Protecting Your Most Essential Acoustical Tester

Richard Honeycutt reminds us the importance of protecting our most essential acoustical tester – our own ears.

Personal Experience
I first became aware of hearing-damage risk from loud sounds in the mid-1960s, in the context of emerging loud rock bands. My own rock band was considered loud by my parents. My neighbors never complained, although they did comment on hearing us practice. Since we did not mic the drums, trumpet, sax, or trombone, and my 50-watt (advertised power) Silvertone Twin-Twelve guitar amp comprised the sound system for the electric guitars, bass, and, yes, vocals as well, I can say with some confidence that the maximum level heard by band members was not over about LZF = 114 dB. (This was back when distorted guitar sound was not considered a good thing.) We always had to keep the drummer in check, since a trap drum kit averages about LZF = 119 dB at one meter, which would drown us out. In fact, the level was probably well below that, since the average level of a 75-piece orchestra was measured at 87 dB, as shown in Figure 1 below. This measurement was made at Bell Telephone Laboratories in 1290. The measuring instrument was a custom-designed, hand-built vacuum-tube-based system that may well have been the first real-time analyzer. (Full details rare given in the JASA paper listed in the footnote.) No frequency weighting was applied, and the “ballistics” corresponded to today’s SLOW time weighting. The levels were measured in a large movie theater (the paper dos not give the dimensions), and the measurement mic was located at the conductor’s stand—probably about 3 meters from the front row of musicians.

Figure 1: The average level of a 75-piece orchestra varies with frequency, with a maximum at about 87 dB at 600 Hz (after Sivian, Dunn, and White)[i].

Measurements on specific pieces of music performed by a symphony orchestra indicate a level of 116 dB* on stage during the conclusion of Shostakovich’s 7th symphony. Stan Kenton’s jazz band was reportedly louder.

*Unfortunately, the frequency and time weighting for these measurements were not given in the articles, nor was the distance from the band to the mic.

Since my group was certainly not as loud as a full orchestra (and nowhere near the levels produced by some bands in the later 60s and beyond), I didn’t worry much about hearing damage from our music.
However, my rock band was not the earliest source of loud noise to which I was exposed. I began mowing our lawn using a push-type gasoline-powered mower when I was in high school. Those could generate LZS=95 dB at the user’s ears. But I only ran the throttle as high as necessary, not fully open, where the noise would have been maximum. When my dad plowed our garden using his Standard Engine Company Twin walk-behind tractor (Figure 2), I was exposed to noise levels that—while not actually painful—were definitely uncomfortable. I’d guess that 2-cylinder unmuffled engine produced about LZS = 100 dB at my ears.

Figure 2: Dad’s Standard Twin tractor probably produced a level of about 110 dB.

The articles on hearing-damage risk I read in the mid-60s indicated that some hearing threshold shift could be expected from loud noise, due to muscles in the ears constricting the motion of the ossicles (hammer, anvil, and stirrup), but hearing sensitivity would usually return to normal within a couple of hours. If it did, it was called “primary hearing threshold shift” and was nothing to worry about. If not, it was “secondary hearing threshold shift” and indicated some degree of permanent damage.

Dad bought his first chain saw while I was in high school, and it was even louder than the tractor. I tried to avoid being around when he used it.

Increasing Public Awareness
Shortly after I joined the NC Regional chapter of the ASA in 1969, OSHA began rule-making to protect workers from hearing damage. They requested comments from the ASA, and some members pointed out that the proposed LAS=85 dB maximum time-weighted average (TWA) noise exposure over an 8-hour period would result in hearing damage for about 10% of workers, as would the proposed 5 dB “doubling rate.” (This is the mandated decrease in average noise level when exposure time is doubled). These members advocated an LAS=80 dB maximum for 8 hours, and a 3 dB doubling rate. This proposed modification of the regulation was intended to take into account the known fact that people differ in their susceptibility to hearing damage from a given noise level and duration. Ultimately, OSHA adopted the LAS = 85 dB maximum TWA with a 5 dB doubling rate.

It is pretty well-known today that loud music, industrial noise, “recreational noise” (snowmobiles and auto races) constitute dangers to hearing. The most common form of hearing loss is sensorineural loss, in which the stereocilia or “hair cells” which sense the motion in the cochlea become damaged. The other common cause of hearing loss is immobilization of the ossicles, which causes a loss of low-frequency sensitivity. This problem can often be corrected by surgery called a “stapes mobilization.” The ears are most susceptible to damage at about 4 kHz, which is the frequency range in which they are most sensitive (Figure 3.)

 

Figure 3: Perceived Loudness vs. Frequency (after Robinson and Dadson)[ii].

Sensorineural hearing damage is cumulative, and medical science presently knows of no way to repair damaged stereocilia. Sensorineural hearing loss can occur instantaneously from exposure to very loud sounds. One example is an auto mechanic who was permanently deafened when he was working on a car with no muffler, and it backfired. Another potential danger is “irrigation” treatments used by some doctors to treat ear-canal infections. I suffered about a 30 dB loss in sensitivity in my left ear due to such a treatment. As Jim Sorensen mentioned in his excellent Syn-Aud-Con blog, Hearing Loss…After ‘A Soundman’s Biggest Nightmare’ :

“…We really can’t trust our ears…they’ve been ‘jiggered with’…but we can learn to work with that jiggering. It’s like any form of pre-distortion. If you know it’s there and can describe it you can make a “filter” that will undo it.”

My own personal “filter” consists of automatic brain reprogramming that tells me to ignore the fact that sounds from my right seem to be about 30 dB louder, especially at high frequencies.

Stealth Loudness
We must also beware of “stealth loudness.” With peak levels typically ranging from LZPK= 120-150 dB rock bands are likely the loudest music source. But the second-loudest — a symphony orchestra at LZPK =  120-137 dB is often regarded as a “tame” sound source.

Another instrument usually considered “tame” holds the Guinness record for the world’s loudest single musical instrument. The Boardwalk Hall Auditorium Organ, which is housed in the Main Auditorium of the Boardwalk Hall in Atlantic City, New Jersey, has over 33,000 pipes, seven keyboards, and 1439 keys.
Although I was unable to find the maximum SPL, this organ is said to be six times louder than a steam train’s whistle! There being no “standard steam train whistle,” and the reported levels of such devices varying from 90-120 dB (measured at an unstated weighting and distance), we can only estimate the level of this organ. A factor of 6 corresponds to about 16 dB SPL increase, which would lead us to estimate the Boardwalk Hall organ’s maximum level at LZS = 106-136 dB.

The loudest sound source I ever had to support was a real surprise. A local church held a special worship service featuring a 200-voice choir accompanied by electrical instruments and drums. I expected to need a lot of sound to balance the choir with those instruments. What I did not expect was the sound power needed to balance the vocal soloists with the choir! During an a Capella selection, the choir reached LZS = 120 dB in the sanctuary, 50 feet from the front of the choir. The choir was not miked, although the drums had to be miked to keep up with the choir!

Most people would say that a solo trumpet is louder than a solo trombone. However, this is a misconception based upon the trumpet’s sound covering higher frequencies, to which the human ear is more sensitive. A solo trombone’s maximum at 1 meter from the bell is about LZS = 115 dB, which is about the same as the maximum SPL of a marching band at normal listening distances. At this level, the solo trombone beats out a solo trumpet by about 5 dB.

Many people consider bagpipe bands to be extremely loud, but this reputation may be undeserved, caused by the fact that much of the bagpipe’s sound is concentrated in harmonics that fall within the ear’s most sensitive frequency range. The perceptual loudness is likely part of the reason that bagpipe corps have been at the forefront of most Scottish infantry for centuries. In World War I, the Germans called the kilted Scots soldiers “the ladies from Hell!” A bagpipe band’s maximum is only about LZS = 102 dB for close listeners.

The Take-Away
So, we want to protect our ears by avoiding very loud music, unmuffled internal combustion engines, auto races, and even sports events with large audiences. If we have to expose our ears to those insults, we should at least use good hearing protection. Several companies offer earplugs designed for musicians, so they can hear their performances while avoiding damage. In addition, re-calibrating our “filters” by regularly listening to good acoustical music is important so that we know what original musical sources sound like. Amplified music essentially has no “original source” since instrument amplifiers and live sound systems create specific sound qualities and textures that may never have existed before and are un-reproducible.  rh

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[1] Sivian, L. J.; Dunn, H. K>; and White, S. D.:”Absolute Amplitudes and Spectra of Certain Musical Instruments and Orchestras”; Bell System Technical Journal, October, 1929.

[1] Robinson, D. W. and Dadson, R. S : “Re-determination of the EqualLloudness Relations for Pure Tones”; British Journal of Applied PsychologyI, 7 (1956), 166-181.