Research on the mechanism of mildew contamination affecting the sound quality of analog tape archives

In 1898, people began to attempt to record sound using magnetic materials. In 1935, AEG in Germany introduced the first practical magnetic tape recorders. This innovation significantly enhanced the recording and playback quality of magnetic tape, is regarded as a significant milestone in the development of magnetic tape recording technology¹. Analog audio tapes convert sound signals into continuously varying analog electrical signals and subsequently employ magnetic heads to record these electrical signals as variations in magnetism on the coating of the magnetic tape. Since 1947, it has been widely used in music dissemination, cultural popularization, scientific research, news gathering, etc., and has played an indispensable role in all aspects of people’s lives and work². The primary components of the tape are the magnetic layer and the substrate, and a back coating was introduced in the 1960s to enhance wear resistance³. The magnetic layer is coated with a polyester-polyurethane adhesive with uniform distribution of magnetic particles. The substrate is the support of the magnetic layer, which provides mechanical properties and is responsible for the physical integrity of the tape, being mostly polyester film (polyethylene terephthalate)⁴.

The polyester polyurethane binders in the tapes, together with other organic substances such as fatty acid esters, paraffin oils, siloxanes, and fluorinated lubricants, provide nutrients for mildew^5,6,7. Aspergillus and Penicillium are among the most common varieties in audiovisual materials^8,9. Other species of mildew including Alternaria, Chaetomium, Stemphylium, Cladosporium, and Streptomyces, are also frequently found on audiovisual materials. Most of the mildew is distributed on the edge of the substrate and on the surface of the magnetic layer (Fig. 1). Mildew growth can significantly affect the deterioration of magnetic media, thereby impairing the retrieval of information¹⁰. Additionally, mildew has the potential to contaminate other archival materials by releasing spores, which may also pose a health risk to archivists¹¹.

Current research on magnetic tapes has centered on the assessment of their chemical and physical properties. For instance, the morphology of the tapes, layer thickness, and physical damage were analyzed using Scanning Electron Microscopy (SEM) and Environmental Scanning Electron Microscopy (ESEM). This approach enabled a preliminary analysis of tapes with incomplete documentation^12,13,14.

Tape degradation is generally attributed to multiple mechanisms, including binder hydrolysis, oxidation of magnetic particles, and mechanical wear¹⁵. Acetone can used to extract soluble compounds formed by binder hydrolysis, thereby studying the kinetics of polyester polyurethane binder breakdown^16,17. Headspace Solid-Phase Microextraction-Gas Chromatography-Mass Spectrometry (SPME-GC-MS) serves for profiling the Volatile Organic Compounds (VOCs) emitted by tapes following decades of both natural and artificial aging¹⁸. An extensive library of ATR spectral data was established to identify characteristic bands for tape substrates and binders^19,20,21,22. Hobai combined with Principal Component Analysis (PCA) provides a rapid method for predicting the playability of magnetic tapes with no obvious signs of aging. TGA degradation temperatures can assist in identifying the materials of unknown tapes. Davis used DSC to observe an irreversible endothermic transition at ~50 °C, which is typically absent in playable tapes. Despite these advanced diagnostic techniques, they primarily focus on material characterization and provide limited information on the direct impact of degradation on audio quality. A critical gap remains: the impact of various degradation types on the actual audio quality is severely under-researched. Specially, sound recording archives research on the effects of mildew on audio quality is still very limited. There is an urgent need to establish a parameter system for objectively evaluating sound quality changes. Such a system is crucial for assessing the effectiveness of tape restoration methods and, ultimately, for improving the recovery of our audio heritage. Building upon existing research, this study employs an integrated approach combining chemical characterization and acoustic measurement techniques to systematically investigate mildew erosion in audio materials. Through controlled simulation of mildew-contaminated media samples, the research establishes a scientific framework for objectively assessing mildew covers on sound quality preservation.

Material

An analog magnetic audiotape, distributed by Tianjin Audiovisual Co., Ltd. in 1991, was used. Potato Dextrose Agar (PDA) medium from Beijing Aobo Star Biotechnology Co., LTD. Lactic acid phenol cotton blue staining agent from Beijing Solaibao Technology Co., LTD. Penicillium and aspergillus versicolor were isolated and purified from the mildew on the surface of the audio tape.

Preparation for mildew simulation samples

Penicillium and Aspergillus were cultured on PDA medium for a duration of 5 days. The mildew colony, along with a small amount of agar, was carefully scraped into a sample tube using an inoculation loop. Fungal colonies were scraped, suspended in normal saline, and the conidial suspension was obtained by vigorous shaking and simple filtration through gauze. The spore concentration of suspension is 2.5 × 10⁹ spores/mL. Estimate about 0.16 mL suspension liquid was on average applied on top of the tape per model sample in total (sum of 5 days). The inclusion of a small amount of agar serves to provide essential nutrients, promote mildew growth on the tape, and reduce the preparation time required for creating mock samples intended for mildew attack testing.

First, play the audio tape and select playback time than 5 s and label D0. The audio sample is labeled R-D0. Subsequently, the fungal spore suspension was applied to sample D0. The tape was laid flat with the magnetic layer facing upward, and a controlled volume of the conidial suspension was applied with a cotton swab; after drying, samples with progressively increasing mildew coverage were labeled D1–D5 and their audio recordings R–D1-R–D5are illustrated in Fig. 2

Surface topography characterization of mildew simulation samples

Epson scanner (Epson Perfection V850 Pro Seiko Epson Co., Ltd.) An Epson Perfection V850 Pro scanner was used to perform 8 bit scans of D0–D5 surface morphology against a black cardboard background, and grayscale histograms were extracted from the scanned images.

Laser confocal scanning microscopy was used to characterize the mildew covered samples by roughness analysis. The VK-H1XMC multi-file analysis software was used to measure surface roughness parameters of mildew covered samples D0, D1, D2, D3, D4 and D5. Five measurement points were taken on each sample at intervals of one sixth of the sample length, avoiding the tape edges, and the resulting data were analyzed for Str, Spc, Sdr, Sz, and Sa. In the equations, z (x, y) denotes the signed normal distance between the reference surface and the scale-limited surface. Symbol A denotes the evaluated area’s numerical measure.

Arithmetic Mean Height (Sa) is defined as the arithmetic mean of the absolute deviations in vertical distance between the measured surface profile and the reference plane within the sampling area. As a height parameter, a higher Sa value indicates greater amplitude variation and increased surface roughness.

Maximum Height (Sz), represents the sum of the highest peak and deepest valley within the sampling area. This parameter quantifies the extreme vertical deviations of the surface.

Texture Aspect Ratio (Str) is a spatial parameter that characterizes surface isotropy or anisotropy, where λ_min and λ_max denote the dominant texture wavelengths perpendicular and parallel to the lay direction. Values approaching 0 indicate striated patterns, Str ≈ 1 denotes isotropic topography.

Arithmetic Mean Peak Curvature (Spc) calculates the average principal curvature of the surface summits. As a feature shape parameter, lower Spc values indicate blunt or rounded asperities, whereas higher values correspond to sharp surface features that influence contact mechanics.

Developed Interfacial Area Ratio (Sdr) is a hybrid parameter that quantifies the percentage increase in surface area relative to an ideal flat plane:

Where A actual is the true surface area and A _projected is the projected area. An Sdr value > 0 reflects surface porosity and roughness complexity, while Sdr = 0 indicates perfect flatness.

Acoustic characterization of simulation samples

Portable tape recorder (6503, Nanjing Panda Electronics Co., Ltd.) was used to record audio in MP3 format before and after processing.

Praat speech analysis software (version Intel64) developed by the Institute of Phonetic Sciences at the University of Amsterdam’s Faculty of Humanities was used. Praat is a comprehensive speech analysis software package developed and maintained by the Institute of Phonetic Sciences at the University of Amsterdam²³. Praat (Intel64) was used to compute wideband spectrograms, formants, center of gravity, standard deviation, skewness, kurtosis, and band energy difference for the recorded audio samples. It enables a wide range of acoustic-phonetic analyses, including but not limited to fundamental frequency (F0), spectral characteristics, and formant tracking, in addition to voice analysis. Wideband spectrogram is a type of spectrogram optimized for high time resolution at the expense of frequency resolution.

The center of gravity (\({{\rm{f}}}_{{\rm{c}}}\)) is a measure for how high the frequencies in a spectrum are on average, expressed as \({{\rm{f}}}_{{\rm{c}}}\) in Hz.

The standard deviation (\({\rm{SD}}\))is a measure for how much the frequencies in a spectrum can deviate from the center of gravity. Standard deviation represents the standard deviation in the spectrum, denoted by SD, and is used to measure the degree of dispersion of the frequency with respect to.

The skewness (\({{\rm{S}}}_{{\rm{k}}}\)) is a measure for how much the shape of the spectrum below the center of gravity is different from the shape above the mean frequency. Skewness represents the skewness in the spectrum, and is used to measure the direction and degree of skewness of the data distribution (relative to the standard normal distribution).

The kurtosis (\({K}_{u}\)) is a measure for how much the shape of the spectrum around the center of gravity is different from a Gaussian shape. Kurtosis denotes the degree of kurtosis in the spectrum, and is used as a measure of the sharpness of the data distribution (relative to the standard normal distribution), with a high kurtosis usually indicating a sharper distribution curve, often with more extreme values.

The band energy difference (\(\text{BED}\)) represents the energy difference between the low-frequency band (20–3000 Hz) and the high-frequency band (3000–6000 Hz).

Samples structural and morphological characterization

In Fig. 3a, the FTIR peak at 630.8 cm⁻¹ is attributed to Fe-O lattice vibrations, indicating the presence of γ-Fe₂O₃ particles in the magnetic layer. The peak at 1060.4 cm⁻¹ is due to the stretching vibration of the C-O bond, and the peak at 1170.4 cm⁻¹ is caused by the stretching vibration of the C-O bond connected to the acetyl group. The peak at 1725.5 cm⁻¹ is attributed to the stretching vibration of the ester group (C=O), and the peak at 2921.6 cm⁻¹ is the absorption peak caused by the -CH₂ in the polyester, suggesting that the binder is a polyester type. Combined with the manufacturing process of the audio tape, it is speculated that the audio tape selected for this experiment contains a polyester polyurethane binder. It can be seen from Fig. 3b, the peak at 723.6 cm⁻¹ is attributed to the out-of-plane bending vibration of the benzene ring, the peak at 1097.5 cm⁻¹ is due to the symmetrical stretching vibration of the C-O bond in the ethylene glycol segment, the peak at 1244.5 cm⁻¹ is attributed to the asymmetrical stretching vibration of C-O-C, and the peak at 1714.1 cm⁻¹ is due to the stretching vibration of the ester group (C=O), indicating that the audio tape selected for this experiment is made of PET (polyethylene terephthalate). Figure 3c, d shows the SEM cross sections show three layers with average thicknesses of ~3.5 μm, 8.0 μm, and 2.5 μm, corresponding to the magnetic layer, base layer, and back coating, respectively.

Figure 4 shows the 2D surface morphology of localized areas of audio samples after mildew erosion during different stages of fungal erosion. The surface of the magnetic layer without mildew erosion (D0) is relatively smooth and contains fine black magnetic particles. When the mildew begins to erode the magnetic layer (D1), long and thin hyphae emerge, arranged in a grid pattern²⁰. As time progresses and spore concentration increases (D2–D5), patchy aggregated colonies gradually form, leading to an increase in coverage of the magnetic layer. Notably, the mildew colonies on the D5 sample nearly encompass the entire surface of the magnetic layer.

Surface and acoustic characterization of simulated samples

As illustrated in Fig. 5, the grayscale histograms representing the gray levels of the scanned image and the corresponding number of pixels was used to represent the varying coverage degrees of the mildew simulation samples. The pixel distribution of the sample U0 was the most concentrated, mainly around the gray level of 25–50, the corresponding number of pixels was about 1.4 × 10⁶. When the sample is covered with mildew (D1), there is a noticeable decrease in the number of pixels within the gray level range of 25–50. This change is accompanied by a shift in the peak shape towards higher gray values, while an increase in the number of pixels within the gray level range of 50–100 becomes evident. These observations indicate that the overall image brightness increases, resulting in diminished detail in darker areas. As the degree of coverage progresses from slight (D2) to severe (D5), there is a gradual decline in pixel count for the gray level range of 25–50, contrasted by a steady rise in pixel count for the gray level range of 50–100. This phenomenon can be attributed to the higher whiteness associated with mildew compared to that of the black magnetic layer found on audio tape. Consequently, as mildew coverage intensifies, there is an overall increase in sample whiteness²⁰.

As shown in Fig. 6, the trend of average surface roughness change of mildew-covered samples at varying coverage levels can be clearly seen. As the degree of mildew coverage increases from slight (D1) to severe (D5), the Sa value exhibits a monotonically increasing trend, rising from 2.34 µm at D0, indicating progressive surface roughening of the magnetic layer. Similar effects of surface degradation on magnetic performance have been reported in previous studies; however, the present results further suggest a direct relationship between surface roughness and acoustic signal deterioration. Figure 6b illustrates the variation in Sz values for samples with differing degrees of mildew coverage. As the degree of mildew coverage progresses from D0 to D5, Sz demonstrates a clear upward trend. This suggests that increased mildew coverage results in an elevation in both the maximum height and deepest valley on the surface, revealing a strong positive correlation between Sz, Sa. Upon examining Fig. 6c, it is evident that the Str values for all mildew-covered samples (D0 through D5) exceed 0.5²⁴, signifying that these sample surfaces exhibit more isotropic characteristics. Figure 6d, e has similar variation patterns. The changes from D1 to D3 are relatively gentle, and the overall trend is monotonically increasing. These results suggest that the growth of mildew on the magnetic layer surface is uneven, and the masking effect on the local magnetic layer is different²⁵.

Figure 7 illustrates the wideband spectrograms of audio samples subjected to varying degrees of mildew coverage. These spectrograms are commonly employed to depict the energy distribution of sound signals across different frequencies over time. The depth of color typically signifies the intensity of energy, with darker hues indicating higher energy levels and lighter shades representing lower energy levels. From the wideband spectrogram corresponding to R-D0, it is evident that color depth fluctuates across various frequency bands. Specifically, in the low-frequency range of 1000–2000 Hz, the coloration is darker compared to that in the high-frequency range, suggesting a predominant concentration of energy within this lower frequency spectrum. This phenomenon can be attributed to low-frequency sound waves possessing longer wavelengths, which render them more susceptible to diffraction or reflection; thus, they retain their energy during propagation²⁶. When mildew begins to cover the sample (R-D1), all frequency bands exhibit a noticeable lightening in color within a span of 5 s, accompanied by a decrease in total or average sound energy. This observation indicates that mildew coverage diminishes both overall amplitude and volume of sound-transforming it from “full-bodied” (characterized by rich harmonics typical in music) into a “thin” quality-and results in reduced speech clarity. As surface coverage escalates from slight (R-D0) to severe (R-D5), there is an increasingly pronounced lightening effect observed across all frequency bands within 5 seconds. Notably, for sample R-D5, nearly all high-frequency energies ranging from 2000 to 6000 Hz dissipate entirely while some degree of intensity persists within the low-frequency range. This observation is consistent with general acoustic principles, but the present study provides experimental evidence linking mildew coverage directly to this selective high-frequency loss. This suggests that high-frequency waves-with their shorter wavelengths are more readily obstructed or scattered by mildew particles than their low-frequency counterparts, consequently leading to a swifter loss of energy²⁷. High-frequency sounds contribute significantly to attributes such as brightness, clarity, and detail within auditory perception. The absence of energetic presence in this high-frequency domain manifests as continued existence but with timbral alterations shifting from “bright” towards “dull,” further accompanied by diminished clarity and loss of intricate details.

As illustrated in Fig. 8, the intensity curves of audio samples were exhibited with varying mildew-covered levels. In comparison to R-D0, the presence of mildew coverage diminishes the amplitude of the sound wave. The severity of this coverage correlates with a reduction in audio energy, with these changes being particularly pronounced at both peaks and troughs. This phenomenon occurs because as the magnetized audio tape moves toward the playback head, the fluctuating magnetic field generates magnetic flux, resulting in an output electrical signal that continuously varies to record information. When there is close contact between the head and the magnetic tape, magnetic coupling is enhanced. Consequently, more magnetic field lines traverse through the head coil, generating a stronger induced signal. However, increased mildew coverage elevates the distance between the playback head and the magnetic layer, thereby decreasing the number of magnetic field lines passing through the head coil. This results in a weakened induced signal and subsequently reduces sound volume. This effect can be attributed to reduced magnetic coupling efficiency caused by increased head-tape spacing, which limits effective signal transfer during playback and is consistent with signal attenuation mechanisms reported in degraded magnetic media. Furthermore, mildew coverage can alter both peak height and shape within intensity curves. For instance, as mildew coverage intensifies, a curve peak occurring around 1 s becomes higher and sharper. It was indicated that sound transitions from low to high levels during this interval which may lead to noise formation. Moreover, the concentrated frequency point (peak) of energy at 2 s shifts to the low-frequency direction, which may cause the attack time to be advanced.

Using Praat software to perform a Fourier transform allows for the decomposition of complex sound waves into simpler components, which are then distributed across various frequency bands to create a formant chart²³. The horizontal axis represents time, while the vertical axis indicates the frequency of the sound wave. Formants correspond to specific regions in the sound spectrum where energy is relatively concentrated, and they appear as thick black “horizontal bars” in the spectrogram²⁸. Figure 9 illustrates the formant charts of audio samples exhibiting varying degrees of mildew coverage. The audio sample R-D0 displays relatively concentrated distributions of sound wave frequencies at 900–1100 Hz, 2900–3100 Hz, 3900–4100 Hz, and 5000–5200 Hz. As mildew coverage increases (D1–D5), the high-frequency changes are obvious, but the shape and distribution range of the formants do not change much overall. On the one hand, mildew presence obstructs direct contact between the magnetic head and magnetic layer, leading to attenuation of remanent magnetic signals read by the magnetic head. Given that high-frequency signals possess shorter wavelengths, these changes become more pronounced at higher frequencies. On the other hand, the presence of hypha reduces the smoothness of the surface of the audio tape, and the local magnetic field distribution changes during playback, causing changes in the frequency distribution. However, since the magnetic domain disorder is random, the impact on signals of different frequencies is relatively consistent, so the shape and distribution range of the formants do not change much overall.

Figure 10a illustrates the variations in 5 s spectrogram parameters (center of gravity, standard deviation, skewness, kurtosis and band energy difference) of mildew-covered audio samples under varying coverage levels. As the coverage changes from slight (D0) to severe (D5), the center of gravity and standard deviation of the 5 s audio spectrogram decrease monotonically, while skewness and kurtosis increase monotonically, indicating a progressive redistribution of spectral energy. This is because the overlay process attenuates the remanence signal read by the magnetic head, some of the sound information lost. The loss of part of the frequency information during reading directly leads to a decreasing trend in the standard deviation of the spectrogram, and the curve becomes sharper and sharper, which is manifested as higher and higher kurtosis. Due to the short wavelength of the high frequency, the coverage treatment has a greater impact on the high frequency region, so the average frequency shows a decreasing trend with the increase of mildew coverage²⁹. This trend is consistent with the higher sensitivity of high-frequency signals to surface irregularities. In addition, due to the higher energy in the low-frequency region, more information in the low-frequency region is lost due to the coverage processing, so the peak area in the spectrogram is positively biased and the skewness increases. As evidence from the Fig. 10b, with the increase of coverage, the energy difference between the low frequency region (20–3000 Hz) and the high frequency region (3000–6000 Hz) becomes larger, indicating that the treatment of surface anisotropy will increase the energy difference to a certain extent, which is consistent with frequency-dependent attenuation observed in degraded magnetic media.

Effect of mold removal treatment on surface morphology

As shown in Fig. 11, surface morphology of the mildew corrosion sample (Fig. 11a, a1) and sample after removing mildew (Fig. 11b, b1) reveal notable. In Fig. 10a, a1, fungal hyphae proliferate across the tape surface, forming an interconnected network. The colonies form pitting on the surface of the magnetic layer. It is worth noting that after removing the mildew, we observe not only localized pitting but also extensive irregular line-like corrosion in severely affected areas on the magnetic layer’s surface (Fig. 11b). As can be seen from the 3D surface morphology of sample after removing mildew (Fig. 11b1), these pitting or linear corrosion patterns exhibit a concave structure. Based on the SEM cross-section image of mildew sample (Fig. 11a2), it can be concluded that during its reproductive process, mildew mycelium penetrates into the magnetic layer, resulting in both pitting and linear concave structures due to corrosion. This suggests that as mildew metabolizes, it produces organic acids which corrode the recording medium, thereby compromising the durability of the magnetic carrier. Mildew degradation affects components of this medium leading to structural damage. Additionally, it causes surface pitting and movement of magnetic particles which ultimately results in distortion and attenuation of magnetic recording signals.

In this study, a combination of chemical characterization and acoustic measurement to systematically evaluate the impact of mildew contamination on audio materials was explored. The common strain Penicillium and Aspergillus was used to prepare mildew corrosive samples with different coverage degrees, and the corresponding audio samples were analyzed by Praat. Refine the data on surface structure and changes in acoustic information, it was found that the coverage of mycelium on the magnetic layer surface was uneven and would cause the image to brighten. On the one hand, the energy was mainly concentrated in the low-frequency range (1000–2000). The coverage of mildew would cause the total energy or average energy of the sound would decrease. This indicates that the covering effect of mildew would reduce the overall amplitude or volume of the sound and decrease the clarity of speech. Notably, the energy loss in the high-frequency range was faster than that in the low-frequency range, resulting in detail loss and reduced intelligibility. On the other hand, it would cause changes in the peak values of the sound intensity curve or shifts in the peak shape, leading to noise or alterations in the attack time.

From the parameter change graph of the 5-second spectrum of the sample, it can be concluded that the average frequency and standard deviation monotonically decreased, while the skewness and kurtosis monotonically increased. The above information indicates that the covering effect of mildew increases the distance between the magnetic head and the magnetic layer, resulting in weakened induction signals and the loss of some sound information. Combining the surface and cross-sectional morphology and roughness information, mildew degradation affects the components of this medium, leading to surface pitting and movement of magnetic particles, which ultimately results in distortion and attenuation of magnetic recording signals. Based on the information presented above, the dual effects of mildew coverage and corrosion cause a decrease in sound clarity and distortion. This work highlights the necessity of mildew removal and provides valuable theoretical references for the evaluation of sound information after the removal of contaminants from magnetic materials in the future.

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Authors and Affiliations

1. Engineering Research Center of Historical Cultural Heritage Conservation, Ministry of Education, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, China

   Zhihui Jia, Yanan Wang, Shujiao Yu, Quanfeng Cai, Bodian Liu, Huiping Xing & Yuhu Li

2. State Key Laboratory of Loess Science, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China

   Long Chen

3. Hubei Provincial Museum, Wuhan, China

   Yanhong Zhao

Contributions

J.Z.H. and C.L. provided research design, research guidance, data analysis and writing Original Draft; X.H.P. and L.Y.H. provided research guidance; W.Y.N. and Z.Y.H. participated in data analysis; Y.S.J. was responsible for research design and data collection; C.Q.F. and L.B.D involved in data collection.

Corresponding authors

Correspondence to Zhihui Jia or Long Chen.

Competing interests

The authors declare no competing interests.

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