sound dampening and tuning comes the reality that nothing is covering up your mix. If you mix poorly, no reverberations, reflections, or resonant frequencies will cover it up. Everything will be heard as it truly is. This can be startling to novice engineers and a welcome pleasure to seasoned technicians.

Comb Filtering

Another aspect of reflections in sound is the concept of comb filtering. Comb filtering is simply the way in which audio waves of similar frequencies combine or subtract energy in their amplitudes to cause an overall perceived sound which is a combination of the original sounds.

Take a look at the following green sine wave which demonstrates how sound two sound waves combine as the “phase” of the waveforms is shifted.

Phase has to do with the placement of the wavelength on the axis of time compared to other waveforms. Manipulating phase intentionally can lead to widening perceived stereo imaging in mixes, can alter tone and timbre, and can even give the perception of the center of the mixing moving between your ears.

Phase Issues with two mics

One extremely important aspect of phase is when you use two or more mics to pickup the same source. You can often hear phase issues in interviews where two individuals are wearing mics and they overlap in their pickup range. You will hear the primary presenter’s mic catch the audio signal first, and then mere milliseconds later a second signal comes through the other participants mic, causing “phasing issues” as you have nearly identical signals out of phase, causing comb filtering and duplicates in transients. It can cause quite the problem in studio recordings and broadcasting! The other occurrence is when you double mic a kick drum or snare from two sides of the drum head. If you do not invert the phase of one signal, then what you get is two signals with mostly identical sonic shape, but 180 degrees out of phase. They will nearly cancel out and only a shallow outline of the sound will be left. You essentially have created this resulting comb filtering by trying to get a better, more dynamic sound with two microphones!

Many sound engineers without training in waveforms, experience, or hundreds of hours of learning from the pros will lack this understanding. It is vitally important to mixes, especially broadcast feeds. Comparing an A/B mix with headphones of two mics on a kick drum, where you invert the phase of one mic, will show the remarkable difference. You will never go back. The reason for this inversion is that when the drum head is hit, one side compresses the sound waves while the other causes rarefaction. This causes the sound waves to be opposite.

Inverting Phase in monitors

Another application of phasing is intentionally reversing polarity in floor wedge monitors in troublesome signals. I am often able to nearly double the output of my choir mics when I used this approach. Choir mics and floor monitors don’t play well, but if you invert the monitor wedge single, then the majority of what was picked up by the mics from the wedges would cancel in the house mix rather than combine and cause feedback. Since the choir members couldn’t tell the signal polarity was changed, they sung louder because of the confidence that loud background music   fom the monitors brings, and thus more signal for the choir mics.

Conversion to electrical signals

Did you know that since the beginning of time, sound has been translated to electrical signals? It’s occurred naturally in nature through the ears and how they take sonic waves and translate them to electric signals in the brain. In mixing, air pressure is captured with a ribbon or coil that vibrates with the air. The vibrations in the magnetic coil cause an electrical signal from the variation in the magnetic flux field. This signal is naturally an alternating current, AC. It is modeled as a sine wave. This sine wave is then transported over wires to a mixer and is then processed. In digital boards, there is a conversion that takes place, from AC to a digital signal made up of bytes. The quality of the signal converter, bit depth and sampling features of your digital mixer thus are very important to helping maintain the accuracy and clarity of your original signal. This is also why many audio enthusiasts prefer mixing with high quality analog equipment in the studio.