Drilling Noise Research Articles

An optimized observation system and inversion method for fault detection based on surface-wave while tunneling

Understanding geological structures ahead of the tunnel face is important for safe and efficient construction of the urban tunnel. The surface-wave while tunneling (SWT) method, using drilling noise by shield machine as source, is expected to dynamically predict the adverse geologies in front of the tunnel face. Observation system and inversion method are keys for SWT. To improve the imaging accuracy of the geological conditions, it is urgent to optimize the observation system for data acquisition and inversion method for velocity inversion, especially for the utilization of multi-modes surface-waves. For observation system, several key parameters (minimum source-geophone distance, length and interval of survey line) are optimized to obtain sufficient information of dispersion curves. Then observation systems for source at different depth were optimized, supporting for geological detection using surface-waves generated by underground drilling noise. For velocity imaging, numerical simulations are studied to reveal the applicability of typical inversion methods for multi-modes of surface wave, and particle swarm optimization (PSO) algorithm is optimized for velocity inversion due to its advantages of stable calculation and good accuracy. On this basis, SWT was optimized both in data acquisition and velocity inversion for better understanding geological condition both in buried depth and detection distance. Then the improved method was applied in the Jinan tunnel and successfully detected a fault, providing geological information for construction safety and verifying the feasibility.

Effect of age-related extended high frequency hearing loss on the subjective impressions of dental drill noise

Fear and anxiety among patients are sometimes evoked in dental clinics due to the sound of dental drills. This study aimed to explore the impact of age-related hearing loss in the extended high frequency (EHF) range above 8 kHz on individuals' subjective discomfort towards dental drill noise. After measuring pure-tone audiometric thresholds at both conventional and extended high frequencies, we used a psychoacoustic approach to evaluate subjective impressions of four dental drill sound stimuli, which featured varying frequency components, in 62 participants (aged 12–67 years). We found a significant decrease in hearing sensitivity within the EHF range as age increased, with notable differences in hearing thresholds at 14 kHz between teenage and older adults exceeding 65 dB. Furthermore, significant differences were observed between younger and older (above 40 years) participants in the subjective impressions of dental drill noise, emphasizing age as a critical factor in the perception of high frequency components. Consequently, age may influence the unpleasantness of dental drilling noise. Compared to older individuals, young participants may exhibit increased fear of dental procedures owing to physiological factors. These results underscore the need for age-appropriate noise control strategies in dental clinics to mitigate anxiety and improve patient comfort.

Analyzing drilling noise in rotational atherectomy: Improving safety and effectiveness through visualization and anomaly detection using autoencoder-A preclinical study.

As the population of aging societies continues to grow, the prevalence of complex coronary artery diseases, including calcification, is expected to increase. Rotational atherectomy (RA) is an essential technique for treating calcified lesions. This study aimed to assess the usefulness of the drilling noise produced during rotablation as a parameter for evaluating the safety and effectiveness of the procedure. A human body model mimicking calcified stenotic coronary lesions was constructed using plastic resin, and burrs of sizes 1.25 and 1.5 mm were utilized. To identify the noise source during rotablation, we activated the ROTAPRO™ rotablator at a rotational speed of 180,000 rpm, recording the noise near the burr (inside the mock model) and advancer (outside). In addition to regular operation, we simulated two major complications: burr entrapment and guidewire transection. The drilling noise recorded in Waveform Audio File Format files was converted into spectrograms for analysis and an autoencoder analyzed the image data for anomalies. The drilling noise from both inside and outside the mock model was predominantly within the 3000 Hz frequency domain. During standard operation, intermittent noise within this range was observed. However, during simulated complications, there were noticeable changes: a drop to 2000 Hz during burr entrapment and a distinct squealing noise during guidewire transection. The autoencoder effectively reduced the spectrogram data into a two-dimensional representation suitable for anomaly detection in potential clinical applications. By analyzing drilling noise, the evaluation of procedural safety and efficacy during RA can be enhanced.

Geological ahead-prospecting method while TBM drilling using improved seismic interferometry method: A case

Tunnel Boring Machines (TBMs) are widely applied in tunnel construction due to their advantages of fast drilling, good cave shape, safety, and economics. To guarantee the safety of TBM, it is necessary to understand the geological conditions ahead of the tunnel face. Seismic ahead-prospecting is an important method for predicting adverse geologies while TBM drilling. Seismic interferometry is the core to achieving the utilization of TBM drilling noise and dynamic detection. To better retrieve wavefield from TBM drilling noise, improved seismic interferometry based- seismic ahead-prospecting method was proposed in this paper. Firstly, the typical methods of cross-correlation, deconvolution, and cross-coherence are analyzed and selected cross-coherence for wavefield retrieval due to its advantages of less noise/artifacts and better stability. Then, the improved seismic interferometry is proposed by combining the smoothing constraint base on cross-coherence and singular spectral decomposition. Numerical simulations prove that more clear and accurate reflected waves can be retrieved by the improved method. And the improved method has been used in the Gaoligongshan tunnel and successfully detected two fractured zones ahead, supporting the safety of TBM drilling.

Seismic ahead-prospecting based on deep learning of retrieving seismic wavefield

Unknown geology ahead of the tunnel boring machine (TBM) brings a large safety risk for tunnel construction. Seismic ahead-prospecting using TBM drilling noise as a source can achieve near-real-time detection, meeting the requirements of TBM rapid drilling. Seismic wavefield retrieval is the key data processing step for the efficient utilization of TBM drilling noise. The traditional solution is based on cross-correlation to extract reflected waves, but the reference waves remain in the result, disturbing the imaging and interpretation of the adverse geology. To solve this problem, the deep learning method was introduced in wavefield retrieval to improve the accuracy of geological prospecting. We trained a deep neural network (DNN) with its strong nonlinear mapping capability to transform seismic data from TBM drilling noise to data from the active source. The issue lies in its features for this specific tunnel task, including the decay of the seismic signal with time and the incomplete spatial correspondence. Thus, we improved a classical DNN with the time constraint as an additional input, and an additional pre-decoder to enlarge the receptive field. Additionally, a loss function weighted by the ground truth and time constraint is improved to achieve an accurate retrieval of the effective signal, considering the little effective information in tunnel data. Finally, the workflow of the proposed method was given, and a dataset designed with reference to the field case was employed to train the network. The proposed method accurately retrieved the reflection signal with higher dominant frequencies, which helped improve the accuracy of imaging. Numerical simulations and imaging on typical geological models show that the proposed method can suppress reference waves and get more accurate results with fewer artifacts. The proposed method has been applied in the Gaoligongshan Tunnel and imaged two abnormal zones, providing meaningful geological information for TBM drilling and tunnel construction.

Effect of sound masking using child-friendly masker sounds on the perception of the dental drill noise in pediatric dental practice

Background: Dental anxiety is significantly influenced by the fear and annoyance brought on by the sound of a dental drill. As a result, the welcoming atmosphere at pediatric dental offices is likely to entice young children to come in for examination and treatment. Aim: The aim of this study was to compare the masking effect of dental drill noise with the sound maskers on the annoyance rate of pediatric dental patients. Materials and Methods: To evaluate the stimuli in the annoyance experiment, 40 children (20 boys and 20 girls) were enlisted. As masker sounds, dragonfly and whistle sounds were chosen, and the functioning dental drill noise was blended with the original sound level. The participants rated each audio stimulus signal delivered to them in a random order, with the level of annoyance indicated by a color scale corresponding to the intensity of their psychological sense. The mean comparison between the acoustic stimuli was made using the Friedman's test. Based on gender, the mean comparison between acoustic stimuli was made using the Mann–Whitney U-test. Results: The mean annoyance rate of dental drill noise dragonfly sound and whistle sound was 4.83, 1.15, and 3.82, respectively. Gender has a significant effect on masking effect in nonacoustic factor outcomes. Conclusion: The dragonfly sound had the best impact and the most significant reduction in annoyance compared to the sound of the drill. Typically, girls felt more at ease around the dragonfly sound than boys did.

Dental drill noise reduction using a commercially-available earplug device.

The dental handpiece noise in a dental surgery is concerning to both patients and staff as a major cause of dental phobia in patients and potential hearing loss in clinical staff. High-frequency noise generated by dental handpieces is considered to be the worst of the many noises in a dental clinic. Methods to reduce this noise have been proposed and either passive or active noise reduction headphones are often suggested. However, in a dental surgery environment, the need for good verbal communication with the patient needs to be maintained. As a result, this paper aims to evaluate one proprietary anti-noise device considered suitable for this specific purpose. Lab-based experiments were set up and carried out to evaluate QuietOn, using GRAS 43AG-1 Ear and Cheek Simulator to mimic a section of the human head and ear to represent the acoustic characteristics of an actual ear. Two types of dental drill noise recordings, one for electric motor-driven and another one for air turbine-driven, were played back through high-definition speakers. Sound data captured by the simulator are then visualised and plotted using MATLAB for analysis. QuietOn is effective at low frequencies (< 1kHz). However, when dealing with high-frequency noise such as dental handpieces it is ineffective yet subdues verbal communication. Further development of passive or active noise cancellation earplugs is still needed to target dental handpiece noise while maintaining verbal communication.

A Passive Noise Attenuation Earplug Designed to Minimise Unwanted Air Turbine Driven High-Speed Dental Drill Noise.

Air turbine dental drill noise contains high-frequency components that are of concern for patients and dental staff. Meanwhile, verbal communication between the dentist and patient is essential. Conventional active noise-cancelling headphones are ineffective for dental drill noise and simply suppress all sound and hinder communication. A compact passive earplug device was designed specifically to attenuate broadband high-frequency noise ranges from 5 to 8 kHz employing an array of quarter wavelength (QW) resonators. This device was 3D printed and tested against white noise to enhance the objectivity of analysis, using a calibrated ear and cheek simulator to effectively measure its performance. The results showed that the resonators produced an average reduction of 27 dB across the targeted frequency range. When compared with two proprietary passive earplugs, this developed passive device prototype was able to attenuate an average of 9 dB more across the target frequency range whilst delivering louder speech signals of 14 dB more. The results also show that using an array of resonators exhibits an accumulated effect of individual resonator performance. This low-cost passive device could have a place in the dental clinic to reduce unwanted drill noise equivalent to the white noise high frequency spectra tested.

Ground measurement of noise characteristics in drill string during drilling

Using the drill string in the wellbore as the information transmission channel and acoustic wave as the carrier of information transmission to realize the acoustic transmission of information in the well is an important means to realize the high-speed and accurate transmission of downhole information. However, the existence of noise in the drill string during the drilling process seriously affects the transmission rate and transmission accuracy of acoustic wave. In order to explore the characteristics of drilling noise in the drill string, the drilling noise under six different working conditions was measured on site. The results show that the frequency and intensity of drilling noise change with the change of drilling conditions and drilling parameters; During directional drilling, when drilling in normal formation, the noise frequency is mainly concentrated at 1000 Hz, when drilling in hard formation, the WOB increases, and the noise frequency will be concentrated at 6000 ∼ 8000 Hz; When combined drilling, the average intensity of low-frequency drilling noise can reach at 50 dB, while when the drilling is stopped and the pump is stopped, the intensity of low-frequency drilling noise is only 20 dB; The drilling noise is mainly concentrated in the low-frequency band of about 2000 Hz, and the strongest noise is concentrated in 800 ∼ 1200 Hz; Calculate and compare the noise intensity, select the condition of stopping drilling and pump as the best transmission condition, analyze and compare the noise frequency, and select 400 ∼ 600 Hz as the best acoustic carrier transmission frequency.

A multi‐objective approach for improving real‐time audio peak reduction and speech clarity in a dental drill noise reduction device

A multi‐objective approach for improving real‐time audio peak reduction and speech clarity in a dental drill noise reduction device

Effect of processing of dental drill noise on subjective impression

The sound produced by a dental air turbine handpiece (dental drill) can markedly influence the sound environment in a dental clinic, and many patients have reported that the sound of a dental drill elicits unpleasant feelings. A comfortable sound environment for patients is desirable in medical care. To identify approaches for improving the sound quality of dental drills, we need to determine systematically the effects of the physical properties of the sounds of dental drills on their impressions. We, therefore, prepared 40 sound stimuli by processing different sound levels or frequency components comprising the sounds emitted by four kinds of dental drills. This study is designed to use psychoacoustic experiments to determine the effects of dental drill sounds on subjective impressions. The analysis resulted in the following findings: the overall attenuation of each original sound stimulus by 5 dB or 10 dB had statistically significant effects and improved the unpleasant impression. Attenuation of the frequency components above 9 kHz by 10 dB also improved the impressions, relative to those for the original sound stimuli. When the prominent frequency components at approximately 6 kHz were attenuated by 10 dB, no statistically significant differences were observed between the impressions corresponding to the original and processed stimuli. The Comfort Index (CI), which consists of the equivalent continuous A-weighted sound pressure level (LAeq) and calculated sharpness, showed a good correlation with unpleasant impressions. The present findings suggest that the consideration of acoustical characteristics based on the CI is useful in reducing the sound pressure level and refining the frequency characteristics of sounds emitted by dental drills, and this can aid in the development of new dental devices to create a comfortable sound environment in dental clinics.

Extended high frequency hearing sensitivity in normal contralateral ears exposed to mastoid drill noise in patients undergoing mastoid surgery

&lt;p class="abstract"&gt;&lt;strong&gt;Background: &lt;/strong&gt;This study aimed at evaluation of extended high frequency hearing in non operated normal ear in patients undergoing mastoid surgery using otological micro drill.&lt;/p&gt;&lt;p class="abstract"&gt;&lt;strong&gt;Methods:&lt;/strong&gt; A hospital based prospective observational study was carried out. Patients up to 40 years of age with unilateral ear disease were recruited and divided into two groups- mastoidectomy group comprising 30 patients who underwent mastoid surgery using high speed microdrill and tympanoplasty group who underwent surgery without using microdrill. Pure tone audiometry (conventional and extended high frequency) was performed preoperatively, on 3rd and 10th postoperative day.&lt;/p&gt;&lt;p class="abstract"&gt;&lt;strong&gt;Results:&lt;/strong&gt; In mastoidectomy group, Air conduction thresholds in extended high frequency showed significant changes on 3rd postoperative day and showed recovery by 10th postoperative day. Recovery started by 3rd postoperative day and in majority of cases there was a complete recovery by 10th postoperative day.&lt;/p&gt;&lt;p class="abstract"&gt;&lt;strong&gt;Conclusions: &lt;/strong&gt;Otological drilling has a significant effect on the extended high frequency hearing of normal contralateral ear. Extended high frequency audiometry till 16000 Hz is a sensitive modality for evaluation to monitor these cases on post-operative follow up.&lt;/p&gt;

Hearing Outcomes after Microvascular Decompression for Hemifacial Spasm: An Institutional Experience.

Introduction:Hearing loss following microvascular decompression (MVD) for hemifacial spasm is one of the most dreaded complications. Several factors such as stretching of VIII cranial nerve, vasospasm of labyrinthine artery, and acoustic trauma due to drill noise may be considered in its causation. We evaluated the incidence and severity of hearing loss following MVD in hemifacial spasm and the factors which might be responsible for this complication.Methods:A retrospective analysis of 30 patients operated for hemifacial spasm between January 1, 2014, and December 31, 2018, with at least 3 months of follow-up were included in the study. Retromastoid craniotomy was made, and Teflon was placed between involved vessel and VII nerve.Results:Freedom from hemifacial spasm was noted in 27 of 30 patients. Moderate spasm persisted in one patient, which was controlled with medications. The recurrence was noted in 3 patients at 6 months follow-up. Postoperatively, hearing loss was found in one female patient. The offending vessel was both anterior inferior cerebellar artery (AICA) and posterior inferior cerebellar artery (PICA) loop, which was transpositioned during surgery, and the patient was spasm free postoperatively.Conclusion:The incidence of hearing loss following MVD can be minimized using proper surgical techniques and various intraoperative adjuncts such as brainstem auditory evoked responses monitoring, use of endoscope, and indocyanine green or dual-image video angiography.

Automated seismic waveform location using Multichannel Coherency Migration (MCM)—II. Application to induced and volcano-tectonic seismicity

Locating microseismic events is essential for many areas of seismology including volcano and earthquake monitoring and reservoir engineering. Due to the large number of microseismic events in these settings, an automated seismic location method is required to perform real time seismic monitoring. The measurement environment requires a precise and noise-resistant event location method for seismic monitoring. In this paper, we apply Multichannel Coherency Migration (MCM) to automatically locate microseismic events of induced and volcano-tectonic seismicity using sparse and irregular monitoring arrays. Compared to other migration-based methods, in spite of the often sparse and irregular distribution of the monitoring arrays, the MCM can show better location performance and obtain more consistent location results with the catalogue obtained by manual picking. Our MCM method successfully locates many triggered volcano-tectonic events with local magnitude smaller that 0, which demonstrates its applicability on locating very small earthquakes. Our synthetic event location example at a carbon capture and storage site shows that continuous and coherent drilling noise in industrial settings will pose great challenges for source imaging. However, automatic quality control techniques including filtering in the frequency domain and weighting are used to automatically select high quality data, and can thus effectively reduce the effects of continuous drilling noise and improve source imaging quality. The location performance of the MCM method for synthetic and real microseismic datasets demonstrates that the MCM method can perform as a reliable and automatic seismic waveform analysis tool to locate microseismic events.

Demographic profile, Oral Health Impact Profile and Dental Anxiety Scale in patients with chronic periodontitis: a case–control study

To examine whether dental anxiety (DA) and oral health-related quality of life (OHRQoL) differ between persons with and those without chronic periodontitis. One-hundred patients with chronic periodontitis and 50 age- and- sex-matched controls were included. Data were collected on the following: demographics; smoking habits; Numerical Rating Scale (NRS) for dental pain assessment; Corah's Dental Anxiety Scale (DAS); Oral Health Impact Profile 14 (OHIP-14); Decayed, Missing and Filled Teeth (DMFT) index; Plaque Index (PI); probing depth (PD); bleeding on probing (BOP); and radiographic bone loss. Patients with chronic periodontitis exhibited a significantly higher percentage of high anxiety and phobia compared with subjects in the control group. Furthermore, patients with chronic periodontitis were statistically significantly more likely to consider themselves as suffering from dental anxiety (68.7% vs. 14.3%, P < 0.001) as well as more likely to have fear of receiving dental injections, hearing the dental drill noise and feeling a foreign object in the mouth. Patients with chronic periodontitis exhibited worse OHIP-14 global scores as well as worse scores in the following individual domains: functional limitation (P = 0.005); physical disability (P = 0.003); psychological disability (P = 0.010); social disability (P = 0.011); and handicap (P = 0.012). Compared with controls, patients with chronic periodontitis had higher levels of dental anxiety and worse OHRQoL. It is important to consider dental anxiety and OHRQoL assessment as an integral component of the evaluation of patients with chronic periodontitis. Communication between dental and behavioral health professionals is needed to implement a multidisciplinary team approach involving behavioural and psychological interventions.

Effect of drill noise on contralateral hearing after mastoidectomy in cases of unilateral Chronic Otitis Media

Introduction: The mainstay of treatment of Chronic Otitis Media (COM) is Tympanoplasty with or without Mastoidectomy. This study was conducted in patients undergoing Tympanoplasty with Mastoidectomy to assess Sensorineural Hearing Loss (if any) due to drill noise on the contralateral ear. Materials and Methods: A total of 25 patients with unilateral COM scheduled to undergo Mastoidectomy and Tympanoplasty who fulfilled inclusion criteria were recruited in the study, and a prospective, observational pilot study was carried out. Pure Tone Audiometry (PTA) and Transient Evoked Otoacoustic Emissions (TEOAE) were recorded preoperatively and postoperatively to assess hearing loss if any on the contralateral side. Results: There was no change in bone-conduction thresholds in the contralateral ear by PTA postoperatively. However, there was a significant worsening in the values of signal-to-noise ratio of TEOAE at all the frequencies in the postoperative period. The mean TEOAE recordings were, however, above 3 dB SPL. Conclusion: There are statistically significant effects of drill noise on the inner ear function on the contralateral ear as detected by TEOAE; however, the effects are not detectable on PTA. Larger studies with a longer follow-up period will be required to determine if the early postoperative effects on TEOAE are reversible or persist.

Anthropogenic noise compromises the anti-predator behaviour of the European seabass, Dicentrarchus labrax (L.)

Anthropogenic noise is a significant pollutant of the world's oceans, affecting behavioural and physiological traits in a range of species, including anti-predator behaviours. Using the open field test, we investigated the effects of recordings of piling and drilling noise on the anti-predator behaviour of captive juvenile European seabass in response to a visual stimulus (a predatory mimic). The impulsive nature of piling noise triggered a reflexive startle response, which contrasted the behaviour elicited by the continuous drilling noise. When presented with the predatory mimic, fish exposed to both piling and drilling noise explored the experimental arena more extensively than control fish exposed to ambient noise. Fish under drilling and piling conditions also exhibited reduced predator inspection behaviour. Piling and drilling noise induced stress as measured by ventilation rate. This study provides further evidence that the behaviour and physiology of European seabass is significantly affected by exposure to elevated noise levels.

LWD-DATC: Logging-While-Drilling Dipole Shear Anisotropy estimation from Two-Component waveform rotation

Sonic dipole shear anisotropy orientation in subsurface formations is key information for a complete characterization of rock mechanical models for well planning. In Wireline logging, it is estimated with the well-known Alford rotation on four-component inline and crossline waveforms from two orthogonal dipole firings. In contrast, shear orientation estimation is more challenging in Logging-While-Drilling (LWD) operations because of the following reasons:—No exactly orthogonal cross-dipole firings in LWD tools due to tool rotation while drilling;—Coupling between the formation flexural mode and the strong collar flexural mode propagating through the stiff drill collar;—Unavoidable tool eccentering;—Strong drilling noise from vibration, shock, and the turbulent mud flow around the tool; To overcome the aforementioned challenges, this paper describes a new technique to estimate the LWD dipole shear orientation with two-component waveforms obtained from a single dipole firing. It is accomplished by maximizing the projected energy of the two-component waveforms into a subspace defined by two eigenfunctions (the Bessel functions of the first- and second-kinds) accounting for the propagation of the two coupled flexural modes over multiple frequency points. By resorting to the subspace estimation, the new technique has been successfully validated on synthetic data and tested on field data sets.

Evaluation of Sensorineural Hearing Loss Caused by Drill Burr (Types, Size and Speed of Drilling) In Ipsilateral and Contra Lateral Ear after Middle Ear Surgery

Background and Objective- It has been observed that drilling during ear surgery can lead to noise induced sensorineural hearing loss in diseased and non-operated ear both. Hearing loss is caused by drill noise conducted to the non-operated ear by vibrations of the intact skull. This noise exposure results in dysfunction of the outer hair cells, which may produce hearing loss. Aim of our study is to evaluate the sensorineural hearing loss caused by various types of drill bit, various size of drill bit, speed of in diseased and non-operated ear during ear surgery. Method- Study design was prospective. Hearing assessment of all patients was done by tuning fork test and pure tone audiometry and patients with only mild, moderate and moderately severe conductive hearing loss were selected for study. Patients underwent middle ear surgeries like cortical mastoidectomy, radical mastoidectomy, modified radical mastoidectomy combined approached tympanoplasty and plain tympanoplasty after taking detailed informed consent. All patients underwent post-operative hearing assessment on 10 th post-operative day and after 2 months of operation. Result and Conclusions- Drilling at high speed (rpm) cause more sensorineural hearing loss thus according to our study best speed of drilling is 15000 rpm-25000 rpm. Larger sized burr produces more sensorineural hearing loss.always avoid the use of burrs of more than 7 mm size. We recommend use of <3 mm burr. Cutting burr produces more sensorineural hearing loss than diamond burr.

Simulated drilling noise affects the space use of a large terrestrial mammal

Wildlife is exposed to increasing anthropogenic disturbances related to shale oil and gas extraction in response to rising worldwide demands. As these disturbances increase in intensity and occurrence across the landscape, understanding their impacts is essential for management. On Anticosti Island (Québec, Canada), we equipped six white‐tailed deer Odocoileus virginianus with GPS collars taking hourly locations. We then designed a playback experiment by simulating constant drilling noise emitted by generators to which half of the collared deer were exposed for a three‐week period. Deer tolerated noise levels up to 70 dB(C). However, the number of locations recorded in areas where the noise was above 70 dB(C) was on average 73% (SE ± 18%) lower than before the disturbance, which suggests that deer experienced fine scale functional habitat loss. This loss of habitat occurred up to 200 m from the noise source. The size of home ranges and movement rates did not appear to be affected by the noise disturbance. In addition, during the experiment, deer were able to relocate in areas of their home range where food availability was similar to that of sites used before the disturbance. These results show that drilling noise can affect the habitat use of white‐tailed deer. However, future research is needed to better understand the cumulative impacts of shale mining on large mammals, as this study isolated only one of the many disturbances present near mining sites and for a limited period.