Acoustics in Twenty Words or Less

What is Acoustics? The Science of Sound!

What is Sound? Minute changes in air pressure Longitudinal waves traveling outwards from their source. These waves consist of alternating areas of high and low pressure (compression and rarefaction) Sound travels at 1130 feet per second (345 meters per second or 770 miles per hour) in dry air at room temperature. As temperature increases, so does the speed of sound. Sound can reflect off of some surfaces and be absorbed or blocked by others. This behaviour is completely predictable! Sound has two major components - frequency and intensity - which happen over time

What is frequency? Harmonic waveforms have a repetitive cycle or pattern (called a period) Frequency refers to the number of times this pattern is repeated over a certain length of time Frequency is generally measured in Hertz (Hz) - cycles (repetitions) per second. Frequency gives us Pitch and Timbre

Pitch The “highness” or “lowness” of a sound Pitch is how our brain interprets the “fundamental frequency,” usually the lowest frequency in the spectrum of a sound. Higher frequency generally means higher pitch Doubling the frequency gives us the musical interval of an octave Humans interpret frequency logarithmically: the distance between 200 and 400 Hz sounds the same as the distance between 2000 and 4000 Hz.

Timbre Almost every sound is made up of a combination of different individual “pitches” (frequencies) which are happening simultaneously These multiple frequencies are referred to as partials, harmonics or overtones. Sine wave is made of 1 pure frequency (no overtones) This combination is often referred to as a “frequency spectrum” (white light/prism analogy) Each instrument and voice has a unique frequency spectrum which our ears interpret as “timbre” (i.e. the spectrum of a violin is different than the spectrum of a flute) “the quality of a sound that that distinguishes it from other sounds of the same pitch and volume” – Bruce Swedien

Intensity = loudness or volume

Intensity - Sound Pressure Level The intensity of a sound is directly related to the amount of pressure it generates (measured in Watts per meter 2 ). Higher pressure = more intense = “louder” Not all frequencies sound equally loud to us, even though the actual intensity may be the same. Our ears are much more sensitive in the range between 100 Hz and 10 kHz and especially so between about 1000 and 5000 Hz. The “loudness” of sound is expressed in decibels (dB SPL).

Analyzing an instrument’s intensity over time A instrument’s sound can be analyzed by how it behaves over time. Attack - the onset of the sound, sometimes called an attack transient Decay - after the initial transient, there is often a drop-off in the amount of energy in the sound. Sustain - After the energy from the attack is spent, the instrument may ring on for some time Release - the behaviour of the instrument after the note is no longer being held by the musician These terms are derived from synthesis and are not always easily applicable to acoustic instruments, but can be useful if one would attempt to duplicate the sound electronically

A bit about the decibel A decibel is a logarithmic ratio. Ratio: “a relationship between the amounts or sizes of two things” - Webster’s II New Riverside Dictionary. Houghton Mifflin Co Zero dB means that there is no difference between the levels that are being compared - not that there are no levels. This type of ratio is used to compare many things - voltages, volume units, etc. including sound pressure levels With SPL, 0 dB = the threshold of human hearing - the softest sound the average human can hear at 1000 Hz (measured as W/m 2). The intensity of every sound is measured against this specific intensity and the ratio is expressed in dB SPL.

Turning Sound Waves into Brain Waves The ear is a transducer Sound waves reflect off of the outer ear (pinna) into the ear canal. They set the ear drum into motion which is attached to three small bones - sound energy is transformed into mechanical energy. These three bones transmit their motion to fluid in the cochlea. Thousands of tiny hair cells, which are hard wired to the brain, pick up the fluid motion. The response of these hair cells is frequency dependant. Their location along the cochlea determines their frequency response. Depending on where in the cochlea the waves are strongest, those hair cells will transmit the strongest signal to the brain.

The Human Ear

How loud is too loud? Level and duration of exposure are related. As SPL increases from 85 dB(A-weighted) in 3 dB increments, safe exposure time is cut in half. “Exposure to 115 dB for any duration may pose a serious health risk.” - HEI If you must raise your voice to be heard in normal conversation, you may be risking permanent hearing loss.

Save your ears! Damage to your ears is permanent. There is no cure for hearing loss!!!! Avoid excessive exposure Wear ear plugs Take 15 minute “quiet breaks” every hour or two Have your hearing checked annually

NIOSH-Recommended SPL Exposure Guidelines Level (dB(A) SPL)Maximum Exposure 85 dB8 hrs. 88 dB4 hrs. 91 dB2 hrs. 94 dB1 hr. 97 dB1/2 hr. 100 dB15 minutes 103 dB7.5 minutes 106 dB3.8 Minutes Source: House Ear Institute

Your room as an instrument Room Resonance Speed of sound = frequency * wavelength Since the speed of sound is constant, as frequency increases wavelength must decrease (and vice versa) The dimensions of your environment have a large impact on the sound Those frequencies whose wavelengths are exact multiples of the dimensions of the room are known as room resonances or room modes