The War Between Noise Reduction Circuits and Vehicle Noise

Automobiles are manufactured all over the world and some cars are quiet, some are noisy, some offer a sporty (throaty) sound, some are quiet; some are  squeaky, others have rattles and bangs.      Historically, there has been a war between the use of hearing and communicating in vehicles.  Over the past few years hearing aid processors, background noise reduction, directional microphones and fitting techniques have worked miracles with reducing the efficts of this type of noise on communication.  The combination of fitting techniques and technology have made vehicle noise much more tolerable for our hearing impaired patients these days.  Although patients do better these days, clinicians still get complaints in frompatients around the world about the use of hearing instrument in vehicles. What is really the problem? Why is more difficult to difficult to hear in cars? It would seem that the same technology that made amplification circuitry perform at a much higher level should have contributed to significant noise reduction in vehicles allowing for better communication.  This week Hearing International investigates the vehicle interior noise.  Interior noise has been part of automotive engineering for a very long time and sound (or the lack of it) may even be a sign of luxury.  Thompson (2011) presents that the frequency-intensity spectrum of auto noise has a heavy low frequency emphasis as presented in the frequency-intensity analysis(Right).   Although the problem of vehicle noise has improved greatly over the last couple of decades, considering the upward spread of masking, no wondere patients have difficulty communicating in this environment.  Obviously, 50 dB of background noise in the lows, 35-40 dB noise input at 1 kHz and 20 dB at 2 kHz can create significant difficulty for a hearing aid circuit performance, especially when other extraneous noises are added to the equation.  Patients report this as “background noise” or the “rumble” or the “tire noise” that keeps them from  hearing what they want to hear in the car. Quite a chore for today’s circuitry to facilitate hearing under these conditions.

Noise Help (2011) has found that many vehicles still have a level of interior noise that is noticeable and sometimes annoying:

  • The constant monotonous roar is mentally fatiguing and saps your energy.
  • If you are trying to talk with others in the car, you have to raise your voice to be heard. Conversation becomes an effort, and some of the subtleties of communication are lost.
  • If you want to listen to music, you have to play it loud just so you can hear it over the noise, and you can’t hear the nuances in the music.

Noise Help (2011) has also found that a noisy car can even be a safety issue:

  • The fatigue caused by the constant noise reduces your alertness and can cause drowsiness, increasing the chances of an accident.
  • You may not hear sounds such as an emergency vehicle’s siren or the warning blast of a train’s horn, which can lead to an accident.
  • You may not notice changes in the sound of the car’s operation, which can be useful warning signs of auto trouble. Car sounds are used to help diagnose problems, if you are able to hear them.
  • Some car noise can damage your hearing over the long term. If you are riding in a convertible with the top down, it’s common for the noise level to average 85–90 decibels, which over time is loud enough to cause hearing loss or ringing in the ears.

Some car noise is expected, acceptable, and even desirable. In fact, if you drive a sporty car, or one of today’s “muscle cars”, it just wouldn’t seem right to have a whisper-quiet ride. If you have a powerful engine, you want to hear it!  Car noise might be exhilarating in a muscle car, at a NASCAR race, or the car is at the track, but to those with hieiaring impairment using amplification it is not a desireable situation.  Often, the sign of a high quality vehicle, such as a Rolls-Royce is a quiet ride where you can hear a whisper. Noise Help (2011) feels that maybe steady background noise these days isn’t loud enough to really be a problem, but its possibly the the rattle and clatter of loose or misaligned parts clinking together that is more of a problem.  Twenty years ago, the engine was the main noise factor in automobiles.  As engineers reduced engine noise so that it no longer masked the other ancellary noises, such as the creaking and squeaking of leather, vinyl, and plastic parts rubbing together noises that have become more noticiable to drivers and passengers [and probably more noticible to patients using amplification].   Obviously, reducing interior vehicle noise is a complex problem involving a number of variables.  Harrison (2004) presents that interior vehicle noise is a combination of a number of issues that include:

  • Engine Noise
  • Road Noise
  • Intake Noise
  • Exhaust Noise
  • Aerodynamic Noise
  • Noise from components and ancillaries
  • Brake Noise
  • Squeaks, Rattles, and Tizzes

Harrison (2004)  further offers that apart from squeaks, rattles, and tizzes; most noise and vibration  originates from outside the vehicle. The interaction of the vehicle structure with the environment and the combination of noise from various sources produce radiated complex sound within the vehicle’s passenger compartment.  Genuit (2009) discusses the importance of interior noise over the past few years, stating that interior noise has been an important vehicle quality task for acoustical engineers in the automotive industry for more than 30 years. However, the noise reduction goals have changed during this period.  Initially, automotive acoustical engineers  were confronted with the task to make the interior noise of a vehicle tolerable and to reduce the overall SPL as much as possible. By reducing passenger cabin noise over the years, the engine sound provided less masking to other sounds.  As a consequence, sound from other sound sources could be heard more easily and cause interference with communication [and with the use of amplification]. Since vehicles have continuously become quieter, the customer’s sensitivity to acoustical comfort has increased. Genuit (2009) summarizes the automotive customer’s sensitivity with the diagram (below) and further states that on the one hand, new noise sources have become more perceptible due to reduced overall sound pressure levels. According to Genuit (2011), human hearing adapts to an average level and becomes more sensitive for any changes in the time and frequency domains.  As demonstrated in the poor quality visual above, by reducing engine noise,  the perception of tire noise increases.  With the reduction of tire noise, wind noise is more perceptible; with the reduction of wind noise ancellary noise, such as rattles, squeaks, etc. become more noticable. Schwoerer (2009) states he spent a number hours perusing the decibel stats of various vehicles measuring vehicle interior noise at 80 mph (130 km/h). He feels that when riding in a vehicle it should be possible to carry on a conversation and for whatever reason, noise levels less than 70 dB(A) seem to be hard to crack in cars for regular folks. Schwoerer says that this seems to be  a pretty tolerable loudness that was  unreachable a few decades ago. It appears that if you are well heeled enough to afford a luxury car, then at 80 mph (130 km/h) the overall noise levels are about 66 dB(A), if you have less purchase power, you might have to settle for 76 dB(A) of background noise in your vehicle.

Obviously, there is a war between noise reduction circuits and interior vehicle noise.  Engineers are winning the fight, however in that there are less problems in 2011 in vehicles than in the past.  Automotive engineers are reducing the interior noise, while simulateously hearing aid engineers are inventing better noise reduction circuits, processing, directional microphone strategies and both are contributing better hearing in vehicles.  Until the vehicle noise reduction gets even better and the hearing instrument circuits become even better, increases in SNR, FM and other technologies will still be necessary to facilitate communication in vehicles.

References:

Genuit, K. (2009).  Vehicle interior noise: A combination of sound, vibration, and interactivity.  Sound and Vibration, 12/2009.  SandV.com.  Retrieved December 18, 2011:  http://www.sandv.com/downloads/0912genu.pdf

Harrison, M., (2004).  Vehicle Refinement Controlling Noise and Vibration in Road Vehicles.  Elsevier, Butterworth-Heinemann:  Linacre House, Jordan Hill, Oxford:  http://books.google.com/booksid=SppqPydUAFQC&pg=PA145&lpg=PA145&dq=methods+of+assessment,+interior+noise+levels+in+vehicles&source=bl&ots=ZzGA_9ml3&sig=Q0kKyi5wNnFbXegf9bvBEDfUic&hl=en&sa=X&ei=cC_uTqj0BY202AWH8vikDw&ved=0CCcQ6AEwAA#v=onepage&q=methods%20of%20assessment%2C%20interior%20noise%20levels%20in%20vehicles&f=false

Noise Help. (2011).  Car noise.  Retrieved December 20, 2011:  http://www.noisehelp.com/car-noise.html

Schwoerer, M. (2009).  The art of noise.  The Truth about Cars.  Retrieved December 18, 2011:  http://www.thetruthaboutcars.com/2009/02/the-art-ofnoise/

Thompson, J., (2011).  Automotive noise control:  Thirty years of changing perspectives.  NIOSH Department of Health and Human Services.  Retireved December 20, 2011: http://www.sae.org/events/nvc/workshops/2011/keynote-thompson.pdf

About Robert Traynor

Robert M. Traynor, Ed.D., MBA is the CEO and practicing audiologist at Audiology Associates, Inc., in Greeley, Colorado with particular emphasis in amplification and operative monitoring, offering all general audiological services to patients of all ages. Dr. Traynor holds degrees from the University of Northern Colorado (BA, 1972, MA 1973, Ed.D., 1975), the University of Phoenix (MBA, 2006) as well as Post Doctoral Study at Northwestern University (1984). He taught Audiology at the University of Northern Colorado (1973-1982), University of Arkansas for Medical Sciences (1976-77) and Colorado State University (1982-1993). Dr. Traynor is a retired Lt. Colonel from the US Army Reserve Medical Service Corps and currently serves as an Adjunct Professor of Audiology at the University of Florida, the University of Colorado, and the University of Northern Colorado. For 17 years he was Senior International Audiology Consultant to a major hearing instrument manufacturer traveling all over the world providing academic audiological and product orientation for distributors and staff. A clinician and practice manager for over 35 years, Dr. Traynor has lectured on most aspects of the field of Audiology in over 40 countries. Dr. Traynor is the current President of the Colorado Academy of Audiology and co-author of Strategic Practice Management a text used in most universities to train audiologists in practice management, now being updated to a 2nd edition.