Identifying and Treating Room Resonances: How do you find resonant frequencies?

In this video, you’ll learn about how resonances are affecting your room, as well as how to find and treat them. Get free advice from one of our talented acoustic designers: “Resonances are frequencies that linger too long in a room, often with unpleasant consequences for the sound happening in the room. Bass resonances can produce muddiness or boominess, where midrange and treble resonances can produce harshness or intelligibility or clarity issues. One of the main goals in acoustically treating a space is to reduce the negative impact resonances have on sound in the room. Resonances usually happen at specific frequencies. Frequency is a measurement based off time. The Hz indicates how many times per second the wave cycles. If we multiply that by the speed of sound we can derive the wavelength which is the physical distance for the complete evolution of the wave. If we play a sound wave in a space with the same dimensions as the wavelength of the sound then that sound will reflect off the surfaces of the room and overlap with itself. When sound waves overlap the energy gets combined and creates phase interference. An in phase sound wave overlapping itself will suffer from constructive interference where the peaks and troughs of the wave are enhanced and the sound becomes stronger. Every Rectangular room will have three main axial room modes that correspond to the depth, width and height of the room, but other modes & resonances can develop based on more complex geometry. We have a whole video on room modes if you’d like to learn more. Keep in mind that your room isn’t the only thing that can cause resonances. Certain instruments will have notes that resonate more when played based on the geometry of the instrument. Vocal recordings often show resonances through harshness or muddiness in the midrange, with certain frequencies being too loud. These resonances then affect compression, saturation, and EQ that are typically applied to vocal recordings in a mix, and make it harder to process them for desired tones. So how do you know where a resonance is coming from? The first step in finding and eliminating a resonance is to test your room. This will give you a better understanding of where (in terms of frequency) the resonances are in the room in general, and also at specific listening spots. For low end resonances, look at time-based data that shows decay time per frequency, like a waterfall graph or a spectrogram. For a simple, more practical real-world test, you can do a listening test with filtered pink noise. GIK provides one online, where all the frequencies from 400Hz on down are playing at once. With filtered pink noise playing through your system, you can move around the room and listen (or measure with an SPL meter/app) for louder frequencies that indicate resonances. Good placement (for speakers, listening position, etc) in a room can help avoid resonances, but a comprehensive bass trapping strategy is the real way to combat them. Typically broadband and/or range limited bass traps are the best place to begin, though we can also use tuned Scopus traps to target specific resonances under 100Hz. Tuned traps are meant to treat resonances that remain once the main bass trapping strategy is in place. GIK a lot more information on Bass trapping if you’d like to know more. For resonances in the midrange and treble, often the culprit has to do with early reflections. This topic has also been well covered in GIK educational materials, but when reflected sound waves interfere with new sound waves being created, you will get cancellation and resonances causing some frequencies to be louder and linger longer in the room. So treating the room for early reflections -- both in recording and in monitoring scenarios -- can reduce this interference and give a more natural and accurate sound.“