For the sound pressure level (SPL) indicator of a loudspeaker, the most common reference distance is 1 m (3.28 ft). This reference distance appears only for ease of calculation, and in fact the reference distance can be any value.
First, setting the reference distance to 1 m simplifies the division in the following sound pressure level attenuation calculation formula:


ΔdB=20log(Dx / 1) ideal point source ΔdB=10log(Dx / 1) ideal line source

Dx indicates the distance of the listener, in meters

The distance measured by the loudspeaker must be set in the area where the shape of the acoustic wave wavefront radiated by one loudspeaker no longer changes. The change in the shape of the spherical wavefront is due to the difference in path length over which the sound waves arrive at different points on the surface of the device.

As the distance from the sound source increases, the impact of these differences continues to decrease. In principle, when we are observing an object, if we continue to move away from the object, the object will become visually "small".

If the distance of the transmission path does not affect the shape of the spherical wavefront when it reaches a certain distance, then the distance is the starting point of the near-field region of the device and the starting point of the far-field region.

An infinitesimal sound source (point source) can be measured at any distance, and the measurement data can be used to accurately estimate the sound pressure level at a greater distance according to the inverse square law.

A very small speaker may measure at a distance of 1 m, but for a larger speaker, the situation is completely different. For larger speakers, it is important to determine the starting point of the far field region, which is the minimum distance at which the acoustic radiation parameters can be measured.

The data measured at this point can be used to estimate the sound pressure level at 1 m from the inverse square law (Figure 1). We can use this calculated reference value to estimate the sound pressure level at a greater distance within an acceptable error range.

General method

An effective rule of thumb for determining the critical points of the near-field and far-field regions is to set the minimum measurement distance to three times the maximum side length of the loudspeaker.
This distance estimation method is usually an acceptable method in actual measurement work, regardless of whether the waveform variation characteristics of the near-field and far-field critical point transition regions are frequency dependent. More accurate estimation of the far field location can be done in several ways:

1. The path length difference from the point of view perpendicular to the surface of the speaker to any point on the surface of the speaker is equal. Unfortunately, this can only be achieved when the observation point is infinitely far from the speaker, while the sound pressure at the observation point is zero.
2. When a certain distance is reached, the spherical wavefront radiated by the loudspeaker no longer exhibits frequency-dependent shape changes as the transmission distance increases.
3. When a certain distance is reached, the sound pressure decay of all frequencies begins to follow the inverse square law. This is a practical way of defining measuring distances.

4. When a certain distance is reached, the path length difference from any point on the speaker plane perpendicular to the observation point to the observation point is less than the 1/4 wavelength of the highest frequency to be measured (Figure 2).
As can be seen from the above definitions, the starting point of the far field region is related to the wavelength (frequency).

Note

As we mentioned earlier, the measurement in the far field is due to the need to estimate the sound pressure level of the far distance according to the inverse square law through the measurement data, which is also the sound pressure level calculation method of the acoustic simulation software.

If the purpose of the measurement data is not for the calculation of sound pressure levels at a greater distance, then the measurements can be made in the near field region. This data is accurate for the location of the measurement points, but is not suitable for extrapolation of sound pressure levels over longer distances using inverse square law.

Due to the wavelength, it is generally believed that the subwoofer needs to be measured at a greater distance. In fact, for a device that radiates high-frequency sound waves, determining the far-field region is a more difficult task. Since the wavelength of the high-frequency sound wave is shorter, it is very difficult to satisfy the above-mentioned fourth standard.

From a data measurement perspective, the most challenging loudspeakers are large-scale devices that radiate high-frequency sound waves over large areas of the surface. The near-field area of ​​this type of loudspeaker can extend up to hundreds of feet away, so using traditional measurement techniques to obtain accurate spherical map data is almost impossible.

Acquiring sonic diffusion data for such devices can be done in other ways, including acoustic modeling and acoustic holography – a new technology developed by Duran Audio. Fulcrum Acoustic's David Gunness has also published several very important papers on this issue.

Some factors have prompted all system engineers to meet the needs of measuring distance expansion as well, and there are also some factors that reduce the measurement distance expansion requirements. These factors include:

1. Large-size speakers that provide extended high-frequency response typically do not radiate high-frequency sound waves through the entire front panel. The natural properties of high-frequency sound waves are strongly directional, so the radiation of high-frequency sound waves is more likely to pass through the HF driver. That is to say, only the size of the high frequency driver needs to be considered when determining the far field region of the high frequency sound wave.

2. A beam-controllable line source speaker (such as the EAW DSA) does not radiate high frequency energy from the entire box. The length of the speaker cabinet is related to the operating frequency by configuring a bandpass filter for each driver. This type of speaker can be measured at a closer distance.

The most difficult to measure is a passive linear array system, especially a speaker array consisting of multiple speakers. Each speaker in the array is a full-range device, so the difference in transmission path between the speaker in the middle position and the speaker in the bottom position can sometimes be very large. A compromise is to measure the spherical map data of a single speaker and then use simulation software to predict the response characteristics of the speaker array.

Bandwidth treble sources and flat panel loudspeakers are comparable to those of passive linear array systems because these types of loudspeakers radiate high frequency energy through larger components.

Obviously, these speakers require a long measurement distance. The longer measurement distance solves the problem of far field measurement, but it also has some other problems:

1. Air absorption loss increases with distance. Although this loss can be corrected by the equalizer, the HF gain boost creates additional load pressure on the device under test.

2. As the distance increases, the ability to control environmental factors (flow and temperature gradients, etc.) is also reduced. These environmental factors can change the measurement data, making the collection of phase response data quite difficult or impossible.

   3. In an indoor measurement environment, the no-time window decreases as the measured distance increases. This is because the time difference between the sound wave radiating to the ceiling, the ground or the side wall is shortened as the distance between the test microphone and the sound source increases. This effect increases the lower frequency limit of the unvoiced measurement (the frequency resolution is impaired).

4. Direct sound attenuation is increased by 10 dB from the 9m (30 ft) position at 30m (100 ft). The increase in attenuation reduces the signal-to-noise ratio of the measured data by 10 dB, or if you want to maintain the same signal-to-noise ratio as the 30 ft position, you need to increase the power fed into the device under test by a factor of 10.

5. Measurements in outdoor environments are very difficult due to unstable noise levels and environmental factors during the measurement period (up to 8 hours).

Long-distance measurements can also be achieved if the above problems can be solved. A large hangar that measures the impulse response of the inserted time window is a good place to measure long-range spherical map data.

Our studio at ETC, Inc. allows us to make data measurements at 9m. This distance is suitable for most mainstream commercial speakers, but not for all commercial speakers.

The speaker flipping device can be moved, so devices that cannot be measured at a distance of 9 m are transferred to a larger space for measurement at a distance of 30 m, and the interference of the sound field reflection is eliminated by inserting a time window. The measurement distance is determined based on the device under test.

The far field area is determined using the above criteria and the measurement distance is fixed at 30 feet (9 m), which in turn determines the highest frequency that can be measured by the device under test of different sizes (Figure 3). It should be noted that this refers to the maximum size of the HF component. Normally, if this far-field region is suitable for high frequencies, it is also suitable for lower frequency bands.
For speaker attenuation characteristic data measurements, determining the far field region is a prerequisite. The measured data in the far field region can be applied to the sound pressure attenuation calculation from 1 m to the audience in an acceptable error range. This condition is easily met for small size speakers (eg bookshelf speakers). Since the physical size of commercial speakers is usually large, there is an upper limit of the measurable frequency at a fixed measurement distance.

Ideally, data not measured in the far field should be discarded or indicated in the product data sheet or system design software. Unfortunately, these data are usually not labeled with measurement methods, so we need to rely on intuition to judge to some extent when performing high-frequency coverage simulation in an auditorium.

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