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Accuracy Applied: Part 1 In A Series On The Keys Of Loudspeaker Measurements

"Frequency response is the single most important aspect of the performance of any audio device. If it is wrong, nothing else matters." – Floyd Toole, 2009

Compared to electronic audio tests, measuring loudspeakers is complicated. First, to measure sound levels accurately requires precision measurement microphones and supporting electronics, which have a known, stable sensitivity and flat frequency response over the frequency range of interest.

Measurements are further complicated by the interaction of the loudspeaker under test with the test environment. Ideally, we want to measure the direct sound radiated from the device under test (DUT) without any contamination caused by reflections from walls, floors, or ceilings, etc. Special rooms called anechoic chambers are available for this purpose, but they’re very expensive. And even the best ones are usually not fully anechoic at the lowest frequencies of interest.

Physical dimensions are also extremely important in acoustics. The audible frequency range is generally considered to be 20 Hz to 20 kHz. The corresponding range of wavelengths of sound in air at room temperature is 56 feet to 0.68 inch (17.2 meters to 17.2 millimeters).

So, at 20 Hz, a typical loudspeaker is tiny compared to the wavelength of sound, and it behaves like a point source, radiating uniformly in all directions. At 20 kHz, the opposite is true; a typical loudspeaker is large compared to the wavelength and its radiation pattern is radically different in all directions. Further, the wavelength at 20 kHz is close to the diameter of a typical measurement microphone (0.5 inch or 12.7 millimeters), making measurements highly sensitive to small changes in mic position.

Despite the challenges, when recommended procedures are followed using quality test equipment, it is possible to make good, repeatable loudspeaker measurements. Over the course of this article series, the following loudspeaker measurements will be covered:

— Frequency response
— Sensitivity
— Input voltage/power
— Impedance & Thiele-Small parameters
— Directivity
— Distortion
— Industry Standards

Ideally, standards represent consensus among industry experts concerning measurement conditions and recommended practices that will help to ensure that devices are tested in a meaningful and repeatable way. The key international standard covering loudspeaker measurements is IEC 60268-5, Sound system equipment, Part 5: Loudspeakers [1]. This standard applies to passive loudspeaker drive units and passive loudspeaker systems only; it does not apply to active loudspeakers (built-in amplifiers).

Given the widespread use of powered loudspeakers today, it seems like IEC 60268-5 is due for revision, and indeed, work in that direction is in process. In the meantime, it serves as a useful reference for conducting acoustical and electrical measurements of loudspeaker drive units and loudspeaker systems.

Frequency Response

Frequency response is the single most important aspect of the performance of any audio device. If it is wrong, nothing else matters. – Floyd Toole, 2009 [3]

Frequency response is a “transfer function” measurement. For a device under test (DUT), it represents the magnitude and phase of the output from the DUT per unit input, as a function of frequency. Devices are often compared in terms of the “shape” of their frequency response curves, which typically refers to the magnitude response only (not phase), and in addition normalizes the magnitude to a reference value.

For example, the response magnitude might be normalized to its value at some reference frequency, say 1 kHz, such that the normalized curve passes through 0 dB at 1 kHz. In the case of loudspeakers, the output from the DUT is the sound pressure as measured at a point in space. For passive loudspeakers, the input to the DUT is an amplified voltage signal, and for powered loudspeakers it could be an unamplified voltage or a digital audio signal (transmitted over a digital audio interface such as S/PDIF, HDMI, or Bluetooth, etc.).

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Loudspeaker design engineers usually strive for flat frequency response in their designs, to help ensure that source material is faithfully recorded and reproduced without spectral coloration. For electronic audio components, flat frequency response is the norm and is easily achievable. For example, almost any audio amplifier will have a flatness of less than ±1 dB within the audio band (20 Hz to 20 kHz); even ±0.1 dB is not uncommon.

In the case of loudspeakers, achieving flat frequency response is challenging, for a variety of reasons. Multiple drive units of different sizes (and sensitivities) with crossover circuits must be combined to cover a wide frequency range. At some frequencies, drivers interact with the enclosure acoustically. And mechanical resonances in the drive units and/or the enclosure can cause sharp peaks or troughs in the response curve.

As a result, it’s not uncommon to find consumer loudspeakers with deviations from flatness of up to 20 dB over their useful frequency range. Nevertheless, it is possible to achieve a relatively smooth and flat response for loudspeaker systems, and a deviation from flatness of ±3 dB is considered “respectable.” Multiple studies of listener preferences for loudspeakers have shown that trained listeners prefer systems with smooth flat frequency response, both on and off axis, and deep bass [4].

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