Here are the easy and the hard parts of controlling the acoustics of rooms.

By Marshall Chasin, AuD

Pictured: Figure 1. 130 Hz resonance

From time to time, audiologists are asked about strategies (and products) that can be used to reduce the echoes in rooms in hopes of improving speech intelligibility or to “flatten” the room’s acoustics for music listening and playing. Other than seeking out the assistance of an acoustic or audio engineer (which actually can be well worth the expenditure), this is a primer on how sound behaves in a room, and how it can be modified.

An audiologist may be approached by an “audiophile” who has done a spectral sweep of a room and noticed an unwanted resonance at 120 Hz or 130 Hz, for example. How can we best respond to this inquiry?

Low Frequency vs. High Frequency

All sound—including speech and music—can be classified as “low frequency” or “high frequency.” These gross categories are different for speech and music and sometimes lead to confusion.

“Low frequency” for speech means 250- 500 Hz, whereas low frequency for music is on the left side of the piano keyboard (below 250 Hz). But suffice it to say that “in general” low-frequency sounds have long wavelengths and are acoustically “blind.” Low-frequency sounds do not “see” obstructions such as walls and furniture, and can be heard in the next room with minimal attenuation. The longer wavelengths associated with lower-frequency sounds provide us with the best explanation. As long as the obstruction (such as a wall) is less than 1⁄2 the wavelength of the sound, that obstruction will have no effect. That is, long-wavelength low-frequency sounds will pass through the obstruction as if it were not even there.

The wavelength of 250 Hz (middle C on the piano) is 136 cm or around 4 1⁄2 feet. A wall or obstruction would have to be at least 1⁄2 of 136 cm (68 cm or 2 1⁄4 feet) to be attenuated, and there are very few walls that are at least 2 or 3 feet thick. Understandably, then, it is very difficult to rid a room of unwanted resonances and sound in the lower frequency region.

In contrast, higher-frequency sounds have shorter wavelengths, and using the “1/2 wavelength rule,” it may only require an obstruction of a few cm (or even the width of a face mask) to attenuate these higher-frequency sounds.

Understandably, there need to be different acoustic treatment strategies for the attenuation or flattening of unwanted sounds or room resonances for the lower frequency bass notes than for the higher frequency treble notes. This is as much the case for a classroom as it is for a concert hall, or a music production room in someone’s basement. And the acoustic treatment can (and initially should be) as simple as relocating a desk, or even using a separate bass speaker so that the main speaker doesn’t have to work as hard.

The Very Easy Part

It may very well be the case that the location of a desk or listening area in a room is the culprit- perhaps being too close to a wall or a corner where unwanted sound can build-up. So before making any significant changes, first just try moving around the furniture.

The Easy Part

In general, the presence of certain obstructions such as wall coverings, furniture, people in the room, acoustic tiling for the ceiling, and room carpeting can all serve to attenuate unwanted higher frequency sound reflections and sound buildups. For example, libraries are ideal in the sense that unwanted reflections are attenuated by the presence of books, bookshelves, carpeting, and soft cushy chairs. In contrast, school gymnasiums can be an acoustic nightmare.

The More Difficult Part

The control of low frequencies in a room is a bit more difficult since they are myopically blind to obstructions. For low-frequency sounds, placing a baffle in front of a loudspeaker, or indeed even placing a loudspeaker upside down and backwards and behind a couch, will have no effect on the sound. And low frequency sounds coming from a loudspeaker are also non-direc- tional, so the loudspeaker can be aimed in any direction without much of an effect.

This is why home theater stereo systems are called “5.1” despite consisting of six individual loudspeakers. You can place the bass speaker—the sixth speaker—any-where, and the placement does not contribute to the spatial separation of the sound.

There can be many reasons for an unwanted build-up of low frequency sounds in a room. It may be that a person’s desk or where they happen to be sitting is too close to a corner, or a wall. Moving the chair and/or desk away from corners could improve the sound quality, as this will decrease the bass response slightly. There is no such thing as a bass frequency “baffle.” It would need to be at least 4 to 5 feet in thickness and would take up an appre- ciable amount of the room space. Instead, a Helmholtz resonator would be needed.

Table 1. Typical Helmholtz resonances for a triangular shaped box that is 4 feet tall and 15 inches on each side that can be placed in the cor- ner of a room. To build a Helmholtz resonator that will remove 120 Hz, the hole needs to be 1.5 inches in diameter (or 3.81 cm). Larger holes (or more holes adding to the same cross-sectional area) will create a box with a higher resonant frequency. These data are only approximate, and the actual resonance may be slightly lower due to an “end correction,” which is related to the thickness of the (3/4-inch) wood used.

Low Frequencies and a Helmholtz Resonator

We have seen (heard?) Helmholtz resonators whenever we blow across the top of a pop bottle. Essentially two things are required: a narrowing or small hole (such as the pop bottle neck) and a volume of semi-trapped air behind the hole. And by varying the hole size and/or the volume of trapped air, we can change the Helmholtz resonant frequency. Sounds in the room that are near that Helmholtz resonant frequency will be “sucked” out of the room and into the resonator. Helmholtz resonators are quite easy to build and require a fairly large-sized box with a “tuned” hole.

We run into this phenomenon in the study of speech acoustics. High vowels such as [i] and [u] have a narrow constriction between the blade of the tongue and the roof of the mouth, and due to this narrowing (or constriction), have a very low-frequency first formant (F1). Less constricted vowels, such as [e] or [a], have higher frequency first formants… a greater cross-sectional area of air flow between the blade of the tongue and the roof of the mouth.

Here is a formula for how you can tune a Helmholtz resonator to remove unwanted lower-frequency resonances:

F = speed of sound/pi (area of hole/ volume)1⁄2

This equation is actually quite simple. It merely says that the resonant frequency (that will suck away any unwanted low frequency sound) is proportional to the cross-sectional area of a hole (or series of smaller holes) and inversely proportional to the volume of the wooden enclosure. I typically use 3/4-inch plywood to make a box that is around 4 feet high with sides measuring about 15 inches to 20 inches, screwed together, and caulked along the seams.

To take up less room, you can create a triangular box that traverses any corner of a room diagonally. Table 1 shows typical Helmholtz resonances for such a box (4 feet tall x 15 inches x 15 inches on a side, with the front of the triangle being 21.5 inches), but with different sized and/or equivalently, number of smaller holes. This box can be placed in the corner of a room and, with baffles and wall treatment to handle the higher frequencies, can “smooth” out the acoustics quite nicely.

When it comes to “conventional wisdom” (e.g. a Google search), you may see mention of something referred to as a “bass trap.” These are boxes that are simply filled with foam rubber and may have a slight bias toward the bass notes, but are essen- tially just sound dampers. These can work well for attenuating echoes in rooms (as can furniture and the presence of people), but they are misnamed, and in many cases merely a marketing name.

For such a box to have any specificity toward only attenuating unwanted low-frequency resonances, you would instead need to make the aforementioned Helmholtz resonator, with a tuned hole in the box. For the example of an audiophile who wants to remove a 120 Hz resonance, a hole measuring 1.5 inches (or 3.81 cm) in diameter would be required for a box of this size.

About the author: Marshall Chasin, AuD, is an audiologist and the director of auditory research at the Musicians’ Clinics of Canada, adjunct professor at the University of Toronto, and adjunct associate professor at Western University. You can contact him at [email protected]

Original citation for this article: Chasin M, Improving Room Acoustics. Hearing Review. 2024;31(6):08-09.