Acoustics 101


Please Share

Sound level

The term most often used in measuring the magnitude of sound is the Sound Pressure Level (SPL), which provides a measure of the intensity of a sound and is measured in decibels (dB). The logarithmic decibel scale is used because the human ear is sensitive to an extremely wide range of intensities – from the loudest (1 watt/m2) to the quietest (10-12 watt/m2 = 0 dB). On the decibel scale a 10 dB increase in the SPL represents a 10-times increase in sound intensity, 20 dB a 100-times increase, and 30 dB a 1000-times increase in sound intensity.

Pitch and frequency

Like light, sound travels in waves. Sound waves with high frequencies (short wavelength) produce high-pitched sounds (e.g. a baby’s cry, a flute), while sound waves with low frequencies (long wavelengths) produce low-pitched sounds (e.g. baritone, thunder, drum beats). On a piano, while facing the piano keyboard, the last key on the far left would be the lowest pitch and the one on the far right the highest pitch.

 

Like most sounds, human voices, shouts, cries, screams, etc., comprise many different frequencies. The Figure below shows the frequency spectrum of a typical female scream.

figure1

Every person’s voice has a unique frequency spectrum and is one of several voice attributes that allows us to tell one person’s voice from another.

The Figure below shows the frequency spectrum of a typical male scream.

Figure2

There are clear differences between the male and female profiles.

Most significantly, the female profile peaks at a higher frequency than the male profile, and decreases more rapidly thereafter. There are therefore noticeable differences in the pitch and in the variation of the pitch.

Measuring sound

The human ear does not have the same sensitivity to all frequencies in a spectrum. In younger people it is sensitive to frequencies between 20 Hz to a maximum of around 22 000 Hz (it drops down to about 20 000 Hz with age). However, largely due to resonance of the ear canal, the human ear is most sensitive to frequencies between 2000 and 5000 Hz. It is particularly insensitive to frequencies below 100 Hz. This means, for example, that a 50 dB, 2000 Hz sound will be perceived as significantly louder than a 50 dB, 100 Hz sound, which will be barely audible.

 

Therefore, a straight dB measurement, where all frequency ranges are treated the same, is not a reliable indication of the sound level (loudness) the human ear will hear. One way around this problem is to measure sound using a filter to make adjustments to the dB levels of different frequency ranges in order to most closely mimic the human ear’s sensitivities to different frequencies. The A-weighted filter is the most commonly used. The A-weighted filter emphasises the dB levels of the higher frequency ranges to which the human ear is most sensitive, while reducing the dB levels of low frequencies to which the ear is most insensitive. The aim is to imitate as closely as possible what the human ear hears. The result of this filtering process is the sound level measured in dBA (A-weighted decibels).

 

In Figure 3 below the blue line represents the frequency spectrum for night-time ambient noise in a typical suburban residential area, away from a traffic noise source (e.g. a major freeway). The total sound pressure level for this profile is 49 dB. However, the ambient noise has a lot of its energy concentrated in the lower frequencies to which the human ear is not very sensitive. The red line represents the result of the A-filter and is more representative of what the human ear will hear. The total A-adjusted noise level is 35 dBA.

 

Figure3

Sound level meters are programmed to perform this filtering automatically if required. You can set a sound level meter to measure the dB level or the dBA level of a sound.

 

The Figure below shows the the A-adjusted noise levels (dBA) of different types of sounds:

Figure4


Please Share