This column originally appeared in three parts in the Sound Advice section of the February, April, and June 2020 issues of Professional Sound magazine.
By Joe Begin
In comparison to electronic audio test, the electroacoustic measurement of loudspeakers presents a number of challenges. In addition to the need for precision microphones, the interactions between the DUT (device under test) and the environment creates unique measurement issues. This article discusses relevant international standards for loudspeaker measurements and describes different test environments, providing some basic requirements to achieve good, repeatable loudspeaker measurements.
IEC60268-5 is an international standard intended to ensure loudspeakers are tested in a meaningful and repeatable way. It applies to passive loudspeaker drive units and passive loudspeaker systems. Standards for powered loudspeakers are in development, and so will not be addressed here. The IEC60268-5 standard specifies mounting, acoustical environment, loudspeaker and microphone position, and the test signal and rated conditions for conducting measurements.
The performance of a loudspeaker drive unit (or driver) depends on the properties of the unit itself and its acoustic loading, which in turn, depends on its mounting arrangement. Drive units may be mounted in one of three configurations, with the selected configuration clearly described in the test results:
- A standard baffle or in one of two specified standard measuring enclosures.
- In free air without a baffle or enclosure.
- In a half-space free field, flush with the reflecting plane.
Loudspeaker systems are usually measured without any additional baffle. The manufacturer can specify that a baffle be used, in which case a description of the mounting arrangement should be included with the test results.
IEC 60268-5 requires that measurements are made in one of five specified acoustic fields: free-field; half-space free-field; diffuse; simulated free-field and simulated half-space free-field conditions.
Generally speaking, acoustic loudspeaker measurements should be conducted in a free field. To achieve free-field conditions, testing can be done outdoors, where sound may propagate freely in all directions, or in an anechoic chamber, which approximates a free-field. However, both have their challenges. For free-field conditions outdoors, the loudspeaker and microphone would have to be placed high above the ground to minimize the influence of ground reflections, and ambient noise must be mitigated. Conversely, controlling the environment using an anechoic chamber can be an expensive proposition.
In an anechoic chamber, the minimum requirement for a free-field is that sound propagation from the source follows the “6 dB/dd” rule within ±10% on the axis between the reference point and the measurement microphone. The 6 dB/dd rule states that the sound pressure level radiating from a source decreases by 6 dB per doubling of the distance from the source.
An inexpensive alternative to an anechoic chamber (to approximate the free-field response of a loudspeaker) is ground plane measurement (see Figure 1), where the loudspeaker and microphone are placed on a hard surface in an open area. When set up correctly, the direct and reflected sound waves will be in phase and will have a combined level 6 dB higher than the level of the direct sound. There are some issues to bear in mind, however, such as the baffle appears to be twice as high as it really is, due to the reflected image of the loudspeaker, causing a different diffraction response along the edge that is in contact with the ground. It also has the same issues as the standard free-field outdoor measurements in that it is affected by ambient outdoor noise from vehicles, machinery, aircraft and wind, especially for low-frequency measurements.
A half-space free-field condition is specified for testing a loudspeaker driver alone. In a half-space free field, the three-dimensional free-field space is split in half, usually by a hard, reflecting plane. For example, a sound source located outside on hard ground, away from any other reflective surfaces constitutes a half-space free field. A hemi-anechoic chamber (an anechoic chamber with one of its 6 interior surfaces being a hard, reflective plane) can also be used. A hemi-anechoic chamber should meet the 6 dB/dd rule within ±10% between the surface and the measurement microphone.
An example of a diffuse sound field is a reverberation chamber, in which all of the interior surfaces are made of hard, reflective material. Here, measurements should be conducted with 1/3-octave band limited noise.
Simulated free-field conditions use quasi-anechoic or time-selective techniques, which involve windowing out the reflected sound from a measurement such that only the direct sound from the speaker is analyzed. If used carefully, this enables measurements in an ordinary room, but limits the lower frequency range of the measurement. A large, unobstructed room can help to extend the low frequency range of the measurement by increasing the time between arrival of the direct sound and reflections.
Simulated Half-Space Free-Field
Simulated half-space free-field conditions are identical to simulated free-field conditions, except that the quasi-anechoic technique simulates a half-space free-field (i.e., the driver is tested in a baffle flush with a hard, reflecting plane).
Loudspeaker & Microphone Position
An important concept in acoustics is the near field and far field. Far away from a source (relative to its size), the inverse square law (or 6 dB/dd rule) mentioned above applies. At this distance the sound field has become stable and radiates from the source in a predictable way. Close to the source, however, sound waves behave in a much more complex fashion and there is no fixed relationship between pressure and distance. In this near field, the sound level is uncertain. Therefore, measurements should be conducted in the far field. The distance from the source to the far field depends on the size of the source. In one often used “rule of thumb”, it is considered to begin at a distance of 3 times the largest dimension of the source .
In all cases and environments, use of a microphone with a known calibration is specified by IEC 60268-1. Measurement microphones, which are stable and have a flat frequency response over a wide frequency range, should be used. A free-field microphone is recommended for free-field measurements; for diffuse field measurements, a random incidence measurement microphone should be used. If needed, an audio analyzer’s input equalization (EQ) feature can eliminate any deviations of the microphone’s measured response from the required response characteristic.
Ideally, measurements in free-field and half-space free-field conditions should be conducted with the measurement microphone in the far field of the loudspeaker. In practice, imperfections in the anechoic chamber and the effects of background noise may impose an upper limit on this distance.
For simulated free-field conditions, IEC 60268-5 specifies the same measurement distance as for free-field conditions, and that the loudspeaker and microphone should be positioned within the measurement room to maximize the time between the direct sound and the first reflection. Errors in the measured frequency response due to the windowing out of reflections should not exceed 1 dB over the frequency range of interest.
The Test Signal & Rated Conditions
A test signal may be chosen from one of the four signal types specified by IEC 60268-5. They are a sinusoidal signal not exceeding the rated sinusoidal voltage at any frequency; a broadband noise signal, with a crest factor between three and four; a narrow-band noise signal, consisting of pink noise filtered in 1/3-octave bands; and an impulsive signal, for impulse testing.
IEC 60268-5 also specifies several properties, known as Rated Conditions, that should be taken from manufacturer’s specifications. These properties do not need to be measured, but other measurement characteristics are based on their values. The Rated Conditions include: Rated Impedance, Rated Sinusoidal Voltage or Power, Rated Noise Voltage or power, Rated Frequency Range, Reference Plane, Reference Point, and Reference Axis.
Impedance & Derivative Characteristics
The rated impedance of a loudspeaker is the nominal value of pure resistance used to define the power required to drive the speaker. Although a nominal resistance value is used, a loudspeaker’s impedance is a phasor or complex quantity (it has both magnitude and phase), and it varies significantly over the audio frequency range. For example, Figure 2 shows the impedance magnitude of a three-way loudspeaker system from 20 Hz to 20 kHz. The three peaks in the curve are resonant frequencies associated with the three drivers in the system.
IEC 60268-5 specifies that the lowest value of the impedance magnitude within the rated frequency range shall not be less than 80% of the rated impedance. It also requires that the impedance at any frequency outside the rated frequency range (including DC) be less than 80% of nominal impedance.
The standard also requires that the impedance magnitude curve is measured over the standard audio frequency range (20 Hz to 20 kHz). The log-swept chirp signal is a good stimulus for impedance measurements. To make impedance measurement easier, the APx1701 Transducer Test Interface has a built-in current sense resistor in the ground leg of each power amplifier circuit. Alternatively, an external sense resistor can connect the loudspeaker and power amplifier to the audio analyzer. Figure 3 shows a test accessory with two selectable sense resistors for this purpose.
Characterizing Drive Units
IEC 60268-5 defines three loudspeaker drive unit characteristics that are derived from the impedance curve. They are the resonance frequency (FS), the total Q-factor (QTS) or the ratio of the inertial part of the acoustic or mechanical impedance to the resistive part of the impedance at the resonance frequency, and the equivalent air volume of compliance (VAS). FS, QTS and VAS are a subset of driver characteristics known as the Thiele-Small parameters.
IEC 60268-5 specifies some simple procedures for calculating QTS and VAS from the resonance region of the measured impedance magnitude. In Audio Precision’s APx500 audio analyzer platform, these and many more of the Thiele-Small parameters are derived by fitting a model to the measured impedance data. Three driver models are supported—Standard, LR-2, and Wright.
Input Voltage & Power
Section 17 of IEC 60268-5 specifies loudspeaker input voltage characteristics which should be measured to determine voltage levels that the loudspeaker can withstand without suffering any thermal or mechanical damage. Three voltage characteristics - input voltage, rated noise voltage and short-term maximum input voltage - use a program simulation noise (PSN) signal obtained by filtering pink noise with a bandpass filter specified in IEC 60268-1. For the rated noise voltage test, the signal is further adjusted to have a crest factor between 1.8 and 2.2. Both versions of this PSN signal are built into APx500 audio analyzers.
Section 18 of IEC 60268-5 specifies corresponding input power characteristics. These are calculated using P_i= 〖U_i〗^2⁄R, where U_i is the measured input voltage characteristic and R is the nominal input impedance of the loudspeaker.
Section 21 indicates that the frequency response shall be specified as measured under free-field or half-space, free-field conditions at a stated position with respect to the reference axis and reference point, at a specified constant voltage.
Section 21 also defines the effective frequency range (EFR). This is determined from a frequency response measurement conducted on the reference axis of the loudspeaker. It is derived by finding the maximum sound pressure level averaged over a bandwidth of one octave and centered on the frequency of maximum sensitivity. The lower and higher limiting frequencies are found at which the response is no more than 10 dB below the maximum sound pressure level. Sharp troughs narrower than 1/9 octave are ignored when determining the EFR.
Section 24 of IEC 60268-5 covers distortion measurements. For harmonic distortion measurements, the loudspeaker must be driven by a series of sinusoidal voltages with increasing frequencies up to 5 kHz. Measurements are conducted with a microphone on-axis in a free-field for loudspeaker systems or a half-space, free-field for drive units. The sound levels of individual harmonics are measured, as well as the overall sound level to determine total harmonic distortion (THD). The standard also specifies harmonic distortion of the second and third order, where the 2nd and 3rd harmonics are expressed as a ratio to the overall sound level.
Frequency response measurements conducted using the log-swept sine chirp stimulus provide harmonic components simultaneously with the main response, in one measurement. In the APx platform, chirp measurements can track individual harmonics up to the 20th order, as well as the sum of any combination of harmonics H2 through H20.
To test at signal levels higher than the rated sinusoidal voltage, IEC 60268-5 specifies a method whereby the loudspeaker is stimulated with a tone burst. A subset of the waveform acquired during the high-level portion of the signal can then be extracted and subjected to fast Fourier transform (FFT) analysis. The harmonic distortion components are extracted from the FFT spectrum and used to calculate the THD.
Rub & Buzz
Rub and buzz distortion is usually caused by mechanical defects such as the driver voice coil rubbing or loose particles in the gap. Annex D of IEC 60268-5 describes a listening test that involves manually sweeping the frequency of a sine signal applied to the speaker at the rated sinusoidal voltage. Problems occur in a highly repetitive situation like production test, where operator performance is likely to decline rapidly. In addition the distortion could be below the threshold of audibility in a production test environment.
For an objective, repeatable means of detecting rub and buzz distortion, the Acoustic Response measurement in the APx500 audio analyzer platform has an optional rub and buzz function.
There are many factors to consider when testing loudspeakers. Adhering to the IEC 60268-5 standard, or at least using it as a guideline, will help produce meaningful test results. The next article in this series will cover the specific challenges in the test environments.
Joe Begin, PE (Acoustical Engineering), a graduate of McGill University (B.Sc.) and the University of Canterbury (M.Sc.), has over 35 years’ experience in test and measurement. As Director of Applications and Technical Support at Audio Precision, he is involved in product management, audio and electroacoustic test applications engineering, and technical support.