What are speakers measured in

Measure loudspeakers and explain measurement data

Studio monitor understand measurement data
by Anselm Goertz,

How important are technical data and measured values? If you have the appropriate knowledge for the interpretation and meaning of the measured values, measured values ​​allow a full evaluation of a loudspeaker - which does not have to mean that you are spared the need to listen to it.

Of course, when measuring loudspeakers, you shouldn't just look at a frequency response or just a distortion value, you always have to interpret the entirety of all measured values. The next important question is therefore: Which measured values ​​are required? These are the frequency response and values ​​derived from it, such as B. a spectrogram, the spatial radiation behavior and the distortion values ​​or the achievable level. Here is a small calculation example: The components of a monitor have a typical sensitivity of 86 dB 1W / 1m. This means that with one watt of electrical power supplied at a distance of 1 m, a sound pressure of 86 dB is achieved. If you want to achieve a typical listening level of 85 dB at a distance of 2 m - which corresponds to 91 dB at 1 m - then a power of 3.16 W is required. However, the level measurement is an average level.

The same applies to the power value, which is an effective value here. For typical music material, the peak level in relation to this can be 15 dB or significantly more. So if these peaks are to be transmitted undistorted by the amplifier, then not 3.16 W, but already 100 W, at least for a short time, are required. For every 3 dB more in level, the necessary amplifier power then doubles further. This small example shows how quickly you reach your limits here. Many manufacturers indicate the achievable peak level of their monitors based on a distance of 1 m. For each doubling of the distance, 6 dB must be deducted and another 15 dB for the estimation of the average level. The typical listening level is 85 dBA Leq (eq = energy-equivalent continuous sound level, average level) with a stereo set. The A-weighting causes the level value to drop by a further 3 dB on average, which is then compensated for by two loudspeakers instead of one (+3 dB). A near-field monitor should therefore be able to deliver a peak level of 106 dB without any problems.

Understand measurement data

It is important that all measurement results are always taken into account when evaluating monitors. Nevertheless, the measurement cannot spare you the hearing test and the hearing comparison with other monitors

Frequency response of a very good studio monitor in an anechoic room (red). This monitor was measured in an acoustically good listening room at three positions around the listening position (below in green, yellow and light blue) and averaged over 30 positions in the vicinity of the listening position (dark blue). Right: Individual measurements at the listening position in an enlarged view for the 1 kHz octave

Frequency responses of various studio monitors (left) and examples of filter functions for location adjustment (right)

Spectrograms from two studio monitors: on the left with some resonances, on the right with an almost perfect decay behavior

Vertical isobars with a constriction in the transition area at 3 kHz (left). Very even horizontal isobars of a 3-way monitor with waveguides to improve the radiation behavior (right)

Distortion measurement at 85 dBA Leq at a distance of 4 m or 2 m. A signal with a 12 dB crest factor and a spectral distribution according to EIA-426B was used for the measurement (turquoise curve). The left monitor shows significantly more distortion components.

Maximum level with a maximum of 3% distortion for two monitor models. The monitor shown in red is uniform and free of weak points. The blue one is louder at certain points, but shows pronounced fluctuations.

The frequency response is the most considered and also one of the most important measured variables for loudspeakers.

Figure 2 shows some examples. The top three curves (blue, red and green) show what is commonly expected from a monitor: almost perfectly straight frequency responses with only slight fluctuations and a few small peculiarities. The red monitor helps itself with a little trick: At the upper and lower end of the frequency range there is a slight level increase of 2 dB, which could spontaneously provide a little more favor in a direct listening comparison. Another special feature is the tweeter, which works flawlessly up to 40 kHz.

If you compare that with the blue curve of a monitor with an aluminum dome, which is even more perfect up to 20 kHz, the difference becomes clear. Above 20 kHz, the limit of what is feasible has been reached for almost all spherical caps. The red monitor, on the other hand, works with an air-motion transformer, which is also able to cover the octave from 20 to 40 kHz in a relaxed manner. Everyone has to decide for themselves whether and how important this aspect is; but one could argue that the tweeter, which still works perfectly at 40 kHz, is also the better one at 20 kHz.

The green curve also shows a monitor with a dome, but in this case with a tissue membrane that does not have such pronounced resonances and therefore tends to say goodbye more inconspicuously above 30 kHz.

You can find the test reports of over 80 studio monitors in our studio monitor special.

All three monitors (blue, red and green) are so good in terms of frequency response that, despite the small differences, they would not result in an important decision criterion. The situation is completely different for the other two monitors with the orange and pink curves in Figure 2. On the one hand, there is a small 2-way monitor with a short transmission line for the woofer and, on the other hand, a broadband system. The sharp drop at 190 Hz is inherent in a transmission line and cannot be avoided. In terms of sound, this should be less critical, as the break-in is very narrow.

In the broadband system, on the other hand, the early level drop at both ends of the transmission range cannot be overlooked and also cannot be overheard; Such monitors are often used as listening devices as a second instance to simulate the hearing impression with a small radio or car radio - whether that still makes sense today remains to be seen, because small ghetto blasters or car radios in particular are now more likely to have a bass and Treble overemphasis is set and are therefore no longer adequately represented by the full range driver. Nevertheless, as a second instance, the broadband monitor can of course be a useful addition to other monitors.

The spectrogram should be studied as an important addition to the frequency response. The spectrogram can be displayed in two or three dimensions (waterfall diagram) and shows the decay behavior of the loudspeaker. The y-axis of the diagram is the frequency axis, the z-axis - or as here the color graduation - represents the level, and the x-axis is added as a time scale. Figure 3 shows two examples of this. Ideally, the spectrogram should fade evenly and quickly. If there are tailings above the time axis, then these are resonances of the loudspeaker, which can have various causes and should be avoided if possible.

The interrelationships between the ripples and the resonances can be easily seen from the frequency responses shown next to the spectrograms. In both examples, a strong resonance can be seen at the low frequencies, which is inherent to the system in loudspeakers. This is the basic resonance of the woofer in the housing or, in the case of bass reflex speakers, the desired tuning frequency of the housing resonator. In addition, in the ideal case, no further resonances should occur, as at least one of the two sample loudspeakers does perfectly well. If there is no spectrogram from a monitor, then with a little experience the problem areas with resonances can already be recognized in the frequency response. Narrow peaks, followed by similar dips, are almost always due to resonance effects. The reasons for this are mostly found in housing modes (modes = natural frequencies of the volume) or in partial vibrations of the membranes.

Research into the causes and avoidance of these problem areas are usually time-consuming and lengthy work, which is a very good way of distinguishing between how much effort a manufacturer puts into the development of its monitors or not. In fact, if you take a look behind the scenes, you can quickly see where a new monitor will be created in just a few days or in years of meticulous development work. The directivity or the spatial radiation behavior of a loudspeaker is also determined using frequency response measurements. Here frequency responses are measured depending on the direction. This can be done in one plane (horizontal or vertical) or very complex in a complete spherical grid around the loudspeaker - the latter is also required as a database for simulation programs. In data sheets or for test reports, diagrams with the horizontal and vertical directivity are typically shown.

The optimal form of representation for directivity are isobar diagrams, as you can see in Figure 4 using two examples. Depending on the frequency (x-axis) you can see how much the level drops compared to the value on the central axis (0 °) at angles of −180 ° to + 180 °. A relatively broad and uniform course of the isobars over the frequency would be ideal, especially for the horizontal plane. Towards the lower frequencies, all loudspeakers begin to radiate more and more broadly to the point of spherical behavior.

In our test reports, the mean opening angle from 1 kHz upwards is therefore always given, as well as the standard deviation for this range, which serves as a reference point for the evenness. A desired behavior could therefore be such that the monitor radiates horizontally wide (100 ° - 120 °) for a lot of freedom of movement at the workplace and rather narrow vertically in order to avoid unnecessary reflections from the work surface. In the diagrams from Figure 4, the transition from yellow to light green is the −6 dB isobar, where the level has dropped 6 dB from the center line. With the help of waveguides, the isobars can be designed evenly over wide frequency ranges, as shown in the example on the right in Figure 4.

The second example shows an unavoidable problem in the vertical for a multi-path system, where the paths are one above the other. There, where both paths are involved in the sound radiation - namely in the transition area at the crossover frequency - so-called "interference effects" with alternating cancellation and peaks occur due to angle-dependent transit time differences. If you rotated the box by 90 °, the problem would be solved for the vertical, but now it would have been moved to the horizontal, where it would be even more critical. This is exactly why 2-way monitors should always be set up so that the woofer and tweeter are on top of each other. In the case of a 3-way system, the midrange and tweeter should be on top of each other, because the problem is less dramatic for the transition from the woofer to the midrange, since at a lower crossover frequency the wavelength is already so large that the small loudness time differences do not have any significant effect show more.

All measurements discussed so far concern the linear transmission behavior of loudspeakers. In addition, there are also non-linear effects, commonly referred to as distortion, which are also very important for the quality of a loudspeaker. "Non-linear distortion" means that the loudspeaker receives a signal with a certain spectral distribution, reproduces it and adds further spectral components to the signal, which are referred to as distortions - which you really don't want.

If you send z. B. a pure sine tone over the loudspeaker, then in the spectrum next to the pure fundamental wave of the original there are also various harmonics, the harmonic distortion (THD). Loudspeakers generate significantly more harmonic distortion than any other device in the signal chain. While the electronics usually have distortion values ​​of 0.01% (= −80 dB) or even less, values ​​of 3% (= −30 dB) are already very good for loudspeakers. Many manufacturers therefore knowingly remain silent about the distortion values ​​of their loudspeakers. In addition, it is not exactly easy to measure this because the method of measurement is not clearly defined.

Two measurement methods are used for our monitor tests in SOUND & RECORDING. To do this, it is best to first look at the two series of measurements in Figure 6, in which a distortion limit value of 3% was initially specified; Then we determined what maximum sound pressure the loudspeaker achieved in relation to a distance of 1 m under free field conditions. For passive loudspeakers there is an additional power limitation in this measurement algorithm so that low-distortion loudspeakers are not destroyed by overload at some point - in active systems we assume that the internal electronics contain appropriate protective functions.

The measurement is carried out with 185 ms long sinus burst signals. The two curves show two monitors that are completely different in concept. The red curve runs very nicely and evenly without dips, i.e. without weak points. The blue curve, on the other hand, is rather restless. In some frequency ranges the loudspeaker reaches very high levels with 3% distortion, in other ranges the curve collapses completely. For the monitor with the red curve, one can conclude from this measurement that with all types of signals and also at low frequencies for sound pressures of up to 103 dB at a distance of 1 m, one can always be quite certain that there will be no more than 3% distortion produced.

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