What Is SPL (Sound Pressure Level) ?

Sound, on a basic level, can be perceived differently by different people – hence it is important to have an objective means with which to measure sound level. This is defined as Sound Pressure Level – which can be a technically difficult concept to grasp.

Here, we explore what exactly is sound, sound pressure and more importantly sound pressure level (SPL). This has notable applications ranging from loudspeaker placement to home theater.

spl - sound pressure level

What Is Sound Pressure

The term “sound pressure” applies to the degree of air pressure increase caused by a sound output. Sound pressure is what we “hear” or “feel” as loudness. When softly struck, the drum’s surface moves a short distance, causing small differential pressure and a mild sound. The surface starts to change away from its resting state when the drum is hammered harder. As a result, the sound appears louder to the listener.

Sound pressure is influenced by the surroundings in which the origin is situated, as well as the range between the source and the listener. Hitting the drum in a small bathroom from two meters away produces a louder sound than hitting it in the middle of a football field.

From Canadian OHS

As one travels further away from the drum, it grows quieter. If there are hard things in the environment that can reflect sound, the sound will appear to be louder than if you heard the identical sound from the same distance in a large field.

Sound pressure is frequently expressed in pascals (Pa). Sound pressures as little as 0.00002 Pa can be heard by a healthy young individual. The sound pressure produced by a general conversation is 0.02 Pa. A lawn mower fueled by gasoline generates roughly 1 Pa. At roughly 20 Pa, the sound is incredibly loud. As a result, we experience a wide range of sound pressure levels in the noises we hear on a daily basis (0.00002 Pa – 20 Pa).

What Is SPL (Sound Pressure Level)?

Sound pressure levels, shortened as dB, are a way of quantifying the intensity of a sound occurrence in decibels. Because of the aforementioned, decibels are a peculiar unit of measurement: They’re a kind of metric for comparing one sound to another. The pascal, a considerably lesser unit generated from one newton (1 N) of force exerted over a one-square-meter area, is the basis for representing the absolute value of sound pressure (1 m2).

The weakest sound a person can generally hear – commonly known as the human hearing threshold – is around 0.00002 pascal or 20 micro pascal (Pa) – a minuscule change in air pressure, demonstrating how delicate the human is meant to be.

Noise in offices - Noise Guide
From Noise Guide

A chainsaw, on the other hand, produces a sound of about 20 pascal. The gap between 0.00002 and 20 pascal is a millionth of a percent — that’s a lot of decimal places! Consider what a graph would look like if it attempted to depict this disparity. Because portraying such a vast range is difficult, an algorithmic scale is employed.

What Is Sound Power?

The sound power is the amount of sound energy supplied into the air by the noise source every second. The sound power of a noise source, such as a drum or compressor, stays unchanged when it is moved to a different location.

Watts are the units of measurement for power (W). A whisper produces 0.0000001 watts (0.1 microwatt (W) of sound, a truck horn produces 0.1 W of sound, and a turbojet engine produces 100,000 W of sound. Sound power (in W) is commonly stated as sound power levels in dB, equivalent to sound pressure.

What Is the Purpose of Sound Power Levels?

It’s quite helpful to know how loud an instrument is. It enables us to measure the sound output of multiple devices accurately, independent of the surroundings or measurement range employed.

To make sure that products are labeled, device noise emission limits are set, and that devices meet those limits, sound power levels must be enough.

If we know the acoustical environment and the measurement site, we can figure out the sound pressure level emissions from a device to certain places in that environment. This is because sound power levels don’t change with the acoustical environment or where the measurement is done.

For instance, an acoustical expert might use the sound power level of an item of machinery to determine the sound pressure level it would generate if put in a particular area. The specialist can then assess whether the noise levels at the home will reach acceptable standards, or if measures to mitigate or new, quieter machines should be recommended.

Relation Between Sound Pressure and Sound Power

The relation between sound pressure and sound power can be explained using heat as an example.

An electric heater generates a specific amount of heat that it distributes around a room to raise the temperature. The room in which the heater is located has no effect on the heater’s energy output. On the contrary, the temperature in the room will fluctuate based on our proximity to the heater and the space’s qualities, such as its size and the number of heat absorbed or carried through the floor and walls.

SPL vs SWL – Sound Pressure Level and Sound Power Level
From Parker Jones Acoustics

A similar link exists between the sound power output of a sound source and the sound pressure levels in a room. People can hear more clearly in a room because of how much energy the source of the sound has. However, even though the source’s power level is fixed, the sound pressure levels change based on our distance from the source and the room’s characteristics.

The size of the room, as well as how much sound reflects or is absorbed by the room’s surfaces, are all factors to consider.

How Sound Pressure is Measured

Decibels are the unit of measurement for SPL (dB). A sound pressure level meter, such as the Pulsar Nova, is used to measure it. Sound reaches the meter using a microphone, after which the meter assesses the sound values by changing them to electrical signals and displaying the decibel measurement readings. SPL meters are often portable, mobile, and lightweight, and are built to international standards like IEC 61672.

Pulsar Instruments produces a high-quality variety of SPL meters that meet these requirements. The meters will instantly capture noise readings and help the user to detect sources or circumstances where noise may be a concern, such as for economic purposes (noise at work), air degradation, or noisy neighbor conflicts.

Because of the microphones used in Class 1 (Type 1), the measurements are more detailed than in Class 2 (Type 2), but both will meet international noise measurement standards and be accurate.

Pulsar Nova Sound Level Meter | Accurate Instruments
From Accurate Instruments

The Technical Difference

A noise level meter’s Class indicates its accuracy in accordance with applicable international standards. International standards such as IEC 61672-1:2013 define sound level meters. In order to be appropriate for the purpose, devices must meet a broad range of complex precision, reliability, and measurement requirements stipulated by these standards.

The Standard distinguishes between two levels of tolerance: Class 1 and Class 2. The Class 1 accuracy rating is higher than the Class 2 accuracy rating.

To account for discrepancies in the instruments, these Class 1 and Class 2 tolerances are required. The variations are attributable to the diverse electrical components used in sound level meters, as well as the different ways in which different meters have been manufactured and validated. Even the sound level meter inspection equipment utilized during production will make a difference.

Due to its numerous tolerance levels, a Class 1 sound level meter is usually known as a ‘precision’ grade meter, whilst a Class 2 sound level meter is known as a ‘general grade’ meter.

Tolerance levels are wider at the low and high corners of the sound frequency spectrum*, whereas they are tighter at higher frequencies. Class 1 meters feature tighter tolerances, resulting in a more exact reaction at the extremities of the frequency spectrum.

A sound level meter that is class 1 must be capable of measuring sound at a higher frequency and must satisfy tighter control conditions than a sound level meter that is class 2. As a result, Class 1 is thought to be more precise than Class 2.

As a result, a Class 2 sound level meter has larger tolerance levels and thus is a little less accurate, but the distinction isn’t visible in most instances, and because Class 2 sound level meters are less costly than Class 1, most people will find that a Class 2 meter satisfies the criteria, is precise enough, and is fit for service.

What Kind of Sound Level Meter Do You Require?

The kind of meter you require is determined by the sort of measurement, the measurement criteria that must be followed, and whether or not your findings will be used as valid proof. For instance, if you want to present your readings as proper proof, you may opt to use a Class 1 meter due to its enhanced precision. Class 1 and Class 2 noise level meters are classified into two parts, each with its own set of characteristics based on what you wish to measure.

Importance of Sound Pressure for Home Cinemas and Hi-Fi Systems

Decibels are useful because they enable us to represent sound pressure differences with only three digits. For example, the utmost softest sound a human ear can perceive, or 0.00002 pascal, is symbolized as 0 dB, but a supersonic jet producing one of the loudest sounds on Earth is only 150 dB.

Because decibels are measured on a human scale, they can be used to quickly and readily determine the sound pressure levels that a device is susceptible of by simply looking at the technical data. The “sensitivity” of the system, or how much of the system’s power is actually utilized to make sound, is generally reported as the appropriate number.

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From Tech4

A microphone is placed one meter from the loudspeaker and given one watt of power to determine this value. The more decibels a speaker has, the more effective it is.

Sound strength, or sound pressure per unit area, is also used in hi-fi and home cinema systems, and is commonly expressed in watts/m2. This measurement does not merely give a figure for the sound’s power, but also for the sound’s power in terms of area.

Measuring the variations in sound wave intensity per distance traveled when sound radiates from the source (in this case, the speakers) allows us to determine how loud the sound is at different distances from the origin. This is handy for calculating how far away from speakers that are playing at full volume one can safely sit.

Matching Your System’s Sound Pressure Levels to the Room

Most high-end setups now include an inbuilt microphone that converts sound from loudspeakers to electrical energy. This lets the device to collect data about the listening environment and alter the sound appropriately. When choosing a system, keep in mind the following general rule: “The larger the space, the more high-performance the device needs to be.” Of course, there are additional considerations besides space size.

The location of the loudspeakers in relation to the walls, as well as the presence of slanting walls, will affect how sound is transmitted from the source to the listening position. When adjusting the system, several parameters must be taken into account.

Conclusion

Sound pressure level is a useful metric which uses a logarithmic scale to represent the sound pressure of a sound relative to a reference pressure.

It is worth noting that there are various warning from listening to high SPLs and its ability to cause hearing damage – hence opting for a noise level meter may come in handy. The Fletcher Munson Curve also illustrates that we perceive various frequencies differently depending on their sound pressure level.

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