Directivity pattern: how to interpret the radiation pattern of speakers

This is how you read polar plot and isobaric charts for your speaker selection

In which direction should the speaker radiate? A question which is part of the daily routine for expert planners can cause headaches time and again for newcomers and users. The answer: that depends on radiation pattern and requirements. The term directivity pattern is used more and more for describing the directivity of speakers. Given the international nature of the speaker market, this term is also used in German. However, what exactly is meant by directivity pattern? How is it displayed and which conclusions can interested people draw from it? Here you can find an overview.

Is the directivity pattern useful when planning and selecting PA speakers?

You are only able to precisely plan a PA system if you know the radiation pattern. For only then it is clear which area the speaker covers within its radiation angle. Expert planners often speak of a 'nominal radiation angle', because the radiation angle always relates to a specific frequency. This is necessary, as low frequencies radiate differently from high frequencies. A ground rule is, the higher the frequency, the more directional is the radiation pattern of the speaker.

The radiation pattern of clusters

The radiation pattern is particularly important if several speakers are supposed to form a cluster. This is almost always the case when several speakers have to cover a specific area from one direction. If you know the radiation pattern you can arrange the speakers in a way that ensures continuous volume on as broad a front as possible. The radiation angles overlap in a way that the sound adds up and no gaps can occur in the cover. For this, a constructive (not destructive) interference is required. Sound waves must not cancel each other out, e.g. because speakers are offset on stage. A couple of centimetres can make the difference between constructive and destructive interferences. Here, the so-called sweet spot is particularly important. That is the area in which people can move about without significantly changing the sound of a speaker. The most important methods to display the radiation pattern of speakers are polar plots and isobaric charts.

In the acoustics room with Frank Kuhl

Planning ceiling speakers¶

The polar plot, also called polar diagram

One version is the so-called polar plot as shown here in the image. This image shows the speaker from above, located in the small circle in the middle. In this top view, the speaker is directed to the north. That is, the northernmost part of the circle, here marked with 0°/0 dB, is the reference point towards which the speaker is directed.

Figure on the left: polar plot for the speaker EUL-2/SW.

In the polar plot, we see 3 values whose interaction is interesting for the positioning of wall speakers.

The (in this case horizontal) radiation angle: 360°, circular
The volume in dB: the further out on the circle, the louder the speaker. In our example, each additional inner circle represents a decrease of 6 dB.
The frequencies: each colour indicates one test frequency.

How to interpret the polar plot

At the northernmost point, i.e. precisely horizontal central in front of the device, speakers are the loudest. Our example shows that even at an angle of 30° to both sides almost all tested frequencies are of consistent volume. Only at 4,000 Hertz the volume decreases slightly. Typically, the low frequencies radiate specially wide. To be precise, if a person stands about 60° to the right of the speaker they will hear a sound with the frequency of 1,000 Hertz just about 5 dB quieter than someone standing right in front of the speaker. A ground rule indicates, a loss of more than 6 dB is too high to say a speaker covers that area. Thus, the speaker in our example has an effective radiation angle of 140° (70° in both directions on the circle). In this case, you can specifically use sketch and protractor when planning PA systems in rooms. The vertical radiation angle can be neglected if the ceiling is of normal height. When dealing with acoustically challenging rooms (halls, houses of worship etc.), it can be beneficial to use speakers which are vertically highly focussed, because they induce fewer sound reflections from floor and ceiling.

Please pay attention when comparing products and check the labelling of diagrams. Diagrams can only be compared if the same measurements are used. There are no legal requirements. Similar to the frequency response for speakers, some retailers try to show an improved picture of the radiation pattern by changing measurements in diagrams. Typically, the dB-intervals regarding the volume are smaller. So please take care that the intervals are 6 dB as usual.

The basics on wall speakers

Planning PA systems

The isobaric chart

In an isobaric chart, we can see the angles on the Y-axis, often marked as 'deg' for degree. The X-axis shows the frequencies. Coloured areas indicate how strong frequencies are in individual radiation angles. In this case, the colour code is graded in steps of 3 dB, from warm colours (no or little loss of sound level) to cold colours (significant loss of sound level).

Once you understood isobaric charts, you can quickly compare them. If the X-axis and Y-axis are labelled consistently, you can see in the isobaric chart at first glance where different speakers have their strengths and weaknesses compared to others. Similar to the frequency response, an image as even as possible would be desirable here.

Figure on the left: isobaric chart for the speaker EUL-2/SW.

Once you understood isobaric charts, you can quickly compare them. If the X-axis and Y-axis are labelled consistently, you can see in the isobaric chart at first glance where different speakers have their strengths and weaknesses compared to others. Similar to the frequency response, an image as even as possible would be desirable here.

Figure on the left: isobaric chart for the speaker EUL-2/SW.

How to interpret an isobaric chart

In our specific example you can see that frequencies before 400 Hz are the lower midrange. We perceive this range as specially warm and pleasant. In the example mentioned above, the speaker here only loses 6 dB at a radiation angle of 200°. Therefore, the tested EUL-80 is clearly a speaker with strengths in the bass range and lower-midrange and the lower and middle treble between 4,000 and 6,000 Hertz.

With our magazine for PA technology we hope to make the work of installers, expert planners and newcomers easier and provide inspiration for practical applications and planning. You can also browse through PA-related topics or read here how our colleague Frank Kuhl optimises speaker series for you.

Contact us now, we look forward to receiving your enquiry.