The physics of a great recording - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 3 Old 12-05-2019, 01:45 AM - Thread Starter
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The physics of a great recording

This concept has been bothering me all day. I want to know more about how sound travels, and how microphones actually respond to sound.

Let's assume the mic we're using in this thought experiment is an industry standard mic. Neumann, RODE, AKG. Etc.

2 rooms are treated for recording vocals with quality acoustic panels, foam, and diffusers.

-One is 15x20 ft and has exceptional treatment.
-The other is 30x25 ft and has 25% less room treatment than room 1.

Which recording will noticeably, and unanimously sound better if all other variables are identical?
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post #2 of 3 Old 12-05-2019, 07:34 AM
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Originally Posted by Bearcat42 View Post
This concept has been bothering me all day. I want to know more about how sound travels, and how microphones actually respond to sound.

Let's assume the mic we're using in this thought experiment is an industry standard mic. Neumann, RODE, AKG. Etc.

2 rooms are treated for recording vocals with quality acoustic panels, foam, and diffusers.

-One is 15x20 ft and has exceptional treatment.
-The other is 30x25 ft and has 25% less room treatment than room 1.

Which recording will noticeably, and unanimously sound better if all other variables are identical?

Sound is a pressure wave. Sound is produced when something vibrates. The vibrating body causes the medium (air, water, etc.) around it to vibrate. Vibrations in air are called traveling longitudinal waves, which we can hear. Sound waves consist of areas of high and low pressure called compressions and rarefactions, respectively. Shown in the diagram below is a traveling wave. The shaded bar above it represents the varying pressure of the wave. Lighter areas are low pressure (rarefactions) and darker areas are high pressure (compressions). One wavelength of the wave is highlighted in red. This pattern repeats indefinitely until it’s energy has dissipated. The wavelength and the speed of the wave determine the pitch, or frequency of the sound. Wavelength, frequency, and speed are related by the equation speed = frequency * wavelength. Since sound travels at 1130 feet per second at standard temperature and pressure (STP), speed is a constant. Thus, frequency is determined by speed / wavelength. The longer the wavelength, the lower the pitch. The 'height' of the wave is its amplitude. The amplitude determines how loud a sound will be. Greater amplitude means the sound will be louder. Additionally, as sound rises in frequency, it becomes more directional. Lower frequency sounds with longer wavelengths become more omni directional as frequency decreases. Higher frequency sounds (with shorter wavelengths) contain less energy and will dissipate more quickly than lower frequency sounds.


With regards to microphones, I'll provide a generic description of how they work, so we don't get too far off in the weeds here. Microphones, while they don't look anything like speakers, are basically tiny speakers in reverse. That is, a small, low mass diaphragm is connected to a coil surrounded by a magnetic field. When sound waves hit the diaphragm, it vibrates the coil in the magnetic field, which produces electrical current. This signal is extremely weak, on the order of 0.001 - 0.1V. This signal is then amplified by a device called a preamplifier which brings the signal strength up to a level that can be used by the electronics in a mixing console for processing.

There are many different types of microphones, but the three most commonly used in recording are dynamics, condensers and ribbons. I won't go into detail about the differences, other than dynamic mics are passive devices - which is to say that they don't require a power source to function. Condenser mics use phantom power (48 volts provided by the mic preamplifier or an external phantom power supply to charge a capacitor) and most ribbon mics are passive devices, although some modern ribbons also use phantom power. The other difference worth mentioning here, is that different microphone designs have different pickup or directional characteristics. The most common are cardioid, omni and figure eight. There are variations on these patterns, but these are the most common (thumbnails attached below). One important thing to note is that microphones are dumb. They don't know what to pick up. They can't exercise selective attention like humans can. They only respond to pressure, so they will pick up any and all sounds that create pressure on the diaphragm.

With regards to your question about which recording would sound better, there is no correct answer. There are simply too many variables to draw a
definitive conclusion. Some of these variables include the quality of the source, the acoustic characteristics of the space that the recording is made in, the type of microphone in use, the directional characteristics of the microphone in use, the other associated equipment in the signal path (mic preamps, EQ, compression, analog to digital converters, etc), the genre of music (if it is music) that's being recorded and of course, the technical and creative decision making of the people creating the recording. If everything other than the room was theoretically identical, then the room itself would be the only difference in the two recordings. The question then becomes "which room sounds better?"

For this discussion, let's assume that we're going for the simplest signal path possible. which would be:

1. Source
2. Mic
3. Mic preamplifier (we'll assume the most linear and transparent available)
4. Capture device (likely some digital recorder in this day and age / analog to digital converters included here)

Depending on the type of microphone used, the directional characteristics of the mic will pick up more or less of the ambient sound of the room in which the recording is taking place. Also, the distance of the microphone from the source will drastically change the ratio of direct / reflected sound at the microphone. Directional microphones also exhibit what's known as proximity effect - which is an increase in lower frequency response the closer they are placed to a source. All of these variables come into play before the signal ever reaches the recording device.

These variables are used to great effect depending on the purpose of the recording. For example, audio books are recorded using close micing (within a foot or so of the source) in acoustically dead spaces. (small, non reflective rooms known as iso booths). The result is a recording free of ambient room sound, and reflections that would cause the audio to sound more distant. Compare that to an orchestral recording, which is mic'd from greater distance to pick up the scale of the orchestra in the performance space. Imagine listening to an opera singer, recorded in an iso booth like an audio book narrator would be. On second thought, don't imagine that. It hurts just thinking about it.


I'm currently in the early stages of a long term recording project involving a handful of world class guitarists, playing hand made, one of a kind, stupid expensive acoustic and acoustic / electric guitars. In order to create a baseline in the studio I'm working in, I spent an afternoon recording one guitarist playing a few different guitars using multiple stereo microphone arrays simultaneously. I set up 5 different stereo mic configurations and recorded each piece with each stereo mic array recorded to a pair of tracks (left and right for each array). The guitarist looked like some sci-fi test subject surrounded by all the mics and stands. On playback, this gives me the ability to switch between mic arrays, in real time, on the exact same performance to make detailed notes on the differences in the results.. I can say that while all of the microphone arrays sound different, which one sounds best, is a matter of personal preference. Barring obvious technical errors, there is no conclusive way to say that one recording sounds best. I could go on for days about all of this, but I'll spare everyone the tedium. I do hope that this addressed your questions to some satisfaction.
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Last edited by garygreyh; 12-05-2019 at 11:55 AM.
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post #3 of 3 Old 12-06-2019, 12:25 AM
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The answer depends on the recording. It isn't unusual for recordings in he past to be done in certain rooms because the acoustics sounded good. In fact, there are some studios which have nothing but speakers and microphones and the sole purpose is to play back the recording on the speakers while recording how the room affects it.

So either room could sound better depending on the artist, composer, and performers feel make their work sound good.

And sometimes things are such that serendipity strikes such that if you left the studio door open and let the audio reverberate in the adjoining hallway, it adds that extra something to the sound.

It's music, and creating it can require breaking all sorts of rules to communicate the feelings the artist desires.

Think of it this way - Moog synthesizers have a filter called the Moog ladder filter. It's famous, but in reality a terrible low pass filter because at certain configurations, it will self oscillate, at others it will boost frequencies near cutoff. But it's famous because when creating the sound, it makes it sound more musical. It's the antithesis to good sound reproduction - you break all sorts of rules and use all sorts of distortion devices to capture the sound.

So you'll find one room is better for some music, while the other room is better for other music. You would be best to make sure each has a distinct sound so musicians can pick their desired sound.
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