Measure frequency response - how it works

The frequency response of loudspeakers cannot be easily measured. Manufacturers of loudspeakers and research institutions have the necessary equipment, from microphones and other transducers to a special room. We show you how the frequency response of a loudspeaker is measured professionally and then show you applicable compromises to roughly determine the frequency response of a loudspeaker at home and in the recording studio.

Measure frequency response under ideal conditions

To measure the frequency response of a loudspeaker, all other sizes of your signal chain should be frequency-independent if possible. The ideal conditions are only given in scientific laboratories. The frequency response of a loudspeaker is measured taking into account the following aspects:

1. Microphones themselves have a frequency response that never forms a perfect horizontal line. However, for mics from the middle price segment, the frequency response is often included in the manual. You can use this to calculate the influence of the microphone frequency response.
2. Rooms also have a kind of frequency response. These are called room modes or resonance frequencies. Ideally, you are not in a room during the measurement, but in an open field. Special free-field spaces, as in the photo, simulate a free field in that the walls absorb most of the sound and hardly reflect it.
3. High-quality microphone preamplifiers and A / D converters for the micro and D / A converters for the loudspeaker ensure that the converters do not falsify the measurement.
4. A special test signal such as the "exponentially swept sine signal" is particularly suitable as an input signal for the loudspeaker for weakly nonlinear systems from Angelo Farina.
5. A typical distance for measuring is a distance of one meter. There may be overemphasis on low frequencies, which have high amplitudes in the near field of the loudspeaker, but do not spread into the far field, i.e. to the listener. At larger distances, especially high frequencies show strong fluctuations with only slight changes in the position of the microphone.
6. Speakers do not emit every frequency equally in all directions. Therefore, the measurement is often repeated for dozens of different angles. This gives you three-dimensional polar plots for each frequency that show the amplitude per solid angle.
7. Alternative measurement methods work with lasers or with ultra-light accelerometers.

Measure frequency response at home

To measure the frequency response of your speakers at home, you have to make significant cuts. Even in the recording studio with professional recording equipment, the conditions are not ideal. The accuracy of your measurement will be very low. Nevertheless, you can find special over- or underemphasis on frequency ranges. Make the following compromises for the approximate measurement of a frequency response. We show you an exemplary implementation with Audacity in the following picture gallery.

1. Before you prepare to measure a frequency response, be sure to check the operating instructions for your loudspeaker and the Internet to see whether there is any information about the frequency response of your loudspeaker model.
2. If a frequency response was supplied for your microphone, you can later add the mirrored frequency response to your measurement. Otherwise, assume that the heights, in particular, are rather weaker.
3. To minimize the influence of the room on your frequency response measurement, you should dampen it as much as possible. Draw heavy curtains, possibly lay out a few blankets and, if you live in a quiet environment, open all the windows.
4. Use a Kronecker delta pulse, an exponential sine sweep or white noise as a test signal, which you can find on YouTube, for example.
5. If necessary, measure at a distance of less than one meter from the loudspeaker so that the ratio of direct sound from the loudspeaker to the reverberation of the room is as high as possible.
6. It is best to move the loudspeaker after each measurement. Vary the distance from the closest walls, floor, and surface where the speaker is located. If you average the recordings at the end, you have a somewhat more reliable measurement of the frequency response.

With the right equalizer settings on your power amplifier, you can reduce narrowband resonances and amplify or attenuate broadband frequency ranges. On the following pages we show how to use an equalizer and how to set your amplifier correctly. In further practical tips, we will introduce you to the advantages of different types of loudspeaker connections and explain how you can connect loudspeakers in series and in parallel.

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Prepare your measurement by going to "Edit" -> "Settings in Audacity

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Switch on only one loudspeaker and avoid noise. Now start a recording by pressing the [R] key. A mono recording is sufficient.

Now play the YouTube video with the test signal. Mark the entire test signal after the end of the recording and analyze it via the menu item "Analysis" -> "Frequency Analysis".

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The spectrum is now the frequency response of your signal chain. It contains in particular the spectrum of the input signal, the loudspeaker, the room acoustics and the microphone. So-called "comb filter effects", ie regular peaks and valleys, are typical signs of overlapping of the loudspeaker signal with reflections, especially from the next wall.

Reposition the speaker a few times and repeat the measurement. Finally mark all recordings and carry out a frequency analysis again. In this case the frequency response has smoothed out. Nevertheless, the lack of frequencies above 17 kHz and the unsightly resonance peaks in the low range are still clear. Unfortunately, under these conditions, you can never be sure whether the speaker frequency response or your room is causing these stresses.