Science of Sound and Recording

Sounds• Sounds are nothing more than waves of air

pressure. We are able to hear sounds because our ears are sensitive to these waves of pressure.

Sound Waves

• A sound wave can be represented as a waveform which graphically describes the shape of the wave.

• Sound waves can be either:• Non-periodic

• Consists of a single wavefront that quickly dies out • An example is a sound wave created by a hand clap.

• Periodic• Created by the vibration of an object such as a guitar string or tuning fork

Non-periodic SoundThe easiest type of sound wave to understand is a non-periodic sound wave. An example is one that is created by a short, sudden event like a hand clap. When you clap your hands, the air that was between your hands is pushed aside increasing the air pressure in the space near your hands. This increase in pressure occurs because more air molecules are temporarily compressed into less space. The increased pressure pushes the air molecules outwards in all directions. When the pressure wave reaches your ear, it pushes on your eardrum causing you to hear the clap.

The image below shows the waveform for a typical hand clap. In the waveform, the horizontal axis represents time and the vertical axis represents pressure. The initial high pressure is followed by a low pressure. Notice that the oscillation from high to low pressure quickly dies out.

Periodic Sound

Another common type of sound wave is a periodic wave. A good example of a periodic wave occurs when you ring a bell. After the initial strike (which is a little like a hand clap), the sound comes from the vibration of the bell.

While the bell is ringing, it vibrates at a particular frequency (depending on the size and shape of the bell) causing the nearby air to vibrate with the same frequency. Waves of air pressure travel outwards from the bell.

Pressure waves from continuous vibration resemble the image shown:

Parts of a Sound Wave

Wavelength describes the horizontal length of one cycle of a wave or the distance from one peak or trough to the next, as shown.

Peak describes the highest point of a wave and trough describes the lowest point.

Amplitude describes the height of the wave or the maximum value of the wave function. The higher the amplitude, the louder the sound.

Pitch

Pitch is sometimes referred to as frequency. Pitch is a term that describes the number of waves that pass a point in one second and is the characteristic of sound that makes it sound high or low.

The image compares an AM radio wave, FM radio/television wave and a radar wave.

Intensity and Loudness

Intensity is measured in decibels (dB). The decibel scale represents how much a sound level varies from a reference point--the reference point being 0 dB.

While the intensity of a sound is a very objective quantity that can be measured, the loudness of a sound is more of a subjective response which will vary with a number of factors including age.

Amplitude & Frequency

• Amplitude is the size of the vibration, and this determines how loud the sound is.• important when balancing and controlling the loudness of sounds

• Frequency is the speed of the vibration, and this determines the pitch of the sound.• measured as the number of wave cycles that occur in one

second. The unit of frequency measurement is Hertz (Hz)• A frequency of 1 Hz means one wave cycle per second. A

frequency of 10 Hz means ten wave cycles per second,

How is Sound Recorded?

Analog recording is the first way that sound was able to be recorded. It was first used successfully by Thomas Edison. Modern examples of analog recording are:

•Vinyl disc (LP) •Magnetic tape •Reel-to-reel recorders •Compact cassette recorders

In 1957, Max Matthews of Bell Telephone Laboratories developed the process to digitally record sound via the computer. In digital recording, the analog signal of a sound is converted into a stream of discrete numbers that represent the changes in air pressure over time.

Digital Recording

Digital recordings are recordings that have been sampled so that they can be used by a digital computer.

Digital recording has many benefits over analog recording. These benefits include the ability to:

•Copy digital files as many times as you want without any loss of quality •Burn digital files to a CD •Share the files on the Internet •Edit the digital file (digital files are edited much more easily than analog tapes)

Recording Sound Digitally

The main device used in digital recording is an Analog-to-Digital Converter (ADC). The ADC captures a snapshot of the electric voltage on an audio line. This snapshot is then represented as a digital number that can be sent to a computer.

By capturing the voltage thousands of times per second, you can get a very good approximation to the original audio signal.

Each dot in the image shown below represents one audio sample.

Factors Affecting the Quality of a Digital Recording

There are two factors that determine the quality of a digital recording:

Sample rate

The rate at which the samples are captured or played back. Sample rate is measured in Hertz (Hz) or samples per second. An audio CD has a sample rate of 44,100 Hz which is often stated as 44 KHz for short.

Sample format or sample size

This is the number of digits in the digital representation of each sample. Basically, the sample rate describes the horizontal precision of the digital waveform. The sample format describes the vertical precision of the waveform. An audio CD has a precision of 16 bits. This corresponds to approximately five decimal digits. Higher sample sizes allow for more dynamic range. In other words, loud sounds are louder and soft sounds are softer.

Digitizing on the Computer

The computer that you are using has a soundcard. Your soundcard comes with an Analog-to-Digital Converter (ADC) for recording and a Digital-to-Analog Converter (DAC) for playing back the recorded audio.

You can also use a digital recording interface to connect via USB or firewire.

Digitizing on the Computer

• Your operating system communicates with the sound card to actually handle the recording and playback. Audio-editing software such as Audacity, Logic Pro, Cubase, communicates with your operating system to allow you to do things such as capture sounds to a file, edit the sounds and mix multiple tracks while playing the audio.

Problems with Digital Audio

Digital audio can create large files that quickly use up hard disk capacity. They also require a huge amount of bandwidth to transmit over a network.

Several things can be done to control the size of digital audio files including lowering the:

•Sampling rate (resulting in lower maximum frequency response). •Resolution (resulting in more noise and distortion being introduced).

There will always be a trade-off between file size and sound quality.

Pros and Cons of Digital Recording

• Pros – • Much easier for the average person.• Much less cumbersome• Will not deteriorate over time• Easily manipulated.

• Cons – • Technology and software changes rapidly.• Question over the quality and longevity of the computer audio

recordings.• Turn tables, tape machines, and other hardware devices are

still being used. Software trying to duplicate the warm natural sounds of the original devices.

Compression

The major driving factors behind the development of compressed audio formats are limited bandwidth and hard disk capacity.

Audio and electronics engineers have been working on the bandwidth issue by increasing bandwidth (the size of the pipe) and compressing data. High speed Internet connections have been developed to increase the size of the pipe and compression schemes such as JPEG and MPEG have been developed to "squeeze" more data through the pipe.

Lossy and Lossless Compression

LosslessLossless compression works by encoding repetitive pieces of information with symbols and equations that take up less space but that provide all the information needed to reconstruct an exact copy of the original.

Lossy and Lossless Compression

Lossy•Lossy compression works by discarding unnecessary and repetitive information (for example, sounds that most people cannot hear) and then applying lossless compression techniques for further file size reduction.•With lossy compression, there is always some loss of fidelity. The loss becomes more noticeable as the compression ratio is increased.•Generally speaking, most people will not be able to detect any differences at higher bit-rates and on good quality sound systems.

File Formats and File Types

The format of a digital audio file refers to the type of audio data within the file. The two main formats are PCM (Pulse Code Modulation) and Compressed Files.

PCM is the technique where each number in the digital audio file represents exactly one sample in the waveform. This is a non-compressed audio file.

Modern compressed audio files use sophisticated algorithms to represent the most important frequencies of the audio signal in a small amount of space. Examples include MP3, Ogg Vorbis, and WMA (Windows Media Audio).

The file type refers to the structure of the data within the file. It is common for the same format to be used by more than one file type. Examples discussed further include wave, MP3, MIDI, & WMA

File Types

WAVThe Microsoft WAV format is commonly used for computer audio on the Windows platform. It is a non-compressed format so the sound quality is very high. However, the file size is enormous compared to other audio formats. WAV files are not copy-protected. Because WAV format is so standard, many programs will change the digital audio file of music CDs into WAVE files before changing them into other file formats such as MP3 (a process known as "ripping"). WAV files are best used for sound effects

MIDIThe MIDI (Musical Instrument Digital Interface) is not really an audio recording. It is really just the instructions required for a sound card to play the music or song. MIDI files originate in electronic instruments like keyboards. Although MIDI files are very tiny, they are used mostly in original instrumental works, online greeting cards and video game soundtracks. Voice cannot be produced from a MIDI file.

File Types Contd.

MP3The MP3 format (Moving Pictures Expert Group, Audio Layer 3) was made famous by the legal battle between Napster and the Recording Industry Association of America. A 17 to 1 compression ration is commonly used in the MP3 format and at this ratio, very little sound quality is lost. This means that MP3 files can be 17 times smaller than the regular digital audio files on music CDs. This makes the MP3 format useful in transferring audio files between computers, portable digital devices and, of course, between people via the Internet. The MP3 format is not copy protected.

WMAThe WMA (Windows Media Audio ) format by Microsoft is very similar to the MP3 format with a few exceptions. It can store the same quality music as the MP3 format into an even smaller sized file. This is especially significant for people with portable digital devices. Another difference is that the WMA format is copy protected. This feature makes it attractive to individuals or companies that do not want their material traded for free over the Internet.