Low Frequency Harmonic Distortion Sound Test

Background

This sound test aims at evaluating the quality of your audio equipment's lowest frequency range. Use this test to subjectively compare the output of different subwoofers.

Subwoofers - and speakers in general - vary in regard to their frequency range, sound pressure and distortion levels. One subwoofer may reproduce lower frequencies than another, or output a higher sound pressure level, but at the expense of a higher distortion. Choosing the right subwoofer needs more investigation than just picking the lowest pitch specification or the highest output level!

Distortion adds new spectral components to the original signal and thereby falsifies the sound (exactly as digital aliasing does at the other end of the spectrum). As distortion increases with the speaker's cone excursion, it limits the maximum usable output of the loudspeaker.

Use our Subwoofer and Low Frequency Response Test to roughly evaluate the lowest bound of your speaker system, then switch back to this page's test to judge how well it performs in terms of (the absence of) harmonic distortion.

The sound files

Each sound file plays a two-second sine tone that gradually fades in then out. Rows span one octave (a factor two in the frequency domain) and columns over four octaves, all the way down to subsonic frequencies.

           
80Hz 96Hz 112Hz 128Hz 144Hz 160Hz
           
70Hz 84Hz 98Hz 112Hz 126Hz 140Hz
           
60Hz 72Hz 84Hz 96Hz 108Hz 120Hz
           
50Hz 60Hz 70Hz 80Hz 90Hz 100Hz
           
40Hz 48Hz 56Hz 64Hz 72Hz 80Hz
           
30Hz 36Hz 42Hz 48Hz 54Hz 60Hz
           
20Hz 24Hz 28Hz 32Hz 36Hz 40Hz
           
10Hz 12Hz 14Hz 16Hz 18Hz 20Hz

The table must be interpreted as follows:

  • The top row (80-160 Hz) represents the highest frequencies a subwoofer is supposed to reproduce, though some frequencies may already be located above your subwoofer's cross-over frequency and be played back by your main speakers. This is fine.

  • The bottom row (10-20 Hz) consists of frequencies that are below the human hearing range (infrasound). Ideally, these frequencies should remain inaudible. Outstanding subwoofers coupled with excellent hearing will reach 18 Hz maybe 16Hz, but never 10 Hz. 10 Hz is located one octave below our lowest limit: there is no way humans can hear such a frequency.

  • As you play with infrasound, you may experience a shaking of your physical environment, sometimes worse :o) . This should be the only tangible sign that your subwoofer is working. No sound should be produced.

  • Do not turn your amplifier's level up while listening to these inaudible sounds, as you may end up in blowing your speakers. Always set your maximum listening level using the top row audio files.

  • The table is arranged so that each file represents the overtone of one of the files located below.

  • Since all these test files are made of pure sinusoidal tones, there is no overtone present in any of them. If you hear something that sounds similar to the sound of a file that is located above the file you are currently auditioning, it must be Harmonic Distortion generated by your sound system!

In other words, to check for Harmonic Distortion, proceed as follows:

  • Start from the top row, set your listening levels and get used to the sound of pure tones: the top row frequencies should be free from audible harmonic distortion (even when played back though cheaper sound systems).

  • Listen to the bottom row: these files should remain inaudible (*). If not, proceed to the next step.

  • Explore the intermediate rows and try to figure out when harmonic distortion starts to appear. If your system suffers from massive harmonic distortion, the main sine tone will fade away as you move down to lower frequencies, while overtones will gradually appear. How will you figure this out? The overtone will sound exactly as one of files located on the top of the file you are currently auditioning... hopefully at a lower level.

Properly measuring harmonic distortion requires a calibrated measurement system and testing software (see our external link section below). Our subjective test only highlights audible distortion, that is to say a severe distortion beyond any acceptable tolerance!

The acceptable benchmark for loudpseaker is less than 10% Total Harmonic Distortion (THD) across its working frequency range. In such a case, distortion products are located 20 dB below the main signal at least, and should be masked to our ears.

(*) Due to the mp3 compression that is used to preserve the bandwidth on this site, you may hear a faint rustling noise when playing infrasonic files. To get rid of the mp3 encoding, simply download the associated .wav files by clicking the download arrow next to each sound, and play the downloaded files straight from your hard disk. Using the .wav files will remove the rustling artifacts.

Related pages

Other subwoofer tests

Other distortion test

External Links

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