Exploring Common DIY Terminology - SPL (Sound Pressure Level)

SPL (Sound Pressure Level)

SPL stands for sound pressure level, and it is a measure of the sound intensity or loudness at a specific location relative to a reference level. It is commonly used to quantify and describe the loudness of sound in various audio applications, including loudspeaker design, room acoustics, and sound engineering. Sound pressure level is expressed in decibels (dB) and is calculated using the following formula:

SPL (in dB) = 20 * log₁₀(P / P_ref)

where:

P is the measured sound pressure at a specific location.
P_ref is the reference sound pressure, which is usually set to 20 micropascals (μPa) for airborne sound (this is the threshold of human hearing at 1 kHz).

Key Points about SPL:

• Relative Measurement: SPL is a relative measurement that compares the sound pressure level at a specific location to the reference sound pressure. It quantifies the difference in pressure between the measured sound and the reference sound, providing a standardized way to describe the loudness.
• Logarithmic Scale: SPL is measured on a logarithmic scale due to the vast range of sound pressures that humans can perceive. This means that each 3 dB increase in SPL represents a twofold increase in sound intensity. For example, 83 dB SPL is twice as intense as 80 dB SPL.
• Human Perception: SPL is closely related to human perception of loudness. A small increase in SPL can be perceived as a significant change in loudness by the human ear.
• Hearing Threshold: The threshold of human hearing, also known as the reference sound pressure (Pref), is set to 20 μPa. This is the minimum sound pressure level that can be heard by a person with normal hearing at 1 kHz.
• Loudspeaker Sensitivity: In loudspeaker specifications, sensitivity is often given as the sound pressure level (in dB SPL) produced by the loudspeaker when driven with a specific input power at a defined distance - often 1 watt (W) of power at a distance of 1 meter (m).

It should also be noted that if one were to know SPL at a known distance, then the sensitivity at other distances can be calculated. To determine the SPL at a different distance from the sound source, you can use the inverse square law. The inverse square law states that the intensity (and consequently, the SPL) of a sound decreases proportionally to the square of the distance from the sound source. Mathematically, the formula for the Inverse Square Law is as follows:

SPL₂ = SPL₁ + 20 * log₁₀(d₁ / d₂)

where:

SPL₁ is the original SPL measured at distance d₁ from the sound source.
SPL₂ is the SPL you want to calculate at a new distance d₂ from the sound source.

Here's how you can use the formula to calculate the SPL at a different distance:

1. Start with the original SPL (SPL₁) that is measured at the reference distance (d₁). For example, if SPL₁ is 90 dB at 1 meter (1W/1m).
2. Measure the new distance (d₂) from the sound source to where you want to determine the SPL. For example, let's say the new distance (d₂) is 2 meters from the sound source.
3. Plug the values into the formula:

SPL₂ = 90 dB + 20 * log₁₀(1m / 2m)
SPL₂ = 90 dB + 20 * log₁₀(0.5)
SPL₂ = 90 dB + 20 * (-0.3010)
SPL₂ = 90 dB - 6.02 dB
SPL₂ ≈ 83.98 dB

So, the SPL at a distance of 2 meters from the sound source would be approximately 83.98 dB SPL, assuming the sound source remains at a constant power of 1W. The SPL decreases as you move further away from the sound source due to the inverse square relationship between distance and sound intensity.



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