What Are Low Frequencies?

Low frequencies are big sound waves. Because of their size, they present some particular problems for us in the recording business. Both in terms of loudspeaker design and room design, low frequencies exhibit particular behaviors that require a different way of thinking, vis-a-vis mid or high frequencies. To make matters worse, we perceive low frequencies differently than high frequencies as well. But that part of it is a different problem for another time. Onward!

Wavelength

Actually low frequency waves are long waves. The speed of sound is more or less constant (at 1130 feet per second), so it is obvious, when you stop to think about it, that low frequencies, which take longer to happen, generate waves that travel further before the next wave begins. A 1 Hertz wavefront travels 1130 feet before the next wavefront departs. A 1 kHz. wavefront travels 1.13 feet before the next wavefront departs. See the table below.

Frequency in Hz.	Wavelength	quarter wavelength
20	56.50	14.13	feet
25	45.2	11.3	feet
31.5	35.87	8.97	feet
40	28.25	7.06	feet
50	22.6	5.65	feet
63	17.94	4.48	feet
80	14.13	3.53	feet
100	11.3	2.83	feet
125	9.04	2.26	feet
160	7.06	1.77	feet
200	5.45	1.41	feet
250	4.52	1.13	feet
320	3.53	0.88	feet
400	2.83	0.71	feet
500	27.12	6.78	inches
640	21.19	5.3	inches
800	16.95	4.24	inches
1000	13.56	3.39	inches
1280	10.59	2.65	inches
1600	8.48	2.12	inches
2000	6.78	1.7	inches
2560	5.3	1.32	inches
3200	4.24	1.06	inches
4000	3.39	0.85	inches
5120	2.65	0.66	inches
6400	2.12	0.53	inches
8000	1.7	0.42	inches
10240	1.32	0.33	inches
12800	1.06	0.26	inches
16000	0.85	0.21	inches
20480	0.66	0.17	inches

Table 1. The wavelengths and quarter-wavelengths for each 1/3rd octave center frequency of the audible spectrum. Note that wavelengths below 500 Hz. are given in feet, while from 500 Hz. on up they are expressed in inches. These magnitudes are worthy of your careful study and contemplation.

In any case, a typical room of the sort in which we live and work is usually no more than about nine feet high, with a longest usual dimension of something around 35 feet (20 feet is probably average for residential rooms). I consider the frequency whose wavelength equals the smallest room dimension as the frequency that begins to present room problems as a “low” frequency, or about 140 Hz., whose wavelength is eight feet (equal to our typical residential ceiling height).

Thank about it. If we have a twenty-foot long living room, and a loudspeaker at one end of it plays a 20 Hz. sine wave (56.5 feet long), that wave will go the length of the room, reflect back, and then reflect back again about 3/4s of the way before the next wavefront in that 20 Hz. wavetrain is generated. It is intuitively obvious that the room is not large enough to allow the wave to fully propagate without interference. No such wave can be free of significant constructive and destructive interference in that room.

This characteristic of low frequencies, that their wavelengths are larger than the dimensions of the rooms they are being performed in is, in fact, a problem.