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Pump up the Bass?

Most car audio enthusiasts are aware they can hear sound at frequencies between 20 and 20,000 cycles per second, or hertz (Hz), depending on age. The other thing we all acknowledge is that prolonged exposure to loud noises within the audible range causes hearing loss over time.

So, what about that bass then?

Let’s get specific, I am talking about music frequencies between 0 and 80Hz.

Recently I suggested to a young guy that he may want to consider wearing ear-plugs when giving earth-shaking demos at high 150dBs at 20Hz. His response was “Bass doesn’t hurt your ears”, which is something I have heard promulgated by my fellow bass-heads over the years.

The latest science, however, does not support these statements.

We are dealing with two elements here, that is

  • The (low) frequency of the music, and
  • The sound pressure level.

Let’s focus just on the low-frequency (bass) music for now.

Having spent a good part of my career in the oil and gas industry on large ocean-going oil tankers, I am very familiar with the direct impacts of subsonic and infrasonic sound on the body. Large ship’s propulsion and propeller systems create frequencies between 2 and 20 Hz (depending on the RPM) but the intensity is low.

Studies have found there is a general inconvenience and irritability caused to humans with an increase in fatigue and concentration problems. Acute exposure hampers precise movement and accurate prehension with hands and fingers.

There is an exhilarating feeling when sitting in a car playing low-bass music at sound pressures of 150dB+. Why? That may have something to do with the resonant frequency of the human body.

The human body can be considered as a series suspended elements (head, thorax, pelvis) linked by shock absorber systems (ligaments, muscles, intervertebral discs). The internal organs are considered to be not heavily damped. The resonant frequencies of the human body when sitting in a car are:

  • Head: 20 – 30 Hz
  • Eyeballs: 60 – 90Hz
  • Thorax: 3 – 7Hz
  • Heart: 4 – 8Hz
  • Abdominal and thoracic organs: 4 – 9Hz
  • Spine: 2 – 6Hz
  • Pelvis: 4 – 9Hz

The important point to consider here is the effect of vibration on the human body. Latest research refutes the popular opinion that vibration doesn’t create trauma in the cochlea. In fact, whole body vibration can agitate the body to the point of causing microfractures in the vertebrae, disc protrusion, nerve damage and acute lower back pain.

It’s important to understand that just because you can’t “hear” the low-frequency components of these sounds (you feel them more than hear them) doesn’t mean they have no impact on your hearing. Listening to just 90 seconds of low-frequency sound can change the way your inner ear works for minutes after the music stops. A recent study by audiology researcher Jeffery Lichtenhan of the Washington University School of Medicine in St. Louis states:

“Low-frequency sound exposure has long been thought to be innocuous, and this study suggests that it’s not.”

Neurobiologist Markus Drexl and colleagues at the Ludwig Maximilian University in Munich, had volunteers with normal hearing sit inside soundproof booths and then played a 30Hz sound for 90 seconds and used probes to record the natural activity of the ear after the noise ended,

The study observes the spontaneous otoacoustic emissions (SOAEs) in which a healthy human ear itself emits faint whistling sounds. SOAEs change when a person’s hearing changes and disappear in conjunction with hearing loss.

People’s SOAEs are normally stable over short time periods, however after around 90 seconds of low-frequency sound the SOAEs started oscillating, becoming alternately stronger and weaker, and this means that the ear may be more prone to damage.

Hearing loss researcher M. Charles Liberman of Harvard Medical School in Boston states:

“The unfortunate thing about our ears is that we can be doing terrible things to them with sounds that aren’t necessarily painful.”

Low-frequency vibrations (4 – 12 Hz) increases general muscular tension in the lower back, chest, and abdomen and an increase in heart rate are observed. There are also reported cases of myocardial infarction in young people with no history of arteriosclerosis or coronary artery disease linked to low-frequency vibration.

Put simply, long-term exposure to loud noise is the most common preventable cause of hearing loss. Although bass demos are fun, it is important to consider the impact of exposing yourself and others to low frequency sound at high sound pressure levels. In particular, young children who are still in the developmental stages.

Craig Harrison

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