WO2012140435A1

WO2012140435A1 - Microphone assembly

Info

Publication number: WO2012140435A1
Application number: PCT/GB2012/050814
Authority: WO
Inventors: Edward Fletcher
Original assignee: Orbitsound Limited
Priority date: 2011-04-14
Filing date: 2012-04-13
Publication date: 2012-10-18
Also published as: GB2494849A; GB201106320D0

Abstract

A microphone assembly having an array of sound transducers electrically connected in parallel with the same polarity and arranged to detect sound in substantially the same direction. The sound transducers may be substantially aligned in a plane substantially normal to the direction of sound detection.

Description

MICROPHONE ASSEMBLY

Technical Field of the Invention

This invention relates to a microphone assembly for recording stereo and/or spatial sound.

Background to , the Invention

While a single small diaphragm microphone will produce an effective and good quality output, a large diapliragm microphone (approx. 25mm diameter or more) generally records a sound signal that is of superior subjective quality when output through a loud speaker.

This is due to upper mid frequency distortions that occur when recording sound with a large diaphragm microphone because the single large diaphragm responds unevenly to higher frequencies, owing to 'rippling' effects across the diaphragm. When the recorded signal is output through a loud speaker the human ear detects these distortions as 'pleasant'. This effect does not occur when a sound signal recorded by a small diaphragm microphone is output through a loud speaker.

However, although recording a sound signal with a large diameter microphone produces a subjectively better quality recording, large diameter microphones are generally expensive.

An object of embodiments of the present invention is to provide a microphone assembly that retains the quality of sound recorded by a large diameter microphone, but that is substantially less expensive. Stereo sound recording and reproduction employs acoustic projection to encode trie relative position of sound sources recorded, and aims to reproduce the sound with a sense of those relative positions. A stereo system can involve two or more channels, but two channels systems dominate for audio recording. The two channels (usually known as left and right) convey information relating to sound fields located to the left and right of a listener.

Many methods have been used to attempt accurately to record position and depth (spatial information). These include panning a mono microphone between left and right, using various types of microphone to the left and right, using a co-incident pair of microphones (Blumlein pair), and even using microphones that form part of a 'dummy head' with microphone elements in artificial ear canals in a model of a human head. While a recording made with a 'dummy head' microphone can be reproduced moderately well through headphones, reproduction through loudspeakers is less successful.

A further object of embodiments of the present invention is to provide a single microphone assembly that is capable of recording spatial information in two-channel format, so that the intensity of recorded depth or spatial information may produce a superior listening^' experience, when compared with a recording made using a conventional two-channel microphone assembly, in that both the positioning of recorded sources and the distances within the recorded sound field may be clearly reproduced both on headphones and loudspeakers.

Summary of the Invention According to the present invention there is provided a microphone assembly comprising an array of sound transducers electrically connected in parallel with the same polarity and arranged to detect sound in substantially the same direction.

Such a microphone assembly is advantageous in that, as the array of sound transducers are electrically connected in parallel with the same polarity and arranged to detect sound in substantially the same direction, the array of sound transducers emulates a single capacitor microphone diaphragm of a similar diameter to the distance across the array.

The parallel connection sums the electrical output and minimises noise because coherent signals sum arithmetically while incoherent signals (noise) sum logarithmically,

This allows multiple small sound transducers, such as electret microphone capsules, to be used in place of a single large diameter microphone (approx. 25mm diameter or more). As such small transducers are inexpensive, this provides a large saving in cost. Preferably the sound transducers are substantially aligned in a plane substantially normal to the direction of sound detection.

Preferably the sound transducers are mounted on a surface substantially normal to the direction of sound direction.

Preferably the array comprises a cluster of adjacent sound transducers. As the individual diaphragms of the small transducers are of low mass (relative to the diaphragms of large transducers), are mounted on a single plane and clustered together, they will respond precisely in unison to an audio wave front in the audible band. Accordingly, this further increases the extent to which the array of sound transducers emulates a single capacitor microphone diaphragm of a similar diameter to the distance across the array.

Preferably the sound transducers each have a diameter that is less than 25mm. Preferably the sound transducers each have a diameter in the range 2mm to 5 mm.

Preferably the diameter of the array of sound transducers is greater than or equal to 20mm. Preferably the diameter of the array of sound transducers is in the range 20mm to 35 mm.

Preferably one or more of the sound transducers is an electret microphone capsule. Preferably all of the sound transducers are electret microphone capsules. This is advantageous in that they are inexpensive, consistent and physically small. Preferably the microphone assembly comprises a first said array of sound transducers arranged to detect sound in a first direction and a second said array of sound transducers arranged to detect sound in a second direction substantially perpendicular to the first direction.

Preferably the microphone assembly comprises a third said array of sound transducers arranged to detect sound in a third direction substantially opposite to the second direction. Preferably the sound transducers in the second array are connected in parallel with the opposite polarity to the sound transducers in the third array.

Preferably at least half of the transducers of the microphone assembly are provided in the first array. Preferably the sound transducers are all contained within a single housing unit.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a front elevation of a microphone assembly according to an embodiment of the present invention;

Figure 2 is a side elevation of the microphone array of Figure 1;

Figure 3 is a plan view of the microphone array of Figure 1 ;

Figure 4 is a circuit diagram showing a circuit connecting transducers in a front transducer array, according to an embodiment of the present invention; and

Figure 5 is a circuit diagram showing a circuit connecting transducers in a left and a right transducer array, according to an embodiment of the present invention.

The microphone is in principle a 'sum-and-difference' type, where one ('mid') element responds to all sounds, and another ('side') element, or pair of opposed elements, works as a dual element responding to sounds to the left and sounds to the right at reverse polarity. The microphone produces left and right signals by combining the outputs of the two elements via an electronic 'sum-and-difference' matrix. This may be achieved using software or hardware, and this signal processing nlay occur within the microphone unit or externally. The mid element can be thought of as 'left plus right' (L + R), as it is receptive to all sounds, or to sounds arriving from a wide central area that spans the left, right, and centre directions. The side element can be considered as 'left minus right' (L - R), as it is receptive to sounds arriving from the left and right directions, one of which is transduced in reverse polarity (i.e. phase reversed) relative to the other. The matrix performs a sum-and-difference function where the sum of the mid and side signals (L + R) + (L - R) resolves to 2 L, and their difference (L + R) - (L - R) resolves to 2 R, thus producing a left signal and a right signal.

As shown in Figures 1 to 3, the microphone consists of a front-facing array (1) of five electret microphone capsules (4) arranged close together to be sensitive to sounds from the front of the microphone, and two further side-facing arrays (2, 3), each of two electret microphone capsules arranged at or close to 90 degrees relative to the front array (1), arranged to be sensitive to sounds from the left and right sides of the microphone. The electret microphone capsules (4) are nominally omni-directional. In a preferred embodiment, multiple identical electret microphone capsules are used in all of the arrays, Alternatively, other types of sound transducer or directional microphone may be used. For example, each array may comprise one or more cardioid or bidirectional microphones. A combination of different electret capsule sizes may be used. For example, multiple capsules of one size in the front- facing array and multiple capsules of another, smaller size in the side-facing arrays.

The electret capsules of each array are arranged in a planar configuration by attaching them to a respective surface of a square-U-shaped plate (5). Other suitable attachment surfaces may be used, where sound transducers in the side-facing arrays are attached to side-facing surfaces that are substantially opposite-facing and substantially perpendicular to a front-facing surface to which the front-facing transducers are attached. For example, sound transducers may be attached to faces of a T-shaped or L-shaped plate, or a substantially cuboidal box or block. Whilst in the illustrated embodiment, the left and right arrays (2, 3) each have two electret capsules (4) and the front array (1) has five, as shown in Figures 1 to 3, any number of electret capsules may be used in each array, although the left and right arrays are preferably identical or symmetrical to one another.

The electret capsules in each array are wired in parallel. The front array electret capsules are all connected in parallel with the same polarity, as in the circuit shown in Figure 4. The electret capsules in each of the left and right arrays are connected in parallel within the same circuit, but electret capsules in the left array (2) are wired with the opposite polarity to those in the right array (3), as illustrated in Figure 5. The electret capsules in the left array are connected in parallel between the positive (V₊) and neutral (V₀) terminals of the phantom-powered microphone input/output, while those in the right array are connected in parallel between the negative (V_) and neutral (V₀) terminals. Other suitable circuits that achieve a similar effect may be used, as would be apparent to the skilled person. An effect of the clustering of multiple parallel-connected electret capsules per array is that each array emulates a single microphone diaphragm of a similar diameter to the distance across the array of electret capsules. As a result, the circuit connected to the front array produces a 'mid' (M) audio signal and the circuit connecting the left and right arrays produces a ' side ' (S) audio signal approximating that detected by mid-side (M-S) microphone pairs.

In certain embodiments, the left and right arrays are separated by a selected distance. This results in a small time delay for sounds from the left side to reach the right array and vice versa, which may improve the spatial depth of the recorded sound. In one embodiment, the distance separating the left and right arrays is 100-200 mm, to approximate the breadth of a human head. In another, a separation of 50-100 mm is used, to approximate the distance between the human inner ears. In an embodiment suitable as a studio microphone, a separation of 20-50 mm provides a convenient balance of compact size and spatial depth. A further compact embodiment more suitable for discreet or portable use has a left-right separation of 10-20 mm.

In addition to the spacing of the left and right arrays, the distance across each array may be sufficient for phase differences between electret capsule signals to cause interference and cancellations in the signal. The size of each array in the illustrated embodiment is such that the interference and cancellations primarily affect mid to high audio frequencies. So, the sensitivity of the front array to mid or high frequencies varies depending on the direction from which the sound originates. Similar effects may be seen on the 'side' signal derived from the left and right arrays. These effects may also cause further interference and cancellations when the 'mid' and 'side' signals are combined to form left and right stereo signals. Any one of these effects, or any combination of these effects, may further enhance the ability of the microphone to capture source direction and spatial distance during recording, and/or may provide an enhanced impression of depth and space to the listener upon playback.

As previously explained, the mid and side signals from the arrays are used to derive discrete left and right stereo electronic signals via a sum-and- difference matrix function. In a preferred embodiment, this is a function of circuitry included within the microphone unit. In other embodiments, this is the function of an external signal processing unit provided for connection to the microphone, either by means of software or hardware. In further embodiments, this function is carried out remotely using a computer or mixing desk to which the microphone is connected via the mid and side audio output channels. In some embodiments, cabling connected to the microphone conveys the derived 'left' and 'right' stereo channel signals, while in others 'mid' and 'side' channel signals may be directly conveyed; alternatively, a switch may be provided to allow the user to select either of these two signal pairs. As each array (1 , 2, 3) of sound transducers is electrically connected in parallel, with the sound transducers within each array (1 , 2, 3) connected with the same polarity, and arranged to detect sound in, substantially the same direction, each array (1, 2, 3) emulates a single capacitor microphone diaphragm of a similar diameter to the distance across the array. The parallel connection sums the electrical output and minimises noise because coherent signals sum arithmetically while incoherent signals (noise) sum logarithmically,

This allows multiple small sound transducers, such as electret microphone capsules (4), to be used in place of a single large diameter microphone (approx. 25mm diameter or more). As such small transducers are inexpensive, this provides a large saving in cost.

Accordingly, the present invention provides a microphone assembly that retains the quality of sound recorded by a large diameter microphone, but that is substantially less expensive.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A microphone assembly comprising an array of sound transducers electrically connected in parallel with the same polarity and arranged to detect sound in substantially the same direction.

2. A microphone assembly according to claim 1 wherein the sound transducers are substantially aligned in a plane substantially normal to the direction of sound detection.

3. A microphone assembly according to either of claims 1 or 2 wherein the sound transducers are mounted on a surface substantially normal to the direction of sound detection.

4. A microphone assembly according to any preceding claim wherein the array comprises a cluster of adjacent sound transducers.

5. A microphone assembly according to any preceding claim wherein the diameter of the array of sound transducers is greater than or equal to 20mm.

6. A microphone assembly according to claim 5 wherein the diameter of the an'ay of sound transducers is in the range 20mm to 35mm.

7. A microphone assembly according to any preceding claim wherein the sound transducers each have a diameter that is less than 25 mm.

8. A microphone assembly according to claim 7 wherein the sound transducers each have a diameter in the range 2mm to 5mm.

9. A microphone assembly according to any preceding claim wherein one or more of the sound transducers is an electret microphone capsule.

10. A microphone assembly according to any preceding claim comprising a first said array of sound transducers arranged to detect sound in a first direction and a second said array of sound transducers arranged to detect sound in a second direction substantially perpendicular to the first direction.

11. A microphone assembly according to claim 10 comprising a third said array of sound transducers arranged to detect sound in a third direction substantially opposite to the second direction.

12. A microphone assembly according to claim 1 1 wherein the sound transducers in the second array are connected in parallel with the opposite polarity to the sound transducers in the third array.

13. A microphone assembly according to any of claims 10 to 12 wherein at least half of the transducers of the microphone assembly are provided in the first array.

14. A microphone assembly according to any preceding claim wherein the sound transducers are all contained within a single housing unit.

15. A microphone assembly substantially as described herein with reference to the accompanying drawings.