Tinnitus as a cause of Low Frequency Noise complaints

Frits (G.P.) van den Berg, Science Shop for Physics, University of Groningen
Nijenborgh 4, 9747 AG Groningen, the Netherlands - nawi@phys.rug.nl

Low Frequency (LF) noise perception in dwellings causes health problems, especially problems related to sleep. Actual sources cannot always be identified: in some cases the LF sound levels are so low that it is improbable or even impossible that any LF sound can be heard at all, in others a (low level) LF sound that may be audible has no apparent relation to the complaints. In these cases the source of complaints seems not to be a sound in the dwelling, but a sound originating within the complainant. It apparently sounds so much like a "real" sound that complainants find it hard or impossible to believe that it is not coming from some machine. The fact that it is not recognised as tinnitus may be the reason that this type of tinnitus (i.e. at low frequencies) has received little attention.
The fact that LFN complaints can be caused by two very different kinds of sources -real machines or tinnitus- should be considered when trying to solve complaints

Complaints about a persistent humming, 'dieseling' or engine-like sound at home have become known as Low Frequency (LF) noise complaints, 'LF' usually taken as frequencies below 100 to 200 Hz. In the Netherlands most complainants are older than 40 (63 - 91%) and female (76 - 78%) [Gielkens, see also van den Berg99A - NSG]. The complaints cannot always be confirmed by sound measurements; in some cases the indoor LF sound level is so low it is improbable or even impossible that LF sound can be heard at all. Sound measurements in dwellings of complainants showed that in 5 out of 19 cases the sound levels at frequencies below 100 Hz were such (each 1/3 octave bands at least 6 dB below the ISO-226 hearing threshold, no tonal components) that the indoor LF sound had to be considered inaudible [van den Berg99B]. According to a recent evaluation, reporting 54 LF sound measurements, in 28 cases no LF sound could be identified as a cause for the complaints (in 12 a LF sound could be identified, in 14 cases no definite conclusion was possible) [NSG]. Below two examples will be given of complaints where a LF sound could not be confirmed by measurements. In [van den Berg98] another example was given of complaints about factory fan noise (tonal components -obvious near factory- were absent in indoor sound; indoor 1/3 octave levels at frequencies < 160 Hz below ISO-226 hearing threshold).

As in some cases the origin of LF noise complaints is unclear, we developed a new line of investigation by presenting sounds produced with a sound generator and a loudspeaker or headphones to complainants in their homes. The purpose of this matching experiment is to identify, if possible, the frequency of the sound that causes the complaints. There is less emphasis on identifying the level of the sound.
First a pure tone is presented and the frequency of it is varied so as the complainant can match it to the LF noise. Use has been made of already available (no special) equipment; the frequency range is limited to 60 Hz when using the loudspeaker, 40 Hz when using the headphones (at low frequencies even 1% distortion produces harmonics that are predominant with respect to normal hearing, due to the steep LF hearing threshold). Following the pure tone a complex tone (sawtooth, triangle) and finally bandwidth limited pink noise (octave or 2/3 octave band) is presented with the same purpose of matching. The complainants do not know the frequencies of the presented sounds before they have decided on the best match.

2.1 Case A
In a matching experiment at her home Mrs A (aged 57) could not match any of the sounds (pure and complex tones, octave band noise) to the two sounds she perceived (a low (S1) and higher-pitched (S2) sound). She rated pink noise in the 125 Hz octave band as lower-pitched than S1, 250 Hz octave band noise as higher-pitched. When she imitated S1 by voice, she produced sound in the 200 en 400 Hz 1/3 octave bands. Imitating S2 increased the 400 and 800 Hz levels. Her husband, who also heard LF sounds at home, matched 'his' sound with a 315 Hz pure tone. His imitation by voice of the sound he heard was a 330 Hz tone.
The best match of the LF sounds with notes from an electronic organ in the house (a matching experiment Mr and Mrs A had thought of themselves) are an Es (156 Hz) and As (105 Hz). Mrs A more often perceives an As, the husband more often an Es. Thus, both find a note played on the organ one octave lower best resembling "their' sound. When 'on' but not played the organ produces a low, but clearly audible hum of 103 Hz.
In earlier measurements in this house (figure 1A), at a time the LF sound was perceived, the equivalent sound level (over at least 5 minutes per recording) of all 1/3 octave bands were below the ISO-226 hearing threshold. Turning off the electricity in the house lead to a de-crease of the 100 Hz band level (in all recordings equal to or less then the ISO-226 threshold). The absence or presence of this tone however did not correlate with the complaints.

2.2 Case B
Ms B (aged 48) said that a 50 Hz pure tone presented to her in a matching experiment at home was quite like the LF sound that caused her complaints. Repeated tests gave the same match within a few Hz. Other sounds did not match. She could positively hear the 50 Hz tone if it was presented at a level of at least 65 dB. At a level below 60 dB she was not able to tell whether the tone was indeed produced in the room or not. At the time of testing the indoor sound level of all 1/3 octave bands was less than 30 dB. The 60-65 dB hearing threshold corresponds to the previously audiologically determined hearing loss at 125 Hz (15/20 dB at left/right ear), added to the ISO-226 hearing threshold at 50 Hz (44 dB). In earlier measurements in Ms B's home (see figure 1B; then another house), the equivalent sound level (over at least 5 minutes per recording) of the 50 Hz 1/3 octave band was 16 dB in one recording, 31 dB in another (the increase may be caused by an electric motor). Both (!) recordings were made at a time she did perceive the LF sound causing her complaints.

2.3 Cases with internal causes
In these and other, comparable cases complainants are convinced they hear a real (external) noise from an outdoor or maybe indoor sound source. Measurements do not support this conclusion, as no (audible) sound measured can be related to the complaints. It seems more likely there is an internal cause for these complaints, i.e. originating within the complainants.

Tinnitus, in one or both ears, is "an otological condition in which a sound is perceived by a person without an external auditory stimulation" [ANSI]. According to the Encyclopaedia Britannica it is a ringing or buzzing in the ears: "Tinnitus may be caused by any of a number of ear conditions, including the clogging of the external auditory canal with earwax (cerumen) or inflammation of the eardrum membrane, the middle ear, or the inner ear. Tinnitus may also result from an overdose of drugs such as aspirin or from excessive use of the telephone, and it may accompany hearing loss, particularly in the high-frequency range. Ringing in the ears also sometimes accompanies vertigo (dizziness)." The Tinnitus Informa- tion Network (www.mixolplix.com/tinnitus) states: "The presence of tinnitus, or head noise, is a very common and annoying occurrence that affects about 17% of the general population and 33% of the elderly. (....) It may be intermittent or constant in character, mild or severe in intensity, vary from a low roar or throbbing to a high-pitch sound so deafening the individual may hear nothing else. It may be subjective (audible only to the patient) or objective (audible to others). It may or may not be associated with a hearing impairment. Currently, over 50 million adults (in the USA - vdB) suffer from tinnitus. For 12 million, the problem is so severe they may become incapacitated. While tinnitus does not cause hearing disorders, it may accompany decreased hearing and other symptoms such as ear pressure or dizziness."
ANSI states that tinnitus is sometimes caused by sound originating in the circulatory system. Also, an examiner may hear the sound causing the tinnitus (then called a bruit), presumed to come from vibrations in the head or neck [Penner]. Lastly, the ear can spontaneously generate and emit sound. Such a Spontaneous OtoAcoustic Emission (SOAE) can be measured with a microphone in the ear canal. It is generated in the cochlea of both healthy and impaired ears. So, in some cases an internal acoustic stimulus can be found for tinnitus. In most cases however, tinnitus is thought of as a malfunctioning of the inner ear or of the auditory system in the brain. In such a case, tinnitus cannot be confirmed by physical measurement.

3.1 Tinnitus pitch and level
Tinnitus in a person can be quantified by the frequency and sound level of a tone that is said by that person to match the tinnitus. Tinnitus usually is high-pitched (2 - 4 kHz), but it can occur as a low-pitched sound, such as a "low roar" or "throbbing sound". SOAE's occur mainly at frequencies between 1 and 2 kHz, but may be below 500 Hz [Sisto]. Penner has described several cases where tinnitus could be related to SOAE's [Penner]. In two cases she demonstrated that the SOAE's were connected to the tinnitus: when the tinnitus was masked, the masker coincided with the SOAE frequencies; when the SOAE's were suppressed (i.e. vanish due to a sound presented that is near the SOAE in frequency), the tinnitus was inaudible. In one of these cases the tinnitus was described as a "changing tone", "more than one tone", and as a "hum". In the second case the tinnitus was a "steady tone" and a "hum".
When tinnitus is matched to a low level sound, it may nevertheless be described as loud. Why, asks Penner, should a low level sound be loud ? This "paradox" is not unfamiliar in LF noise complaints where complainants may perceive a sound that no one else can hear as "loud". Perhaps this is explained by 'recruitment': an abnormal strong increase of loudness with sound level due to a decreased dynamic hearing range (an elevated threshold and/or reduced upper limit).

When complaints about Low Frequency noise occcur, sometimes a LF sound source is obvious or the complaints can be supported by measurements that indicate a relatively high LF sound level. In many cases, however, the evidence is less secure, and in some cases the measurements lead to the conclusion that there is no audible LF sound present at a time such a sound is perceived. An internal source then may explain the perception of the sound.
There is no doubt that LF noise complainants do perceive a sound that is very much like a real, external sound. Indeed, common experience has learned us that real LF sources can be heard from considerable distance and are difficult to track because of the distance and the lack of directional information (implied by the long wavelength). This experience may explain why complainants find it hard to believe 'their' LF noise is not such a real sound. However, when measurements indicate LF sound levels below the (individual) hearing level, an internal origin of the LF noise seems a possible and even likely explanation. Perception of sounds without a external stimulus usually is known as tinnitus. At frequencies in the 'normal' frequency range (from some hundreds of Hz upward) it is accepted that sounds can be perceived without an obvious acoustic stimulus: millions of people hear such sounds or even suffer from them.

ANSI (Acoustical Society of America, Standards Secretariat), "American National Standard Bioacoustical Terminology", 1995, ANSI S3.20-1995

M.J. Penner and P.J. Jastreboff, "Tinnitus: Psychophysical Observations in Humans and an Animal Model", in: Clinical Aspects of Hearing, 1996

R. Sisto, A. Moleti and M. Lucertini, "Spontaneous otoacoustic emissions and relaxation dynamics of long decay time OAEs in audiometrically normal and impaired subjects", JASA, Vol. 109 (2), pp. 638-647, 2001

G.P. van den Berg, "Sound exposure measurements in cases of low frequency noise complaints", proc. Internoise 1998, Christchurch

G.P. van den Berg (A), W. Passchier-Vermeer, "Assessment of Low Frequency noise complaints", proc. Internoise 1999, Fort Lauderdale

G.P. van den Berg (B), "Case control study in low frequency sound measurements", proc. Internoise 1999, Fort Lauderdale

C. Gielkens-Sijstermans, T.H. Collijn, A.W. Jongmans-Liedekerken, "Sensitivity for low frequency sounds: a study into possible factors" (Gevoeligheid voor laagfrequent geluid; een studie naar mogelijke factoren), GGD Oostelijk Zuid-Limburg, 1998 (in Dutch). Some results are in [van den BergA and -B].

NSG (Nederlandse Stichting Geluidhinder), "Evaluation NSG Guideline for Low Frequency Noise" (Evaluatie NSG-Richtlijn Laagfrequent Geluid), Delft, 2001 (in Dutch)