SENIOR THESIS

Noise exposure associated with marching bands

PREPARED FOR MUSIC EDUCATORS

Joseph Keefe

Dr. Dewey Lawson, faculty advisor

Duke University Department of Physics

Everyone knows marching bands are loud. Students and teachers joke about hearing loss and going deaf at rehearsals, and no one wants to stand next to the drums because you can’t hear yourself think. Perhaps you know that marching bands are dangerously loud – enough to cause permanent hearing loss. If so, I hope your band students know, too; they have the right to safe rehearsals and performances. Read on to find out how to protect everyone’s ears.

During the 2003 marching band season, I used a sound level meter to record the decibel levels of the Duke University Marching and the Riverside High School Marching Band (Durham, NC). These bands are 85 (78 instrumentalists) and 61 (42 instrumentalists) members in size, respectively. Interfering as little as possible (I told everyone to treat me like a competition judge), I tried to get the microphone as close as possible to band members’ heads during all measurements. Since I’ve been in marching band for nine years, I used my experience to help me take measurements as realistically as possible.

So, how loud is unsafe? There is plenty of debate over that question; I have chosen to use recommendations from the National Institute of Occupational Safety and Health¹. While these numbers are designed to be used for noise exposure in the workplace, they do a reasonable job of letting us know when exposures from a band are dangerous.  

To adjust the response of a sound level meter, frequency-weighting scales are used. The most often used weighting scale (and the scale used in almost all noise standards) is the A scale, expressed as dBA or dB(A); it approximates the response of the human ear to moderate level sounds and includes a large low-frequency drop-off. The C scale approximates the response to high level sounds, and incorporates small drop-offs at both low and high frequencies. Despite the fact that this study deals with high level sounds, A weighting is used throughout for comparison with noise exposure standards. Below are the combinations of exposure and duration that are considered safe.

            The above numbers mean nothing if you are unfamiliar with decibel measurements. Scientists and acousticians use a measure called sound pressure level to gauge how loud noises are. SPL is a scientific quantity closely related to what is usually referred to as loudness, and is measured in dB (decibels). Below are some typical SPLs of easily recognized sound sources².

For all of my measurements at Duke and Riverside, exposures are colored green (no risk for the duration of the measurement or the estimated exposure time), orange (risk for the estimated exposure time but not the duration of the measurement), or red (risk for both the duration of the measurement and the estimated exposure time). Estimated exposure times are in parentheses.

In some cases Riverside High School used an amplified Dr. Beat metronome during rehearsal. Exposure from this metronome, or click track, is denoted “CT”.

The measurements show that the safest locations are inside without the drumline and outside in the woodwind section. The most dangerous locations are both inside and outside near drums or brass. Since typically dangerous situations occur in every band, chances are that most band members will need some form of hearing protection to remain safe.

            The challenge for you as a band director is to retain the level of musical quality while protecting your students. Thankfully, this is not difficult with musician’s earplugs. Musician’s earplugs are specially designed to reduce music to comfortable levels without significantly distorting speech or the music itself. Sound good? It did to me when I first heard about it. Read on to find out more.

            Most of you have probably worn foam earplugs at some point in your life; you can get them at drugs stores and pharmacies. Foam earplugs are very good at reducing sound – too good, in fact. They’re great for using power tools or mowing the lawn, but foam earplugs reduce high frequency sounds a lot more than they reduce low frequency sounds. High frequency means anything over 2000 Hz, which includes lots of information found in speech and lots of information found in the upper overtones of musical instruments. In a human ear, there is a resonance at 2700 Hz, and when you stick an earplug into that ear, the resonance is eliminated, distorting speech and music.

Figure 1: Attenuation characteristics of a typical industrial foam earplug that is deeply inserted into the ear canal.³

As you can see, the response is not “flat” at all, which makes everything sound strange.

So, are foam earplugs better than nothing? Yes and no. Obviously, if people are being exposed to dangerous levels, foam earplugs will offer protection. I used foam earplugs for three years when I was in high school. I’m sure they protected my hearing, but I was constantly pulling them out so I could understand what people were saying and putting them back in to play. This is quite an annoyance, plus it gets the earplugs dirty and wastes time. Also, it’s definitely not a good idea to have your entire band wear foam earplugs. Generally wind players wearing foam earplugs tend to overblow to replace lost harmonics and percussionists tend to overstrike to replace stick strike and head slap noises.

            Musician’s earplugs differ from foam earplugs because they have a flat response. Musician’s earplugs eliminate the effects of closing off your ear canal, so that the spectrum of the sound is reduced overall, but the shape doesn’t change. Etymotic Research, Inc. has produced two flat-response earplugs, the ER-15 and the ER-25, respectively providing 15 and 25 dB of sound attenuation regardless of frequency.

Figure 2: The effect of the ER-15 earplug on a violin playing at 440 Hz. The white spectrum in the un-attenuated sound level and the black spectrum is the attenuated sound level.³

When I learned of ER-15 and ER-25 earplugs, I thought the idea was fantastic, so I got a pair of ER-25s (I’m a percussionist), and I started wearing them to football games, basketball games, indoor band rehearsals, and individual practice sessions. Immediately my headaches after playing went away, as did the muffled feeling I would get in my ears. If you ever watch people as they leave a loud band rehearsal, chances are they’ll be yawning. It’s not because they’re tired, it’s because they can’t hear as well and their brains think their ears need to pop. It doesn’t work, of course; only time and rest can heal temporary hearing loss. Once the loss becomes permanent, nothing can reverse it.

Figure 3: A pair of ER-15 / -25 earplugs.⁴

I wear my ER-25s whenever things get loud (including throughout my measurements). Unfortunately, ER-15s ands -25 are rather expensive (between $125 and $200, including production and initial fitting of the necessary earmolds). Personally, I feel it is well worth the one-time cost for any musician who plans to participate in ensembles for a long time. After all, they are no more expensive than a set of long-use contacts or a pair of glasses, and I feel they have the same medical benefit. If you or your students are purchasing their own individual earplugs, I would get ER-15s or -25s.

The procedure for ER-15/25 fitting is very simple. You must visit an audiologist, who will take a mold of our ear canal. This mold gets sent out for production, and in about two weeks you get a set of custom-fit earplugs with the ER-15 or 25 inserts. You can even order both sets of inserts and switch them back and forth in the same mold for different levels of protection. If you haven’t had an audiogram in a while, or if you don’t have one on record at all, you should also have that done.

            So how do these ER earplugs actually work? Some of you may be content to accept the fact that they just do. For the rest of you, here’s a short explanation. Familiarity with basic physics and the understanding of an RLC circuit will help.

A schematic of the ER-15 is shown below. The ER-15 and -25 earplugs use a small button containing a thin plastic diaphragm and an acoustic resistance (R1 in the schematic). The compliance (C1) of the diaphragm is selected to produce the desired 15 dB of attenuation at low frequencies.  At high frequencies, because the normal open ear produces a boost of about 15 dB at 2700 Hz, 15 dB of protection at 2700 Hz requires 0 dB of attenuation through the earplug at that frequency.  In order to produce that 0 dB of attenuation, the dimensions of the sound channel in the earmold are adjusted so that the acoustic mass (L1) of the air in that channel resonates with the diaphragm compliance and forms a peak at 2700 Hz. Finally, an additional tuned resistive element (R2) is added to smooth the peak. For an ER-25 earplug, the compliance is reduced to increase attenuation and the diaphragm and resistors are adjusted to maintain the shape of the response.⁶

Figure 4: The ER-15 earplug; C stands for compliance, R for resistance, and L for inductance.⁷

            If you or your students can’t afford ER-15 or -25 earplugs, there is an alternative, the ER-20. It is neither as flat in response nor nearly as expensive as the ER-15 or -25. This is not a custom plug; it is one size fits most, and retails for less than $15. ER-20s are available via the internet and in an increasing number of retail stores serving musicians. The ER-20 consists of a triple flange design that when fully inserted into the ear provides a semi-flat response with 20dB of attenuation.

Figure 5: A pair of ER-20 earplugs.⁸

Figure 6: Attenuations of ER-15, ER-20, ER-25, and foam earplugs.⁹

An ideal situation would be for your school’s music program to provide ER-20 earplugs to all of its participants and encourage their use. ER-20s can be purchased in bulk for a discounted price, and hopefully the expenditure is quite justifiable either to your school administrators or to your band parents or band booster organization. Using ER-25 earplugs, every decibel level measured would be reduced to levels below 85 dB. ER-20s or -15s would reduce exposure to safe levels for the durations of exposures.

♪ ACKNOWLEDGEMENTS ♪

Dewey Lawson

Riverside High School Marching Band Members

Ken Davis, Riverside Band Director

Duke University Marching Band Members

Neil Boumpani, Duke Marching Band Director