Eilex Prism™

Eilex PRISM™ equalizes acoustic power volume density frequency response and corrects time and phase alignment errors of a speaker system. Speakers with Eilex PRISM perform as near-perfect electro-acoustic transducers providing true-to-original sound with the highest musicality and intelligibility.

Acoustic Power Volume Density Measurement and Equalization Using VIR Filter

VIR Filter LogoEilex PRISM™ (Primary Sound Measurement) technology measures Acoustic Power Volume Density (APVD) frequency response and equalizes it using our newly developed VIR™ (Variable-resolution Impulse Response) filter.

APVD frequency response is measured in the frontal space of a speaker. This space contains interference in the sound waves coming from single or multiple drivers, creating inconsistencies in the sound intensity. PRISM corrects the APVD frequency response to desired characteristics. PRISM simultaneously corrects time/phase alignment, especially in the low frequency range. As a result, the reproduced sound is extremely close to that of the source material.

PRISM delivers fully objective and reliable measurement results, regardless of operator or environment. It offers immense freedom to industrial and structural designers in terms of speaker layout and acoustic situations. It saves substantial development time.

VIR filter executes the PRISM equalization. This filter is superior to any other filter for audio applications, including FIR filter. FIR filter has the critical drawback of insufficient resolution at low frequencies, which often prohibits the audio designer from effectively using it for low frequencies. By comparison, VIR filter has consistent resolution across the entire bandwidth. Moreover, if necessary, VIR filter can allocate more correction points to critical frequency ranges by borrowing them from less important ranges. Thus, VIR filter is suitable for equalizing sub-woofers as well as higher sample-rate systems.

VIR filter uses less than 25% of the computation resources (cycles or MIPS) that FIR filter would use.

PRISM imbues a system with the highest fidelity and intelligibility while maintaining realism and naturalism in its sound. It reduces “sweet-spots” and “dead-spots,” effectively unifying the sound throughout the room.


The correction of acoustic power frequency response of a speaker dramatically improves its performance. Thanks to the wide availability and cost reduction of electronic devices such as DSP and CPU consumer electronic products like TV and home audio have started to apply acoustic power equalization.

Traditionally, a speaker’s frequency response is equalized only at one point on-axis (typically 1 meter from the high frequency driver), while the off-axis sound has been ignored. The equalized sound will never be heard unless a listener sits exactly at the measurement point. In reality we hear off-axis sound as direct sound, while the rest of the off-axis sound reaches our ears as indirect sound after reflecting off the walls, floor, ceiling and furniture. Indirect sound makes up a nearly half of the sound we hear in a room with fair acoustics. Therefore, the equalization of off-axis sound is more important than one-point on-axis sound. Acoustic power measurement is a solution for this. Its measurement is made from the multiple sample data on an imaginary plane in front of the speaker. Acoustic power equalization dramatically improves the sound actually heard.

However, acoustic power equalization is ideal only when the origin of sound is a small point. Speakers are not points, and they often have multiple drivers, which create interference (frequency-dependent peaks and dips) in their near field space. Acoustic power measurement, which is made only on a single plane in front of a speaker, does not detect this interference.

Eilex PRISM™ measures the acoustic power in the frontal space of a speaker. The result of this measurement is called Acoustic Power Volume Density (APVD) frequency response. APVD measurement captures the interference. Equalization of APVD to any desired characteristics improves the sound beyond the capability of acoustic power equalization. (Ref. 2)

PRISM’s measurement process is fully automated and easy to perform using single microphone connected to a laptop computer with PRISM Composer software. The accumulated measurement data of about 400 points in 16,384 frequency bands is analyzed and calculated to generate coefficients, which are later used to build an equalization curve. Realization uses a VIR (Variable-resolution Impulse Response) filter which typically resides in a DSP, CPU, MCU or SoC. The number of bands of the VIR filter is selected from 10 to 200 sections depending on the application. PRISM delivers fully objective and reliable measurement results, regardless of operator and environment.

Compared to the “room acoustic compensation method,” which measures the audio signal at only a single listening point, PRISM provides more natural and realistic sound. PRISM imbues a system with the highest fidelity and intelligibility while maintaining realism and naturalism in its sound. It reduces “sweet spots” and “dead spots,” effectively unifying the sound throughout the room.

Unlike conventional equalizers, PRISM is also suitable for subwoofer equalization. Conventional equalizers are not capable of this because of the difficulty of low frequency measurement itself and their inadequate frequency resolution. PRISM provides the highest possible low frequency resolution within the given VIR filter section numbers by dynamically allocating the sections along the frequency range. (Ref. 1)

PRISM not only works on speakers, but also works on microphones. Their irregular frequency responses can be corrected to an ideal curve by PRISM. MEMS microphones in particular get the full benefit of PRISM correction.

PRISM offers extreme flexibility to industrial and structural designers in terms of speaker layout and acoustic situations. Plus, it saves an invaluable amount of development time.

Acoustic Power Volume Density Measurement

Eilex PRISM™ application begins with measurement of APVD (Acoustic Power Volume Density) frequency response. This measurement process requires a PC with PRISM Composer software, a measurement microphone and microphone amplifier (audio I/O).

PRISM Composer has an integrated wide-range test signal (0Hz–24KHz for 48KHz sampling, and 0Hz–48KHz for 96KHz sampling). While playing the test signal through a speaker, Composer measures the reproduced sound. One takes the measurement by moving the handheld microphone slowly in the frontal space of the speaker (0.5–2.5 meters, depending on the size of the speaker and the room). Cover the entire frontal space, first from side to side and then from top to bottom for a consistent grid pattern. If there is a floor or a wall in the proximity, move the microphone close to it. Fig. 1 illustrates the desired measurement space. A measurement of over a few hundred points is completed in a few minutes.

Figure 1
Fig. 1 – APVD Measurement Concept

Based on this measurement result, PRISM Composer will calculate the APVD frequency response of the speaker. Fig. 2 and 3 show the part of measurement and calculation process.

Figure 2
Fig. 2 – APVD Measurement
Figure 3
Fig. 3 APVD – Frequency Response Calculation

The red curve in Fig. 4 below is the calculated APVD frequency response. The blue curve is the reversed curve of APVD, which is built into a filter. When applied, this effectively flattens the speaker’s APVD response to as flat as the yellow target curve.

Figure 4
Fig. 4 – Smoothed and Inverted Data (Flat Equalization)

Fig. 5 illustrates the example of target curve application to obtain a specific APVD frequency response. The blue correction curve changes accordingly. The expected frequency response after equalization is shown in gray (although it is mostly overlapped by the yellow curve).

Figure 5
Fig. 5 – PRISM Equalized APVD Frequency Response (Target Curve is used)

Eilex PRISM performs equalization quite accurately. Take, for example, the equalization of the red APVD curve in Fig. 4. Fig.6 shows the expected flat APVD result.

Figure 6
Fig. 6 – Expected Result of Flat APVD Curve

Fig. 7 is the actual speaker measurement result after equalization is applied. The actual result is very close to the expectation.

Figure 7
Fig. 7 – Measured APVD Curve Result

VIR filter is a minimum-phase filter. The correction of speaker frequency response compensates for its group delay. As a result, phase characteristics are corrected. (Phase correction frequency range is limited by the frequency response compensation range.)

Fig. 8 is the original phase characteristics of the speaker shown in Fig. 4. Fig. 9 shows the corrected phase characteristics after Eilex PRISM is applied. The phase is nicely corrected in the mid to high frequency range.

Figure 8
Fig. 8 – Original Phase Characteristics
Figure 9
Fig. 9 – Phase Characteristics after Eilex PRISM applied

The phase characteristics correction delivers perceptible improvements in accurate sound stage reproduction, ambience and depth.

To realize the filter, a VIR (Variable-resolution Impulse Response) filter is used. A VIR filter is composed mathematically and the coefficients are loaded onto a physical (firmware) filter in a DSP, CPU, MCU or SoC for real-time calculation. Its resolution can be chosen from the range of 10–300 sections depending on the application. For example, high-end audio uses 100 sections, a TV uses 20 to 30 sections and personal portable audio uses 40 sections. Super high-end audio can use 100, 150 or 200 sections.

What is Acoustic Power Volume Density?

Every sound has a source. The sound emitted from the source travels in all directions through air, similar to ripples on the surface of water after dropping a stone. Just as the ripples carry kinetic energy, the sound waves carry acoustic energy. The acoustic energy at a moment is acoustic power. The sound we hear in the natural world is the sum of the acoustic energy arriving from all directions, converted to sound pressure at our ears.

It is easier to understand acoustic power if the acoustic terms and formulas are explained in electric equivalents as below. (The units for the acoustic terms are not assigned here to maintain simplicity.)


Acoustical Electrical Equivalent
Sound Pressure Voltage (V)
Particle Velocity Current (A)
Acoustic Power Electric Power (W)


Acoustical: Sound Pressure × Particle Velocity = Acoustic Power
Electrical: Voltage (V) × Current (A) = Electric Power
Acoustical: Acoustic Power × Time = Acoustic Energy
Electrical: Electric Power (W) × Time (H) = Electric Energy (WH)

The Acoustic Power of a loudspeaker is the acoustic energy emitted from the speaker passing through a specific plane which covers speaker’s entire frontal angle, over one second. More specifically, acoustic power is the product of particle velocity, which is perpendicular to the plane, and sound pressure. Acoustic power values are obtained by calculation after measuring sound pressure frequency response at multiple points on a specific plane in front of a speaker.

Acoustic Power Volume Density (APVD) is obtained from the sound pressure measurements at multiple points within a specific volume which covers speaker’s entire frontal angle. The measurement includes the peaks and dips caused by interference between drivers.

The Necessity of Equalizing APVD

When we enjoy music from an audio system, the total sound energy that reaches our ears consists of direct sound, which reaches us directly from the speakers, and indirect sound which is deflected off the walls, floor, ceiling and furniture. In a room with fair acoustics, indirect sound comprises nearly as much of the total sound as direct sound.

In an anechoic room—in which only direct sound is heard—the sound can be harsh, with no vivacity or taste, no matter how good the speakers are. This is because indirect sound is an integral part of the total sound our ears are used to hearing. Thus, the indirect sound must be equalized along with the direct sound so that the total sound in the listening environment is clear, natural and lifelike.

It is fair to say that ideal audio reproduction is the exact reproduction of the original (source) sound. To accomplish that, the speaker must have flat frequency response across the full frequency range, no directionality, no time alignment and phase errors, and most importantly it must be a point source. Of course no such speaker exists. All speakers have uneven frequency response, directionality, time alignment and phase errors, and way larger than a point source (even relative to the wave length).

Under such conditions, acoustic power measurement and equalization greatly improves the speaker system performance compared to the classic on-axis sound pressure measurement method. However, because of the effective size and multiple drivers which a speaker normally has, noticeable interference occurs. This interference generates peaks and dips in the near-field frontal space of the speaker. Acoustic power measurement does not capture the interferences in this space.

To capture the interference as the sound travels, acoustic power measurement must be conducted in the specific frontal space. The result is APVD (Acoustic Power Volume Density).

Every speaker has a different sound emission pattern which varies with distance from the speaker and frequency. Fortunately, the total acoustic power is proportional to the APVD which contains the sound emission pattern. After analysis, the APVD frequency response quite accurately represents the sound heard in a room.