Reaching The Maximum
The program I use most for modeling the directivity of loudspeakers and for crossover design is SpeakerLab (from AFMG). A nice feature of SpeakerLab is that it calculates the broad band level of the signal driving the individual pass bands.
If the maximum input voltage (MIV) has been entered in the data files for these pass bands, then the program is able to determine the maximum input level for the complete system.
The maximum level for the loudspeaker is based on the spectral content of the input signal to the loudspeaker, as well as the crossover and EQ filters used for each of the pass bands. This feature can help to give a good indication of the maximum continuous SPL that the loudspeaker can produce.
Using our two-way loudspeaker example, the MIV level for the HF pass band is 23 dBV. This is an rms voltage of about 14.1 volts, equivalent to approximately 25 watts at 8 ohms (the rated impedance of the HF driver).
Note that MIV is not the same as “power handling” for a loudspeaker. The MIV level for the LF pass band is 31 dBV, an rms voltage of about 35.5 volts, which is equivalent to approximately 300 watts into 4 ohms (the rated impedance of the LF driver).
Let’s look at some screen shots from SpeakerLab (Figure 8 and Figure 9).
From these we can see what the maximum input level to the system (upstream of the filters) is when the effects of the crossover and EQ filters are taken into account. The maximum input level to the system based only on the HF pass band and its filters is 39.3 dBV (92.3 volts). We have basically muted (disconnected) the LF pass band from the system to determine what the HF pass band can do all by itself.
We can then reverse this and mute the HF pass band so that only the LF pass band is being used. The maximum input level to the system based only on the LF pass band and its filters is 32.5 dBV (42.2 volts). This is about 7 dB lower than the maximum system input level based on the HF pass band. The LF pass band will be the limiting factor for the maximum input for the system, which will be 32.5 dBV.
What It Means
This analysis is based on an input signal having the spectral content specified for the IEC 60268 noise test signal. There is a roll off of the low-frequency and high-frequency energy in this signal (blue curve in Figure 8 and Figure 9).
It approximates the spectral content for an average of many different types of music and speech. The results of our analysis might be different for other input signals with different spectral content, such as pink noise.
With the IEC 60268 signal at an input level of 32.5 dBV to the loudspeaker system, the maximum continuous SPL that it should be able produce is about 122 dB, referenced to 1 meter (Figure 10).
Hopefully this has given you some insight into a few of the possible benefits of using full directivity loudspeaker modeling data for each individual pass band.
Next time we’ll look at what kind of testing is required to determine the MIV for the individual pass bands of a loudspeaker.
In the meantime, for additional information on modeling the directivity of loudspeakers, check out this AES paper: S. Feistel, W. Ahnert, C. Hughes, and B. Olson. “Simulating the Directivity Behavior of Loudspeakers with Crossover Filters”, 123rd Audio Eng. Soc. Convention preprint #7254, 2007 October.