Vibration Classroom · Issue 8

Vibration Frequency vs. Sensor Frequency Response

Many people confuse vibration frequency with frequency response and end up buying sensors haphazardly based on specifications: they get no data when measuring low-speed equipment, and the data fluctuates wildly when measuring high-speed equipment. The root cause is the confusion between these two concepts. Today, we’ll use everyday examples to break down these two key terms in simple terms.

I. What Is Vibration Frequency? (The Object Being Measured Itself)

Vibration Frequency: The number of times the equipment vibrates per second. During operation, it generates multiple vibration components at different frequencies, the most fundamental of which is typically the rotational frequency.

For example:

Standard motor at 3,000 rpm: 50 revolutions per second, fundamental frequency of 50 Hz;

Large rotary kiln at 20 rpm: 1/3 revolution per second, fundamental frequency less than 1 Hz;

High-speed gear meshing: tens of thousands of impacts per second, with frequencies ranging from several thousand to tens of thousands of Hz.

Quick Note: Frequency reflects how quickly vibrations change and is primarily determined by the equipment’s structure and operating conditions.

Failures can generate additional high-frequency signals: bearing wear, gear tooth breakage, and other issues can all produce high-frequency fault signals.

II. What Is a Sensor’s Frequency Response? (A Sensor’s Receiving Capability)

Frequency response = The range of vibration frequencies that a sensor can accurately detect; it is equivalent to the sensor’s “listening range.”

Think of a radio: It can only pick up stations within a specific frequency range—frequencies that are too low won’t come in, and those that are too high result in static and distortion. The same principle applies to sensors: measurement data is only accurate and reliable within their specified frequency response range.

Below the lower frequency limit: The signal gradually weakens, and the data values are low (low-frequency drop-off)

Above the upper frequency limit: The signal becomes distorted and spikes sharply, resulting in false peaks (resonance distortion)

Frequency response is divided into two bands.

Low-frequency and high-frequency performance are two key metrics. For example, modal testing, shock testing, product reliability testing, aircraft testing, automotive testing, and so on—many of these applications cannot avoid low- and high-frequency requirements.

①Low-frequency characteristics: Captures slow vibrations (0.1 Hz to several tens of Hz)

1. The Principle in Plain Language

Piezoelectric crystals generate electric charges when subjected to pressure, but these charges naturally leak away. The slower the vibration and the longer the interval between pressure applications, the greater the leakage, causing the signal to gradually decay—which is known as low-frequency roll-off.

IEPE sensors with built-in circuitry are limited by their internal capacitance; their typical lower frequency limit is approximately 0.5 Hz to 5 Hz, depending on the design of the built-in circuitry, and signals below this frequency are gradually lost;

Sensors with charge-signal output, when paired with a constant-charge amplifier, can achieve measurement capabilities down to 0.1 Hz or even lower, thereby extending the lower frequency limit.

②High-frequency characteristics: Captures high-speed transients and high-frequency oscillations (thousands of Hz to 20 kHz)

1. The Principle in Plain Language

Inside the sensor = mass block + spring + piezoelectric element, which has its own natural resonant frequency. When the frequency of the vibration being measured is close to this resonant frequency, sensitivity spikes abnormally, and readings become artificially high.

Industry Standard: In engineering applications, the measurement frequency is typically required to be significantly lower than the sensor’s resonance frequency; a common rule of thumb is to keep it within 1/3 to 1/5 of the resonance frequency.

The smaller the sensor and the lighter the mass, the higher the resonance frequency and the better the high-frequency performance; the higher the sensitivity and the larger the mass, the lower the upper limit of the high-frequency range.

III. The Logic Behind Matching Frequency and Frequency Response (Key to Selection)

The full frequency range of equipment vibration must fall within the sensor's frequency response range.

For example:

Low-speed ball mill: Minimum equipment frequency is 0.3 Hz → A 2 Hz start-up sensor cannot be used; a 0.1 Hz low-frequency sensor must be used instead.

High-speed gearbox: Maximum fault frequency is 8 kHz → The sensor’s high-frequency response must be >12 kHz to allow for a safety margin.

Rule of thumb: The device’s lowest frequency > the sensor’s lower limit, and the device’s highest fault frequency < the sensor’s upper limit.

IV. The Two Most Common Mistakes on Site

Insufficient low-frequency response: In large reduction gearboxes and rotary kilns, the lower limit of the sensor is set too high, causing low-frequency wear signals to be lost entirely, making it impossible to provide early warning of failures;

Excessive high-frequency response: When narrow-band sensors are used on high-speed gears, they exceed the upper limit of the frequency response, triggering unexplained vibration alarms, even though the equipment is found to be in good condition upon disassembly.

Additional Note: The installation method affects the actual frequency response. Rigid bolt mounting = nominal frequency response; magnetic mounting with rubber padding results in approximately a 30% attenuation of high frequencies. In high-frequency impact conditions, install a mechanical filter to remove excess noise.

V. Xiyuan’s Targeted Product Matching

Vibration frequency is the sound emitted by equipment, while frequency response is the sensor’s “ear.” Only when the sensor’s frequency range covers the entire sound spectrum can measurements be accurate. Low frequencies determine whether slow-moving equipment can be monitored, while high frequencies determine whether high-speed impacts can be detected. Insufficient low-frequency coverage means low-speed faults go undetected; insufficient high-frequency coverage results in false alarms from high-speed data. Only by matching parameters to the equipment’s vibration speed can potential equipment hazards be accurately monitored.

When selecting a model, first verify the equipment’s full frequency range; Xiyuan Electronics will match sensor parameters on a one-to-one basis according to operating conditions.

Yangzhou Xiyuan Electronic Technology Co.,Ltd.

#77 Yeqiao Road, Liandong U Valley slender West Lake Innovation port, building 10-2,Hanjiang District, Yangzhou,Jiangsu,China

Mobie Phone: +86 180-5105-8377

Tel: +86 514-82885589

Whatsapp:+86 182-6066-6867

Email: sale1@yzxyt.com

Fax: +86 514-82885089