Sound Pressure, Frequency and Density

Our primary goal in understanding glazing acoustic properties is to evaluate the influence of sound on occupants within a building and to establish conditions in which negative influences may be prevented or reduced. Such negative influences, in this context, are referred to as noise.

In its broader definition, noise describes any unwanted and possibly harmful sound. While subjective and based on individual preferences, legislation often considers any sound produced by humans and their device as noise and aim to control its quantity. When harmful, physiological effects (hearing impairment, sleep disturbance, hypertension, etc.) and psychological effects (stress, aggression and other behaviours) are well documented.

Sound Pressure and Frequency

We perceive sound and potential noise within our eardrums. Audible hearing frequencies lie in the range of about 16 to 20,000 vibrations per second, specified in Hertz (Hz) and determine the pitch the listener perceives. This covers an extremely high range of sound pressures, with the perception of loudness dependent on a combination of sound pressure and frequency. Our eardrums harness this change in pressure, and the ear’s neuro-acoustic system transforms it into a sound sensation.

• Sound pressure, expressed in Pascal, or more generally, the level of sound pressure, expressed in decibels (dB)

• Frequency depends on the duration of a complete vibration; the higher the frequency, the more high-pitched the sound. Low frequencies, below 300 Hz, medium frequencies, between 300 and 1,200 Hz and high frequencies, above 1,200 Hz.

In practice, given the range of sound pressure is too great and the relationship is not linear, we convert sound pressure to acoustic pressure, expressed in decibels (dB) and ranging between 0 – 200 dB. Maximum acoustic pressure levels for comfort, depending on the type of area or activity performed, range from 20 – 55 dB.

Effect	Example	Sound pressure p (Pa)	Acoustic pressure Lp(dB)
Blackout		200,000	200
			190
		20,000	180
			170
		20,00	160
			150
Pain Threshold		200	140
	Aircraft engine		130
	Klaxon	20	120
	Lawnmower		110
	Metro train arriving	2	100
	Large orchestra		90
	Heavy traffic	0.2	80
	Busy street		70
	Loud voices	0.02	60
	Quiet apartment		50
	Normal voices	0.002	40
	Quite in the mountains		30
	Whispers	0.0002	20
	Silence in the desert		10
Hearing threshold	Total silence	0.00002	0

Area	Level of acoustic pressure (dB)
Bedrooms, libraries	20 - 30
Apartments, living areas	20 - 40
Schools	25 - 40
Cinemas and conference rooms	30 - 40
Individual offices	30 - 45
Shared office	40 - 50
Typing room, large shops, restaurants	45 - 55

In reality, the sounds we hear are not made up of repeated frequency cycles and identical pressure levels but of different frequencies and sound pressures superimposed on each other, which create a continuous spectrum containing all the frequencies.

Density

When we refer to the acoustic performance of an insulating material and strategy to produce a high-performance building envelope, we generally refer to its density. Measured in kg/m³, while defined as the quotient of the mass of the material and the volume occupied by its mass, density is critical for both thermal and acoustic performance. A low density generally also implies a high porosity or a high volume of voids, which leads to a decrease in thermal conductivity.